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diff --git a/external/jsr166/java/util/AbstractQueue.java b/external/jsr166/java/util/AbstractQueue.java
new file mode 100644
index 000000000..644df6c49
--- /dev/null
+++ b/external/jsr166/java/util/AbstractQueue.java
@@ -0,0 +1,166 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util;
+
+/**
+ * This class provides skeletal implementations of some {@link Queue}
+ * operations. The implementations in this class are appropriate when
+ * the base implementation does <em>not</em> allow <tt>null</tt>
+ * elements.  Methods {@link #add add}, {@link #remove remove}, and
+ * {@link #element element} are based on {@link #offer offer}, {@link
+ * #poll poll}, and {@link #peek peek}, respectively but throw
+ * exceptions instead of indicating failure via <tt>false</tt> or
+ * <tt>null</tt> returns.
+ *
+ * <p> A <tt>Queue</tt> implementation that extends this class must
+ * minimally define a method {@link Queue#offer} which does not permit
+ * insertion of <tt>null</tt> elements, along with methods {@link
+ * Queue#peek}, {@link Queue#poll}, {@link Collection#size}, and a
+ * {@link Collection#iterator} supporting {@link
+ * Iterator#remove}. Typically, additional methods will be overridden
+ * as well. If these requirements cannot be met, consider instead
+ * subclassing {@link AbstractCollection}.
+ *
+ * <p>This class is a member of the
+ * <a href="{@docRoot}/../technotes/guides/collections/index.html">
+ * Java Collections Framework</a>.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ * @param <E> the type of elements held in this collection
+ */
+public abstract class AbstractQueue<E>
+    extends AbstractCollection<E>
+    implements Queue<E> {
+
+    /**
+     * Constructor for use by subclasses.
+     */
+    protected AbstractQueue() {
+    }
+
+    /**
+     * Inserts the specified element into this queue if it is possible to do so
+     * immediately without violating capacity restrictions, returning
+     * <tt>true</tt> upon success and throwing an <tt>IllegalStateException</tt>
+     * if no space is currently available.
+     *
+     * <p>This implementation returns <tt>true</tt> if <tt>offer</tt> succeeds,
+     * else throws an <tt>IllegalStateException</tt>.
+     *
+     * @param e the element to add
+     * @return <tt>true</tt> (as specified by {@link Collection#add})
+     * @throws IllegalStateException if the element cannot be added at this
+     *         time due to capacity restrictions
+     * @throws ClassCastException if the class of the specified element
+     *         prevents it from being added to this queue
+     * @throws NullPointerException if the specified element is null and
+     *         this queue does not permit null elements
+     * @throws IllegalArgumentException if some property of this element
+     *         prevents it from being added to this queue
+     */
+    public boolean add(E e) {
+        if (offer(e))
+            return true;
+        else
+            throw new IllegalStateException("Queue full");
+    }
+
+    /**
+     * Retrieves and removes the head of this queue.  This method differs
+     * from {@link #poll poll} only in that it throws an exception if this
+     * queue is empty.
+     *
+     * <p>This implementation returns the result of <tt>poll</tt>
+     * unless the queue is empty.
+     *
+     * @return the head of this queue
+     * @throws NoSuchElementException if this queue is empty
+     */
+    public E remove() {
+        E x = poll();
+        if (x != null)
+            return x;
+        else
+            throw new NoSuchElementException();
+    }
+
+    /**
+     * Retrieves, but does not remove, the head of this queue.  This method
+     * differs from {@link #peek peek} only in that it throws an exception if
+     * this queue is empty.
+     *
+     * <p>This implementation returns the result of <tt>peek</tt>
+     * unless the queue is empty.
+     *
+     * @return the head of this queue
+     * @throws NoSuchElementException if this queue is empty
+     */
+    public E element() {
+        E x = peek();
+        if (x != null)
+            return x;
+        else
+            throw new NoSuchElementException();
+    }
+
+    /**
+     * Removes all of the elements from this queue.
+     * The queue will be empty after this call returns.
+     *
+     * <p>This implementation repeatedly invokes {@link #poll poll} until it
+     * returns <tt>null</tt>.
+     */
+    public void clear() {
+        while (poll() != null)
+            ;
+    }
+
+    /**
+     * Adds all of the elements in the specified collection to this
+     * queue.  Attempts to addAll of a queue to itself result in
+     * <tt>IllegalArgumentException</tt>. Further, the behavior of
+     * this operation is undefined if the specified collection is
+     * modified while the operation is in progress.
+     *
+     * <p>This implementation iterates over the specified collection,
+     * and adds each element returned by the iterator to this
+     * queue, in turn.  A runtime exception encountered while
+     * trying to add an element (including, in particular, a
+     * <tt>null</tt> element) may result in only some of the elements
+     * having been successfully added when the associated exception is
+     * thrown.
+     *
+     * @param c collection containing elements to be added to this queue
+     * @return <tt>true</tt> if this queue changed as a result of the call
+     * @throws ClassCastException if the class of an element of the specified
+     *         collection prevents it from being added to this queue
+     * @throws NullPointerException if the specified collection contains a
+     *         null element and this queue does not permit null elements,
+     *         or if the specified collection is null
+     * @throws IllegalArgumentException if some property of an element of the
+     *         specified collection prevents it from being added to this
+     *         queue, or if the specified collection is this queue
+     * @throws IllegalStateException if not all the elements can be added at
+     *         this time due to insertion restrictions
+     * @see #add(Object)
+     */
+    public boolean addAll(Collection<? extends E> c) {
+        if (c == null)
+            throw new NullPointerException();
+        if (c == this)
+            throw new IllegalArgumentException();
+        boolean modified = false;
+        Iterator<? extends E> e = c.iterator();
+        while (e.hasNext()) {
+            if (add(e.next()))
+                modified = true;
+        }
+        return modified;
+    }
+
+}
diff --git a/external/jsr166/java/util/ArrayDeque.java b/external/jsr166/java/util/ArrayDeque.java
new file mode 100644
index 000000000..c94986937
--- /dev/null
+++ b/external/jsr166/java/util/ArrayDeque.java
@@ -0,0 +1,837 @@
+/*
+ * Written by Josh Bloch of Google Inc. and released to the public domain,
+ * as explained at http://creativecommons.org/licenses/publicdomain.
+ */
+
+package java.util;
+import java.io.*;
+
+/**
+ * Resizable-array implementation of the {@link Deque} interface.  Array
+ * deques have no capacity restrictions; they grow as necessary to support
+ * usage.  They are not thread-safe; in the absence of external
+ * synchronization, they do not support concurrent access by multiple threads.
+ * Null elements are prohibited.  This class is likely to be faster than
+ * {@link Stack} when used as a stack, and faster than {@link LinkedList}
+ * when used as a queue.
+ *
+ * <p>Most <tt>ArrayDeque</tt> operations run in amortized constant time.
+ * Exceptions include {@link #remove(Object) remove}, {@link
+ * #removeFirstOccurrence removeFirstOccurrence}, {@link #removeLastOccurrence
+ * removeLastOccurrence}, {@link #contains contains}, {@link #iterator
+ * iterator.remove()}, and the bulk operations, all of which run in linear
+ * time.
+ *
+ * <p>The iterators returned by this class's <tt>iterator</tt> method are
+ * <i>fail-fast</i>: If the deque is modified at any time after the iterator
+ * is created, in any way except through the iterator's own <tt>remove</tt>
+ * method, the iterator will generally throw a {@link
+ * ConcurrentModificationException}.  Thus, in the face of concurrent
+ * modification, the iterator fails quickly and cleanly, rather than risking
+ * arbitrary, non-deterministic behavior at an undetermined time in the
+ * future.
+ *
+ * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
+ * as it is, generally speaking, impossible to make any hard guarantees in the
+ * presence of unsynchronized concurrent modification.  Fail-fast iterators
+ * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
+ * Therefore, it would be wrong to write a program that depended on this
+ * exception for its correctness: <i>the fail-fast behavior of iterators
+ * should be used only to detect bugs.</i>
+ *
+ * <p>This class and its iterator implement all of the
+ * <em>optional</em> methods of the {@link Collection} and {@link
+ * Iterator} interfaces.
+ *
+ * <p>This class is a member of the
+ * <a href="{@docRoot}/../technotes/guides/collections/index.html">
+ * Java Collections Framework</a>.
+ *
+ * @author  Josh Bloch and Doug Lea
+ * @since   1.6
+ * @param <E> the type of elements held in this collection
+ */
+public class ArrayDeque<E> extends AbstractCollection<E>
+                           implements Deque<E>, Cloneable, Serializable
+{
+    /**
+     * The array in which the elements of the deque are stored.
+     * The capacity of the deque is the length of this array, which is
+     * always a power of two. The array is never allowed to become
+     * full, except transiently within an addX method where it is
+     * resized (see doubleCapacity) immediately upon becoming full,
+     * thus avoiding head and tail wrapping around to equal each
+     * other.  We also guarantee that all array cells not holding
+     * deque elements are always null.
+     */
+    private transient E[] elements;
+
+    /**
+     * The index of the element at the head of the deque (which is the
+     * element that would be removed by remove() or pop()); or an
+     * arbitrary number equal to tail if the deque is empty.
+     */
+    private transient int head;
+
+    /**
+     * The index at which the next element would be added to the tail
+     * of the deque (via addLast(E), add(E), or push(E)).
+     */
+    private transient int tail;
+
+    /**
+     * The minimum capacity that we'll use for a newly created deque.
+     * Must be a power of 2.
+     */
+    private static final int MIN_INITIAL_CAPACITY = 8;
+
+    // ******  Array allocation and resizing utilities ******
+
+    /**
+     * Allocate empty array to hold the given number of elements.
+     *
+     * @param numElements  the number of elements to hold
+     */
+    private void allocateElements(int numElements) {
+        int initialCapacity = MIN_INITIAL_CAPACITY;
+        // Find the best power of two to hold elements.
+        // Tests "<=" because arrays aren't kept full.
+        if (numElements >= initialCapacity) {
+            initialCapacity = numElements;
+            initialCapacity |= (initialCapacity >>>  1);
+            initialCapacity |= (initialCapacity >>>  2);
+            initialCapacity |= (initialCapacity >>>  4);
+            initialCapacity |= (initialCapacity >>>  8);
+            initialCapacity |= (initialCapacity >>> 16);
+            initialCapacity++;
+
+            if (initialCapacity < 0)   // Too many elements, must back off
+                initialCapacity >>>= 1;// Good luck allocating 2 ^ 30 elements
+        }
+        elements = (E[]) new Object[initialCapacity];
+    }
+
+    /**
+     * Double the capacity of this deque.  Call only when full, i.e.,
+     * when head and tail have wrapped around to become equal.
+     */
+    private void doubleCapacity() {
+        assert head == tail;
+        int p = head;
+        int n = elements.length;
+        int r = n - p; // number of elements to the right of p
+        int newCapacity = n << 1;
+        if (newCapacity < 0)
+            throw new IllegalStateException("Sorry, deque too big");
+        Object[] a = new Object[newCapacity];
+        System.arraycopy(elements, p, a, 0, r);
+        System.arraycopy(elements, 0, a, r, p);
+        elements = (E[])a;
+        head = 0;
+        tail = n;
+    }
+
+    /**
+     * Copies the elements from our element array into the specified array,
+     * in order (from first to last element in the deque).  It is assumed
+     * that the array is large enough to hold all elements in the deque.
+     *
+     * @return its argument
+     */
+    private <T> T[] copyElements(T[] a) {
+        if (head < tail) {
+            System.arraycopy(elements, head, a, 0, size());
+        } else if (head > tail) {
+            int headPortionLen = elements.length - head;
+            System.arraycopy(elements, head, a, 0, headPortionLen);
+            System.arraycopy(elements, 0, a, headPortionLen, tail);
+        }
+        return a;
+    }
+
+    /**
+     * Constructs an empty array deque with an initial capacity
+     * sufficient to hold 16 elements.
+     */
+    public ArrayDeque() {
+        elements = (E[]) new Object[16];
+    }
+
+    /**
+     * Constructs an empty array deque with an initial capacity
+     * sufficient to hold the specified number of elements.
+     *
+     * @param numElements  lower bound on initial capacity of the deque
+     */
+    public ArrayDeque(int numElements) {
+        allocateElements(numElements);
+    }
+
+    /**
+     * Constructs a deque containing the elements of the specified
+     * collection, in the order they are returned by the collection's
+     * iterator.  (The first element returned by the collection's
+     * iterator becomes the first element, or <i>front</i> of the
+     * deque.)
+     *
+     * @param c the collection whose elements are to be placed into the deque
+     * @throws NullPointerException if the specified collection is null
+     */
+    public ArrayDeque(Collection<? extends E> c) {
+        allocateElements(c.size());
+        addAll(c);
+    }
+
+    // The main insertion and extraction methods are addFirst,
+    // addLast, pollFirst, pollLast. The other methods are defined in
+    // terms of these.
+
+    /**
+     * Inserts the specified element at the front of this deque.
+     *
+     * @param e the element to add
+     * @throws NullPointerException if the specified element is null
+     */
+    public void addFirst(E e) {
+        if (e == null)
+            throw new NullPointerException();
+        elements[head = (head - 1) & (elements.length - 1)] = e;
+        if (head == tail)
+            doubleCapacity();
+    }
+
+    /**
+     * Inserts the specified element at the end of this deque.
+     *
+     * <p>This method is equivalent to {@link #add}.
+     *
+     * @param e the element to add
+     * @throws NullPointerException if the specified element is null
+     */
+    public void addLast(E e) {
+        if (e == null)
+            throw new NullPointerException();
+        elements[tail] = e;
+        if ( (tail = (tail + 1) & (elements.length - 1)) == head)
+            doubleCapacity();
+    }
+
+    /**
+     * Inserts the specified element at the front of this deque.
+     *
+     * @param e the element to add
+     * @return <tt>true</tt> (as specified by {@link Deque#offerFirst})
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean offerFirst(E e) {
+        addFirst(e);
+        return true;
+    }
+
+    /**
+     * Inserts the specified element at the end of this deque.
+     *
+     * @param e the element to add
+     * @return <tt>true</tt> (as specified by {@link Deque#offerLast})
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean offerLast(E e) {
+        addLast(e);
+        return true;
+    }
+
+    /**
+     * @throws NoSuchElementException {@inheritDoc}
+     */
+    public E removeFirst() {
+        E x = pollFirst();
+        if (x == null)
+            throw new NoSuchElementException();
+        return x;
+    }
+
+    /**
+     * @throws NoSuchElementException {@inheritDoc}
+     */
+    public E removeLast() {
+        E x = pollLast();
+        if (x == null)
+            throw new NoSuchElementException();
+        return x;
+    }
+
+    public E pollFirst() {
+        int h = head;
+        E result = elements[h]; // Element is null if deque empty
+        if (result == null)
+            return null;
+        elements[h] = null;     // Must null out slot
+        head = (h + 1) & (elements.length - 1);
+        return result;
+    }
+
+    public E pollLast() {
+        int t = (tail - 1) & (elements.length - 1);
+        E result = elements[t];
+        if (result == null)
+            return null;
+        elements[t] = null;
+        tail = t;
+        return result;
+    }
+
+    /**
+     * @throws NoSuchElementException {@inheritDoc}
+     */
+    public E getFirst() {
+        E x = elements[head];
+        if (x == null)
+            throw new NoSuchElementException();
+        return x;
+    }
+
+    /**
+     * @throws NoSuchElementException {@inheritDoc}
+     */
+    public E getLast() {
+        E x = elements[(tail - 1) & (elements.length - 1)];
+        if (x == null)
+            throw new NoSuchElementException();
+        return x;
+    }
+
+    public E peekFirst() {
+        return elements[head]; // elements[head] is null if deque empty
+    }
+
+    public E peekLast() {
+        return elements[(tail - 1) & (elements.length - 1)];
+    }
+
+    /**
+     * Removes the first occurrence of the specified element in this
+     * deque (when traversing the deque from head to tail).
+     * If the deque does not contain the element, it is unchanged.
+     * More formally, removes the first element <tt>e</tt> such that
+     * <tt>o.equals(e)</tt> (if such an element exists).
+     * Returns <tt>true</tt> if this deque contained the specified element
+     * (or equivalently, if this deque changed as a result of the call).
+     *
+     * @param o element to be removed from this deque, if present
+     * @return <tt>true</tt> if the deque contained the specified element
+     */
+    public boolean removeFirstOccurrence(Object o) {
+        if (o == null)
+            return false;
+        int mask = elements.length - 1;
+        int i = head;
+        E x;
+        while ( (x = elements[i]) != null) {
+            if (o.equals(x)) {
+                delete(i);
+                return true;
+            }
+            i = (i + 1) & mask;
+        }
+        return false;
+    }
+
+    /**
+     * Removes the last occurrence of the specified element in this
+     * deque (when traversing the deque from head to tail).
+     * If the deque does not contain the element, it is unchanged.
+     * More formally, removes the last element <tt>e</tt> such that
+     * <tt>o.equals(e)</tt> (if such an element exists).
+     * Returns <tt>true</tt> if this deque contained the specified element
+     * (or equivalently, if this deque changed as a result of the call).
+     *
+     * @param o element to be removed from this deque, if present
+     * @return <tt>true</tt> if the deque contained the specified element
+     */
+    public boolean removeLastOccurrence(Object o) {
+        if (o == null)
+            return false;
+        int mask = elements.length - 1;
+        int i = (tail - 1) & mask;
+        E x;
+        while ( (x = elements[i]) != null) {
+            if (o.equals(x)) {
+                delete(i);
+                return true;
+            }
+            i = (i - 1) & mask;
+        }
+        return false;
+    }
+
+    // *** Queue methods ***
+
+    /**
+     * Inserts the specified element at the end of this deque.
+     *
+     * <p>This method is equivalent to {@link #addLast}.
+     *
+     * @param e the element to add
+     * @return <tt>true</tt> (as specified by {@link Collection#add})
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean add(E e) {
+        addLast(e);
+        return true;
+    }
+
+    /**
+     * Inserts the specified element at the end of this deque.
+     *
+     * <p>This method is equivalent to {@link #offerLast}.
+     *
+     * @param e the element to add
+     * @return <tt>true</tt> (as specified by {@link Queue#offer})
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean offer(E e) {
+        return offerLast(e);
+    }
+
+    /**
+     * Retrieves and removes the head of the queue represented by this deque.
+     *
+     * This method differs from {@link #poll poll} only in that it throws an
+     * exception if this deque is empty.
+     *
+     * <p>This method is equivalent to {@link #removeFirst}.
+     *
+     * @return the head of the queue represented by this deque
+     * @throws NoSuchElementException {@inheritDoc}
+     */
+    public E remove() {
+        return removeFirst();
+    }
+
+    /**
+     * Retrieves and removes the head of the queue represented by this deque
+     * (in other words, the first element of this deque), or returns
+     * <tt>null</tt> if this deque is empty.
+     *
+     * <p>This method is equivalent to {@link #pollFirst}.
+     *
+     * @return the head of the queue represented by this deque, or
+     *         <tt>null</tt> if this deque is empty
+     */
+    public E poll() {
+        return pollFirst();
+    }
+
+    /**
+     * Retrieves, but does not remove, the head of the queue represented by
+     * this deque.  This method differs from {@link #peek peek} only in
+     * that it throws an exception if this deque is empty.
+     *
+     * <p>This method is equivalent to {@link #getFirst}.
+     *
+     * @return the head of the queue represented by this deque
+     * @throws NoSuchElementException {@inheritDoc}
+     */
+    public E element() {
+        return getFirst();
+    }
+
+    /**
+     * Retrieves, but does not remove, the head of the queue represented by
+     * this deque, or returns <tt>null</tt> if this deque is empty.
+     *
+     * <p>This method is equivalent to {@link #peekFirst}.
+     *
+     * @return the head of the queue represented by this deque, or
+     *         <tt>null</tt> if this deque is empty
+     */
+    public E peek() {
+        return peekFirst();
+    }
+
+    // *** Stack methods ***
+
+    /**
+     * Pushes an element onto the stack represented by this deque.  In other
+     * words, inserts the element at the front of this deque.
+     *
+     * <p>This method is equivalent to {@link #addFirst}.
+     *
+     * @param e the element to push
+     * @throws NullPointerException if the specified element is null
+     */
+    public void push(E e) {
+        addFirst(e);
+    }
+
+    /**
+     * Pops an element from the stack represented by this deque.  In other
+     * words, removes and returns the first element of this deque.
+     *
+     * <p>This method is equivalent to {@link #removeFirst()}.
+     *
+     * @return the element at the front of this deque (which is the top
+     *         of the stack represented by this deque)
+     * @throws NoSuchElementException {@inheritDoc}
+     */
+    public E pop() {
+        return removeFirst();
+    }
+
+    private void checkInvariants() {
+	assert elements[tail] == null;
+	assert head == tail ? elements[head] == null :
+	    (elements[head] != null &&
+	     elements[(tail - 1) & (elements.length - 1)] != null);
+	assert elements[(head - 1) & (elements.length - 1)] == null;
+    }
+
+    /**
+     * Removes the element at the specified position in the elements array,
+     * adjusting head and tail as necessary.  This can result in motion of
+     * elements backwards or forwards in the array.
+     *
+     * <p>This method is called delete rather than remove to emphasize
+     * that its semantics differ from those of {@link List#remove(int)}.
+     *
+     * @return true if elements moved backwards
+     */
+    private boolean delete(int i) {
+ 	checkInvariants();
+	final E[] elements = this.elements;
+	final int mask = elements.length - 1;
+	final int h = head;
+	final int t = tail;
+	final int front = (i - h) & mask;
+	final int back  = (t - i) & mask;
+
+	// Invariant: head <= i < tail mod circularity
+	if (front >= ((t - h) & mask))
+	    throw new ConcurrentModificationException();
+
+	// Optimize for least element motion
+	if (front < back) {
+	    if (h <= i) {
+		System.arraycopy(elements, h, elements, h + 1, front);
+	    } else { // Wrap around
+		System.arraycopy(elements, 0, elements, 1, i);
+		elements[0] = elements[mask];
+		System.arraycopy(elements, h, elements, h + 1, mask - h);
+	    }
+	    elements[h] = null;
+	    head = (h + 1) & mask;
+	    return false;
+	} else {
+	    if (i < t) { // Copy the null tail as well
+		System.arraycopy(elements, i + 1, elements, i, back);
+		tail = t - 1;
+	    } else { // Wrap around
+		System.arraycopy(elements, i + 1, elements, i, mask - i);
+		elements[mask] = elements[0];
+		System.arraycopy(elements, 1, elements, 0, t);
+		tail = (t - 1) & mask;
+	    }
+	    return true;
+	}
+    }
+
+    // *** Collection Methods ***
+
+    /**
+     * Returns the number of elements in this deque.
+     *
+     * @return the number of elements in this deque
+     */
+    public int size() {
+        return (tail - head) & (elements.length - 1);
+    }
+
+    /**
+     * Returns <tt>true</tt> if this deque contains no elements.
+     *
+     * @return <tt>true</tt> if this deque contains no elements
+     */
+    public boolean isEmpty() {
+        return head == tail;
+    }
+
+    /**
+     * Returns an iterator over the elements in this deque.  The elements
+     * will be ordered from first (head) to last (tail).  This is the same
+     * order that elements would be dequeued (via successive calls to
+     * {@link #remove} or popped (via successive calls to {@link #pop}).
+     *
+     * @return an iterator over the elements in this deque
+     */
+    public Iterator<E> iterator() {
+        return new DeqIterator();
+    }
+
+    public Iterator<E> descendingIterator() {
+        return new DescendingIterator();
+    }
+
+    private class DeqIterator implements Iterator<E> {
+        /**
+         * Index of element to be returned by subsequent call to next.
+         */
+        private int cursor = head;
+
+        /**
+         * Tail recorded at construction (also in remove), to stop
+         * iterator and also to check for comodification.
+         */
+        private int fence = tail;
+
+        /**
+         * Index of element returned by most recent call to next.
+         * Reset to -1 if element is deleted by a call to remove.
+         */
+        private int lastRet = -1;
+
+        public boolean hasNext() {
+            return cursor != fence;
+        }
+
+        public E next() {
+            if (cursor == fence)
+                throw new NoSuchElementException();
+            E result = elements[cursor];
+            // This check doesn't catch all possible comodifications,
+            // but does catch the ones that corrupt traversal
+            if (tail != fence || result == null)
+                throw new ConcurrentModificationException();
+            lastRet = cursor;
+            cursor = (cursor + 1) & (elements.length - 1);
+            return result;
+        }
+
+        public void remove() {
+            if (lastRet < 0)
+                throw new IllegalStateException();
+            if (delete(lastRet)) { // if left-shifted, undo increment in next()
+                cursor = (cursor - 1) & (elements.length - 1);
+		fence = tail;
+	    }
+            lastRet = -1;
+        }
+    }
+
+    private class DescendingIterator implements Iterator<E> {
+        /*
+         * This class is nearly a mirror-image of DeqIterator, using
+         * tail instead of head for initial cursor, and head instead of
+         * tail for fence.
+         */
+        private int cursor = tail;
+        private int fence = head;
+        private int lastRet = -1;
+
+        public boolean hasNext() {
+            return cursor != fence;
+        }
+
+        public E next() {
+            if (cursor == fence)
+                throw new NoSuchElementException();
+            cursor = (cursor - 1) & (elements.length - 1);
+	    E result = elements[cursor];
+            if (head != fence || result == null)
+                throw new ConcurrentModificationException();
+            lastRet = cursor;
+            return result;
+        }
+
+        public void remove() {
+            if (lastRet < 0)
+                throw new IllegalStateException();
+            if (!delete(lastRet)) {
+                cursor = (cursor + 1) & (elements.length - 1);
+		fence = head;
+	    }
+            lastRet = -1;
+        }
+    }
+
+    /**
+     * Returns <tt>true</tt> if this deque contains the specified element.
+     * More formally, returns <tt>true</tt> if and only if this deque contains
+     * at least one element <tt>e</tt> such that <tt>o.equals(e)</tt>.
+     *
+     * @param o object to be checked for containment in this deque
+     * @return <tt>true</tt> if this deque contains the specified element
+     */
+    public boolean contains(Object o) {
+        if (o == null)
+            return false;
+        int mask = elements.length - 1;
+        int i = head;
+        E x;
+        while ( (x = elements[i]) != null) {
+            if (o.equals(x))
+                return true;
+            i = (i + 1) & mask;
+        }
+        return false;
+    }
+
+    /**
+     * Removes a single instance of the specified element from this deque.
+     * If the deque does not contain the element, it is unchanged.
+     * More formally, removes the first element <tt>e</tt> such that
+     * <tt>o.equals(e)</tt> (if such an element exists).
+     * Returns <tt>true</tt> if this deque contained the specified element
+     * (or equivalently, if this deque changed as a result of the call).
+     *
+     * <p>This method is equivalent to {@link #removeFirstOccurrence}.
+     *
+     * @param o element to be removed from this deque, if present
+     * @return <tt>true</tt> if this deque contained the specified element
+     */
+    public boolean remove(Object o) {
+        return removeFirstOccurrence(o);
+    }
+
+    /**
+     * Removes all of the elements from this deque.
+     * The deque will be empty after this call returns.
+     */
+    public void clear() {
+        int h = head;
+        int t = tail;
+        if (h != t) { // clear all cells
+            head = tail = 0;
+            int i = h;
+            int mask = elements.length - 1;
+            do {
+                elements[i] = null;
+                i = (i + 1) & mask;
+            } while (i != t);
+        }
+    }
+
+    /**
+     * Returns an array containing all of the elements in this deque
+     * in proper sequence (from first to last element).
+     *
+     * <p>The returned array will be "safe" in that no references to it are
+     * maintained by this deque.  (In other words, this method must allocate
+     * a new array).  The caller is thus free to modify the returned array.
+     *
+     * <p>This method acts as bridge between array-based and collection-based
+     * APIs.
+     *
+     * @return an array containing all of the elements in this deque
+     */
+    public Object[] toArray() {
+	return copyElements(new Object[size()]);
+    }
+
+    /**
+     * Returns an array containing all of the elements in this deque in
+     * proper sequence (from first to last element); the runtime type of the
+     * returned array is that of the specified array.  If the deque fits in
+     * the specified array, it is returned therein.  Otherwise, a new array
+     * is allocated with the runtime type of the specified array and the
+     * size of this deque.
+     *
+     * <p>If this deque fits in the specified array with room to spare
+     * (i.e., the array has more elements than this deque), the element in
+     * the array immediately following the end of the deque is set to
+     * <tt>null</tt>.
+     *
+     * <p>Like the {@link #toArray()} method, this method acts as bridge between
+     * array-based and collection-based APIs.  Further, this method allows
+     * precise control over the runtime type of the output array, and may,
+     * under certain circumstances, be used to save allocation costs.
+     *
+     * <p>Suppose <tt>x</tt> is a deque known to contain only strings.
+     * The following code can be used to dump the deque into a newly
+     * allocated array of <tt>String</tt>:
+     *
+     * <pre>
+     *     String[] y = x.toArray(new String[0]);</pre>
+     *
+     * Note that <tt>toArray(new Object[0])</tt> is identical in function to
+     * <tt>toArray()</tt>.
+     *
+     * @param a the array into which the elements of the deque are to
+     *          be stored, if it is big enough; otherwise, a new array of the
+     *          same runtime type is allocated for this purpose
+     * @return an array containing all of the elements in this deque
+     * @throws ArrayStoreException if the runtime type of the specified array
+     *         is not a supertype of the runtime type of every element in
+     *         this deque
+     * @throws NullPointerException if the specified array is null
+     */
+    public <T> T[] toArray(T[] a) {
+        int size = size();
+        if (a.length < size)
+            a = (T[])java.lang.reflect.Array.newInstance(
+                    a.getClass().getComponentType(), size);
+	copyElements(a);
+        if (a.length > size)
+            a[size] = null;
+        return a;
+    }
+
+    // *** Object methods ***
+
+    /**
+     * Returns a copy of this deque.
+     *
+     * @return a copy of this deque
+     */
+    public ArrayDeque<E> clone() {
+        try {
+            ArrayDeque<E> result = (ArrayDeque<E>) super.clone();
+            result.elements = Arrays.copyOf(elements, elements.length);
+            return result;
+
+        } catch (CloneNotSupportedException e) {
+            throw new AssertionError();
+        }
+    }
+
+    /**
+     * Appease the serialization gods.
+     */
+    private static final long serialVersionUID = 2340985798034038923L;
+
+    /**
+     * Serialize this deque.
+     *
+     * @serialData The current size (<tt>int</tt>) of the deque,
+     * followed by all of its elements (each an object reference) in
+     * first-to-last order.
+     */
+    private void writeObject(ObjectOutputStream s) throws IOException {
+        s.defaultWriteObject();
+
+        // Write out size
+        s.writeInt(size());
+
+        // Write out elements in order.
+        int mask = elements.length - 1;
+        for (int i = head; i != tail; i = (i + 1) & mask)
+            s.writeObject(elements[i]);
+    }
+
+    /**
+     * Deserialize this deque.
+     */
+    private void readObject(ObjectInputStream s)
+            throws IOException, ClassNotFoundException {
+        s.defaultReadObject();
+
+        // Read in size and allocate array
+        int size = s.readInt();
+        allocateElements(size);
+        head = 0;
+        tail = size;
+
+        // Read in all elements in the proper order.
+        for (int i = 0; i < size; i++)
+            elements[i] = (E)s.readObject();
+    }
+}
diff --git a/external/jsr166/java/util/Deque.java b/external/jsr166/java/util/Deque.java
new file mode 100644
index 000000000..a769561dc
--- /dev/null
+++ b/external/jsr166/java/util/Deque.java
@@ -0,0 +1,547 @@
+/*
+ * Written by Doug Lea and Josh Bloch with assistance from members of
+ * JCP JSR-166 Expert Group and released to the public domain, as explained
+ * at http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util;
+
+/**
+ * A linear collection that supports element insertion and removal at
+ * both ends.  The name <i>deque</i> is short for "double ended queue"
+ * and is usually pronounced "deck".  Most <tt>Deque</tt>
+ * implementations place no fixed limits on the number of elements
+ * they may contain, but this interface supports capacity-restricted
+ * deques as well as those with no fixed size limit.
+ *
+ * <p>This interface defines methods to access the elements at both
+ * ends of the deque.  Methods are provided to insert, remove, and
+ * examine the element.  Each of these methods exists in two forms:
+ * one throws an exception if the operation fails, the other returns a
+ * special value (either <tt>null</tt> or <tt>false</tt>, depending on
+ * the operation).  The latter form of the insert operation is
+ * designed specifically for use with capacity-restricted
+ * <tt>Deque</tt> implementations; in most implementations, insert
+ * operations cannot fail.
+ *
+ * <p>The twelve methods described above are summarized in the
+ * following table:
+ *
+ * <p>
+ * <table BORDER CELLPADDING=3 CELLSPACING=1>
+ *  <tr>
+ *    <td></td>
+ *    <td ALIGN=CENTER COLSPAN = 2> <b>First Element (Head)</b></td>
+ *    <td ALIGN=CENTER COLSPAN = 2> <b>Last Element (Tail)</b></td>
+ *  </tr>
+ *  <tr>
+ *    <td></td>
+ *    <td ALIGN=CENTER><em>Throws exception</em></td>
+ *    <td ALIGN=CENTER><em>Special value</em></td>
+ *    <td ALIGN=CENTER><em>Throws exception</em></td>
+ *    <td ALIGN=CENTER><em>Special value</em></td>
+ *  </tr>
+ *  <tr>
+ *    <td><b>Insert</b></td>
+ *    <td>{@link #addFirst addFirst(e)}</td>
+ *    <td>{@link #offerFirst offerFirst(e)}</td>
+ *    <td>{@link #addLast addLast(e)}</td>
+ *    <td>{@link #offerLast offerLast(e)}</td>
+ *  </tr>
+ *  <tr>
+ *    <td><b>Remove</b></td>
+ *    <td>{@link #removeFirst removeFirst()}</td>
+ *    <td>{@link #pollFirst pollFirst()}</td>
+ *    <td>{@link #removeLast removeLast()}</td>
+ *    <td>{@link #pollLast pollLast()}</td>
+ *  </tr>
+ *  <tr>
+ *    <td><b>Examine</b></td>
+ *    <td>{@link #getFirst getFirst()}</td>
+ *    <td>{@link #peekFirst peekFirst()}</td>
+ *    <td>{@link #getLast getLast()}</td>
+ *    <td>{@link #peekLast peekLast()}</td>
+ *  </tr>
+ * </table>
+ *
+ * <p>This interface extends the {@link Queue} interface.  When a deque is
+ * used as a queue, FIFO (First-In-First-Out) behavior results.  Elements are
+ * added at the end of the deque and removed from the beginning.  The methods
+ * inherited from the <tt>Queue</tt> interface are precisely equivalent to
+ * <tt>Deque</tt> methods as indicated in the following table:
+ *
+ * <p>
+ * <table BORDER CELLPADDING=3 CELLSPACING=1>
+ *  <tr>
+ *    <td ALIGN=CENTER> <b><tt>Queue</tt> Method</b></td>
+ *    <td ALIGN=CENTER> <b>Equivalent <tt>Deque</tt> Method</b></td>
+ *  </tr>
+ *  <tr>
+ *    <td>{@link java.util.Queue#add add(e)}</td>
+ *    <td>{@link #addLast addLast(e)}</td>
+ *  </tr>
+ *  <tr>
+ *    <td>{@link java.util.Queue#offer offer(e)}</td>
+ *    <td>{@link #offerLast offerLast(e)}</td>
+ *  </tr>
+ *  <tr>
+ *    <td>{@link java.util.Queue#remove remove()}</td>
+ *    <td>{@link #removeFirst removeFirst()}</td>
+ *  </tr>
+ *  <tr>
+ *    <td>{@link java.util.Queue#poll poll()}</td>
+ *    <td>{@link #pollFirst pollFirst()}</td>
+ *  </tr>
+ *  <tr>
+ *    <td>{@link java.util.Queue#element element()}</td>
+ *    <td>{@link #getFirst getFirst()}</td>
+ *  </tr>
+ *  <tr>
+ *    <td>{@link java.util.Queue#peek peek()}</td>
+ *    <td>{@link #peek peekFirst()}</td>
+ *  </tr>
+ * </table>
+ *
+ * <p>Deques can also be used as LIFO (Last-In-First-Out) stacks.  This
+ * interface should be used in preference to the legacy {@link Stack} class.
+ * When a deque is used as a stack, elements are pushed and popped from the
+ * beginning of the deque.  Stack methods are precisely equivalent to
+ * <tt>Deque</tt> methods as indicated in the table below:
+ *
+ * <p>
+ * <table BORDER CELLPADDING=3 CELLSPACING=1>
+ *  <tr>
+ *    <td ALIGN=CENTER> <b>Stack Method</b></td>
+ *    <td ALIGN=CENTER> <b>Equivalent <tt>Deque</tt> Method</b></td>
+ *  </tr>
+ *  <tr>
+ *    <td>{@link #push push(e)}</td>
+ *    <td>{@link #addFirst addFirst(e)}</td>
+ *  </tr>
+ *  <tr>
+ *    <td>{@link #pop pop()}</td>
+ *    <td>{@link #removeFirst removeFirst()}</td>
+ *  </tr>
+ *  <tr>
+ *    <td>{@link #peek peek()}</td>
+ *    <td>{@link #peekFirst peekFirst()}</td>
+ *  </tr>
+ * </table>
+ *
+ * <p>Note that the {@link #peek peek} method works equally well when
+ * a deque is used as a queue or a stack; in either case, elements are
+ * drawn from the beginning of the deque.
+ *
+ * <p>This interface provides two methods to remove interior
+ * elements, {@link #removeFirstOccurrence removeFirstOccurrence} and
+ * {@link #removeLastOccurrence removeLastOccurrence}.
+ *
+ * <p>Unlike the {@link List} interface, this interface does not
+ * provide support for indexed access to elements.
+ *
+ * <p>While <tt>Deque</tt> implementations are not strictly required
+ * to prohibit the insertion of null elements, they are strongly
+ * encouraged to do so.  Users of any <tt>Deque</tt> implementations
+ * that do allow null elements are strongly encouraged <i>not</i> to
+ * take advantage of the ability to insert nulls.  This is so because
+ * <tt>null</tt> is used as a special return value by various methods
+ * to indicated that the deque is empty.
+ *
+ * <p><tt>Deque</tt> implementations generally do not define
+ * element-based versions of the <tt>equals</tt> and <tt>hashCode</tt>
+ * methods, but instead inherit the identity-based versions from class
+ * <tt>Object</tt>.
+ *
+ * <p>This interface is a member of the <a
+ * href="{@docRoot}/../technotes/guides/collections/index.html"> Java Collections
+ * Framework</a>.
+ *
+ * @author Doug Lea
+ * @author Josh Bloch
+ * @since  1.6
+ * @param <E> the type of elements held in this collection
+ */
+
+public interface Deque<E> extends Queue<E> {
+    /**
+     * Inserts the specified element at the front of this deque if it is
+     * possible to do so immediately without violating capacity restrictions.
+     * When using a capacity-restricted deque, it is generally preferable to
+     * use method {@link #offerFirst}.
+     *
+     * @param e the element to add
+     * @throws IllegalStateException if the element cannot be added at this
+     *         time due to capacity restrictions
+     * @throws ClassCastException if the class of the specified element
+     *         prevents it from being added to this deque
+     * @throws NullPointerException if the specified element is null and this
+     *         deque does not permit null elements
+     * @throws IllegalArgumentException if some property of the specified
+     *         element prevents it from being added to this deque
+     */
+    void addFirst(E e);
+
+    /**
+     * Inserts the specified element at the end of this deque if it is
+     * possible to do so immediately without violating capacity restrictions.
+     * When using a capacity-restricted deque, it is generally preferable to
+     * use method {@link #offerLast}.
+     *
+     * <p>This method is equivalent to {@link #add}.
+     *
+     * @param e the element to add
+     * @throws IllegalStateException if the element cannot be added at this
+     *         time due to capacity restrictions
+     * @throws ClassCastException if the class of the specified element
+     *         prevents it from being added to this deque
+     * @throws NullPointerException if the specified element is null and this
+     *         deque does not permit null elements
+     * @throws IllegalArgumentException if some property of the specified
+     *         element prevents it from being added to this deque
+     */
+    void addLast(E e);
+
+    /**
+     * Inserts the specified element at the front of this deque unless it would
+     * violate capacity restrictions.  When using a capacity-restricted deque,
+     * this method is generally preferable to the {@link #addFirst} method,
+     * which can fail to insert an element only by throwing an exception.
+     *
+     * @param e the element to add
+     * @return <tt>true</tt> if the element was added to this deque, else
+     *         <tt>false</tt>
+     * @throws ClassCastException if the class of the specified element
+     *         prevents it from being added to this deque
+     * @throws NullPointerException if the specified element is null and this
+     *         deque does not permit null elements
+     * @throws IllegalArgumentException if some property of the specified
+     *         element prevents it from being added to this deque
+     */
+    boolean offerFirst(E e);
+
+    /**
+     * Inserts the specified element at the end of this deque unless it would
+     * violate capacity restrictions.  When using a capacity-restricted deque,
+     * this method is generally preferable to the {@link #addLast} method,
+     * which can fail to insert an element only by throwing an exception.
+     *
+     * @param e the element to add
+     * @return <tt>true</tt> if the element was added to this deque, else
+     *         <tt>false</tt>
+     * @throws ClassCastException if the class of the specified element
+     *         prevents it from being added to this deque
+     * @throws NullPointerException if the specified element is null and this
+     *         deque does not permit null elements
+     * @throws IllegalArgumentException if some property of the specified
+     *         element prevents it from being added to this deque
+     */
+    boolean offerLast(E e);
+
+    /**
+     * Retrieves and removes the first element of this deque.  This method
+     * differs from {@link #pollFirst pollFirst} only in that it throws an
+     * exception if this deque is empty.
+     *
+     * @return the head of this deque
+     * @throws NoSuchElementException if this deque is empty
+     */
+    E removeFirst();
+
+    /**
+     * Retrieves and removes the last element of this deque.  This method
+     * differs from {@link #pollLast pollLast} only in that it throws an
+     * exception if this deque is empty.
+     *
+     * @return the tail of this deque
+     * @throws NoSuchElementException if this deque is empty
+     */
+    E removeLast();
+
+    /**
+     * Retrieves and removes the first element of this deque,
+     * or returns <tt>null</tt> if this deque is empty.
+     *
+     * @return the head of this deque, or <tt>null</tt> if this deque is empty
+     */
+    E pollFirst();
+
+    /**
+     * Retrieves and removes the last element of this deque,
+     * or returns <tt>null</tt> if this deque is empty.
+     *
+     * @return the tail of this deque, or <tt>null</tt> if this deque is empty
+     */
+    E pollLast();
+
+    /**
+     * Retrieves, but does not remove, the first element of this deque.
+     *
+     * This method differs from {@link #peekFirst peekFirst} only in that it
+     * throws an exception if this deque is empty.
+     *
+     * @return the head of this deque
+     * @throws NoSuchElementException if this deque is empty
+     */
+    E getFirst();
+
+    /**
+     * Retrieves, but does not remove, the last element of this deque.
+     * This method differs from {@link #peekLast peekLast} only in that it
+     * throws an exception if this deque is empty.
+     *
+     * @return the tail of this deque
+     * @throws NoSuchElementException if this deque is empty
+     */
+    E getLast();
+
+    /**
+     * Retrieves, but does not remove, the first element of this deque,
+     * or returns <tt>null</tt> if this deque is empty.
+     *
+     * @return the head of this deque, or <tt>null</tt> if this deque is empty
+     */
+    E peekFirst();
+
+    /**
+     * Retrieves, but does not remove, the last element of this deque,
+     * or returns <tt>null</tt> if this deque is empty.
+     *
+     * @return the tail of this deque, or <tt>null</tt> if this deque is empty
+     */
+    E peekLast();
+
+    /**
+     * Removes the first occurrence of the specified element from this deque.
+     * If the deque does not contain the element, it is unchanged.
+     * More formally, removes the first element <tt>e</tt> such that
+     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>
+     * (if such an element exists).
+     * Returns <tt>true</tt> if this deque contained the specified element
+     * (or equivalently, if this deque changed as a result of the call).
+     *
+     * @param o element to be removed from this deque, if present
+     * @return <tt>true</tt> if an element was removed as a result of this call
+     * @throws ClassCastException if the class of the specified element
+     *         is incompatible with this deque (optional)
+     * @throws NullPointerException if the specified element is null and this
+     *         deque does not permit null elements (optional)
+     */
+    boolean removeFirstOccurrence(Object o);
+
+    /**
+     * Removes the last occurrence of the specified element from this deque.
+     * If the deque does not contain the element, it is unchanged.
+     * More formally, removes the last element <tt>e</tt> such that
+     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>
+     * (if such an element exists).
+     * Returns <tt>true</tt> if this deque contained the specified element
+     * (or equivalently, if this deque changed as a result of the call).
+     *
+     * @param o element to be removed from this deque, if present
+     * @return <tt>true</tt> if an element was removed as a result of this call
+     * @throws ClassCastException if the class of the specified element
+     *         is incompatible with this deque (optional)
+     * @throws NullPointerException if the specified element is null and this
+     *         deque does not permit null elements (optional)
+     */
+    boolean removeLastOccurrence(Object o);
+
+    // *** Queue methods ***
+
+    /**
+     * Inserts the specified element into the queue represented by this deque
+     * (in other words, at the tail of this deque) if it is possible to do so
+     * immediately without violating capacity restrictions, returning
+     * <tt>true</tt> upon success and throwing an
+     * <tt>IllegalStateException</tt> if no space is currently available.
+     * When using a capacity-restricted deque, it is generally preferable to
+     * use {@link #offer(Object) offer}.
+     *
+     * <p>This method is equivalent to {@link #addLast}.
+     *
+     * @param e the element to add
+     * @return <tt>true</tt> (as specified by {@link Collection#add})
+     * @throws IllegalStateException if the element cannot be added at this
+     *         time due to capacity restrictions
+     * @throws ClassCastException if the class of the specified element
+     *         prevents it from being added to this deque
+     * @throws NullPointerException if the specified element is null and this
+     *         deque does not permit null elements
+     * @throws IllegalArgumentException if some property of the specified
+     *         element prevents it from being added to this deque
+     */
+    boolean add(E e);
+
+    /**
+     * Inserts the specified element into the queue represented by this deque
+     * (in other words, at the tail of this deque) if it is possible to do so
+     * immediately without violating capacity restrictions, returning
+     * <tt>true</tt> upon success and <tt>false</tt> if no space is currently
+     * available.  When using a capacity-restricted deque, this method is
+     * generally preferable to the {@link #add} method, which can fail to
+     * insert an element only by throwing an exception.
+     *
+     * <p>This method is equivalent to {@link #offerLast}.
+     *
+     * @param e the element to add
+     * @return <tt>true</tt> if the element was added to this deque, else
+     *         <tt>false</tt>
+     * @throws ClassCastException if the class of the specified element
+     *         prevents it from being added to this deque
+     * @throws NullPointerException if the specified element is null and this
+     *         deque does not permit null elements
+     * @throws IllegalArgumentException if some property of the specified
+     *         element prevents it from being added to this deque
+     */
+    boolean offer(E e);
+
+    /**
+     * Retrieves and removes the head of the queue represented by this deque
+     * (in other words, the first element of this deque).
+     * This method differs from {@link #poll poll} only in that it throws an
+     * exception if this deque is empty.
+     *
+     * <p>This method is equivalent to {@link #removeFirst()}.
+     *
+     * @return the head of the queue represented by this deque
+     * @throws NoSuchElementException if this deque is empty
+     */
+    E remove();
+
+    /**
+     * Retrieves and removes the head of the queue represented by this deque
+     * (in other words, the first element of this deque), or returns
+     * <tt>null</tt> if this deque is empty.
+     *
+     * <p>This method is equivalent to {@link #pollFirst()}.
+     *
+     * @return the first element of this deque, or <tt>null</tt> if
+     *         this deque is empty
+     */
+    E poll();
+
+    /**
+     * Retrieves, but does not remove, the head of the queue represented by
+     * this deque (in other words, the first element of this deque).
+     * This method differs from {@link #peek peek} only in that it throws an
+     * exception if this deque is empty.
+     *
+     * <p>This method is equivalent to {@link #getFirst()}.
+     *
+     * @return the head of the queue represented by this deque
+     * @throws NoSuchElementException if this deque is empty
+     */
+    E element();
+
+    /**
+     * Retrieves, but does not remove, the head of the queue represented by
+     * this deque (in other words, the first element of this deque), or
+     * returns <tt>null</tt> if this deque is empty.
+     *
+     * <p>This method is equivalent to {@link #peekFirst()}.
+     *
+     * @return the head of the queue represented by this deque, or
+     *         <tt>null</tt> if this deque is empty
+     */
+    E peek();
+
+
+    // *** Stack methods ***
+
+    /**
+     * Pushes an element onto the stack represented by this deque (in other
+     * words, at the head of this deque) if it is possible to do so
+     * immediately without violating capacity restrictions, returning
+     * <tt>true</tt> upon success and throwing an
+     * <tt>IllegalStateException</tt> if no space is currently available.
+     *
+     * <p>This method is equivalent to {@link #addFirst}.
+     *
+     * @param e the element to push
+     * @throws IllegalStateException if the element cannot be added at this
+     *         time due to capacity restrictions
+     * @throws ClassCastException if the class of the specified element
+     *         prevents it from being added to this deque
+     * @throws NullPointerException if the specified element is null and this
+     *         deque does not permit null elements
+     * @throws IllegalArgumentException if some property of the specified
+     *         element prevents it from being added to this deque
+     */
+    void push(E e);
+
+    /**
+     * Pops an element from the stack represented by this deque.  In other
+     * words, removes and returns the first element of this deque.
+     *
+     * <p>This method is equivalent to {@link #removeFirst()}.
+     *
+     * @return the element at the front of this deque (which is the top
+     *         of the stack represented by this deque)
+     * @throws NoSuchElementException if this deque is empty
+     */
+    E pop();
+
+
+    // *** Collection methods ***
+
+    /**
+     * Removes the first occurrence of the specified element from this deque.
+     * If the deque does not contain the element, it is unchanged.
+     * More formally, removes the first element <tt>e</tt> such that
+     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>
+     * (if such an element exists).
+     * Returns <tt>true</tt> if this deque contained the specified element
+     * (or equivalently, if this deque changed as a result of the call).
+     *
+     * <p>This method is equivalent to {@link #removeFirstOccurrence}.
+     *
+     * @param o element to be removed from this deque, if present
+     * @return <tt>true</tt> if an element was removed as a result of this call
+     * @throws ClassCastException if the class of the specified element
+     *         is incompatible with this deque (optional)
+     * @throws NullPointerException if the specified element is null and this
+     *         deque does not permit null elements (optional)
+     */
+    boolean remove(Object o);
+
+    /**
+     * Returns <tt>true</tt> if this deque contains the specified element.
+     * More formally, returns <tt>true</tt> if and only if this deque contains
+     * at least one element <tt>e</tt> such that
+     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
+     *
+     * @param o element whose presence in this deque is to be tested
+     * @return <tt>true</tt> if this deque contains the specified element
+     * @throws ClassCastException if the type of the specified element
+     *         is incompatible with this deque (optional)
+     * @throws NullPointerException if the specified element is null and this
+     *         deque does not permit null elements (optional)
+     */
+    boolean contains(Object o);
+
+    /**
+     * Returns the number of elements in this deque.
+     *
+     * @return the number of elements in this deque
+     */
+    public int size();
+
+    /**
+     * Returns an iterator over the elements in this deque in proper sequence.
+     * The elements will be returned in order from first (head) to last (tail).
+     *
+     * @return an iterator over the elements in this deque in proper sequence
+     */
+    Iterator<E> iterator();
+
+    /**
+     * Returns an iterator over the elements in this deque in reverse
+     * sequential order.  The elements will be returned in order from
+     * last (tail) to first (head).
+     *
+     * @return an iterator over the elements in this deque in reverse
+     * sequence
+     */
+    Iterator<E> descendingIterator();
+
+}
diff --git a/external/jsr166/java/util/NavigableMap.java b/external/jsr166/java/util/NavigableMap.java
new file mode 100644
index 000000000..a55f84bdd
--- /dev/null
+++ b/external/jsr166/java/util/NavigableMap.java
@@ -0,0 +1,395 @@
+/*
+ * Written by Doug Lea and Josh Bloch with assistance from members of JCP
+ * JSR-166 Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util;
+
+/**
+ * A {@link SortedMap} extended with navigation methods returning the
+ * closest matches for given search targets. Methods
+ * {@code lowerEntry}, {@code floorEntry}, {@code ceilingEntry},
+ * and {@code higherEntry} return {@code Map.Entry} objects
+ * associated with keys respectively less than, less than or equal,
+ * greater than or equal, and greater than a given key, returning
+ * {@code null} if there is no such key.  Similarly, methods
+ * {@code lowerKey}, {@code floorKey}, {@code ceilingKey}, and
+ * {@code higherKey} return only the associated keys. All of these
+ * methods are designed for locating, not traversing entries.
+ *
+ * <p>A {@code NavigableMap} may be accessed and traversed in either
+ * ascending or descending key order.  The {@code descendingMap}
+ * method returns a view of the map with the senses of all relational
+ * and directional methods inverted. The performance of ascending
+ * operations and views is likely to be faster than that of descending
+ * ones.  Methods {@code subMap}, {@code headMap},
+ * and {@code tailMap} differ from the like-named {@code
+ * SortedMap} methods in accepting additional arguments describing
+ * whether lower and upper bounds are inclusive versus exclusive.
+ * Submaps of any {@code NavigableMap} must implement the {@code
+ * NavigableMap} interface.
+ *
+ * <p>This interface additionally defines methods {@code firstEntry},
+ * {@code pollFirstEntry}, {@code lastEntry}, and
+ * {@code pollLastEntry} that return and/or remove the least and
+ * greatest mappings, if any exist, else returning {@code null}.
+ *
+ * <p>Implementations of entry-returning methods are expected to
+ * return {@code Map.Entry} pairs representing snapshots of mappings
+ * at the time they were produced, and thus generally do <em>not</em>
+ * support the optional {@code Entry.setValue} method. Note however
+ * that it is possible to change mappings in the associated map using
+ * method {@code put}.
+ *
+ * <p>Methods
+ * {@link #subMap(Object, Object) subMap(K, K)},
+ * {@link #headMap(Object) headMap(K)}, and
+ * {@link #tailMap(Object) tailMap(K)}
+ * are specified to return {@code SortedMap} to allow existing
+ * implementations of {@code SortedMap} to be compatibly retrofitted to
+ * implement {@code NavigableMap}, but extensions and implementations
+ * of this interface are encouraged to override these methods to return
+ * {@code NavigableMap}.  Similarly,
+ * {@link #keySet()} can be overriden to return {@code NavigableSet}.
+ *
+ * <p>This interface is a member of the
+ * <a href="{@docRoot}/../technotes/guides/collections/index.html">
+ * Java Collections Framework</a>.
+ *
+ * @author Doug Lea
+ * @author Josh Bloch
+ * @param <K> the type of keys maintained by this map
+ * @param <V> the type of mapped values
+ * @since 1.6
+ */
+public interface NavigableMap<K,V> extends SortedMap<K,V> {
+    /**
+     * Returns a key-value mapping associated with the greatest key
+     * strictly less than the given key, or {@code null} if there is
+     * no such key.
+     *
+     * @param key the key
+     * @return an entry with the greatest key less than {@code key},
+     *         or {@code null} if there is no such key
+     * @throws ClassCastException if the specified key cannot be compared
+     *         with the keys currently in the map
+     * @throws NullPointerException if the specified key is null
+     *         and this map does not permit null keys
+     */
+    Map.Entry<K,V> lowerEntry(K key);
+
+    /**
+     * Returns the greatest key strictly less than the given key, or
+     * {@code null} if there is no such key.
+     *
+     * @param key the key
+     * @return the greatest key less than {@code key},
+     *         or {@code null} if there is no such key
+     * @throws ClassCastException if the specified key cannot be compared
+     *         with the keys currently in the map
+     * @throws NullPointerException if the specified key is null
+     *         and this map does not permit null keys
+     */
+    K lowerKey(K key);
+
+    /**
+     * Returns a key-value mapping associated with the greatest key
+     * less than or equal to the given key, or {@code null} if there
+     * is no such key.
+     *
+     * @param key the key
+     * @return an entry with the greatest key less than or equal to
+     *         {@code key}, or {@code null} if there is no such key
+     * @throws ClassCastException if the specified key cannot be compared
+     *         with the keys currently in the map
+     * @throws NullPointerException if the specified key is null
+     *         and this map does not permit null keys
+     */
+    Map.Entry<K,V> floorEntry(K key);
+
+    /**
+     * Returns the greatest key less than or equal to the given key,
+     * or {@code null} if there is no such key.
+     *
+     * @param key the key
+     * @return the greatest key less than or equal to {@code key},
+     *         or {@code null} if there is no such key
+     * @throws ClassCastException if the specified key cannot be compared
+     *         with the keys currently in the map
+     * @throws NullPointerException if the specified key is null
+     *         and this map does not permit null keys
+     */
+    K floorKey(K key);
+
+    /**
+     * Returns a key-value mapping associated with the least key
+     * greater than or equal to the given key, or {@code null} if
+     * there is no such key.
+     *
+     * @param key the key
+     * @return an entry with the least key greater than or equal to
+     *         {@code key}, or {@code null} if there is no such key
+     * @throws ClassCastException if the specified key cannot be compared
+     *         with the keys currently in the map
+     * @throws NullPointerException if the specified key is null
+     *         and this map does not permit null keys
+     */
+    Map.Entry<K,V> ceilingEntry(K key);
+
+    /**
+     * Returns the least key greater than or equal to the given key,
+     * or {@code null} if there is no such key.
+     *
+     * @param key the key
+     * @return the least key greater than or equal to {@code key},
+     *         or {@code null} if there is no such key
+     * @throws ClassCastException if the specified key cannot be compared
+     *         with the keys currently in the map
+     * @throws NullPointerException if the specified key is null
+     *         and this map does not permit null keys
+     */
+    K ceilingKey(K key);
+
+    /**
+     * Returns a key-value mapping associated with the least key
+     * strictly greater than the given key, or {@code null} if there
+     * is no such key.
+     *
+     * @param key the key
+     * @return an entry with the least key greater than {@code key},
+     *         or {@code null} if there is no such key
+     * @throws ClassCastException if the specified key cannot be compared
+     *         with the keys currently in the map
+     * @throws NullPointerException if the specified key is null
+     *         and this map does not permit null keys
+     */
+    Map.Entry<K,V> higherEntry(K key);
+
+    /**
+     * Returns the least key strictly greater than the given key, or
+     * {@code null} if there is no such key.
+     *
+     * @param key the key
+     * @return the least key greater than {@code key},
+     *         or {@code null} if there is no such key
+     * @throws ClassCastException if the specified key cannot be compared
+     *         with the keys currently in the map
+     * @throws NullPointerException if the specified key is null
+     *         and this map does not permit null keys
+     */
+    K higherKey(K key);
+
+    /**
+     * Returns a key-value mapping associated with the least
+     * key in this map, or {@code null} if the map is empty.
+     *
+     * @return an entry with the least key,
+     *         or {@code null} if this map is empty
+     */
+    Map.Entry<K,V> firstEntry();
+
+    /**
+     * Returns a key-value mapping associated with the greatest
+     * key in this map, or {@code null} if the map is empty.
+     *
+     * @return an entry with the greatest key,
+     *         or {@code null} if this map is empty
+     */
+    Map.Entry<K,V> lastEntry();
+
+    /**
+     * Removes and returns a key-value mapping associated with
+     * the least key in this map, or {@code null} if the map is empty.
+     *
+     * @return the removed first entry of this map,
+     *         or {@code null} if this map is empty
+     */
+    Map.Entry<K,V> pollFirstEntry();
+
+    /**
+     * Removes and returns a key-value mapping associated with
+     * the greatest key in this map, or {@code null} if the map is empty.
+     *
+     * @return the removed last entry of this map,
+     *         or {@code null} if this map is empty
+     */
+    Map.Entry<K,V> pollLastEntry();
+
+    /**
+     * Returns a reverse order view of the mappings contained in this map.
+     * The descending map is backed by this map, so changes to the map are
+     * reflected in the descending map, and vice-versa.  If either map is
+     * modified while an iteration over a collection view of either map
+     * is in progress (except through the iterator's own {@code remove}
+     * operation), the results of the iteration are undefined.
+     *
+     * <p>The returned map has an ordering equivalent to
+     * <tt>{@link Collections#reverseOrder(Comparator) Collections.reverseOrder}(comparator())</tt>.
+     * The expression {@code m.descendingMap().descendingMap()} returns a
+     * view of {@code m} essentially equivalent to {@code m}.
+     *
+     * @return a reverse order view of this map
+     */
+    NavigableMap<K,V> descendingMap();
+
+    /**
+     * Returns a {@link NavigableSet} view of the keys contained in this map.
+     * The set's iterator returns the keys in ascending order.
+     * The set is backed by the map, so changes to the map are reflected in
+     * the set, and vice-versa.  If the map is modified while an iteration
+     * over the set is in progress (except through the iterator's own {@code
+     * remove} operation), the results of the iteration are undefined.  The
+     * set supports element removal, which removes the corresponding mapping
+     * from the map, via the {@code Iterator.remove}, {@code Set.remove},
+     * {@code removeAll}, {@code retainAll}, and {@code clear} operations.
+     * It does not support the {@code add} or {@code addAll} operations.
+     *
+     * @return a navigable set view of the keys in this map
+     */
+    NavigableSet<K> navigableKeySet();
+
+    /**
+     * Returns a reverse order {@link NavigableSet} view of the keys contained in this map.
+     * The set's iterator returns the keys in descending order.
+     * The set is backed by the map, so changes to the map are reflected in
+     * the set, and vice-versa.  If the map is modified while an iteration
+     * over the set is in progress (except through the iterator's own {@code
+     * remove} operation), the results of the iteration are undefined.  The
+     * set supports element removal, which removes the corresponding mapping
+     * from the map, via the {@code Iterator.remove}, {@code Set.remove},
+     * {@code removeAll}, {@code retainAll}, and {@code clear} operations.
+     * It does not support the {@code add} or {@code addAll} operations.
+     *
+     * @return a reverse order navigable set view of the keys in this map
+     */
+    NavigableSet<K> descendingKeySet();
+
+    /**
+     * Returns a view of the portion of this map whose keys range from
+     * {@code fromKey} to {@code toKey}.  If {@code fromKey} and
+     * {@code toKey} are equal, the returned map is empty unless
+     * {@code fromExclusive} and {@code toExclusive} are both true.  The
+     * returned map is backed by this map, so changes in the returned map are
+     * reflected in this map, and vice-versa.  The returned map supports all
+     * optional map operations that this map supports.
+     *
+     * <p>The returned map will throw an {@code IllegalArgumentException}
+     * on an attempt to insert a key outside of its range, or to construct a
+     * submap either of whose endpoints lie outside its range.
+     *
+     * @param fromKey low endpoint of the keys in the returned map
+     * @param fromInclusive {@code true} if the low endpoint
+     *        is to be included in the returned view
+     * @param toKey high endpoint of the keys in the returned map
+     * @param toInclusive {@code true} if the high endpoint
+     *        is to be included in the returned view
+     * @return a view of the portion of this map whose keys range from
+     *         {@code fromKey} to {@code toKey}
+     * @throws ClassCastException if {@code fromKey} and {@code toKey}
+     *         cannot be compared to one another using this map's comparator
+     *         (or, if the map has no comparator, using natural ordering).
+     *         Implementations may, but are not required to, throw this
+     *         exception if {@code fromKey} or {@code toKey}
+     *         cannot be compared to keys currently in the map.
+     * @throws NullPointerException if {@code fromKey} or {@code toKey}
+     *         is null and this map does not permit null keys
+     * @throws IllegalArgumentException if {@code fromKey} is greater than
+     *         {@code toKey}; or if this map itself has a restricted
+     *         range, and {@code fromKey} or {@code toKey} lies
+     *         outside the bounds of the range
+     */
+    NavigableMap<K,V> subMap(K fromKey, boolean fromInclusive,
+                             K toKey,   boolean toInclusive);
+
+    /**
+     * Returns a view of the portion of this map whose keys are less than (or
+     * equal to, if {@code inclusive} is true) {@code toKey}.  The returned
+     * map is backed by this map, so changes in the returned map are reflected
+     * in this map, and vice-versa.  The returned map supports all optional
+     * map operations that this map supports.
+     *
+     * <p>The returned map will throw an {@code IllegalArgumentException}
+     * on an attempt to insert a key outside its range.
+     *
+     * @param toKey high endpoint of the keys in the returned map
+     * @param inclusive {@code true} if the high endpoint
+     *        is to be included in the returned view
+     * @return a view of the portion of this map whose keys are less than
+     *         (or equal to, if {@code inclusive} is true) {@code toKey}
+     * @throws ClassCastException if {@code toKey} is not compatible
+     *         with this map's comparator (or, if the map has no comparator,
+     *         if {@code toKey} does not implement {@link Comparable}).
+     *         Implementations may, but are not required to, throw this
+     *         exception if {@code toKey} cannot be compared to keys
+     *         currently in the map.
+     * @throws NullPointerException if {@code toKey} is null
+     *         and this map does not permit null keys
+     * @throws IllegalArgumentException if this map itself has a
+     *         restricted range, and {@code toKey} lies outside the
+     *         bounds of the range
+     */
+    NavigableMap<K,V> headMap(K toKey, boolean inclusive);
+
+    /**
+     * Returns a view of the portion of this map whose keys are greater than (or
+     * equal to, if {@code inclusive} is true) {@code fromKey}.  The returned
+     * map is backed by this map, so changes in the returned map are reflected
+     * in this map, and vice-versa.  The returned map supports all optional
+     * map operations that this map supports.
+     *
+     * <p>The returned map will throw an {@code IllegalArgumentException}
+     * on an attempt to insert a key outside its range.
+     *
+     * @param fromKey low endpoint of the keys in the returned map
+     * @param inclusive {@code true} if the low endpoint
+     *        is to be included in the returned view
+     * @return a view of the portion of this map whose keys are greater than
+     *         (or equal to, if {@code inclusive} is true) {@code fromKey}
+     * @throws ClassCastException if {@code fromKey} is not compatible
+     *         with this map's comparator (or, if the map has no comparator,
+     *         if {@code fromKey} does not implement {@link Comparable}).
+     *         Implementations may, but are not required to, throw this
+     *         exception if {@code fromKey} cannot be compared to keys
+     *         currently in the map.
+     * @throws NullPointerException if {@code fromKey} is null
+     *         and this map does not permit null keys
+     * @throws IllegalArgumentException if this map itself has a
+     *         restricted range, and {@code fromKey} lies outside the
+     *         bounds of the range
+     */
+    NavigableMap<K,V> tailMap(K fromKey, boolean inclusive);
+
+    /**
+     * {@inheritDoc}
+     *
+     * <p>Equivalent to {@code subMap(fromKey, true, toKey, false)}.
+     *
+     * @throws ClassCastException       {@inheritDoc}
+     * @throws NullPointerException     {@inheritDoc}
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    SortedMap<K,V> subMap(K fromKey, K toKey);
+
+    /**
+     * {@inheritDoc}
+     *
+     * <p>Equivalent to {@code headMap(toKey, false)}.
+     *
+     * @throws ClassCastException       {@inheritDoc}
+     * @throws NullPointerException     {@inheritDoc}
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    SortedMap<K,V> headMap(K toKey);
+
+    /**
+     * {@inheritDoc}
+     *
+     * <p>Equivalent to {@code tailMap(fromKey, true)}.
+     *
+     * @throws ClassCastException       {@inheritDoc}
+     * @throws NullPointerException     {@inheritDoc}
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    SortedMap<K,V> tailMap(K fromKey);
+}
diff --git a/external/jsr166/java/util/NavigableSet.java b/external/jsr166/java/util/NavigableSet.java
new file mode 100644
index 000000000..e14fe347d
--- /dev/null
+++ b/external/jsr166/java/util/NavigableSet.java
@@ -0,0 +1,290 @@
+/*
+ * Written by Doug Lea and Josh Bloch with assistance from members of JCP
+ * JSR-166 Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util;
+
+/**
+ * A {@link SortedSet} extended with navigation methods reporting
+ * closest matches for given search targets. Methods {@code lower},
+ * {@code floor}, {@code ceiling}, and {@code higher} return elements
+ * respectively less than, less than or equal, greater than or equal,
+ * and greater than a given element, returning {@code null} if there
+ * is no such element.  A {@code NavigableSet} may be accessed and
+ * traversed in either ascending or descending order.  The {@code
+ * descendingSet} method returns a view of the set with the senses of
+ * all relational and directional methods inverted. The performance of
+ * ascending operations and views is likely to be faster than that of
+ * descending ones.  This interface additionally defines methods
+ * {@code pollFirst} and {@code pollLast} that return and remove the
+ * lowest and highest element, if one exists, else returning {@code
+ * null}.  Methods {@code subSet}, {@code headSet},
+ * and {@code tailSet} differ from the like-named {@code
+ * SortedSet} methods in accepting additional arguments describing
+ * whether lower and upper bounds are inclusive versus exclusive.
+ * Subsets of any {@code NavigableSet} must implement the {@code
+ * NavigableSet} interface.
+ *
+ * <p> The return values of navigation methods may be ambiguous in
+ * implementations that permit {@code null} elements. However, even
+ * in this case the result can be disambiguated by checking
+ * {@code contains(null)}. To avoid such issues, implementations of
+ * this interface are encouraged to <em>not</em> permit insertion of
+ * {@code null} elements. (Note that sorted sets of {@link
+ * Comparable} elements intrinsically do not permit {@code null}.)
+ *
+ * <p>Methods
+ * {@link #subSet(Object, Object) subSet(E, E)},
+ * {@link #headSet(Object) headSet(E)}, and
+ * {@link #tailSet(Object) tailSet(E)}
+ * are specified to return {@code SortedSet} to allow existing
+ * implementations of {@code SortedSet} to be compatibly retrofitted to
+ * implement {@code NavigableSet}, but extensions and implementations
+ * of this interface are encouraged to override these methods to return
+ * {@code NavigableSet}.
+ *
+ * <p>This interface is a member of the
+ * <a href="{@docRoot}/../technotes/guides/collections/index.html">
+ * Java Collections Framework</a>.
+ *
+ * @author Doug Lea
+ * @author Josh Bloch
+ * @param <E> the type of elements maintained by this set
+ * @since 1.6
+ */
+public interface NavigableSet<E> extends SortedSet<E> {
+    /**
+     * Returns the greatest element in this set strictly less than the
+     * given element, or {@code null} if there is no such element.
+     *
+     * @param e the value to match
+     * @return the greatest element less than {@code e},
+     *         or {@code null} if there is no such element
+     * @throws ClassCastException if the specified element cannot be
+     *         compared with the elements currently in the set
+     * @throws NullPointerException if the specified element is null
+     *         and this set does not permit null elements
+     */
+    E lower(E e);
+
+    /**
+     * Returns the greatest element in this set less than or equal to
+     * the given element, or {@code null} if there is no such element.
+     *
+     * @param e the value to match
+     * @return the greatest element less than or equal to {@code e},
+     *         or {@code null} if there is no such element
+     * @throws ClassCastException if the specified element cannot be
+     *         compared with the elements currently in the set
+     * @throws NullPointerException if the specified element is null
+     *         and this set does not permit null elements
+     */
+    E floor(E e);
+
+    /**
+     * Returns the least element in this set greater than or equal to
+     * the given element, or {@code null} if there is no such element.
+     *
+     * @param e the value to match
+     * @return the least element greater than or equal to {@code e},
+     *         or {@code null} if there is no such element
+     * @throws ClassCastException if the specified element cannot be
+     *         compared with the elements currently in the set
+     * @throws NullPointerException if the specified element is null
+     *         and this set does not permit null elements
+     */
+    E ceiling(E e);
+
+    /**
+     * Returns the least element in this set strictly greater than the
+     * given element, or {@code null} if there is no such element.
+     *
+     * @param e the value to match
+     * @return the least element greater than {@code e},
+     *         or {@code null} if there is no such element
+     * @throws ClassCastException if the specified element cannot be
+     *         compared with the elements currently in the set
+     * @throws NullPointerException if the specified element is null
+     *         and this set does not permit null elements
+     */
+    E higher(E e);
+
+    /**
+     * Retrieves and removes the first (lowest) element,
+     * or returns {@code null} if this set is empty.
+     *
+     * @return the first element, or {@code null} if this set is empty
+     */
+    E pollFirst();
+
+    /**
+     * Retrieves and removes the last (highest) element,
+     * or returns {@code null} if this set is empty.
+     *
+     * @return the last element, or {@code null} if this set is empty
+     */
+    E pollLast();
+
+    /**
+     * Returns an iterator over the elements in this set, in ascending order.
+     *
+     * @return an iterator over the elements in this set, in ascending order
+     */
+    Iterator<E> iterator();
+
+    /**
+     * Returns a reverse order view of the elements contained in this set.
+     * The descending set is backed by this set, so changes to the set are
+     * reflected in the descending set, and vice-versa.  If either set is
+     * modified while an iteration over either set is in progress (except
+     * through the iterator's own {@code remove} operation), the results of
+     * the iteration are undefined.
+     *
+     * <p>The returned set has an ordering equivalent to
+     * <tt>{@link Collections#reverseOrder(Comparator) Collections.reverseOrder}(comparator())</tt>.
+     * The expression {@code s.descendingSet().descendingSet()} returns a
+     * view of {@code s} essentially equivalent to {@code s}.
+     *
+     * @return a reverse order view of this set
+     */
+    NavigableSet<E> descendingSet();
+
+    /**
+     * Returns an iterator over the elements in this set, in descending order.
+     * Equivalent in effect to {@code descendingSet().iterator()}.
+     *
+     * @return an iterator over the elements in this set, in descending order
+     */
+    Iterator<E> descendingIterator();
+
+    /**
+     * Returns a view of the portion of this set whose elements range from
+     * {@code fromElement} to {@code toElement}.  If {@code fromElement} and
+     * {@code toElement} are equal, the returned set is empty unless {@code
+     * fromExclusive} and {@code toExclusive} are both true.  The returned set
+     * is backed by this set, so changes in the returned set are reflected in
+     * this set, and vice-versa.  The returned set supports all optional set
+     * operations that this set supports.
+     *
+     * <p>The returned set will throw an {@code IllegalArgumentException}
+     * on an attempt to insert an element outside its range.
+     *
+     * @param fromElement low endpoint of the returned set
+     * @param fromInclusive {@code true} if the low endpoint
+     *        is to be included in the returned view
+     * @param toElement high endpoint of the returned set
+     * @param toInclusive {@code true} if the high endpoint
+     *        is to be included in the returned view
+     * @return a view of the portion of this set whose elements range from
+     *         {@code fromElement}, inclusive, to {@code toElement}, exclusive
+     * @throws ClassCastException if {@code fromElement} and
+     *         {@code toElement} cannot be compared to one another using this
+     *         set's comparator (or, if the set has no comparator, using
+     *         natural ordering).  Implementations may, but are not required
+     *         to, throw this exception if {@code fromElement} or
+     *         {@code toElement} cannot be compared to elements currently in
+     *         the set.
+     * @throws NullPointerException if {@code fromElement} or
+     *         {@code toElement} is null and this set does
+     *         not permit null elements
+     * @throws IllegalArgumentException if {@code fromElement} is
+     *         greater than {@code toElement}; or if this set itself
+     *         has a restricted range, and {@code fromElement} or
+     *         {@code toElement} lies outside the bounds of the range.
+     */
+    NavigableSet<E> subSet(E fromElement, boolean fromInclusive,
+                           E toElement,   boolean toInclusive);
+
+    /**
+     * Returns a view of the portion of this set whose elements are less than
+     * (or equal to, if {@code inclusive} is true) {@code toElement}.  The
+     * returned set is backed by this set, so changes in the returned set are
+     * reflected in this set, and vice-versa.  The returned set supports all
+     * optional set operations that this set supports.
+     *
+     * <p>The returned set will throw an {@code IllegalArgumentException}
+     * on an attempt to insert an element outside its range.
+     *
+     * @param toElement high endpoint of the returned set
+     * @param inclusive {@code true} if the high endpoint
+     *        is to be included in the returned view
+     * @return a view of the portion of this set whose elements are less than
+     *         (or equal to, if {@code inclusive} is true) {@code toElement}
+     * @throws ClassCastException if {@code toElement} is not compatible
+     *         with this set's comparator (or, if the set has no comparator,
+     *         if {@code toElement} does not implement {@link Comparable}).
+     *         Implementations may, but are not required to, throw this
+     *         exception if {@code toElement} cannot be compared to elements
+     *         currently in the set.
+     * @throws NullPointerException if {@code toElement} is null and
+     *         this set does not permit null elements
+     * @throws IllegalArgumentException if this set itself has a
+     *         restricted range, and {@code toElement} lies outside the
+     *         bounds of the range
+     */
+    NavigableSet<E> headSet(E toElement, boolean inclusive);
+
+    /**
+     * Returns a view of the portion of this set whose elements are greater
+     * than (or equal to, if {@code inclusive} is true) {@code fromElement}.
+     * The returned set is backed by this set, so changes in the returned set
+     * are reflected in this set, and vice-versa.  The returned set supports
+     * all optional set operations that this set supports.
+     *
+     * <p>The returned set will throw an {@code IllegalArgumentException}
+     * on an attempt to insert an element outside its range.
+     *
+     * @param fromElement low endpoint of the returned set
+     * @param inclusive {@code true} if the low endpoint
+     *        is to be included in the returned view
+     * @return a view of the portion of this set whose elements are greater
+     *         than or equal to {@code fromElement}
+     * @throws ClassCastException if {@code fromElement} is not compatible
+     *         with this set's comparator (or, if the set has no comparator,
+     *         if {@code fromElement} does not implement {@link Comparable}).
+     *         Implementations may, but are not required to, throw this
+     *         exception if {@code fromElement} cannot be compared to elements
+     *         currently in the set.
+     * @throws NullPointerException if {@code fromElement} is null
+     *         and this set does not permit null elements
+     * @throws IllegalArgumentException if this set itself has a
+     *         restricted range, and {@code fromElement} lies outside the
+     *         bounds of the range
+     */
+    NavigableSet<E> tailSet(E fromElement, boolean inclusive);
+
+    /**
+     * {@inheritDoc}
+     *
+     * <p>Equivalent to {@code subSet(fromElement, true, toElement, false)}.
+     *
+     * @throws ClassCastException       {@inheritDoc}
+     * @throws NullPointerException     {@inheritDoc}
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    SortedSet<E> subSet(E fromElement, E toElement);
+
+    /**
+     * {@inheritDoc}
+     *
+     * <p>Equivalent to {@code headSet(toElement, false)}.
+     *
+     * @throws ClassCastException       {@inheritDoc}
+     * @throws NullPointerException     {@inheritDoc}
+     * @throws IllegalArgumentException {@inheritDoc}
+na     */
+    SortedSet<E> headSet(E toElement);
+
+    /**
+     * {@inheritDoc}
+     *
+     * <p>Equivalent to {@code tailSet(fromElement, true)}.
+     *
+     * @throws ClassCastException       {@inheritDoc}
+     * @throws NullPointerException     {@inheritDoc}
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    SortedSet<E> tailSet(E fromElement);
+}
diff --git a/external/jsr166/java/util/Queue.java b/external/jsr166/java/util/Queue.java
new file mode 100644
index 000000000..5711545b0
--- /dev/null
+++ b/external/jsr166/java/util/Queue.java
@@ -0,0 +1,189 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util;
+
+/**
+ * A collection designed for holding elements prior to processing.
+ * Besides basic {@link java.util.Collection Collection} operations,
+ * queues provide additional insertion, extraction, and inspection
+ * operations.  Each of these methods exists in two forms: one throws
+ * an exception if the operation fails, the other returns a special
+ * value (either <tt>null</tt> or <tt>false</tt>, depending on the
+ * operation).  The latter form of the insert operation is designed
+ * specifically for use with capacity-restricted <tt>Queue</tt>
+ * implementations; in most implementations, insert operations cannot
+ * fail.
+ *
+ * <p>
+ * <table BORDER CELLPADDING=3 CELLSPACING=1>
+ *  <tr>
+ *    <td></td>
+ *    <td ALIGN=CENTER><em>Throws exception</em></td>
+ *    <td ALIGN=CENTER><em>Returns special value</em></td>
+ *  </tr>
+ *  <tr>
+ *    <td><b>Insert</b></td>
+ *    <td>{@link #add add(e)}</td>
+ *    <td>{@link #offer offer(e)}</td>
+ *  </tr>
+ *  <tr>
+ *    <td><b>Remove</b></td>
+ *    <td>{@link #remove remove()}</td>
+ *    <td>{@link #poll poll()}</td>
+ *  </tr>
+ *  <tr>
+ *    <td><b>Examine</b></td>
+ *    <td>{@link #element element()}</td>
+ *    <td>{@link #peek peek()}</td>
+ *  </tr>
+ * </table>
+ *
+ * <p>Queues typically, but do not necessarily, order elements in a
+ * FIFO (first-in-first-out) manner.  Among the exceptions are
+ * priority queues, which order elements according to a supplied
+ * comparator, or the elements' natural ordering, and LIFO queues (or
+ * stacks) which order the elements LIFO (last-in-first-out).
+ * Whatever the ordering used, the <em>head</em> of the queue is that
+ * element which would be removed by a call to {@link #remove() } or
+ * {@link #poll()}.  In a FIFO queue, all new elements are inserted at
+ * the <em> tail</em> of the queue. Other kinds of queues may use
+ * different placement rules.  Every <tt>Queue</tt> implementation
+ * must specify its ordering properties.
+ *
+ * <p>The {@link #offer offer} method inserts an element if possible,
+ * otherwise returning <tt>false</tt>.  This differs from the {@link
+ * java.util.Collection#add Collection.add} method, which can fail to
+ * add an element only by throwing an unchecked exception.  The
+ * <tt>offer</tt> method is designed for use when failure is a normal,
+ * rather than exceptional occurrence, for example, in fixed-capacity
+ * (or &quot;bounded&quot;) queues.
+ *
+ * <p>The {@link #remove()} and {@link #poll()} methods remove and
+ * return the head of the queue.
+ * Exactly which element is removed from the queue is a
+ * function of the queue's ordering policy, which differs from
+ * implementation to implementation. The <tt>remove()</tt> and
+ * <tt>poll()</tt> methods differ only in their behavior when the
+ * queue is empty: the <tt>remove()</tt> method throws an exception,
+ * while the <tt>poll()</tt> method returns <tt>null</tt>.
+ *
+ * <p>The {@link #element()} and {@link #peek()} methods return, but do
+ * not remove, the head of the queue.
+ *
+ * <p>The <tt>Queue</tt> interface does not define the <i>blocking queue
+ * methods</i>, which are common in concurrent programming.  These methods,
+ * which wait for elements to appear or for space to become available, are
+ * defined in the {@link java.util.concurrent.BlockingQueue} interface, which
+ * extends this interface.
+ *
+ * <p><tt>Queue</tt> implementations generally do not allow insertion
+ * of <tt>null</tt> elements, although some implementations, such as
+ * {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
+ * Even in the implementations that permit it, <tt>null</tt> should
+ * not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also
+ * used as a special return value by the <tt>poll</tt> method to
+ * indicate that the queue contains no elements.
+ *
+ * <p><tt>Queue</tt> implementations generally do not define
+ * element-based versions of methods <tt>equals</tt> and
+ * <tt>hashCode</tt> but instead inherit the identity based versions
+ * from class <tt>Object</tt>, because element-based equality is not
+ * always well-defined for queues with the same elements but different
+ * ordering properties.
+ *
+ *
+ * <p>This interface is a member of the
+ * <a href="{@docRoot}/../technotes/guides/collections/index.html">
+ * Java Collections Framework</a>.
+ *
+ * @see java.util.Collection
+ * @see LinkedList
+ * @see PriorityQueue
+ * @see java.util.concurrent.LinkedBlockingQueue
+ * @see java.util.concurrent.BlockingQueue
+ * @see java.util.concurrent.ArrayBlockingQueue
+ * @see java.util.concurrent.LinkedBlockingQueue
+ * @see java.util.concurrent.PriorityBlockingQueue
+ * @since 1.5
+ * @author Doug Lea
+ * @param <E> the type of elements held in this collection
+ */
+public interface Queue<E> extends Collection<E> {
+    /**
+     * Inserts the specified element into this queue if it is possible to do so
+     * immediately without violating capacity restrictions, returning
+     * <tt>true</tt> upon success and throwing an <tt>IllegalStateException</tt>
+     * if no space is currently available.
+     *
+     * @param e the element to add
+     * @return <tt>true</tt> (as specified by {@link Collection#add})
+     * @throws IllegalStateException if the element cannot be added at this
+     *         time due to capacity restrictions
+     * @throws ClassCastException if the class of the specified element
+     *         prevents it from being added to this queue
+     * @throws NullPointerException if the specified element is null and
+     *         this queue does not permit null elements
+     * @throws IllegalArgumentException if some property of this element
+     *         prevents it from being added to this queue
+     */
+    boolean add(E e);
+
+    /**
+     * Inserts the specified element into this queue if it is possible to do
+     * so immediately without violating capacity restrictions.
+     * When using a capacity-restricted queue, this method is generally
+     * preferable to {@link #add}, which can fail to insert an element only
+     * by throwing an exception.
+     *
+     * @param e the element to add
+     * @return <tt>true</tt> if the element was added to this queue, else
+     *         <tt>false</tt>
+     * @throws ClassCastException if the class of the specified element
+     *         prevents it from being added to this queue
+     * @throws NullPointerException if the specified element is null and
+     *         this queue does not permit null elements
+     * @throws IllegalArgumentException if some property of this element
+     *         prevents it from being added to this queue
+     */
+    boolean offer(E e);
+
+    /**
+     * Retrieves and removes the head of this queue.  This method differs
+     * from {@link #poll poll} only in that it throws an exception if this
+     * queue is empty.
+     *
+     * @return the head of this queue
+     * @throws NoSuchElementException if this queue is empty
+     */
+    E remove();
+
+    /**
+     * Retrieves and removes the head of this queue,
+     * or returns <tt>null</tt> if this queue is empty.
+     *
+     * @return the head of this queue, or <tt>null</tt> if this queue is empty
+     */
+    E poll();
+
+    /**
+     * Retrieves, but does not remove, the head of this queue.  This method
+     * differs from {@link #peek peek} only in that it throws an exception
+     * if this queue is empty.
+     *
+     * @return the head of this queue
+     * @throws NoSuchElementException if this queue is empty
+     */
+    E element();
+
+    /**
+     * Retrieves, but does not remove, the head of this queue,
+     * or returns <tt>null</tt> if this queue is empty.
+     *
+     * @return the head of this queue, or <tt>null</tt> if this queue is empty
+     */
+    E peek();
+}
diff --git a/external/jsr166/java/util/concurrent/AbstractExecutorService.java b/external/jsr166/java/util/concurrent/AbstractExecutorService.java
new file mode 100644
index 000000000..ac15c5010
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/AbstractExecutorService.java
@@ -0,0 +1,270 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+import java.util.*;
+
+/**
+ * Provides default implementations of {@link ExecutorService}
+ * execution methods. This class implements the <tt>submit</tt>,
+ * <tt>invokeAny</tt> and <tt>invokeAll</tt> methods using a
+ * {@link RunnableFuture} returned by <tt>newTaskFor</tt>, which defaults
+ * to the {@link FutureTask} class provided in this package.  For example,
+ * the implementation of <tt>submit(Runnable)</tt> creates an
+ * associated <tt>RunnableFuture</tt> that is executed and
+ * returned. Subclasses may override the <tt>newTaskFor</tt> methods
+ * to return <tt>RunnableFuture</tt> implementations other than
+ * <tt>FutureTask</tt>.
+ *
+ * <p> <b>Extension example</b>. Here is a sketch of a class
+ * that customizes {@link ThreadPoolExecutor} to use
+ * a <tt>CustomTask</tt> class instead of the default <tt>FutureTask</tt>:
+ * <pre>
+ * public class CustomThreadPoolExecutor extends ThreadPoolExecutor {
+ *
+ *   static class CustomTask&lt;V&gt; implements RunnableFuture&lt;V&gt; {...}
+ *
+ *   protected &lt;V&gt; RunnableFuture&lt;V&gt; newTaskFor(Callable&lt;V&gt; c) {
+ *       return new CustomTask&lt;V&gt;(c);
+ *   }
+ *   protected &lt;V&gt; RunnableFuture&lt;V&gt; newTaskFor(Runnable r, V v) {
+ *       return new CustomTask&lt;V&gt;(r, v);
+ *   }
+ *   // ... add constructors, etc.
+ * }
+ * </pre>
+ * @since 1.5
+ * @author Doug Lea
+ */
+public abstract class AbstractExecutorService implements ExecutorService {
+
+    /**
+     * Returns a <tt>RunnableFuture</tt> for the given runnable and default
+     * value.
+     *
+     * @param runnable the runnable task being wrapped
+     * @param value the default value for the returned future
+     * @return a <tt>RunnableFuture</tt> which when run will run the
+     * underlying runnable and which, as a <tt>Future</tt>, will yield
+     * the given value as its result and provide for cancellation of
+     * the underlying task.
+     * @since 1.6
+     */
+    protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
+        return new FutureTask<T>(runnable, value);
+    }
+
+    /**
+     * Returns a <tt>RunnableFuture</tt> for the given callable task.
+     *
+     * @param callable the callable task being wrapped
+     * @return a <tt>RunnableFuture</tt> which when run will call the
+     * underlying callable and which, as a <tt>Future</tt>, will yield
+     * the callable's result as its result and provide for
+     * cancellation of the underlying task.
+     * @since 1.6
+     */
+    protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
+        return new FutureTask<T>(callable);
+    }
+
+    public Future<?> submit(Runnable task) {
+        if (task == null) throw new NullPointerException();
+        RunnableFuture<Object> ftask = newTaskFor(task, null);
+        execute(ftask);
+        return ftask;
+    }
+
+    public <T> Future<T> submit(Runnable task, T result) {
+        if (task == null) throw new NullPointerException();
+        RunnableFuture<T> ftask = newTaskFor(task, result);
+        execute(ftask);
+        return ftask;
+    }
+
+    public <T> Future<T> submit(Callable<T> task) {
+        if (task == null) throw new NullPointerException();
+        RunnableFuture<T> ftask = newTaskFor(task);
+        execute(ftask);
+        return ftask;
+    }
+
+    /**
+     * the main mechanics of invokeAny.
+     */
+    private <T> T doInvokeAny(Collection<? extends Callable<T>> tasks,
+                            boolean timed, long nanos)
+        throws InterruptedException, ExecutionException, TimeoutException {
+        if (tasks == null)
+            throw new NullPointerException();
+        int ntasks = tasks.size();
+        if (ntasks == 0)
+            throw new IllegalArgumentException();
+        List<Future<T>> futures= new ArrayList<Future<T>>(ntasks);
+        ExecutorCompletionService<T> ecs =
+            new ExecutorCompletionService<T>(this);
+
+        // For efficiency, especially in executors with limited
+        // parallelism, check to see if previously submitted tasks are
+        // done before submitting more of them. This interleaving
+        // plus the exception mechanics account for messiness of main
+        // loop.
+
+        try {
+            // Record exceptions so that if we fail to obtain any
+            // result, we can throw the last exception we got.
+            ExecutionException ee = null;
+            long lastTime = (timed)? System.nanoTime() : 0;
+            Iterator<? extends Callable<T>> it = tasks.iterator();
+
+            // Start one task for sure; the rest incrementally
+            futures.add(ecs.submit(it.next()));
+            --ntasks;
+            int active = 1;
+
+            for (;;) {
+                Future<T> f = ecs.poll();
+                if (f == null) {
+                    if (ntasks > 0) {
+                        --ntasks;
+                        futures.add(ecs.submit(it.next()));
+                        ++active;
+                    }
+                    else if (active == 0)
+                        break;
+                    else if (timed) {
+                        f = ecs.poll(nanos, TimeUnit.NANOSECONDS);
+                        if (f == null)
+                            throw new TimeoutException();
+                        long now = System.nanoTime();
+                        nanos -= now - lastTime;
+                        lastTime = now;
+                    }
+                    else
+                        f = ecs.take();
+                }
+                if (f != null) {
+                    --active;
+                    try {
+                        return f.get();
+                    } catch (InterruptedException ie) {
+                        throw ie;
+                    } catch (ExecutionException eex) {
+                        ee = eex;
+                    } catch (RuntimeException rex) {
+                        ee = new ExecutionException(rex);
+                    }
+                }
+            }
+
+            if (ee == null)
+                ee = new ExecutionException();
+            throw ee;
+
+        } finally {
+            for (Future<T> f : futures)
+                f.cancel(true);
+        }
+    }
+
+    public <T> T invokeAny(Collection<? extends Callable<T>> tasks)
+        throws InterruptedException, ExecutionException {
+        try {
+            return doInvokeAny(tasks, false, 0);
+        } catch (TimeoutException cannotHappen) {
+            assert false;
+            return null;
+        }
+    }
+
+    public <T> T invokeAny(Collection<? extends Callable<T>> tasks,
+                           long timeout, TimeUnit unit)
+        throws InterruptedException, ExecutionException, TimeoutException {
+        return doInvokeAny(tasks, true, unit.toNanos(timeout));
+    }
+
+    public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
+        throws InterruptedException {
+        if (tasks == null)
+            throw new NullPointerException();
+        List<Future<T>> futures = new ArrayList<Future<T>>(tasks.size());
+        boolean done = false;
+        try {
+            for (Callable<T> t : tasks) {
+                RunnableFuture<T> f = newTaskFor(t);
+                futures.add(f);
+                execute(f);
+            }
+            for (Future<T> f : futures) {
+                if (!f.isDone()) {
+                    try {
+                        f.get();
+                    } catch (CancellationException ignore) {
+                    } catch (ExecutionException ignore) {
+                    }
+                }
+            }
+            done = true;
+            return futures;
+        } finally {
+            if (!done)
+                for (Future<T> f : futures)
+                    f.cancel(true);
+        }
+    }
+
+    public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,
+                                         long timeout, TimeUnit unit)
+        throws InterruptedException {
+        if (tasks == null || unit == null)
+            throw new NullPointerException();
+        long nanos = unit.toNanos(timeout);
+        List<Future<T>> futures = new ArrayList<Future<T>>(tasks.size());
+        boolean done = false;
+        try {
+            for (Callable<T> t : tasks)
+                futures.add(newTaskFor(t));
+
+            long lastTime = System.nanoTime();
+
+            // Interleave time checks and calls to execute in case
+            // executor doesn't have any/much parallelism.
+            Iterator<Future<T>> it = futures.iterator();
+            while (it.hasNext()) {
+                execute((Runnable)(it.next()));
+                long now = System.nanoTime();
+                nanos -= now - lastTime;
+                lastTime = now;
+                if (nanos <= 0)
+                    return futures;
+            }
+
+            for (Future<T> f : futures) {
+                if (!f.isDone()) {
+                    if (nanos <= 0)
+                        return futures;
+                    try {
+                        f.get(nanos, TimeUnit.NANOSECONDS);
+                    } catch (CancellationException ignore) {
+                    } catch (ExecutionException ignore) {
+                    } catch (TimeoutException toe) {
+                        return futures;
+                    }
+                    long now = System.nanoTime();
+                    nanos -= now - lastTime;
+                    lastTime = now;
+                }
+            }
+            done = true;
+            return futures;
+        } finally {
+            if (!done)
+                for (Future<T> f : futures)
+                    f.cancel(true);
+        }
+    }
+
+}
diff --git a/external/jsr166/java/util/concurrent/ArrayBlockingQueue.java b/external/jsr166/java/util/concurrent/ArrayBlockingQueue.java
new file mode 100644
index 000000000..bdf34d28a
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/ArrayBlockingQueue.java
@@ -0,0 +1,778 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+import java.util.concurrent.locks.*;
+import java.util.*;
+
+/**
+ * A bounded {@linkplain BlockingQueue blocking queue} backed by an
+ * array.  This queue orders elements FIFO (first-in-first-out).  The
+ * <em>head</em> of the queue is that element that has been on the
+ * queue the longest time.  The <em>tail</em> of the queue is that
+ * element that has been on the queue the shortest time. New elements
+ * are inserted at the tail of the queue, and the queue retrieval
+ * operations obtain elements at the head of the queue.
+ *
+ * <p>This is a classic &quot;bounded buffer&quot;, in which a
+ * fixed-sized array holds elements inserted by producers and
+ * extracted by consumers.  Once created, the capacity cannot be
+ * increased.  Attempts to <tt>put</tt> an element into a full queue
+ * will result in the operation blocking; attempts to <tt>take</tt> an
+ * element from an empty queue will similarly block.
+ *
+ * <p> This class supports an optional fairness policy for ordering
+ * waiting producer and consumer threads.  By default, this ordering
+ * is not guaranteed. However, a queue constructed with fairness set
+ * to <tt>true</tt> grants threads access in FIFO order. Fairness
+ * generally decreases throughput but reduces variability and avoids
+ * starvation.
+ *
+ * <p>This class and its iterator implement all of the
+ * <em>optional</em> methods of the {@link Collection} and {@link
+ * Iterator} interfaces.
+ *
+ * <p>This class is a member of the
+ * <a href="{@docRoot}/../technotes/guides/collections/index.html">
+ * Java Collections Framework</a>.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ * @param <E> the type of elements held in this collection
+ */
+public class ArrayBlockingQueue<E> extends AbstractQueue<E>
+        implements BlockingQueue<E>, java.io.Serializable {
+
+    /**
+     * Serialization ID. This class relies on default serialization
+     * even for the items array, which is default-serialized, even if
+     * it is empty. Otherwise it could not be declared final, which is
+     * necessary here.
+     */
+    private static final long serialVersionUID = -817911632652898426L;
+
+    /** The queued items  */
+    private final E[] items;
+    /** items index for next take, poll or remove */
+    private int takeIndex;
+    /** items index for next put, offer, or add. */
+    private int putIndex;
+    /** Number of items in the queue */
+    private int count;
+
+    /*
+     * Concurrency control uses the classic two-condition algorithm
+     * found in any textbook.
+     */
+
+    /** Main lock guarding all access */
+    private final ReentrantLock lock;
+    /** Condition for waiting takes */
+    private final Condition notEmpty;
+    /** Condition for waiting puts */
+    private final Condition notFull;
+
+    // Internal helper methods
+
+    /**
+     * Circularly increment i.
+     */
+    final int inc(int i) {
+        return (++i == items.length)? 0 : i;
+    }
+
+    /**
+     * Inserts element at current put position, advances, and signals.
+     * Call only when holding lock.
+     */
+    private void insert(E x) {
+        items[putIndex] = x;
+        putIndex = inc(putIndex);
+        ++count;
+        notEmpty.signal();
+    }
+
+    /**
+     * Extracts element at current take position, advances, and signals.
+     * Call only when holding lock.
+     */
+    private E extract() {
+        final E[] items = this.items;
+        E x = items[takeIndex];
+        items[takeIndex] = null;
+        takeIndex = inc(takeIndex);
+        --count;
+        notFull.signal();
+        return x;
+    }
+
+    /**
+     * Utility for remove and iterator.remove: Delete item at position i.
+     * Call only when holding lock.
+     */
+    void removeAt(int i) {
+        final E[] items = this.items;
+        // if removing front item, just advance
+        if (i == takeIndex) {
+            items[takeIndex] = null;
+            takeIndex = inc(takeIndex);
+        } else {
+            // slide over all others up through putIndex.
+            for (;;) {
+                int nexti = inc(i);
+                if (nexti != putIndex) {
+                    items[i] = items[nexti];
+                    i = nexti;
+                } else {
+                    items[i] = null;
+                    putIndex = i;
+                    break;
+                }
+            }
+        }
+        --count;
+        notFull.signal();
+    }
+
+    /**
+     * Creates an <tt>ArrayBlockingQueue</tt> with the given (fixed)
+     * capacity and default access policy.
+     *
+     * @param capacity the capacity of this queue
+     * @throws IllegalArgumentException if <tt>capacity</tt> is less than 1
+     */
+    public ArrayBlockingQueue(int capacity) {
+        this(capacity, false);
+    }
+
+    /**
+     * Creates an <tt>ArrayBlockingQueue</tt> with the given (fixed)
+     * capacity and the specified access policy.
+     *
+     * @param capacity the capacity of this queue
+     * @param fair if <tt>true</tt> then queue accesses for threads blocked
+     *        on insertion or removal, are processed in FIFO order;
+     *        if <tt>false</tt> the access order is unspecified.
+     * @throws IllegalArgumentException if <tt>capacity</tt> is less than 1
+     */
+    public ArrayBlockingQueue(int capacity, boolean fair) {
+        if (capacity <= 0)
+            throw new IllegalArgumentException();
+        this.items = (E[]) new Object[capacity];
+        lock = new ReentrantLock(fair);
+        notEmpty = lock.newCondition();
+        notFull =  lock.newCondition();
+    }
+
+    /**
+     * Creates an <tt>ArrayBlockingQueue</tt> with the given (fixed)
+     * capacity, the specified access policy and initially containing the
+     * elements of the given collection,
+     * added in traversal order of the collection's iterator.
+     *
+     * @param capacity the capacity of this queue
+     * @param fair if <tt>true</tt> then queue accesses for threads blocked
+     *        on insertion or removal, are processed in FIFO order;
+     *        if <tt>false</tt> the access order is unspecified.
+     * @param c the collection of elements to initially contain
+     * @throws IllegalArgumentException if <tt>capacity</tt> is less than
+     *         <tt>c.size()</tt>, or less than 1.
+     * @throws NullPointerException if the specified collection or any
+     *         of its elements are null
+     */
+    public ArrayBlockingQueue(int capacity, boolean fair,
+                              Collection<? extends E> c) {
+        this(capacity, fair);
+        if (capacity < c.size())
+            throw new IllegalArgumentException();
+
+        for (Iterator<? extends E> it = c.iterator(); it.hasNext();)
+            add(it.next());
+    }
+
+    /**
+     * Inserts the specified element at the tail of this queue if it is
+     * possible to do so immediately without exceeding the queue's capacity,
+     * returning <tt>true</tt> upon success and throwing an
+     * <tt>IllegalStateException</tt> if this queue is full.
+     *
+     * @param e the element to add
+     * @return <tt>true</tt> (as specified by {@link Collection#add})
+     * @throws IllegalStateException if this queue is full
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean add(E e) {
+	return super.add(e);
+    }
+
+    /**
+     * Inserts the specified element at the tail of this queue if it is
+     * possible to do so immediately without exceeding the queue's capacity,
+     * returning <tt>true</tt> upon success and <tt>false</tt> if this queue
+     * is full.  This method is generally preferable to method {@link #add},
+     * which can fail to insert an element only by throwing an exception.
+     *
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean offer(E e) {
+        if (e == null) throw new NullPointerException();
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            if (count == items.length)
+                return false;
+            else {
+                insert(e);
+                return true;
+            }
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Inserts the specified element at the tail of this queue, waiting
+     * for space to become available if the queue is full.
+     *
+     * @throws InterruptedException {@inheritDoc}
+     * @throws NullPointerException {@inheritDoc}
+     */
+    public void put(E e) throws InterruptedException {
+        if (e == null) throw new NullPointerException();
+        final E[] items = this.items;
+        final ReentrantLock lock = this.lock;
+        lock.lockInterruptibly();
+        try {
+            try {
+                while (count == items.length)
+                    notFull.await();
+            } catch (InterruptedException ie) {
+                notFull.signal(); // propagate to non-interrupted thread
+                throw ie;
+            }
+            insert(e);
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Inserts the specified element at the tail of this queue, waiting
+     * up to the specified wait time for space to become available if
+     * the queue is full.
+     *
+     * @throws InterruptedException {@inheritDoc}
+     * @throws NullPointerException {@inheritDoc}
+     */
+    public boolean offer(E e, long timeout, TimeUnit unit)
+        throws InterruptedException {
+
+        if (e == null) throw new NullPointerException();
+	long nanos = unit.toNanos(timeout);
+        final ReentrantLock lock = this.lock;
+        lock.lockInterruptibly();
+        try {
+            for (;;) {
+                if (count != items.length) {
+                    insert(e);
+                    return true;
+                }
+                if (nanos <= 0)
+                    return false;
+                try {
+                    nanos = notFull.awaitNanos(nanos);
+                } catch (InterruptedException ie) {
+                    notFull.signal(); // propagate to non-interrupted thread
+                    throw ie;
+                }
+            }
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    public E poll() {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            if (count == 0)
+                return null;
+            E x = extract();
+            return x;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    public E take() throws InterruptedException {
+        final ReentrantLock lock = this.lock;
+        lock.lockInterruptibly();
+        try {
+            try {
+                while (count == 0)
+                    notEmpty.await();
+            } catch (InterruptedException ie) {
+                notEmpty.signal(); // propagate to non-interrupted thread
+                throw ie;
+            }
+            E x = extract();
+            return x;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    public E poll(long timeout, TimeUnit unit) throws InterruptedException {
+	long nanos = unit.toNanos(timeout);
+        final ReentrantLock lock = this.lock;
+        lock.lockInterruptibly();
+        try {
+            for (;;) {
+                if (count != 0) {
+                    E x = extract();
+                    return x;
+                }
+                if (nanos <= 0)
+                    return null;
+                try {
+                    nanos = notEmpty.awaitNanos(nanos);
+                } catch (InterruptedException ie) {
+                    notEmpty.signal(); // propagate to non-interrupted thread
+                    throw ie;
+                }
+
+            }
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    public E peek() {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            return (count == 0) ? null : items[takeIndex];
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    // this doc comment is overridden to remove the reference to collections
+    // greater in size than Integer.MAX_VALUE
+    /**
+     * Returns the number of elements in this queue.
+     *
+     * @return the number of elements in this queue
+     */
+    public int size() {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            return count;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    // this doc comment is a modified copy of the inherited doc comment,
+    // without the reference to unlimited queues.
+    /**
+     * Returns the number of additional elements that this queue can ideally
+     * (in the absence of memory or resource constraints) accept without
+     * blocking. This is always equal to the initial capacity of this queue
+     * less the current <tt>size</tt> of this queue.
+     *
+     * <p>Note that you <em>cannot</em> always tell if an attempt to insert
+     * an element will succeed by inspecting <tt>remainingCapacity</tt>
+     * because it may be the case that another thread is about to
+     * insert or remove an element.
+     */
+    public int remainingCapacity() {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            return items.length - count;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Removes a single instance of the specified element from this queue,
+     * if it is present.  More formally, removes an element <tt>e</tt> such
+     * that <tt>o.equals(e)</tt>, if this queue contains one or more such
+     * elements.
+     * Returns <tt>true</tt> if this queue contained the specified element
+     * (or equivalently, if this queue changed as a result of the call).
+     *
+     * @param o element to be removed from this queue, if present
+     * @return <tt>true</tt> if this queue changed as a result of the call
+     */
+    public boolean remove(Object o) {
+        if (o == null) return false;
+        final E[] items = this.items;
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            int i = takeIndex;
+            int k = 0;
+            for (;;) {
+                if (k++ >= count)
+                    return false;
+                if (o.equals(items[i])) {
+                    removeAt(i);
+                    return true;
+                }
+                i = inc(i);
+            }
+
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Returns <tt>true</tt> if this queue contains the specified element.
+     * More formally, returns <tt>true</tt> if and only if this queue contains
+     * at least one element <tt>e</tt> such that <tt>o.equals(e)</tt>.
+     *
+     * @param o object to be checked for containment in this queue
+     * @return <tt>true</tt> if this queue contains the specified element
+     */
+    public boolean contains(Object o) {
+        if (o == null) return false;
+        final E[] items = this.items;
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            int i = takeIndex;
+            int k = 0;
+            while (k++ < count) {
+                if (o.equals(items[i]))
+                    return true;
+                i = inc(i);
+            }
+            return false;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Returns an array containing all of the elements in this queue, in
+     * proper sequence.
+     *
+     * <p>The returned array will be "safe" in that no references to it are
+     * maintained by this queue.  (In other words, this method must allocate
+     * a new array).  The caller is thus free to modify the returned array.
+     *
+     * <p>This method acts as bridge between array-based and collection-based
+     * APIs.
+     *
+     * @return an array containing all of the elements in this queue
+     */
+    public Object[] toArray() {
+        final E[] items = this.items;
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            Object[] a = new Object[count];
+            int k = 0;
+            int i = takeIndex;
+            while (k < count) {
+                a[k++] = items[i];
+                i = inc(i);
+            }
+            return a;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Returns an array containing all of the elements in this queue, in
+     * proper sequence; the runtime type of the returned array is that of
+     * the specified array.  If the queue fits in the specified array, it
+     * is returned therein.  Otherwise, a new array is allocated with the
+     * runtime type of the specified array and the size of this queue.
+     *
+     * <p>If this queue fits in the specified array with room to spare
+     * (i.e., the array has more elements than this queue), the element in
+     * the array immediately following the end of the queue is set to
+     * <tt>null</tt>.
+     *
+     * <p>Like the {@link #toArray()} method, this method acts as bridge between
+     * array-based and collection-based APIs.  Further, this method allows
+     * precise control over the runtime type of the output array, and may,
+     * under certain circumstances, be used to save allocation costs.
+     *
+     * <p>Suppose <tt>x</tt> is a queue known to contain only strings.
+     * The following code can be used to dump the queue into a newly
+     * allocated array of <tt>String</tt>:
+     *
+     * <pre>
+     *     String[] y = x.toArray(new String[0]);</pre>
+     *
+     * Note that <tt>toArray(new Object[0])</tt> is identical in function to
+     * <tt>toArray()</tt>.
+     *
+     * @param a the array into which the elements of the queue are to
+     *          be stored, if it is big enough; otherwise, a new array of the
+     *          same runtime type is allocated for this purpose
+     * @return an array containing all of the elements in this queue
+     * @throws ArrayStoreException if the runtime type of the specified array
+     *         is not a supertype of the runtime type of every element in
+     *         this queue
+     * @throws NullPointerException if the specified array is null
+     */
+    public <T> T[] toArray(T[] a) {
+        final E[] items = this.items;
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            if (a.length < count)
+                a = (T[])java.lang.reflect.Array.newInstance(
+                    a.getClass().getComponentType(),
+                    count
+                    );
+
+            int k = 0;
+            int i = takeIndex;
+            while (k < count) {
+                a[k++] = (T)items[i];
+                i = inc(i);
+            }
+            if (a.length > count)
+                a[count] = null;
+            return a;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    public String toString() {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            return super.toString();
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Atomically removes all of the elements from this queue.
+     * The queue will be empty after this call returns.
+     */
+    public void clear() {
+        final E[] items = this.items;
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            int i = takeIndex;
+            int k = count;
+            while (k-- > 0) {
+                items[i] = null;
+                i = inc(i);
+            }
+            count = 0;
+            putIndex = 0;
+            takeIndex = 0;
+            notFull.signalAll();
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * @throws UnsupportedOperationException {@inheritDoc}
+     * @throws ClassCastException            {@inheritDoc}
+     * @throws NullPointerException          {@inheritDoc}
+     * @throws IllegalArgumentException      {@inheritDoc}
+     */
+    public int drainTo(Collection<? super E> c) {
+        if (c == null)
+            throw new NullPointerException();
+        if (c == this)
+            throw new IllegalArgumentException();
+        final E[] items = this.items;
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            int i = takeIndex;
+            int n = 0;
+            int max = count;
+            while (n < max) {
+                c.add(items[i]);
+                items[i] = null;
+                i = inc(i);
+                ++n;
+            }
+            if (n > 0) {
+                count = 0;
+                putIndex = 0;
+                takeIndex = 0;
+                notFull.signalAll();
+            }
+            return n;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * @throws UnsupportedOperationException {@inheritDoc}
+     * @throws ClassCastException            {@inheritDoc}
+     * @throws NullPointerException          {@inheritDoc}
+     * @throws IllegalArgumentException      {@inheritDoc}
+     */
+    public int drainTo(Collection<? super E> c, int maxElements) {
+        if (c == null)
+            throw new NullPointerException();
+        if (c == this)
+            throw new IllegalArgumentException();
+        if (maxElements <= 0)
+            return 0;
+        final E[] items = this.items;
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            int i = takeIndex;
+            int n = 0;
+            int sz = count;
+            int max = (maxElements < count)? maxElements : count;
+            while (n < max) {
+                c.add(items[i]);
+                items[i] = null;
+                i = inc(i);
+                ++n;
+            }
+            if (n > 0) {
+                count -= n;
+                takeIndex = i;
+                notFull.signalAll();
+            }
+            return n;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+
+    /**
+     * Returns an iterator over the elements in this queue in proper sequence.
+     * The returned <tt>Iterator</tt> is a "weakly consistent" iterator that
+     * will never throw {@link ConcurrentModificationException},
+     * and guarantees to traverse elements as they existed upon
+     * construction of the iterator, and may (but is not guaranteed to)
+     * reflect any modifications subsequent to construction.
+     *
+     * @return an iterator over the elements in this queue in proper sequence
+     */
+    public Iterator<E> iterator() {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            return new Itr();
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Iterator for ArrayBlockingQueue
+     */
+    private class Itr implements Iterator<E> {
+        /**
+         * Index of element to be returned by next,
+         * or a negative number if no such.
+         */
+        private int nextIndex;
+
+        /**
+         * nextItem holds on to item fields because once we claim
+         * that an element exists in hasNext(), we must return it in
+         * the following next() call even if it was in the process of
+         * being removed when hasNext() was called.
+         */
+        private E nextItem;
+
+        /**
+         * Index of element returned by most recent call to next.
+         * Reset to -1 if this element is deleted by a call to remove.
+         */
+        private int lastRet;
+
+        Itr() {
+            lastRet = -1;
+            if (count == 0)
+                nextIndex = -1;
+            else {
+                nextIndex = takeIndex;
+                nextItem = items[takeIndex];
+            }
+        }
+
+        public boolean hasNext() {
+            /*
+             * No sync. We can return true by mistake here
+             * only if this iterator passed across threads,
+             * which we don't support anyway.
+             */
+            return nextIndex >= 0;
+        }
+
+        /**
+         * Checks whether nextIndex is valid; if so setting nextItem.
+         * Stops iterator when either hits putIndex or sees null item.
+         */
+        private void checkNext() {
+            if (nextIndex == putIndex) {
+                nextIndex = -1;
+                nextItem = null;
+            } else {
+                nextItem = items[nextIndex];
+                if (nextItem == null)
+                    nextIndex = -1;
+            }
+        }
+
+        public E next() {
+            final ReentrantLock lock = ArrayBlockingQueue.this.lock;
+            lock.lock();
+            try {
+                if (nextIndex < 0)
+                    throw new NoSuchElementException();
+                lastRet = nextIndex;
+                E x = nextItem;
+                nextIndex = inc(nextIndex);
+                checkNext();
+                return x;
+            } finally {
+                lock.unlock();
+            }
+        }
+
+        public void remove() {
+            final ReentrantLock lock = ArrayBlockingQueue.this.lock;
+            lock.lock();
+            try {
+                int i = lastRet;
+                if (i == -1)
+                    throw new IllegalStateException();
+                lastRet = -1;
+
+                int ti = takeIndex;
+                removeAt(i);
+                // back up cursor (reset to front if was first element)
+                nextIndex = (i == ti) ? takeIndex : i;
+                checkNext();
+            } finally {
+                lock.unlock();
+            }
+        }
+    }
+}
diff --git a/external/jsr166/java/util/concurrent/BlockingDeque.java b/external/jsr166/java/util/concurrent/BlockingDeque.java
new file mode 100644
index 000000000..d77a96555
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/BlockingDeque.java
@@ -0,0 +1,613 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+import java.util.*;
+
+/**
+ * A {@link Deque} that additionally supports blocking operations that wait
+ * for the deque to become non-empty when retrieving an element, and wait for
+ * space to become available in the deque when storing an element.
+ *
+ * <p><tt>BlockingDeque</tt> methods come in four forms, with different ways
+ * of handling operations that cannot be satisfied immediately, but may be
+ * satisfied at some point in the future:
+ * one throws an exception, the second returns a special value (either
+ * <tt>null</tt> or <tt>false</tt>, depending on the operation), the third
+ * blocks the current thread indefinitely until the operation can succeed,
+ * and the fourth blocks for only a given maximum time limit before giving
+ * up.  These methods are summarized in the following table:
+ *
+ * <p>
+ * <table BORDER CELLPADDING=3 CELLSPACING=1>
+ *  <tr>
+ *    <td ALIGN=CENTER COLSPAN = 5> <b>First Element (Head)</b></td>
+ *  </tr>
+ *  <tr>
+ *    <td></td>
+ *    <td ALIGN=CENTER><em>Throws exception</em></td>
+ *    <td ALIGN=CENTER><em>Special value</em></td>
+ *    <td ALIGN=CENTER><em>Blocks</em></td>
+ *    <td ALIGN=CENTER><em>Times out</em></td>
+ *  </tr>
+ *  <tr>
+ *    <td><b>Insert</b></td>
+ *    <td>{@link #addFirst addFirst(e)}</td>
+ *    <td>{@link #offerFirst(Object) offerFirst(e)}</td>
+ *    <td>{@link #putFirst putFirst(e)}</td>
+ *    <td>{@link #offerFirst(Object, long, TimeUnit) offerFirst(e, time, unit)}</td>
+ *  </tr>
+ *  <tr>
+ *    <td><b>Remove</b></td>
+ *    <td>{@link #removeFirst removeFirst()}</td>
+ *    <td>{@link #pollFirst pollFirst()}</td>
+ *    <td>{@link #takeFirst takeFirst()}</td>
+ *    <td>{@link #pollFirst(long, TimeUnit) pollFirst(time, unit)}</td>
+ *  </tr>
+ *  <tr>
+ *    <td><b>Examine</b></td>
+ *    <td>{@link #getFirst getFirst()}</td>
+ *    <td>{@link #peekFirst peekFirst()}</td>
+ *    <td><em>not applicable</em></td>
+ *    <td><em>not applicable</em></td>
+ *  </tr>
+ *  <tr>
+ *    <td ALIGN=CENTER COLSPAN = 5> <b>Last Element (Tail)</b></td>
+ *  </tr>
+ *  <tr>
+ *    <td></td>
+ *    <td ALIGN=CENTER><em>Throws exception</em></td>
+ *    <td ALIGN=CENTER><em>Special value</em></td>
+ *    <td ALIGN=CENTER><em>Blocks</em></td>
+ *    <td ALIGN=CENTER><em>Times out</em></td>
+ *  </tr>
+ *  <tr>
+ *    <td><b>Insert</b></td>
+ *    <td>{@link #addLast addLast(e)}</td>
+ *    <td>{@link #offerLast(Object) offerLast(e)}</td>
+ *    <td>{@link #putLast putLast(e)}</td>
+ *    <td>{@link #offerLast(Object, long, TimeUnit) offerLast(e, time, unit)}</td>
+ *  </tr>
+ *  <tr>
+ *    <td><b>Remove</b></td>
+ *    <td>{@link #removeLast() removeLast()}</td>
+ *    <td>{@link #pollLast() pollLast()}</td>
+ *    <td>{@link #takeLast takeLast()}</td>
+ *    <td>{@link #pollLast(long, TimeUnit) pollLast(time, unit)}</td>
+ *  </tr>
+ *  <tr>
+ *    <td><b>Examine</b></td>
+ *    <td>{@link #getLast getLast()}</td>
+ *    <td>{@link #peekLast peekLast()}</td>
+ *    <td><em>not applicable</em></td>
+ *    <td><em>not applicable</em></td>
+ *  </tr>
+ * </table>
+ *
+ * <p>Like any {@link BlockingQueue}, a <tt>BlockingDeque</tt> is thread safe,
+ * does not permit null elements, and may (or may not) be
+ * capacity-constrained.
+ *
+ * <p>A <tt>BlockingDeque</tt> implementation may be used directly as a FIFO
+ * <tt>BlockingQueue</tt>. The methods inherited from the
+ * <tt>BlockingQueue</tt> interface are precisely equivalent to
+ * <tt>BlockingDeque</tt> methods as indicated in the following table:
+ *
+ * <p>
+ * <table BORDER CELLPADDING=3 CELLSPACING=1>
+ *  <tr>
+ *    <td ALIGN=CENTER> <b><tt>BlockingQueue</tt> Method</b></td>
+ *    <td ALIGN=CENTER> <b>Equivalent <tt>BlockingDeque</tt> Method</b></td>
+ *  </tr>
+ *  <tr>
+ *    <td ALIGN=CENTER COLSPAN = 2> <b>Insert</b></td>
+ *  </tr>
+ *  <tr>
+ *    <td>{@link #add(Object) add(e)}</td>
+ *    <td>{@link #addLast(Object) addLast(e)}</td>
+ *  </tr>
+ *  <tr>
+ *    <td>{@link #offer(Object) offer(e)}</td>
+ *    <td>{@link #offerLast(Object) offerLast(e)}</td>
+ *  </tr>
+ *  <tr>
+ *    <td>{@link #put(Object) put(e)}</td>
+ *    <td>{@link #putLast(Object) putLast(e)}</td>
+ *  </tr>
+ *  <tr>
+ *    <td>{@link #offer(Object, long, TimeUnit) offer(e, time, unit)}</td>
+ *    <td>{@link #offerLast(Object, long, TimeUnit) offerLast(e, time, unit)}</td>
+ *  </tr>
+ *  <tr>
+ *    <td ALIGN=CENTER COLSPAN = 2> <b>Remove</b></td>
+ *  </tr>
+ *  <tr>
+ *    <td>{@link #remove() remove()}</td>
+ *    <td>{@link #removeFirst() removeFirst()}</td>
+ *  </tr>
+ *  <tr>
+ *    <td>{@link #poll() poll()}</td>
+ *    <td>{@link #pollFirst() pollFirst()}</td>
+ *  </tr>
+ *  <tr>
+ *    <td>{@link #take() take()}</td>
+ *    <td>{@link #takeFirst() takeFirst()}</td>
+ *  </tr>
+ *  <tr>
+ *    <td>{@link #poll(long, TimeUnit) poll(time, unit)}</td>
+ *    <td>{@link #pollFirst(long, TimeUnit) pollFirst(time, unit)}</td>
+ *  </tr>
+ *  <tr>
+ *    <td ALIGN=CENTER COLSPAN = 2> <b>Examine</b></td>
+ *  </tr>
+ *  <tr>
+ *    <td>{@link #element() element()}</td>
+ *    <td>{@link #getFirst() getFirst()}</td>
+ *  </tr>
+ *  <tr>
+ *    <td>{@link #peek() peek()}</td>
+ *    <td>{@link #peekFirst() peekFirst()}</td>
+ *  </tr>
+ * </table>
+ *
+ * <p>Memory consistency effects: As with other concurrent
+ * collections, actions in a thread prior to placing an object into a
+ * {@code BlockingDeque}
+ * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
+ * actions subsequent to the access or removal of that element from
+ * the {@code BlockingDeque} in another thread.
+ *
+ * <p>This interface is a member of the
+ * <a href="{@docRoot}/../technotes/guides/collections/index.html">
+ * Java Collections Framework</a>.
+ *
+ * @since 1.6
+ * @author Doug Lea
+ * @param <E> the type of elements held in this collection
+ */
+public interface BlockingDeque<E> extends BlockingQueue<E>, Deque<E> {
+    /*
+     * We have "diamond" multiple interface inheritance here, and that
+     * introduces ambiguities.  Methods might end up with different
+     * specs depending on the branch chosen by javadoc.  Thus a lot of
+     * methods specs here are copied from superinterfaces.
+     */
+
+    /**
+     * Inserts the specified element at the front of this deque if it is
+     * possible to do so immediately without violating capacity restrictions,
+     * throwing an <tt>IllegalStateException</tt> if no space is currently
+     * available.  When using a capacity-restricted deque, it is generally
+     * preferable to use {@link #offerFirst(Object) offerFirst}.
+     *
+     * @param e the element to add
+     * @throws IllegalStateException {@inheritDoc}
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if the specified element is null
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    void addFirst(E e);
+
+    /**
+     * Inserts the specified element at the end of this deque if it is
+     * possible to do so immediately without violating capacity restrictions,
+     * throwing an <tt>IllegalStateException</tt> if no space is currently
+     * available.  When using a capacity-restricted deque, it is generally
+     * preferable to use {@link #offerLast(Object) offerLast}.
+     *
+     * @param e the element to add
+     * @throws IllegalStateException {@inheritDoc}
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if the specified element is null
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    void addLast(E e);
+
+    /**
+     * Inserts the specified element at the front of this deque if it is
+     * possible to do so immediately without violating capacity restrictions,
+     * returning <tt>true</tt> upon success and <tt>false</tt> if no space is
+     * currently available.
+     * When using a capacity-restricted deque, this method is generally
+     * preferable to the {@link #addFirst(Object) addFirst} method, which can
+     * fail to insert an element only by throwing an exception.
+     *
+     * @param e the element to add
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if the specified element is null
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    boolean offerFirst(E e);
+
+    /**
+     * Inserts the specified element at the end of this deque if it is
+     * possible to do so immediately without violating capacity restrictions,
+     * returning <tt>true</tt> upon success and <tt>false</tt> if no space is
+     * currently available.
+     * When using a capacity-restricted deque, this method is generally
+     * preferable to the {@link #addLast(Object) addLast} method, which can
+     * fail to insert an element only by throwing an exception.
+     *
+     * @param e the element to add
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if the specified element is null
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    boolean offerLast(E e);
+
+    /**
+     * Inserts the specified element at the front of this deque,
+     * waiting if necessary for space to become available.
+     *
+     * @param e the element to add
+     * @throws InterruptedException if interrupted while waiting
+     * @throws ClassCastException if the class of the specified element
+     *         prevents it from being added to this deque
+     * @throws NullPointerException if the specified element is null
+     * @throws IllegalArgumentException if some property of the specified
+     *         element prevents it from being added to this deque
+     */
+    void putFirst(E e) throws InterruptedException;
+
+    /**
+     * Inserts the specified element at the end of this deque,
+     * waiting if necessary for space to become available.
+     *
+     * @param e the element to add
+     * @throws InterruptedException if interrupted while waiting
+     * @throws ClassCastException if the class of the specified element
+     *         prevents it from being added to this deque
+     * @throws NullPointerException if the specified element is null
+     * @throws IllegalArgumentException if some property of the specified
+     *         element prevents it from being added to this deque
+     */
+    void putLast(E e) throws InterruptedException;
+
+    /**
+     * Inserts the specified element at the front of this deque,
+     * waiting up to the specified wait time if necessary for space to
+     * become available.
+     *
+     * @param e the element to add
+     * @param timeout how long to wait before giving up, in units of
+     *        <tt>unit</tt>
+     * @param unit a <tt>TimeUnit</tt> determining how to interpret the
+     *        <tt>timeout</tt> parameter
+     * @return <tt>true</tt> if successful, or <tt>false</tt> if
+     *         the specified waiting time elapses before space is available
+     * @throws InterruptedException if interrupted while waiting
+     * @throws ClassCastException if the class of the specified element
+     *         prevents it from being added to this deque
+     * @throws NullPointerException if the specified element is null
+     * @throws IllegalArgumentException if some property of the specified
+     *         element prevents it from being added to this deque
+     */
+    boolean offerFirst(E e, long timeout, TimeUnit unit)
+        throws InterruptedException;
+
+    /**
+     * Inserts the specified element at the end of this deque,
+     * waiting up to the specified wait time if necessary for space to
+     * become available.
+     *
+     * @param e the element to add
+     * @param timeout how long to wait before giving up, in units of
+     *        <tt>unit</tt>
+     * @param unit a <tt>TimeUnit</tt> determining how to interpret the
+     *        <tt>timeout</tt> parameter
+     * @return <tt>true</tt> if successful, or <tt>false</tt> if
+     *         the specified waiting time elapses before space is available
+     * @throws InterruptedException if interrupted while waiting
+     * @throws ClassCastException if the class of the specified element
+     *         prevents it from being added to this deque
+     * @throws NullPointerException if the specified element is null
+     * @throws IllegalArgumentException if some property of the specified
+     *         element prevents it from being added to this deque
+     */
+    boolean offerLast(E e, long timeout, TimeUnit unit)
+        throws InterruptedException;
+
+    /**
+     * Retrieves and removes the first element of this deque, waiting
+     * if necessary until an element becomes available.
+     *
+     * @return the head of this deque
+     * @throws InterruptedException if interrupted while waiting
+     */
+    E takeFirst() throws InterruptedException;
+
+    /**
+     * Retrieves and removes the last element of this deque, waiting
+     * if necessary until an element becomes available.
+     *
+     * @return the tail of this deque
+     * @throws InterruptedException if interrupted while waiting
+     */
+    E takeLast() throws InterruptedException;
+
+    /**
+     * Retrieves and removes the first element of this deque, waiting
+     * up to the specified wait time if necessary for an element to
+     * become available.
+     *
+     * @param timeout how long to wait before giving up, in units of
+     *        <tt>unit</tt>
+     * @param unit a <tt>TimeUnit</tt> determining how to interpret the
+     *        <tt>timeout</tt> parameter
+     * @return the head of this deque, or <tt>null</tt> if the specified
+     *         waiting time elapses before an element is available
+     * @throws InterruptedException if interrupted while waiting
+     */
+    E pollFirst(long timeout, TimeUnit unit)
+        throws InterruptedException;
+
+    /**
+     * Retrieves and removes the last element of this deque, waiting
+     * up to the specified wait time if necessary for an element to
+     * become available.
+     *
+     * @param timeout how long to wait before giving up, in units of
+     *        <tt>unit</tt>
+     * @param unit a <tt>TimeUnit</tt> determining how to interpret the
+     *        <tt>timeout</tt> parameter
+     * @return the tail of this deque, or <tt>null</tt> if the specified
+     *         waiting time elapses before an element is available
+     * @throws InterruptedException if interrupted while waiting
+     */
+    E pollLast(long timeout, TimeUnit unit)
+        throws InterruptedException;
+
+    /**
+     * Removes the first occurrence of the specified element from this deque.
+     * If the deque does not contain the element, it is unchanged.
+     * More formally, removes the first element <tt>e</tt> such that
+     * <tt>o.equals(e)</tt> (if such an element exists).
+     * Returns <tt>true</tt> if this deque contained the specified element
+     * (or equivalently, if this deque changed as a result of the call).
+     *
+     * @param o element to be removed from this deque, if present
+     * @return <tt>true</tt> if an element was removed as a result of this call
+     * @throws ClassCastException if the class of the specified element
+     *         is incompatible with this deque (optional)
+     * @throws NullPointerException if the specified element is null (optional)
+     */
+    boolean removeFirstOccurrence(Object o);
+
+    /**
+     * Removes the last occurrence of the specified element from this deque.
+     * If the deque does not contain the element, it is unchanged.
+     * More formally, removes the last element <tt>e</tt> such that
+     * <tt>o.equals(e)</tt> (if such an element exists).
+     * Returns <tt>true</tt> if this deque contained the specified element
+     * (or equivalently, if this deque changed as a result of the call).
+     *
+     * @param o element to be removed from this deque, if present
+     * @return <tt>true</tt> if an element was removed as a result of this call
+     * @throws ClassCastException if the class of the specified element
+     *         is incompatible with this deque (optional)
+     * @throws NullPointerException if the specified element is null (optional)
+     */
+    boolean removeLastOccurrence(Object o);
+
+    // *** BlockingQueue methods ***
+
+    /**
+     * Inserts the specified element into the queue represented by this deque
+     * (in other words, at the tail of this deque) if it is possible to do so
+     * immediately without violating capacity restrictions, returning
+     * <tt>true</tt> upon success and throwing an
+     * <tt>IllegalStateException</tt> if no space is currently available.
+     * When using a capacity-restricted deque, it is generally preferable to
+     * use {@link #offer(Object) offer}.
+     *
+     * <p>This method is equivalent to {@link #addLast(Object) addLast}.
+     *
+     * @param e the element to add
+     * @throws IllegalStateException {@inheritDoc}
+     * @throws ClassCastException if the class of the specified element
+     *         prevents it from being added to this deque
+     * @throws NullPointerException if the specified element is null
+     * @throws IllegalArgumentException if some property of the specified
+     *         element prevents it from being added to this deque
+     */
+    boolean add(E e);
+
+    /**
+     * Inserts the specified element into the queue represented by this deque
+     * (in other words, at the tail of this deque) if it is possible to do so
+     * immediately without violating capacity restrictions, returning
+     * <tt>true</tt> upon success and <tt>false</tt> if no space is currently
+     * available.  When using a capacity-restricted deque, this method is
+     * generally preferable to the {@link #add} method, which can fail to
+     * insert an element only by throwing an exception.
+     *
+     * <p>This method is equivalent to {@link #offerLast(Object) offerLast}.
+     *
+     * @param e the element to add
+     * @throws ClassCastException if the class of the specified element
+     *         prevents it from being added to this deque
+     * @throws NullPointerException if the specified element is null
+     * @throws IllegalArgumentException if some property of the specified
+     *         element prevents it from being added to this deque
+     */
+    boolean offer(E e);
+
+    /**
+     * Inserts the specified element into the queue represented by this deque
+     * (in other words, at the tail of this deque), waiting if necessary for
+     * space to become available.
+     *
+     * <p>This method is equivalent to {@link #putLast(Object) putLast}.
+     *
+     * @param e the element to add
+     * @throws InterruptedException {@inheritDoc}
+     * @throws ClassCastException if the class of the specified element
+     *         prevents it from being added to this deque
+     * @throws NullPointerException if the specified element is null
+     * @throws IllegalArgumentException if some property of the specified
+     *         element prevents it from being added to this deque
+     */
+    void put(E e) throws InterruptedException;
+
+    /**
+     * Inserts the specified element into the queue represented by this deque
+     * (in other words, at the tail of this deque), waiting up to the
+     * specified wait time if necessary for space to become available.
+     *
+     * <p>This method is equivalent to
+     * {@link #offerLast(Object,long,TimeUnit) offerLast}.
+     *
+     * @param e the element to add
+     * @return <tt>true</tt> if the element was added to this deque, else
+     *         <tt>false</tt>
+     * @throws InterruptedException {@inheritDoc}
+     * @throws ClassCastException if the class of the specified element
+     *         prevents it from being added to this deque
+     * @throws NullPointerException if the specified element is null
+     * @throws IllegalArgumentException if some property of the specified
+     *         element prevents it from being added to this deque
+     */
+    boolean offer(E e, long timeout, TimeUnit unit)
+        throws InterruptedException;
+
+    /**
+     * Retrieves and removes the head of the queue represented by this deque
+     * (in other words, the first element of this deque).
+     * This method differs from {@link #poll poll} only in that it
+     * throws an exception if this deque is empty.
+     *
+     * <p>This method is equivalent to {@link #removeFirst() removeFirst}.
+     *
+     * @return the head of the queue represented by this deque
+     * @throws NoSuchElementException if this deque is empty
+     */
+    E remove();
+
+    /**
+     * Retrieves and removes the head of the queue represented by this deque
+     * (in other words, the first element of this deque), or returns
+     * <tt>null</tt> if this deque is empty.
+     *
+     * <p>This method is equivalent to {@link #pollFirst()}.
+     *
+     * @return the head of this deque, or <tt>null</tt> if this deque is empty
+     */
+    E poll();
+
+    /**
+     * Retrieves and removes the head of the queue represented by this deque
+     * (in other words, the first element of this deque), waiting if
+     * necessary until an element becomes available.
+     *
+     * <p>This method is equivalent to {@link #takeFirst() takeFirst}.
+     *
+     * @return the head of this deque
+     * @throws InterruptedException if interrupted while waiting
+     */
+    E take() throws InterruptedException;
+
+    /**
+     * Retrieves and removes the head of the queue represented by this deque
+     * (in other words, the first element of this deque), waiting up to the
+     * specified wait time if necessary for an element to become available.
+     *
+     * <p>This method is equivalent to
+     * {@link #pollFirst(long,TimeUnit) pollFirst}.
+     *
+     * @return the head of this deque, or <tt>null</tt> if the
+     *         specified waiting time elapses before an element is available
+     * @throws InterruptedException if interrupted while waiting
+     */
+    E poll(long timeout, TimeUnit unit)
+        throws InterruptedException;
+
+    /**
+     * Retrieves, but does not remove, the head of the queue represented by
+     * this deque (in other words, the first element of this deque).
+     * This method differs from {@link #peek peek} only in that it throws an
+     * exception if this deque is empty.
+     *
+     * <p>This method is equivalent to {@link #getFirst() getFirst}.
+     *
+     * @return the head of this deque
+     * @throws NoSuchElementException if this deque is empty
+     */
+    E element();
+
+    /**
+     * Retrieves, but does not remove, the head of the queue represented by
+     * this deque (in other words, the first element of this deque), or
+     * returns <tt>null</tt> if this deque is empty.
+     *
+     * <p>This method is equivalent to {@link #peekFirst() peekFirst}.
+     *
+     * @return the head of this deque, or <tt>null</tt> if this deque is empty
+     */
+    E peek();
+
+    /**
+     * Removes the first occurrence of the specified element from this deque.
+     * If the deque does not contain the element, it is unchanged.
+     * More formally, removes the first element <tt>e</tt> such that
+     * <tt>o.equals(e)</tt> (if such an element exists).
+     * Returns <tt>true</tt> if this deque contained the specified element
+     * (or equivalently, if this deque changed as a result of the call).
+     *
+     * <p>This method is equivalent to
+     * {@link #removeFirstOccurrence(Object) removeFirstOccurrence}.
+     *
+     * @param o element to be removed from this deque, if present
+     * @return <tt>true</tt> if this deque changed as a result of the call
+     * @throws ClassCastException if the class of the specified element
+     *         is incompatible with this deque (optional)
+     * @throws NullPointerException if the specified element is null (optional)
+     */
+    boolean remove(Object o);
+
+    /**
+     * Returns <tt>true</tt> if this deque contains the specified element.
+     * More formally, returns <tt>true</tt> if and only if this deque contains
+     * at least one element <tt>e</tt> such that <tt>o.equals(e)</tt>.
+     *
+     * @param o object to be checked for containment in this deque
+     * @return <tt>true</tt> if this deque contains the specified element
+     * @throws ClassCastException if the class of the specified element
+     *         is incompatible with this deque (optional)
+     * @throws NullPointerException if the specified element is null (optional)
+     */
+    public boolean contains(Object o);
+
+    /**
+     * Returns the number of elements in this deque.
+     *
+     * @return the number of elements in this deque
+     */
+    public int size();
+
+    /**
+     * Returns an iterator over the elements in this deque in proper sequence.
+     * The elements will be returned in order from first (head) to last (tail).
+     *
+     * @return an iterator over the elements in this deque in proper sequence
+     */
+    Iterator<E> iterator();
+
+    // *** Stack methods ***
+
+    /**
+     * Pushes an element onto the stack represented by this deque.  In other
+     * words, inserts the element at the front of this deque unless it would
+     * violate capacity restrictions.
+     *
+     * <p>This method is equivalent to {@link #addFirst(Object) addFirst}.
+     *
+     * @throws IllegalStateException {@inheritDoc}
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if the specified element is null
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    void push(E e);
+}
diff --git a/external/jsr166/java/util/concurrent/BlockingQueue.java b/external/jsr166/java/util/concurrent/BlockingQueue.java
new file mode 100644
index 000000000..b47cc9842
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/BlockingQueue.java
@@ -0,0 +1,344 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+
+import java.util.Collection;
+import java.util.Queue;
+
+/**
+ * A {@link java.util.Queue} that additionally supports operations
+ * that wait for the queue to become non-empty when retrieving an
+ * element, and wait for space to become available in the queue when
+ * storing an element.
+ *
+ * <p><tt>BlockingQueue</tt> methods come in four forms, with different ways
+ * of handling operations that cannot be satisfied immediately, but may be
+ * satisfied at some point in the future:
+ * one throws an exception, the second returns a special value (either
+ * <tt>null</tt> or <tt>false</tt>, depending on the operation), the third
+ * blocks the current thread indefinitely until the operation can succeed,
+ * and the fourth blocks for only a given maximum time limit before giving
+ * up.  These methods are summarized in the following table:
+ *
+ * <p>
+ * <table BORDER CELLPADDING=3 CELLSPACING=1>
+ *  <tr>
+ *    <td></td>
+ *    <td ALIGN=CENTER><em>Throws exception</em></td>
+ *    <td ALIGN=CENTER><em>Special value</em></td>
+ *    <td ALIGN=CENTER><em>Blocks</em></td>
+ *    <td ALIGN=CENTER><em>Times out</em></td>
+ *  </tr>
+ *  <tr>
+ *    <td><b>Insert</b></td>
+ *    <td>{@link #add add(e)}</td>
+ *    <td>{@link #offer offer(e)}</td>
+ *    <td>{@link #put put(e)}</td>
+ *    <td>{@link #offer(Object, long, TimeUnit) offer(e, time, unit)}</td>
+ *  </tr>
+ *  <tr>
+ *    <td><b>Remove</b></td>
+ *    <td>{@link #remove remove()}</td>
+ *    <td>{@link #poll poll()}</td>
+ *    <td>{@link #take take()}</td>
+ *    <td>{@link #poll(long, TimeUnit) poll(time, unit)}</td>
+ *  </tr>
+ *  <tr>
+ *    <td><b>Examine</b></td>
+ *    <td>{@link #element element()}</td>
+ *    <td>{@link #peek peek()}</td>
+ *    <td><em>not applicable</em></td>
+ *    <td><em>not applicable</em></td>
+ *  </tr>
+ * </table>
+ *
+ * <p>A <tt>BlockingQueue</tt> does not accept <tt>null</tt> elements.
+ * Implementations throw <tt>NullPointerException</tt> on attempts
+ * to <tt>add</tt>, <tt>put</tt> or <tt>offer</tt> a <tt>null</tt>.  A
+ * <tt>null</tt> is used as a sentinel value to indicate failure of
+ * <tt>poll</tt> operations.
+ *
+ * <p>A <tt>BlockingQueue</tt> may be capacity bounded. At any given
+ * time it may have a <tt>remainingCapacity</tt> beyond which no
+ * additional elements can be <tt>put</tt> without blocking.
+ * A <tt>BlockingQueue</tt> without any intrinsic capacity constraints always
+ * reports a remaining capacity of <tt>Integer.MAX_VALUE</tt>.
+ *
+ * <p> <tt>BlockingQueue</tt> implementations are designed to be used
+ * primarily for producer-consumer queues, but additionally support
+ * the {@link java.util.Collection} interface.  So, for example, it is
+ * possible to remove an arbitrary element from a queue using
+ * <tt>remove(x)</tt>. However, such operations are in general
+ * <em>not</em> performed very efficiently, and are intended for only
+ * occasional use, such as when a queued message is cancelled.
+ *
+ * <p> <tt>BlockingQueue</tt> implementations are thread-safe.  All
+ * queuing methods achieve their effects atomically using internal
+ * locks or other forms of concurrency control. However, the
+ * <em>bulk</em> Collection operations <tt>addAll</tt>,
+ * <tt>containsAll</tt>, <tt>retainAll</tt> and <tt>removeAll</tt> are
+ * <em>not</em> necessarily performed atomically unless specified
+ * otherwise in an implementation. So it is possible, for example, for
+ * <tt>addAll(c)</tt> to fail (throwing an exception) after adding
+ * only some of the elements in <tt>c</tt>.
+ *
+ * <p>A <tt>BlockingQueue</tt> does <em>not</em> intrinsically support
+ * any kind of &quot;close&quot; or &quot;shutdown&quot; operation to
+ * indicate that no more items will be added.  The needs and usage of
+ * such features tend to be implementation-dependent. For example, a
+ * common tactic is for producers to insert special
+ * <em>end-of-stream</em> or <em>poison</em> objects, that are
+ * interpreted accordingly when taken by consumers.
+ *
+ * <p>
+ * Usage example, based on a typical producer-consumer scenario.
+ * Note that a <tt>BlockingQueue</tt> can safely be used with multiple
+ * producers and multiple consumers.
+ * <pre>
+ * class Producer implements Runnable {
+ *   private final BlockingQueue queue;
+ *   Producer(BlockingQueue q) { queue = q; }
+ *   public void run() {
+ *     try {
+ *       while (true) { queue.put(produce()); }
+ *     } catch (InterruptedException ex) { ... handle ...}
+ *   }
+ *   Object produce() { ... }
+ * }
+ *
+ * class Consumer implements Runnable {
+ *   private final BlockingQueue queue;
+ *   Consumer(BlockingQueue q) { queue = q; }
+ *   public void run() {
+ *     try {
+ *       while (true) { consume(queue.take()); }
+ *     } catch (InterruptedException ex) { ... handle ...}
+ *   }
+ *   void consume(Object x) { ... }
+ * }
+ *
+ * class Setup {
+ *   void main() {
+ *     BlockingQueue q = new SomeQueueImplementation();
+ *     Producer p = new Producer(q);
+ *     Consumer c1 = new Consumer(q);
+ *     Consumer c2 = new Consumer(q);
+ *     new Thread(p).start();
+ *     new Thread(c1).start();
+ *     new Thread(c2).start();
+ *   }
+ * }
+ * </pre>
+ *
+ * <p>Memory consistency effects: As with other concurrent
+ * collections, actions in a thread prior to placing an object into a
+ * {@code BlockingQueue}
+ * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
+ * actions subsequent to the access or removal of that element from
+ * the {@code BlockingQueue} in another thread.
+ *
+ * <p>This interface is a member of the
+ * <a href="{@docRoot}/../technotes/guides/collections/index.html">
+ * Java Collections Framework</a>.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ * @param <E> the type of elements held in this collection
+ */
+public interface BlockingQueue<E> extends Queue<E> {
+    /**
+     * Inserts the specified element into this queue if it is possible to do
+     * so immediately without violating capacity restrictions, returning
+     * <tt>true</tt> upon success and throwing an
+     * <tt>IllegalStateException</tt> if no space is currently available.
+     * When using a capacity-restricted queue, it is generally preferable to
+     * use {@link #offer(Object) offer}.
+     *
+     * @param e the element to add
+     * @return <tt>true</tt> (as specified by {@link Collection#add})
+     * @throws IllegalStateException if the element cannot be added at this
+     *         time due to capacity restrictions
+     * @throws ClassCastException if the class of the specified element
+     *         prevents it from being added to this queue
+     * @throws NullPointerException if the specified element is null
+     * @throws IllegalArgumentException if some property of the specified
+     *         element prevents it from being added to this queue
+     */
+    boolean add(E e);
+
+    /**
+     * Inserts the specified element into this queue if it is possible to do
+     * so immediately without violating capacity restrictions, returning
+     * <tt>true</tt> upon success and <tt>false</tt> if no space is currently
+     * available.  When using a capacity-restricted queue, this method is
+     * generally preferable to {@link #add}, which can fail to insert an
+     * element only by throwing an exception.
+     *
+     * @param e the element to add
+     * @return <tt>true</tt> if the element was added to this queue, else
+     *         <tt>false</tt>
+     * @throws ClassCastException if the class of the specified element
+     *         prevents it from being added to this queue
+     * @throws NullPointerException if the specified element is null
+     * @throws IllegalArgumentException if some property of the specified
+     *         element prevents it from being added to this queue
+     */
+    boolean offer(E e);
+
+    /**
+     * Inserts the specified element into this queue, waiting if necessary
+     * for space to become available.
+     *
+     * @param e the element to add
+     * @throws InterruptedException if interrupted while waiting
+     * @throws ClassCastException if the class of the specified element
+     *         prevents it from being added to this queue
+     * @throws NullPointerException if the specified element is null
+     * @throws IllegalArgumentException if some property of the specified
+     *         element prevents it from being added to this queue
+     */
+    void put(E e) throws InterruptedException;
+
+    /**
+     * Inserts the specified element into this queue, waiting up to the
+     * specified wait time if necessary for space to become available.
+     *
+     * @param e the element to add
+     * @param timeout how long to wait before giving up, in units of
+     *        <tt>unit</tt>
+     * @param unit a <tt>TimeUnit</tt> determining how to interpret the
+     *        <tt>timeout</tt> parameter
+     * @return <tt>true</tt> if successful, or <tt>false</tt> if
+     *         the specified waiting time elapses before space is available
+     * @throws InterruptedException if interrupted while waiting
+     * @throws ClassCastException if the class of the specified element
+     *         prevents it from being added to this queue
+     * @throws NullPointerException if the specified element is null
+     * @throws IllegalArgumentException if some property of the specified
+     *         element prevents it from being added to this queue
+     */
+    boolean offer(E e, long timeout, TimeUnit unit)
+        throws InterruptedException;
+
+    /**
+     * Retrieves and removes the head of this queue, waiting if necessary
+     * until an element becomes available.
+     *
+     * @return the head of this queue
+     * @throws InterruptedException if interrupted while waiting
+     */
+    E take() throws InterruptedException;
+
+    /**
+     * Retrieves and removes the head of this queue, waiting up to the
+     * specified wait time if necessary for an element to become available.
+     *
+     * @param timeout how long to wait before giving up, in units of
+     *        <tt>unit</tt>
+     * @param unit a <tt>TimeUnit</tt> determining how to interpret the
+     *        <tt>timeout</tt> parameter
+     * @return the head of this queue, or <tt>null</tt> if the
+     *         specified waiting time elapses before an element is available
+     * @throws InterruptedException if interrupted while waiting
+     */
+    E poll(long timeout, TimeUnit unit)
+        throws InterruptedException;
+
+    /**
+     * Returns the number of additional elements that this queue can ideally
+     * (in the absence of memory or resource constraints) accept without
+     * blocking, or <tt>Integer.MAX_VALUE</tt> if there is no intrinsic
+     * limit.
+     *
+     * <p>Note that you <em>cannot</em> always tell if an attempt to insert
+     * an element will succeed by inspecting <tt>remainingCapacity</tt>
+     * because it may be the case that another thread is about to
+     * insert or remove an element.
+     *
+     * @return the remaining capacity
+     */
+    int remainingCapacity();
+
+    /**
+     * Removes a single instance of the specified element from this queue,
+     * if it is present.  More formally, removes an element <tt>e</tt> such
+     * that <tt>o.equals(e)</tt>, if this queue contains one or more such
+     * elements.
+     * Returns <tt>true</tt> if this queue contained the specified element
+     * (or equivalently, if this queue changed as a result of the call).
+     *
+     * @param o element to be removed from this queue, if present
+     * @return <tt>true</tt> if this queue changed as a result of the call
+     * @throws ClassCastException if the class of the specified element
+     *         is incompatible with this queue (optional)
+     * @throws NullPointerException if the specified element is null (optional)
+     */
+    boolean remove(Object o);
+
+    /**
+     * Returns <tt>true</tt> if this queue contains the specified element.
+     * More formally, returns <tt>true</tt> if and only if this queue contains
+     * at least one element <tt>e</tt> such that <tt>o.equals(e)</tt>.
+     *
+     * @param o object to be checked for containment in this queue
+     * @return <tt>true</tt> if this queue contains the specified element
+     * @throws ClassCastException if the class of the specified element
+     *         is incompatible with this queue (optional)
+     * @throws NullPointerException if the specified element is null (optional)
+     */
+    public boolean contains(Object o);
+
+    /**
+     * Removes all available elements from this queue and adds them
+     * to the given collection.  This operation may be more
+     * efficient than repeatedly polling this queue.  A failure
+     * encountered while attempting to add elements to
+     * collection <tt>c</tt> may result in elements being in neither,
+     * either or both collections when the associated exception is
+     * thrown.  Attempts to drain a queue to itself result in
+     * <tt>IllegalArgumentException</tt>. Further, the behavior of
+     * this operation is undefined if the specified collection is
+     * modified while the operation is in progress.
+     *
+     * @param c the collection to transfer elements into
+     * @return the number of elements transferred
+     * @throws UnsupportedOperationException if addition of elements
+     *         is not supported by the specified collection
+     * @throws ClassCastException if the class of an element of this queue
+     *         prevents it from being added to the specified collection
+     * @throws NullPointerException if the specified collection is null
+     * @throws IllegalArgumentException if the specified collection is this
+     *         queue, or some property of an element of this queue prevents
+     *         it from being added to the specified collection
+     */
+    int drainTo(Collection<? super E> c);
+
+    /**
+     * Removes at most the given number of available elements from
+     * this queue and adds them to the given collection.  A failure
+     * encountered while attempting to add elements to
+     * collection <tt>c</tt> may result in elements being in neither,
+     * either or both collections when the associated exception is
+     * thrown.  Attempts to drain a queue to itself result in
+     * <tt>IllegalArgumentException</tt>. Further, the behavior of
+     * this operation is undefined if the specified collection is
+     * modified while the operation is in progress.
+     *
+     * @param c the collection to transfer elements into
+     * @param maxElements the maximum number of elements to transfer
+     * @return the number of elements transferred
+     * @throws UnsupportedOperationException if addition of elements
+     *         is not supported by the specified collection
+     * @throws ClassCastException if the class of an element of this queue
+     *         prevents it from being added to the specified collection
+     * @throws NullPointerException if the specified collection is null
+     * @throws IllegalArgumentException if the specified collection is this
+     *         queue, or some property of an element of this queue prevents
+     *         it from being added to the specified collection
+     */
+    int drainTo(Collection<? super E> c, int maxElements);
+}
diff --git a/external/jsr166/java/util/concurrent/BrokenBarrierException.java b/external/jsr166/java/util/concurrent/BrokenBarrierException.java
new file mode 100644
index 000000000..3f93fbb9d
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/BrokenBarrierException.java
@@ -0,0 +1,38 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+
+/**
+ * Exception thrown when a thread tries to wait upon a barrier that is
+ * in a broken state, or which enters the broken state while the thread
+ * is waiting.
+ *
+ * @see CyclicBarrier
+ *
+ * @since 1.5
+ * @author Doug Lea
+ *
+ */
+public class BrokenBarrierException extends Exception {
+    private static final long serialVersionUID = 7117394618823254244L;
+
+    /**
+     * Constructs a <tt>BrokenBarrierException</tt> with no specified detail
+     * message.
+     */
+    public BrokenBarrierException() {}
+
+    /**
+     * Constructs a <tt>BrokenBarrierException</tt> with the specified
+     * detail message.
+     *
+     * @param message the detail message
+     */
+    public BrokenBarrierException(String message) {
+        super(message);
+    }
+}
diff --git a/external/jsr166/java/util/concurrent/Callable.java b/external/jsr166/java/util/concurrent/Callable.java
new file mode 100644
index 000000000..abc4d0407
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/Callable.java
@@ -0,0 +1,36 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+
+/**
+ * A task that returns a result and may throw an exception.
+ * Implementors define a single method with no arguments called
+ * <tt>call</tt>.
+ *
+ * <p>The <tt>Callable</tt> interface is similar to {@link
+ * java.lang.Runnable}, in that both are designed for classes whose
+ * instances are potentially executed by another thread.  A
+ * <tt>Runnable</tt>, however, does not return a result and cannot
+ * throw a checked exception.
+ *
+ * <p> The {@link Executors} class contains utility methods to
+ * convert from other common forms to <tt>Callable</tt> classes.
+ *
+ * @see Executor
+ * @since 1.5
+ * @author Doug Lea
+ * @param <V> the result type of method <tt>call</tt>
+ */
+public interface Callable<V> {
+    /**
+     * Computes a result, or throws an exception if unable to do so.
+     *
+     * @return computed result
+     * @throws Exception if unable to compute a result
+     */
+    V call() throws Exception;
+}
diff --git a/external/jsr166/java/util/concurrent/CancellationException.java b/external/jsr166/java/util/concurrent/CancellationException.java
new file mode 100644
index 000000000..2c29544a0
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/CancellationException.java
@@ -0,0 +1,34 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+
+/**
+ * Exception indicating that the result of a value-producing task,
+ * such as a {@link FutureTask}, cannot be retrieved because the task
+ * was cancelled.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ */
+public class CancellationException extends IllegalStateException {
+    private static final long serialVersionUID = -9202173006928992231L;
+
+    /**
+     * Constructs a <tt>CancellationException</tt> with no detail message.
+     */
+    public CancellationException() {}
+
+    /**
+     * Constructs a <tt>CancellationException</tt> with the specified detail
+     * message.
+     *
+     * @param message the detail message
+     */
+    public CancellationException(String message) {
+        super(message);
+    }
+}
diff --git a/external/jsr166/java/util/concurrent/CompletionService.java b/external/jsr166/java/util/concurrent/CompletionService.java
new file mode 100644
index 000000000..df9f719c4
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/CompletionService.java
@@ -0,0 +1,97 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+
+/**
+ * A service that decouples the production of new asynchronous tasks
+ * from the consumption of the results of completed tasks.  Producers
+ * <tt>submit</tt> tasks for execution. Consumers <tt>take</tt>
+ * completed tasks and process their results in the order they
+ * complete.  A <tt>CompletionService</tt> can for example be used to
+ * manage asynchronous IO, in which tasks that perform reads are
+ * submitted in one part of a program or system, and then acted upon
+ * in a different part of the program when the reads complete,
+ * possibly in a different order than they were requested.
+ *
+ * <p>Typically, a <tt>CompletionService</tt> relies on a separate
+ * {@link Executor} to actually execute the tasks, in which case the
+ * <tt>CompletionService</tt> only manages an internal completion
+ * queue. The {@link ExecutorCompletionService} class provides an
+ * implementation of this approach.
+ *
+ * <p>Memory consistency effects: Actions in a thread prior to
+ * submitting a task to a {@code CompletionService}
+ * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
+ * actions taken by that task, which in turn <i>happen-before</i>
+ * actions following a successful return from the corresponding {@code take()}.
+ *
+ */
+public interface CompletionService<V> {
+    /**
+     * Submits a value-returning task for execution and returns a Future
+     * representing the pending results of the task.  Upon completion,
+     * this task may be taken or polled.
+     *
+     * @param task the task to submit
+     * @return a Future representing pending completion of the task
+     * @throws RejectedExecutionException if the task cannot be
+     *         scheduled for execution
+     * @throws NullPointerException if the task is null
+     */
+    Future<V> submit(Callable<V> task);
+
+    /**
+     * Submits a Runnable task for execution and returns a Future
+     * representing that task.  Upon completion, this task may be
+     * taken or polled.
+     *
+     * @param task the task to submit
+     * @param result the result to return upon successful completion
+     * @return a Future representing pending completion of the task,
+     *         and whose <tt>get()</tt> method will return the given
+     *         result value upon completion
+     * @throws RejectedExecutionException if the task cannot be
+     *         scheduled for execution
+     * @throws NullPointerException if the task is null
+     */
+    Future<V> submit(Runnable task, V result);
+
+    /**
+     * Retrieves and removes the Future representing the next
+     * completed task, waiting if none are yet present.
+     *
+     * @return the Future representing the next completed task
+     * @throws InterruptedException if interrupted while waiting
+     */
+    Future<V> take() throws InterruptedException;
+
+
+    /**
+     * Retrieves and removes the Future representing the next
+     * completed task or <tt>null</tt> if none are present.
+     *
+     * @return the Future representing the next completed task, or
+     *         <tt>null</tt> if none are present
+     */
+    Future<V> poll();
+
+    /**
+     * Retrieves and removes the Future representing the next
+     * completed task, waiting if necessary up to the specified wait
+     * time if none are yet present.
+     *
+     * @param timeout how long to wait before giving up, in units of
+     *        <tt>unit</tt>
+     * @param unit a <tt>TimeUnit</tt> determining how to interpret the
+     *        <tt>timeout</tt> parameter
+     * @return the Future representing the next completed task or
+     *         <tt>null</tt> if the specified waiting time elapses
+     *         before one is present
+     * @throws InterruptedException if interrupted while waiting
+     */
+    Future<V> poll(long timeout, TimeUnit unit) throws InterruptedException;
+}
diff --git a/external/jsr166/java/util/concurrent/ConcurrentHashMap.java b/external/jsr166/java/util/concurrent/ConcurrentHashMap.java
new file mode 100644
index 000000000..7f6d4493c
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/ConcurrentHashMap.java
@@ -0,0 +1,1277 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+import java.util.concurrent.locks.*;
+import java.util.*;
+import java.io.Serializable;
+import java.io.IOException;
+import java.io.ObjectInputStream;
+import java.io.ObjectOutputStream;
+
+/**
+ * A hash table supporting full concurrency of retrievals and
+ * adjustable expected concurrency for updates. This class obeys the
+ * same functional specification as {@link java.util.Hashtable}, and
+ * includes versions of methods corresponding to each method of
+ * <tt>Hashtable</tt>. However, even though all operations are
+ * thread-safe, retrieval operations do <em>not</em> entail locking,
+ * and there is <em>not</em> any support for locking the entire table
+ * in a way that prevents all access.  This class is fully
+ * interoperable with <tt>Hashtable</tt> in programs that rely on its
+ * thread safety but not on its synchronization details.
+ *
+ * <p> Retrieval operations (including <tt>get</tt>) generally do not
+ * block, so may overlap with update operations (including
+ * <tt>put</tt> and <tt>remove</tt>). Retrievals reflect the results
+ * of the most recently <em>completed</em> update operations holding
+ * upon their onset.  For aggregate operations such as <tt>putAll</tt>
+ * and <tt>clear</tt>, concurrent retrievals may reflect insertion or
+ * removal of only some entries.  Similarly, Iterators and
+ * Enumerations return elements reflecting the state of the hash table
+ * at some point at or since the creation of the iterator/enumeration.
+ * They do <em>not</em> throw {@link ConcurrentModificationException}.
+ * However, iterators are designed to be used by only one thread at a time.
+ *
+ * <p> The allowed concurrency among update operations is guided by
+ * the optional <tt>concurrencyLevel</tt> constructor argument
+ * (default <tt>16</tt>), which is used as a hint for internal sizing.  The
+ * table is internally partitioned to try to permit the indicated
+ * number of concurrent updates without contention. Because placement
+ * in hash tables is essentially random, the actual concurrency will
+ * vary.  Ideally, you should choose a value to accommodate as many
+ * threads as will ever concurrently modify the table. Using a
+ * significantly higher value than you need can waste space and time,
+ * and a significantly lower value can lead to thread contention. But
+ * overestimates and underestimates within an order of magnitude do
+ * not usually have much noticeable impact. A value of one is
+ * appropriate when it is known that only one thread will modify and
+ * all others will only read. Also, resizing this or any other kind of
+ * hash table is a relatively slow operation, so, when possible, it is
+ * a good idea to provide estimates of expected table sizes in
+ * constructors.
+ *
+ * <p>This class and its views and iterators implement all of the
+ * <em>optional</em> methods of the {@link Map} and {@link Iterator}
+ * interfaces.
+ *
+ * <p> Like {@link Hashtable} but unlike {@link HashMap}, this class
+ * does <em>not</em> allow <tt>null</tt> to be used as a key or value.
+ *
+ * <p>This class is a member of the
+ * <a href="{@docRoot}/../technotes/guides/collections/index.html">
+ * Java Collections Framework</a>.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ * @param <K> the type of keys maintained by this map
+ * @param <V> the type of mapped values
+ */
+public class ConcurrentHashMap<K, V> extends AbstractMap<K, V>
+        implements ConcurrentMap<K, V>, Serializable {
+    private static final long serialVersionUID = 7249069246763182397L;
+
+    /*
+     * The basic strategy is to subdivide the table among Segments,
+     * each of which itself is a concurrently readable hash table.
+     */
+
+    /* ---------------- Constants -------------- */
+
+    /**
+     * The default initial capacity for this table,
+     * used when not otherwise specified in a constructor.
+     */
+    static final int DEFAULT_INITIAL_CAPACITY = 16;
+
+    /**
+     * The default load factor for this table, used when not
+     * otherwise specified in a constructor.
+     */
+    static final float DEFAULT_LOAD_FACTOR = 0.75f;
+
+    /**
+     * The default concurrency level for this table, used when not
+     * otherwise specified in a constructor.
+     */
+    static final int DEFAULT_CONCURRENCY_LEVEL = 16;
+
+    /**
+     * The maximum capacity, used if a higher value is implicitly
+     * specified by either of the constructors with arguments.  MUST
+     * be a power of two <= 1<<30 to ensure that entries are indexable
+     * using ints.
+     */
+    static final int MAXIMUM_CAPACITY = 1 << 30;
+
+    /**
+     * The maximum number of segments to allow; used to bound
+     * constructor arguments.
+     */
+    static final int MAX_SEGMENTS = 1 << 16; // slightly conservative
+
+    /**
+     * Number of unsynchronized retries in size and containsValue
+     * methods before resorting to locking. This is used to avoid
+     * unbounded retries if tables undergo continuous modification
+     * which would make it impossible to obtain an accurate result.
+     */
+    static final int RETRIES_BEFORE_LOCK = 2;
+
+    /* ---------------- Fields -------------- */
+
+    /**
+     * Mask value for indexing into segments. The upper bits of a
+     * key's hash code are used to choose the segment.
+     */
+    final int segmentMask;
+
+    /**
+     * Shift value for indexing within segments.
+     */
+    final int segmentShift;
+
+    /**
+     * The segments, each of which is a specialized hash table
+     */
+    final Segment<K,V>[] segments;
+
+    transient Set<K> keySet;
+    transient Set<Map.Entry<K,V>> entrySet;
+    transient Collection<V> values;
+
+    /* ---------------- Small Utilities -------------- */
+
+    /**
+     * Applies a supplemental hash function to a given hashCode, which
+     * defends against poor quality hash functions.  This is critical
+     * because ConcurrentHashMap uses power-of-two length hash tables,
+     * that otherwise encounter collisions for hashCodes that do not
+     * differ in lower bits.
+     */
+    private static int hash(int h) {
+        // This function ensures that hashCodes that differ only by
+        // constant multiples at each bit position have a bounded
+        // number of collisions (approximately 8 at default load factor).
+        h ^= (h >>> 20) ^ (h >>> 12);
+        return h ^ (h >>> 7) ^ (h >>> 4);
+    }
+
+    /**
+     * Returns the segment that should be used for key with given hash
+     * @param hash the hash code for the key
+     * @return the segment
+     */
+    final Segment<K,V> segmentFor(int hash) {
+        return segments[(hash >>> segmentShift) & segmentMask];
+    }
+
+    /* ---------------- Inner Classes -------------- */
+
+    /**
+     * ConcurrentHashMap list entry. Note that this is never exported
+     * out as a user-visible Map.Entry.
+     *
+     * Because the value field is volatile, not final, it is legal wrt
+     * the Java Memory Model for an unsynchronized reader to see null
+     * instead of initial value when read via a data race.  Although a
+     * reordering leading to this is not likely to ever actually
+     * occur, the Segment.readValueUnderLock method is used as a
+     * backup in case a null (pre-initialized) value is ever seen in
+     * an unsynchronized access method.
+     */
+    static final class HashEntry<K,V> {
+        final K key;
+        final int hash;
+        volatile V value;
+        final HashEntry<K,V> next;
+
+        HashEntry(K key, int hash, HashEntry<K,V> next, V value) {
+            this.key = key;
+            this.hash = hash;
+            this.next = next;
+            this.value = value;
+        }
+
+	@SuppressWarnings("unchecked")
+	static final <K,V> HashEntry<K,V>[] newArray(int i) {
+	    return new HashEntry[i];
+	}
+    }
+
+    /**
+     * Segments are specialized versions of hash tables.  This
+     * subclasses from ReentrantLock opportunistically, just to
+     * simplify some locking and avoid separate construction.
+     */
+    static final class Segment<K,V> extends ReentrantLock implements Serializable {
+        /*
+         * Segments maintain a table of entry lists that are ALWAYS
+         * kept in a consistent state, so can be read without locking.
+         * Next fields of nodes are immutable (final).  All list
+         * additions are performed at the front of each bin. This
+         * makes it easy to check changes, and also fast to traverse.
+         * When nodes would otherwise be changed, new nodes are
+         * created to replace them. This works well for hash tables
+         * since the bin lists tend to be short. (The average length
+         * is less than two for the default load factor threshold.)
+         *
+         * Read operations can thus proceed without locking, but rely
+         * on selected uses of volatiles to ensure that completed
+         * write operations performed by other threads are
+         * noticed. For most purposes, the "count" field, tracking the
+         * number of elements, serves as that volatile variable
+         * ensuring visibility.  This is convenient because this field
+         * needs to be read in many read operations anyway:
+         *
+         *   - All (unsynchronized) read operations must first read the
+         *     "count" field, and should not look at table entries if
+         *     it is 0.
+         *
+         *   - All (synchronized) write operations should write to
+         *     the "count" field after structurally changing any bin.
+         *     The operations must not take any action that could even
+         *     momentarily cause a concurrent read operation to see
+         *     inconsistent data. This is made easier by the nature of
+         *     the read operations in Map. For example, no operation
+         *     can reveal that the table has grown but the threshold
+         *     has not yet been updated, so there are no atomicity
+         *     requirements for this with respect to reads.
+         *
+         * As a guide, all critical volatile reads and writes to the
+         * count field are marked in code comments.
+         */
+
+        private static final long serialVersionUID = 2249069246763182397L;
+
+        /**
+         * The number of elements in this segment's region.
+         */
+        transient volatile int count;
+
+        /**
+         * Number of updates that alter the size of the table. This is
+         * used during bulk-read methods to make sure they see a
+         * consistent snapshot: If modCounts change during a traversal
+         * of segments computing size or checking containsValue, then
+         * we might have an inconsistent view of state so (usually)
+         * must retry.
+         */
+        transient int modCount;
+
+        /**
+         * The table is rehashed when its size exceeds this threshold.
+         * (The value of this field is always <tt>(int)(capacity *
+         * loadFactor)</tt>.)
+         */
+        transient int threshold;
+
+        /**
+         * The per-segment table.
+         */
+        transient volatile HashEntry<K,V>[] table;
+
+        /**
+         * The load factor for the hash table.  Even though this value
+         * is same for all segments, it is replicated to avoid needing
+         * links to outer object.
+         * @serial
+         */
+        final float loadFactor;
+
+        Segment(int initialCapacity, float lf) {
+            loadFactor = lf;
+            setTable(HashEntry.<K,V>newArray(initialCapacity));
+        }
+
+	@SuppressWarnings("unchecked")
+        static final <K,V> Segment<K,V>[] newArray(int i) {
+	    return new Segment[i];
+        }
+
+        /**
+         * Sets table to new HashEntry array.
+         * Call only while holding lock or in constructor.
+         */
+        void setTable(HashEntry<K,V>[] newTable) {
+            threshold = (int)(newTable.length * loadFactor);
+            table = newTable;
+        }
+
+        /**
+         * Returns properly casted first entry of bin for given hash.
+         */
+        HashEntry<K,V> getFirst(int hash) {
+            HashEntry<K,V>[] tab = table;
+            return tab[hash & (tab.length - 1)];
+        }
+
+        /**
+         * Reads value field of an entry under lock. Called if value
+         * field ever appears to be null. This is possible only if a
+         * compiler happens to reorder a HashEntry initialization with
+         * its table assignment, which is legal under memory model
+         * but is not known to ever occur.
+         */
+        V readValueUnderLock(HashEntry<K,V> e) {
+            lock();
+            try {
+                return e.value;
+            } finally {
+                unlock();
+            }
+        }
+
+        /* Specialized implementations of map methods */
+
+        V get(Object key, int hash) {
+            if (count != 0) { // read-volatile
+                HashEntry<K,V> e = getFirst(hash);
+                while (e != null) {
+                    if (e.hash == hash && key.equals(e.key)) {
+                        V v = e.value;
+                        if (v != null)
+                            return v;
+                        return readValueUnderLock(e); // recheck
+                    }
+                    e = e.next;
+                }
+            }
+            return null;
+        }
+
+        boolean containsKey(Object key, int hash) {
+            if (count != 0) { // read-volatile
+                HashEntry<K,V> e = getFirst(hash);
+                while (e != null) {
+                    if (e.hash == hash && key.equals(e.key))
+                        return true;
+                    e = e.next;
+                }
+            }
+            return false;
+        }
+
+        boolean containsValue(Object value) {
+            if (count != 0) { // read-volatile
+                HashEntry<K,V>[] tab = table;
+                int len = tab.length;
+                for (int i = 0 ; i < len; i++) {
+                    for (HashEntry<K,V> e = tab[i]; e != null; e = e.next) {
+                        V v = e.value;
+                        if (v == null) // recheck
+                            v = readValueUnderLock(e);
+                        if (value.equals(v))
+                            return true;
+                    }
+                }
+            }
+            return false;
+        }
+
+        boolean replace(K key, int hash, V oldValue, V newValue) {
+            lock();
+            try {
+                HashEntry<K,V> e = getFirst(hash);
+                while (e != null && (e.hash != hash || !key.equals(e.key)))
+                    e = e.next;
+
+                boolean replaced = false;
+                if (e != null && oldValue.equals(e.value)) {
+                    replaced = true;
+                    e.value = newValue;
+                }
+                return replaced;
+            } finally {
+                unlock();
+            }
+        }
+
+        V replace(K key, int hash, V newValue) {
+            lock();
+            try {
+                HashEntry<K,V> e = getFirst(hash);
+                while (e != null && (e.hash != hash || !key.equals(e.key)))
+                    e = e.next;
+
+                V oldValue = null;
+                if (e != null) {
+                    oldValue = e.value;
+                    e.value = newValue;
+                }
+                return oldValue;
+            } finally {
+                unlock();
+            }
+        }
+
+
+        V put(K key, int hash, V value, boolean onlyIfAbsent) {
+            lock();
+            try {
+                int c = count;
+                if (c++ > threshold) // ensure capacity
+                    rehash();
+                HashEntry<K,V>[] tab = table;
+                int index = hash & (tab.length - 1);
+                HashEntry<K,V> first = tab[index];
+                HashEntry<K,V> e = first;
+                while (e != null && (e.hash != hash || !key.equals(e.key)))
+                    e = e.next;
+
+                V oldValue;
+                if (e != null) {
+                    oldValue = e.value;
+                    if (!onlyIfAbsent)
+                        e.value = value;
+                }
+                else {
+                    oldValue = null;
+                    ++modCount;
+                    tab[index] = new HashEntry<K,V>(key, hash, first, value);
+                    count = c; // write-volatile
+                }
+                return oldValue;
+            } finally {
+                unlock();
+            }
+        }
+
+        void rehash() {
+            HashEntry<K,V>[] oldTable = table;
+            int oldCapacity = oldTable.length;
+            if (oldCapacity >= MAXIMUM_CAPACITY)
+                return;
+
+            /*
+             * Reclassify nodes in each list to new Map.  Because we are
+             * using power-of-two expansion, the elements from each bin
+             * must either stay at same index, or move with a power of two
+             * offset. We eliminate unnecessary node creation by catching
+             * cases where old nodes can be reused because their next
+             * fields won't change. Statistically, at the default
+             * threshold, only about one-sixth of them need cloning when
+             * a table doubles. The nodes they replace will be garbage
+             * collectable as soon as they are no longer referenced by any
+             * reader thread that may be in the midst of traversing table
+             * right now.
+             */
+
+            HashEntry<K,V>[] newTable = HashEntry.newArray(oldCapacity<<1);
+            threshold = (int)(newTable.length * loadFactor);
+            int sizeMask = newTable.length - 1;
+            for (int i = 0; i < oldCapacity ; i++) {
+                // We need to guarantee that any existing reads of old Map can
+                //  proceed. So we cannot yet null out each bin.
+                HashEntry<K,V> e = oldTable[i];
+
+                if (e != null) {
+                    HashEntry<K,V> next = e.next;
+                    int idx = e.hash & sizeMask;
+
+                    //  Single node on list
+                    if (next == null)
+                        newTable[idx] = e;
+
+                    else {
+                        // Reuse trailing consecutive sequence at same slot
+                        HashEntry<K,V> lastRun = e;
+                        int lastIdx = idx;
+                        for (HashEntry<K,V> last = next;
+                             last != null;
+                             last = last.next) {
+                            int k = last.hash & sizeMask;
+                            if (k != lastIdx) {
+                                lastIdx = k;
+                                lastRun = last;
+                            }
+                        }
+                        newTable[lastIdx] = lastRun;
+
+                        // Clone all remaining nodes
+                        for (HashEntry<K,V> p = e; p != lastRun; p = p.next) {
+                            int k = p.hash & sizeMask;
+                            HashEntry<K,V> n = newTable[k];
+                            newTable[k] = new HashEntry<K,V>(p.key, p.hash,
+                                                             n, p.value);
+                        }
+                    }
+                }
+            }
+            table = newTable;
+        }
+
+        /**
+         * Remove; match on key only if value null, else match both.
+         */
+        V remove(Object key, int hash, Object value) {
+            lock();
+            try {
+                int c = count - 1;
+                HashEntry<K,V>[] tab = table;
+                int index = hash & (tab.length - 1);
+                HashEntry<K,V> first = tab[index];
+                HashEntry<K,V> e = first;
+                while (e != null && (e.hash != hash || !key.equals(e.key)))
+                    e = e.next;
+
+                V oldValue = null;
+                if (e != null) {
+                    V v = e.value;
+                    if (value == null || value.equals(v)) {
+                        oldValue = v;
+                        // All entries following removed node can stay
+                        // in list, but all preceding ones need to be
+                        // cloned.
+                        ++modCount;
+                        HashEntry<K,V> newFirst = e.next;
+                        for (HashEntry<K,V> p = first; p != e; p = p.next)
+                            newFirst = new HashEntry<K,V>(p.key, p.hash,
+                                                          newFirst, p.value);
+                        tab[index] = newFirst;
+                        count = c; // write-volatile
+                    }
+                }
+                return oldValue;
+            } finally {
+                unlock();
+            }
+        }
+
+        void clear() {
+            if (count != 0) {
+                lock();
+                try {
+                    HashEntry<K,V>[] tab = table;
+                    for (int i = 0; i < tab.length ; i++)
+                        tab[i] = null;
+                    ++modCount;
+                    count = 0; // write-volatile
+                } finally {
+                    unlock();
+                }
+            }
+        }
+    }
+
+
+
+    /* ---------------- Public operations -------------- */
+
+    /**
+     * Creates a new, empty map with the specified initial
+     * capacity, load factor and concurrency level.
+     *
+     * @param initialCapacity the initial capacity. The implementation
+     * performs internal sizing to accommodate this many elements.
+     * @param loadFactor  the load factor threshold, used to control resizing.
+     * Resizing may be performed when the average number of elements per
+     * bin exceeds this threshold.
+     * @param concurrencyLevel the estimated number of concurrently
+     * updating threads. The implementation performs internal sizing
+     * to try to accommodate this many threads.
+     * @throws IllegalArgumentException if the initial capacity is
+     * negative or the load factor or concurrencyLevel are
+     * nonpositive.
+     */
+    public ConcurrentHashMap(int initialCapacity,
+                             float loadFactor, int concurrencyLevel) {
+        if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0)
+            throw new IllegalArgumentException();
+
+        if (concurrencyLevel > MAX_SEGMENTS)
+            concurrencyLevel = MAX_SEGMENTS;
+
+        // Find power-of-two sizes best matching arguments
+        int sshift = 0;
+        int ssize = 1;
+        while (ssize < concurrencyLevel) {
+            ++sshift;
+            ssize <<= 1;
+        }
+        segmentShift = 32 - sshift;
+        segmentMask = ssize - 1;
+        this.segments = Segment.newArray(ssize);
+
+        if (initialCapacity > MAXIMUM_CAPACITY)
+            initialCapacity = MAXIMUM_CAPACITY;
+        int c = initialCapacity / ssize;
+        if (c * ssize < initialCapacity)
+            ++c;
+        int cap = 1;
+        while (cap < c)
+            cap <<= 1;
+
+        for (int i = 0; i < this.segments.length; ++i)
+            this.segments[i] = new Segment<K,V>(cap, loadFactor);
+    }
+
+    /**
+     * Creates a new, empty map with the specified initial capacity
+     * and load factor and with the default concurrencyLevel (16).
+     *
+     * @param initialCapacity The implementation performs internal
+     * sizing to accommodate this many elements.
+     * @param loadFactor  the load factor threshold, used to control resizing.
+     * Resizing may be performed when the average number of elements per
+     * bin exceeds this threshold.
+     * @throws IllegalArgumentException if the initial capacity of
+     * elements is negative or the load factor is nonpositive
+     *
+     * @since 1.6
+     */
+    public ConcurrentHashMap(int initialCapacity, float loadFactor) {
+        this(initialCapacity, loadFactor, DEFAULT_CONCURRENCY_LEVEL);
+    }
+
+    /**
+     * Creates a new, empty map with the specified initial capacity,
+     * and with default load factor (0.75) and concurrencyLevel (16).
+     *
+     * @param initialCapacity the initial capacity. The implementation
+     * performs internal sizing to accommodate this many elements.
+     * @throws IllegalArgumentException if the initial capacity of
+     * elements is negative.
+     */
+    public ConcurrentHashMap(int initialCapacity) {
+        this(initialCapacity, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
+    }
+
+    /**
+     * Creates a new, empty map with a default initial capacity (16),
+     * load factor (0.75) and concurrencyLevel (16).
+     */
+    public ConcurrentHashMap() {
+        this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
+    }
+
+    /**
+     * Creates a new map with the same mappings as the given map.
+     * The map is created with a capacity of 1.5 times the number
+     * of mappings in the given map or 16 (whichever is greater),
+     * and a default load factor (0.75) and concurrencyLevel (16).
+     *
+     * @param m the map
+     */
+    public ConcurrentHashMap(Map<? extends K, ? extends V> m) {
+        this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
+                      DEFAULT_INITIAL_CAPACITY),
+             DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
+        putAll(m);
+    }
+
+    /**
+     * Returns <tt>true</tt> if this map contains no key-value mappings.
+     *
+     * @return <tt>true</tt> if this map contains no key-value mappings
+     */
+    public boolean isEmpty() {
+        final Segment<K,V>[] segments = this.segments;
+        /*
+         * We keep track of per-segment modCounts to avoid ABA
+         * problems in which an element in one segment was added and
+         * in another removed during traversal, in which case the
+         * table was never actually empty at any point. Note the
+         * similar use of modCounts in the size() and containsValue()
+         * methods, which are the only other methods also susceptible
+         * to ABA problems.
+         */
+        int[] mc = new int[segments.length];
+        int mcsum = 0;
+        for (int i = 0; i < segments.length; ++i) {
+            if (segments[i].count != 0)
+                return false;
+            else
+                mcsum += mc[i] = segments[i].modCount;
+        }
+        // If mcsum happens to be zero, then we know we got a snapshot
+        // before any modifications at all were made.  This is
+        // probably common enough to bother tracking.
+        if (mcsum != 0) {
+            for (int i = 0; i < segments.length; ++i) {
+                if (segments[i].count != 0 ||
+                    mc[i] != segments[i].modCount)
+                    return false;
+            }
+        }
+        return true;
+    }
+
+    /**
+     * Returns the number of key-value mappings in this map.  If the
+     * map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
+     * <tt>Integer.MAX_VALUE</tt>.
+     *
+     * @return the number of key-value mappings in this map
+     */
+    public int size() {
+        final Segment<K,V>[] segments = this.segments;
+        long sum = 0;
+        long check = 0;
+        int[] mc = new int[segments.length];
+        // Try a few times to get accurate count. On failure due to
+        // continuous async changes in table, resort to locking.
+        for (int k = 0; k < RETRIES_BEFORE_LOCK; ++k) {
+            check = 0;
+            sum = 0;
+            int mcsum = 0;
+            for (int i = 0; i < segments.length; ++i) {
+                sum += segments[i].count;
+                mcsum += mc[i] = segments[i].modCount;
+            }
+            if (mcsum != 0) {
+                for (int i = 0; i < segments.length; ++i) {
+                    check += segments[i].count;
+                    if (mc[i] != segments[i].modCount) {
+                        check = -1; // force retry
+                        break;
+                    }
+                }
+            }
+            if (check == sum)
+                break;
+        }
+        if (check != sum) { // Resort to locking all segments
+            sum = 0;
+            for (int i = 0; i < segments.length; ++i)
+                segments[i].lock();
+            for (int i = 0; i < segments.length; ++i)
+                sum += segments[i].count;
+            for (int i = 0; i < segments.length; ++i)
+                segments[i].unlock();
+        }
+        if (sum > Integer.MAX_VALUE)
+            return Integer.MAX_VALUE;
+        else
+            return (int)sum;
+    }
+
+    /**
+     * Returns the value to which the specified key is mapped,
+     * or {@code null} if this map contains no mapping for the key.
+     *
+     * <p>More formally, if this map contains a mapping from a key
+     * {@code k} to a value {@code v} such that {@code key.equals(k)},
+     * then this method returns {@code v}; otherwise it returns
+     * {@code null}.  (There can be at most one such mapping.)
+     *
+     * @throws NullPointerException if the specified key is null
+     */
+    public V get(Object key) {
+        int hash = hash(key.hashCode());
+        return segmentFor(hash).get(key, hash);
+    }
+
+    /**
+     * Tests if the specified object is a key in this table.
+     *
+     * @param  key   possible key
+     * @return <tt>true</tt> if and only if the specified object
+     *         is a key in this table, as determined by the
+     *         <tt>equals</tt> method; <tt>false</tt> otherwise.
+     * @throws NullPointerException if the specified key is null
+     */
+    public boolean containsKey(Object key) {
+        int hash = hash(key.hashCode()); 
+        return segmentFor(hash).containsKey(key, hash);
+    }
+
+    /**
+     * Returns <tt>true</tt> if this map maps one or more keys to the
+     * specified value. Note: This method requires a full internal
+     * traversal of the hash table, and so is much slower than
+     * method <tt>containsKey</tt>.
+     *
+     * @param value value whose presence in this map is to be tested
+     * @return <tt>true</tt> if this map maps one or more keys to the
+     *         specified value
+     * @throws NullPointerException if the specified value is null
+     */
+    public boolean containsValue(Object value) {
+        if (value == null)
+            throw new NullPointerException();
+
+        // See explanation of modCount use above
+
+        final Segment<K,V>[] segments = this.segments;
+        int[] mc = new int[segments.length];
+
+        // Try a few times without locking
+        for (int k = 0; k < RETRIES_BEFORE_LOCK; ++k) {
+            int sum = 0;
+            int mcsum = 0;
+            for (int i = 0; i < segments.length; ++i) {
+                int c = segments[i].count;
+                mcsum += mc[i] = segments[i].modCount;
+                if (segments[i].containsValue(value))
+                    return true;
+            }
+            boolean cleanSweep = true;
+            if (mcsum != 0) {
+                for (int i = 0; i < segments.length; ++i) {
+                    int c = segments[i].count;
+                    if (mc[i] != segments[i].modCount) {
+                        cleanSweep = false;
+                        break;
+                    }
+                }
+            }
+            if (cleanSweep)
+                return false;
+        }
+        // Resort to locking all segments
+        for (int i = 0; i < segments.length; ++i)
+            segments[i].lock();
+        boolean found = false;
+        try {
+            for (int i = 0; i < segments.length; ++i) {
+                if (segments[i].containsValue(value)) {
+                    found = true;
+                    break;
+                }
+            }
+        } finally {
+            for (int i = 0; i < segments.length; ++i)
+                segments[i].unlock();
+        }
+        return found;
+    }
+
+    /**
+     * Legacy method testing if some key maps into the specified value
+     * in this table.  This method is identical in functionality to
+     * {@link #containsValue}, and exists solely to ensure
+     * full compatibility with class {@link java.util.Hashtable},
+     * which supported this method prior to introduction of the
+     * Java Collections framework.
+
+     * @param  value a value to search for
+     * @return <tt>true</tt> if and only if some key maps to the
+     *         <tt>value</tt> argument in this table as
+     *         determined by the <tt>equals</tt> method;
+     *         <tt>false</tt> otherwise
+     * @throws NullPointerException if the specified value is null
+     */
+    public boolean contains(Object value) {
+        return containsValue(value);
+    }
+
+    /**
+     * Maps the specified key to the specified value in this table.
+     * Neither the key nor the value can be null.
+     *
+     * <p> The value can be retrieved by calling the <tt>get</tt> method
+     * with a key that is equal to the original key.
+     *
+     * @param key key with which the specified value is to be associated
+     * @param value value to be associated with the specified key
+     * @return the previous value associated with <tt>key</tt>, or
+     *         <tt>null</tt> if there was no mapping for <tt>key</tt>
+     * @throws NullPointerException if the specified key or value is null
+     */
+    public V put(K key, V value) {
+        if (value == null)
+            throw new NullPointerException();
+        int hash = hash(key.hashCode());
+        return segmentFor(hash).put(key, hash, value, false);
+    }
+
+    /**
+     * {@inheritDoc}
+     *
+     * @return the previous value associated with the specified key,
+     *         or <tt>null</tt> if there was no mapping for the key
+     * @throws NullPointerException if the specified key or value is null
+     */
+    public V putIfAbsent(K key, V value) {
+        if (value == null)
+            throw new NullPointerException();
+        int hash = hash(key.hashCode());
+        return segmentFor(hash).put(key, hash, value, true);
+    }
+
+    /**
+     * Copies all of the mappings from the specified map to this one.
+     * These mappings replace any mappings that this map had for any of the
+     * keys currently in the specified map.
+     *
+     * @param m mappings to be stored in this map
+     */
+    public void putAll(Map<? extends K, ? extends V> m) {
+        for (Map.Entry<? extends K, ? extends V> e : m.entrySet())
+            put(e.getKey(), e.getValue());
+    }
+
+    /**
+     * Removes the key (and its corresponding value) from this map.
+     * This method does nothing if the key is not in the map.
+     *
+     * @param  key the key that needs to be removed
+     * @return the previous value associated with <tt>key</tt>, or
+     *         <tt>null</tt> if there was no mapping for <tt>key</tt>
+     * @throws NullPointerException if the specified key is null
+     */
+    public V remove(Object key) {
+	int hash = hash(key.hashCode());
+        return segmentFor(hash).remove(key, hash, null);
+    }
+
+    /**
+     * {@inheritDoc}
+     *
+     * @throws NullPointerException if the specified key is null
+     */
+    public boolean remove(Object key, Object value) {
+        int hash = hash(key.hashCode());
+        if (value == null)
+            return false;
+        return segmentFor(hash).remove(key, hash, value) != null;
+    }
+
+    /**
+     * {@inheritDoc}
+     *
+     * @throws NullPointerException if any of the arguments are null
+     */
+    public boolean replace(K key, V oldValue, V newValue) {
+        if (oldValue == null || newValue == null)
+            throw new NullPointerException();
+        int hash = hash(key.hashCode());
+        return segmentFor(hash).replace(key, hash, oldValue, newValue);
+    }
+
+    /**
+     * {@inheritDoc}
+     *
+     * @return the previous value associated with the specified key,
+     *         or <tt>null</tt> if there was no mapping for the key
+     * @throws NullPointerException if the specified key or value is null
+     */
+    public V replace(K key, V value) {
+        if (value == null)
+            throw new NullPointerException();
+        int hash = hash(key.hashCode());
+        return segmentFor(hash).replace(key, hash, value);
+    }
+
+    /**
+     * Removes all of the mappings from this map.
+     */
+    public void clear() {
+        for (int i = 0; i < segments.length; ++i)
+            segments[i].clear();
+    }
+
+    /**
+     * Returns a {@link Set} view of the keys contained in this map.
+     * The set is backed by the map, so changes to the map are
+     * reflected in the set, and vice-versa.  The set supports element
+     * removal, which removes the corresponding mapping from this map,
+     * via the <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
+     * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
+     * operations.  It does not support the <tt>add</tt> or
+     * <tt>addAll</tt> operations.
+     *
+     * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator
+     * that will never throw {@link ConcurrentModificationException},
+     * and guarantees to traverse elements as they existed upon
+     * construction of the iterator, and may (but is not guaranteed to)
+     * reflect any modifications subsequent to construction.
+     */
+    public Set<K> keySet() {
+        Set<K> ks = keySet;
+        return (ks != null) ? ks : (keySet = new KeySet());
+    }
+
+    /**
+     * Returns a {@link Collection} view of the values contained in this map.
+     * The collection is backed by the map, so changes to the map are
+     * reflected in the collection, and vice-versa.  The collection
+     * supports element removal, which removes the corresponding
+     * mapping from this map, via the <tt>Iterator.remove</tt>,
+     * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
+     * <tt>retainAll</tt>, and <tt>clear</tt> operations.  It does not
+     * support the <tt>add</tt> or <tt>addAll</tt> operations.
+     *
+     * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator
+     * that will never throw {@link ConcurrentModificationException},
+     * and guarantees to traverse elements as they existed upon
+     * construction of the iterator, and may (but is not guaranteed to)
+     * reflect any modifications subsequent to construction.
+     */
+    public Collection<V> values() {
+        Collection<V> vs = values;
+        return (vs != null) ? vs : (values = new Values());
+    }
+
+    /**
+     * Returns a {@link Set} view of the mappings contained in this map.
+     * The set is backed by the map, so changes to the map are
+     * reflected in the set, and vice-versa.  The set supports element
+     * removal, which removes the corresponding mapping from the map,
+     * via the <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
+     * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
+     * operations.  It does not support the <tt>add</tt> or
+     * <tt>addAll</tt> operations.
+     *
+     * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator
+     * that will never throw {@link ConcurrentModificationException},
+     * and guarantees to traverse elements as they existed upon
+     * construction of the iterator, and may (but is not guaranteed to)
+     * reflect any modifications subsequent to construction.
+     */
+    public Set<Map.Entry<K,V>> entrySet() {
+        Set<Map.Entry<K,V>> es = entrySet;
+        return (es != null) ? es : (entrySet = new EntrySet());
+    }
+
+    /**
+     * Returns an enumeration of the keys in this table.
+     *
+     * @return an enumeration of the keys in this table
+     * @see #keySet
+     */
+    public Enumeration<K> keys() {
+        return new KeyIterator();
+    }
+
+    /**
+     * Returns an enumeration of the values in this table.
+     *
+     * @return an enumeration of the values in this table
+     * @see #values
+     */
+    public Enumeration<V> elements() {
+        return new ValueIterator();
+    }
+
+    /* ---------------- Iterator Support -------------- */
+
+    abstract class HashIterator {
+        int nextSegmentIndex;
+        int nextTableIndex;
+        HashEntry<K,V>[] currentTable;
+        HashEntry<K, V> nextEntry;
+        HashEntry<K, V> lastReturned;
+
+        HashIterator() {
+            nextSegmentIndex = segments.length - 1;
+            nextTableIndex = -1;
+            advance();
+        }
+
+        public boolean hasMoreElements() { return hasNext(); }
+
+        final void advance() {
+            if (nextEntry != null && (nextEntry = nextEntry.next) != null)
+                return;
+
+            while (nextTableIndex >= 0) {
+                if ( (nextEntry = currentTable[nextTableIndex--]) != null)
+                    return;
+            }
+
+            while (nextSegmentIndex >= 0) {
+                Segment<K,V> seg = segments[nextSegmentIndex--];
+                if (seg.count != 0) {
+                    currentTable = seg.table;
+                    for (int j = currentTable.length - 1; j >= 0; --j) {
+                        if ( (nextEntry = currentTable[j]) != null) {
+                            nextTableIndex = j - 1;
+                            return;
+                        }
+                    }
+                }
+            }
+        }
+
+        public boolean hasNext() { return nextEntry != null; }
+
+        HashEntry<K,V> nextEntry() {
+            if (nextEntry == null)
+                throw new NoSuchElementException();
+            lastReturned = nextEntry;
+            advance();
+            return lastReturned;
+        }
+
+        public void remove() {
+            if (lastReturned == null)
+                throw new IllegalStateException();
+            ConcurrentHashMap.this.remove(lastReturned.key);
+            lastReturned = null;
+        }
+    }
+
+    final class KeyIterator
+	extends HashIterator
+	implements Iterator<K>, Enumeration<K>
+    {
+        public K next()        { return super.nextEntry().key; }
+        public K nextElement() { return super.nextEntry().key; }
+    }
+
+    final class ValueIterator
+	extends HashIterator
+	implements Iterator<V>, Enumeration<V>
+    {
+        public V next()        { return super.nextEntry().value; }
+        public V nextElement() { return super.nextEntry().value; }
+    }
+
+    /**
+     * Custom Entry class used by EntryIterator.next(), that relays
+     * setValue changes to the underlying map.
+     */
+    final class WriteThroughEntry
+	extends AbstractMap.SimpleEntry<K,V>
+    {
+        WriteThroughEntry(K k, V v) {
+            super(k,v);
+        }
+
+        /**
+         * Set our entry's value and write through to the map. The
+         * value to return is somewhat arbitrary here. Since a
+         * WriteThroughEntry does not necessarily track asynchronous
+         * changes, the most recent "previous" value could be
+         * different from what we return (or could even have been
+         * removed in which case the put will re-establish). We do not
+         * and cannot guarantee more.
+         */
+	public V setValue(V value) {
+            if (value == null) throw new NullPointerException();
+            V v = super.setValue(value);
+            ConcurrentHashMap.this.put(getKey(), value);
+            return v;
+        }
+    }
+
+    final class EntryIterator
+	extends HashIterator
+	implements Iterator<Entry<K,V>>
+    {
+        public Map.Entry<K,V> next() {
+            HashEntry<K,V> e = super.nextEntry();
+            return new WriteThroughEntry(e.key, e.value);
+        }
+    }
+
+    final class KeySet extends AbstractSet<K> {
+        public Iterator<K> iterator() {
+            return new KeyIterator();
+        }
+        public int size() {
+            return ConcurrentHashMap.this.size();
+        }
+        public boolean contains(Object o) {
+            return ConcurrentHashMap.this.containsKey(o);
+        }
+        public boolean remove(Object o) {
+            return ConcurrentHashMap.this.remove(o) != null;
+        }
+        public void clear() {
+            ConcurrentHashMap.this.clear();
+        }
+    }
+
+    final class Values extends AbstractCollection<V> {
+        public Iterator<V> iterator() {
+            return new ValueIterator();
+        }
+        public int size() {
+            return ConcurrentHashMap.this.size();
+        }
+        public boolean contains(Object o) {
+            return ConcurrentHashMap.this.containsValue(o);
+        }
+        public void clear() {
+            ConcurrentHashMap.this.clear();
+        }
+    }
+
+    final class EntrySet extends AbstractSet<Map.Entry<K,V>> {
+        public Iterator<Map.Entry<K,V>> iterator() {
+            return new EntryIterator();
+        }
+        public boolean contains(Object o) {
+            if (!(o instanceof Map.Entry))
+                return false;
+            Map.Entry<?,?> e = (Map.Entry<?,?>)o;
+            V v = ConcurrentHashMap.this.get(e.getKey());
+            return v != null && v.equals(e.getValue());
+        }
+        public boolean remove(Object o) {
+            if (!(o instanceof Map.Entry))
+                return false;
+            Map.Entry<?,?> e = (Map.Entry<?,?>)o;
+            return ConcurrentHashMap.this.remove(e.getKey(), e.getValue());
+        }
+        public int size() {
+            return ConcurrentHashMap.this.size();
+        }
+        public void clear() {
+            ConcurrentHashMap.this.clear();
+        }
+    }
+
+    /* ---------------- Serialization Support -------------- */
+
+    /**
+     * Save the state of the <tt>ConcurrentHashMap</tt> instance to a
+     * stream (i.e., serialize it).
+     * @param s the stream
+     * @serialData
+     * the key (Object) and value (Object)
+     * for each key-value mapping, followed by a null pair.
+     * The key-value mappings are emitted in no particular order.
+     */
+    private void writeObject(java.io.ObjectOutputStream s) throws IOException  {
+        s.defaultWriteObject();
+
+        for (int k = 0; k < segments.length; ++k) {
+            Segment<K,V> seg = segments[k];
+            seg.lock();
+            try {
+                HashEntry<K,V>[] tab = seg.table;
+                for (int i = 0; i < tab.length; ++i) {
+                    for (HashEntry<K,V> e = tab[i]; e != null; e = e.next) {
+                        s.writeObject(e.key);
+                        s.writeObject(e.value);
+                    }
+                }
+            } finally {
+                seg.unlock();
+            }
+        }
+        s.writeObject(null);
+        s.writeObject(null);
+    }
+
+    /**
+     * Reconstitute the <tt>ConcurrentHashMap</tt> instance from a
+     * stream (i.e., deserialize it).
+     * @param s the stream
+     */
+    private void readObject(java.io.ObjectInputStream s)
+        throws IOException, ClassNotFoundException  {
+        s.defaultReadObject();
+
+        // Initialize each segment to be minimally sized, and let grow.
+        for (int i = 0; i < segments.length; ++i) {
+            segments[i].setTable(new HashEntry[1]);
+        }
+
+        // Read the keys and values, and put the mappings in the table
+        for (;;) {
+            K key = (K) s.readObject();
+            V value = (V) s.readObject();
+            if (key == null)
+                break;
+            put(key, value);
+        }
+    }
+}
diff --git a/external/jsr166/java/util/concurrent/ConcurrentLinkedQueue.java b/external/jsr166/java/util/concurrent/ConcurrentLinkedQueue.java
new file mode 100644
index 000000000..000f4a4c9
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/ConcurrentLinkedQueue.java
@@ -0,0 +1,480 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+import java.util.*;
+import java.util.concurrent.atomic.*;
+
+
+/**
+ * An unbounded thread-safe {@linkplain Queue queue} based on linked nodes.
+ * This queue orders elements FIFO (first-in-first-out).
+ * The <em>head</em> of the queue is that element that has been on the
+ * queue the longest time.
+ * The <em>tail</em> of the queue is that element that has been on the
+ * queue the shortest time. New elements
+ * are inserted at the tail of the queue, and the queue retrieval
+ * operations obtain elements at the head of the queue.
+ * A <tt>ConcurrentLinkedQueue</tt> is an appropriate choice when
+ * many threads will share access to a common collection.
+ * This queue does not permit <tt>null</tt> elements.
+ *
+ * <p>This implementation employs an efficient &quot;wait-free&quot;
+ * algorithm based on one described in <a
+ * href="http://www.cs.rochester.edu/u/michael/PODC96.html"> Simple,
+ * Fast, and Practical Non-Blocking and Blocking Concurrent Queue
+ * Algorithms</a> by Maged M. Michael and Michael L. Scott.
+ *
+ * <p>Beware that, unlike in most collections, the <tt>size</tt> method
+ * is <em>NOT</em> a constant-time operation. Because of the
+ * asynchronous nature of these queues, determining the current number
+ * of elements requires a traversal of the elements.
+ *
+ * <p>This class and its iterator implement all of the
+ * <em>optional</em> methods of the {@link Collection} and {@link
+ * Iterator} interfaces.
+ *
+ * <p>Memory consistency effects: As with other concurrent
+ * collections, actions in a thread prior to placing an object into a
+ * {@code ConcurrentLinkedQueue}
+ * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
+ * actions subsequent to the access or removal of that element from
+ * the {@code ConcurrentLinkedQueue} in another thread.
+ *
+ * <p>This class is a member of the
+ * <a href="{@docRoot}/../technotes/guides/collections/index.html">
+ * Java Collections Framework</a>.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ * @param <E> the type of elements held in this collection
+ *
+ */
+public class ConcurrentLinkedQueue<E> extends AbstractQueue<E>
+        implements Queue<E>, java.io.Serializable {
+    private static final long serialVersionUID = 196745693267521676L;
+
+    /*
+     * This is a straight adaptation of Michael & Scott algorithm.
+     * For explanation, read the paper.  The only (minor) algorithmic
+     * difference is that this version supports lazy deletion of
+     * internal nodes (method remove(Object)) -- remove CAS'es item
+     * fields to null. The normal queue operations unlink but then
+     * pass over nodes with null item fields. Similarly, iteration
+     * methods ignore those with nulls.
+     *
+     * Also note that like most non-blocking algorithms in this
+     * package, this implementation relies on the fact that in garbage
+     * collected systems, there is no possibility of ABA problems due
+     * to recycled nodes, so there is no need to use "counted
+     * pointers" or related techniques seen in versions used in
+     * non-GC'ed settings.
+     */
+
+    private static class Node<E> {
+        private volatile E item;
+        private volatile Node<E> next;
+
+        private static final
+            AtomicReferenceFieldUpdater<Node, Node>
+            nextUpdater =
+            AtomicReferenceFieldUpdater.newUpdater
+            (Node.class, Node.class, "next");
+        private static final
+            AtomicReferenceFieldUpdater<Node, Object>
+            itemUpdater =
+            AtomicReferenceFieldUpdater.newUpdater
+            (Node.class, Object.class, "item");
+
+        Node(E x) { item = x; }
+
+        Node(E x, Node<E> n) { item = x; next = n; }
+
+        E getItem() {
+            return item;
+        }
+
+        boolean casItem(E cmp, E val) {
+            return itemUpdater.compareAndSet(this, cmp, val);
+        }
+
+        void setItem(E val) {
+            itemUpdater.set(this, val);
+        }
+
+        Node<E> getNext() {
+            return next;
+        }
+
+        boolean casNext(Node<E> cmp, Node<E> val) {
+            return nextUpdater.compareAndSet(this, cmp, val);
+        }
+
+        void setNext(Node<E> val) {
+            nextUpdater.set(this, val);
+        }
+
+    }
+
+    private static final
+        AtomicReferenceFieldUpdater<ConcurrentLinkedQueue, Node>
+        tailUpdater =
+        AtomicReferenceFieldUpdater.newUpdater
+        (ConcurrentLinkedQueue.class, Node.class, "tail");
+    private static final
+        AtomicReferenceFieldUpdater<ConcurrentLinkedQueue, Node>
+        headUpdater =
+        AtomicReferenceFieldUpdater.newUpdater
+        (ConcurrentLinkedQueue.class,  Node.class, "head");
+
+    private boolean casTail(Node<E> cmp, Node<E> val) {
+        return tailUpdater.compareAndSet(this, cmp, val);
+    }
+
+    private boolean casHead(Node<E> cmp, Node<E> val) {
+        return headUpdater.compareAndSet(this, cmp, val);
+    }
+
+
+    /**
+     * Pointer to header node, initialized to a dummy node.  The first
+     * actual node is at head.getNext().
+     */
+    private transient volatile Node<E> head = new Node<E>(null, null);
+
+    /** Pointer to last node on list **/
+    private transient volatile Node<E> tail = head;
+
+
+    /**
+     * Creates a <tt>ConcurrentLinkedQueue</tt> that is initially empty.
+     */
+    public ConcurrentLinkedQueue() {}
+
+    /**
+     * Creates a <tt>ConcurrentLinkedQueue</tt>
+     * initially containing the elements of the given collection,
+     * added in traversal order of the collection's iterator.
+     * @param c the collection of elements to initially contain
+     * @throws NullPointerException if the specified collection or any
+     *         of its elements are null
+     */
+    public ConcurrentLinkedQueue(Collection<? extends E> c) {
+        for (Iterator<? extends E> it = c.iterator(); it.hasNext();)
+            add(it.next());
+    }
+
+    // Have to override just to update the javadoc
+
+    /**
+     * Inserts the specified element at the tail of this queue.
+     *
+     * @return <tt>true</tt> (as specified by {@link Collection#add})
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean add(E e) {
+        return offer(e);
+    }
+
+    /**
+     * Inserts the specified element at the tail of this queue.
+     *
+     * @return <tt>true</tt> (as specified by {@link Queue#offer})
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean offer(E e) {
+        if (e == null) throw new NullPointerException();
+        Node<E> n = new Node<E>(e, null);
+        for (;;) {
+            Node<E> t = tail;
+            Node<E> s = t.getNext();
+            if (t == tail) {
+                if (s == null) {
+                    if (t.casNext(s, n)) {
+                        casTail(t, n);
+                        return true;
+                    }
+                } else {
+                    casTail(t, s);
+                }
+            }
+        }
+    }
+
+    public E poll() {
+        for (;;) {
+            Node<E> h = head;
+            Node<E> t = tail;
+            Node<E> first = h.getNext();
+            if (h == head) {
+                if (h == t) {
+                    if (first == null)
+                        return null;
+                    else
+                        casTail(t, first);
+                } else if (casHead(h, first)) {
+                    E item = first.getItem();
+                    if (item != null) {
+                        first.setItem(null);
+                        return item;
+                    }
+                    // else skip over deleted item, continue loop,
+                }
+            }
+        }
+    }
+
+    public E peek() { // same as poll except don't remove item
+        for (;;) {
+            Node<E> h = head;
+            Node<E> t = tail;
+            Node<E> first = h.getNext();
+            if (h == head) {
+                if (h == t) {
+                    if (first == null)
+                        return null;
+                    else
+                        casTail(t, first);
+                } else {
+                    E item = first.getItem();
+                    if (item != null)
+                        return item;
+                    else // remove deleted node and continue
+                        casHead(h, first);
+                }
+            }
+        }
+    }
+
+    /**
+     * Returns the first actual (non-header) node on list.  This is yet
+     * another variant of poll/peek; here returning out the first
+     * node, not element (so we cannot collapse with peek() without
+     * introducing race.)
+     */
+    Node<E> first() {
+        for (;;) {
+            Node<E> h = head;
+            Node<E> t = tail;
+            Node<E> first = h.getNext();
+            if (h == head) {
+                if (h == t) {
+                    if (first == null)
+                        return null;
+                    else
+                        casTail(t, first);
+                } else {
+                    if (first.getItem() != null)
+                        return first;
+                    else // remove deleted node and continue
+                        casHead(h, first);
+                }
+            }
+        }
+    }
+
+
+    /**
+     * Returns <tt>true</tt> if this queue contains no elements.
+     *
+     * @return <tt>true</tt> if this queue contains no elements
+     */
+    public boolean isEmpty() {
+        return first() == null;
+    }
+
+    /**
+     * Returns the number of elements in this queue.  If this queue
+     * contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
+     * <tt>Integer.MAX_VALUE</tt>.
+     *
+     * <p>Beware that, unlike in most collections, this method is
+     * <em>NOT</em> a constant-time operation. Because of the
+     * asynchronous nature of these queues, determining the current
+     * number of elements requires an O(n) traversal.
+     *
+     * @return the number of elements in this queue
+     */
+    public int size() {
+        int count = 0;
+        for (Node<E> p = first(); p != null; p = p.getNext()) {
+            if (p.getItem() != null) {
+                // Collections.size() spec says to max out
+                if (++count == Integer.MAX_VALUE)
+                    break;
+            }
+        }
+        return count;
+    }
+
+    /**
+     * Returns <tt>true</tt> if this queue contains the specified element.
+     * More formally, returns <tt>true</tt> if and only if this queue contains
+     * at least one element <tt>e</tt> such that <tt>o.equals(e)</tt>.
+     *
+     * @param o object to be checked for containment in this queue
+     * @return <tt>true</tt> if this queue contains the specified element
+     */
+    public boolean contains(Object o) {
+        if (o == null) return false;
+        for (Node<E> p = first(); p != null; p = p.getNext()) {
+            E item = p.getItem();
+            if (item != null &&
+                o.equals(item))
+                return true;
+        }
+        return false;
+    }
+
+    /**
+     * Removes a single instance of the specified element from this queue,
+     * if it is present.  More formally, removes an element <tt>e</tt> such
+     * that <tt>o.equals(e)</tt>, if this queue contains one or more such
+     * elements.
+     * Returns <tt>true</tt> if this queue contained the specified element
+     * (or equivalently, if this queue changed as a result of the call).
+     *
+     * @param o element to be removed from this queue, if present
+     * @return <tt>true</tt> if this queue changed as a result of the call
+     */
+    public boolean remove(Object o) {
+        if (o == null) return false;
+        for (Node<E> p = first(); p != null; p = p.getNext()) {
+            E item = p.getItem();
+            if (item != null &&
+                o.equals(item) &&
+                p.casItem(item, null))
+                return true;
+        }
+        return false;
+    }
+
+    /**
+     * Returns an iterator over the elements in this queue in proper sequence.
+     * The returned iterator is a "weakly consistent" iterator that
+     * will never throw {@link ConcurrentModificationException},
+     * and guarantees to traverse elements as they existed upon
+     * construction of the iterator, and may (but is not guaranteed to)
+     * reflect any modifications subsequent to construction.
+     *
+     * @return an iterator over the elements in this queue in proper sequence
+     */
+    public Iterator<E> iterator() {
+        return new Itr();
+    }
+
+    private class Itr implements Iterator<E> {
+        /**
+         * Next node to return item for.
+         */
+        private Node<E> nextNode;
+
+        /**
+         * nextItem holds on to item fields because once we claim
+         * that an element exists in hasNext(), we must return it in
+         * the following next() call even if it was in the process of
+         * being removed when hasNext() was called.
+         */
+        private E nextItem;
+
+        /**
+         * Node of the last returned item, to support remove.
+         */
+        private Node<E> lastRet;
+
+        Itr() {
+            advance();
+        }
+
+        /**
+         * Moves to next valid node and returns item to return for
+         * next(), or null if no such.
+         */
+        private E advance() {
+            lastRet = nextNode;
+            E x = nextItem;
+
+            Node<E> p = (nextNode == null)? first() : nextNode.getNext();
+            for (;;) {
+                if (p == null) {
+                    nextNode = null;
+                    nextItem = null;
+                    return x;
+                }
+                E item = p.getItem();
+                if (item != null) {
+                    nextNode = p;
+                    nextItem = item;
+                    return x;
+                } else // skip over nulls
+                    p = p.getNext();
+            }
+        }
+
+        public boolean hasNext() {
+            return nextNode != null;
+        }
+
+        public E next() {
+            if (nextNode == null) throw new NoSuchElementException();
+            return advance();
+        }
+
+        public void remove() {
+            Node<E> l = lastRet;
+            if (l == null) throw new IllegalStateException();
+            // rely on a future traversal to relink.
+            l.setItem(null);
+            lastRet = null;
+        }
+    }
+
+    /**
+     * Save the state to a stream (that is, serialize it).
+     *
+     * @serialData All of the elements (each an <tt>E</tt>) in
+     * the proper order, followed by a null
+     * @param s the stream
+     */
+    private void writeObject(java.io.ObjectOutputStream s)
+        throws java.io.IOException {
+
+        // Write out any hidden stuff
+        s.defaultWriteObject();
+
+        // Write out all elements in the proper order.
+        for (Node<E> p = first(); p != null; p = p.getNext()) {
+            Object item = p.getItem();
+            if (item != null)
+                s.writeObject(item);
+        }
+
+        // Use trailing null as sentinel
+        s.writeObject(null);
+    }
+
+    /**
+     * Reconstitute the Queue instance from a stream (that is,
+     * deserialize it).
+     * @param s the stream
+     */
+    private void readObject(java.io.ObjectInputStream s)
+        throws java.io.IOException, ClassNotFoundException {
+        // Read in capacity, and any hidden stuff
+        s.defaultReadObject();
+        head = new Node<E>(null, null);
+        tail = head;
+        // Read in all elements and place in queue
+        for (;;) {
+            E item = (E)s.readObject();
+            if (item == null)
+                break;
+            else
+                offer(item);
+        }
+    }
+
+}
diff --git a/external/jsr166/java/util/concurrent/ConcurrentMap.java b/external/jsr166/java/util/concurrent/ConcurrentMap.java
new file mode 100644
index 000000000..6e5bd0738
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/ConcurrentMap.java
@@ -0,0 +1,134 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+import java.util.Map;
+
+/**
+ * A {@link java.util.Map} providing additional atomic
+ * <tt>putIfAbsent</tt>, <tt>remove</tt>, and <tt>replace</tt> methods.
+ *
+ * <p>Memory consistency effects: As with other concurrent
+ * collections, actions in a thread prior to placing an object into a
+ * {@code ConcurrentMap} as a key or value
+ * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
+ * actions subsequent to the access or removal of that object from
+ * the {@code ConcurrentMap} in another thread.
+ *
+ * <p>This interface is a member of the
+ * <a href="{@docRoot}/../technotes/guides/collections/index.html">
+ * Java Collections Framework</a>.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ * @param <K> the type of keys maintained by this map
+ * @param <V> the type of mapped values
+ */
+public interface ConcurrentMap<K, V> extends Map<K, V> {
+    /**
+     * If the specified key is not already associated
+     * with a value, associate it with the given value.
+     * This is equivalent to
+     * <pre>
+     *   if (!map.containsKey(key))
+     *       return map.put(key, value);
+     *   else
+     *       return map.get(key);</pre>
+     * except that the action is performed atomically.
+     *
+     * @param key key with which the specified value is to be associated
+     * @param value value to be associated with the specified key
+     * @return the previous value associated with the specified key, or
+     *         <tt>null</tt> if there was no mapping for the key.
+     *         (A <tt>null</tt> return can also indicate that the map
+     *         previously associated <tt>null</tt> with the key,
+     *         if the implementation supports null values.)
+     * @throws UnsupportedOperationException if the <tt>put</tt> operation
+     *         is not supported by this map
+     * @throws ClassCastException if the class of the specified key or value
+     *         prevents it from being stored in this map
+     * @throws NullPointerException if the specified key or value is null,
+     *         and this map does not permit null keys or values
+     * @throws IllegalArgumentException if some property of the specified key
+     *         or value prevents it from being stored in this map
+     *
+     */
+    V putIfAbsent(K key, V value);
+
+    /**
+     * Removes the entry for a key only if currently mapped to a given value.
+     * This is equivalent to
+     * <pre>
+     *   if (map.containsKey(key) &amp;&amp; map.get(key).equals(value)) {
+     *       map.remove(key);
+     *       return true;
+     *   } else return false;</pre>
+     * except that the action is performed atomically.
+     *
+     * @param key key with which the specified value is associated
+     * @param value value expected to be associated with the specified key
+     * @return <tt>true</tt> if the value was removed
+     * @throws UnsupportedOperationException if the <tt>remove</tt> operation
+     *         is not supported by this map
+     * @throws ClassCastException if the key or value is of an inappropriate
+     *         type for this map (optional)
+     * @throws NullPointerException if the specified key or value is null,
+     *         and this map does not permit null keys or values (optional)
+     */
+    boolean remove(Object key, Object value);
+
+    /**
+     * Replaces the entry for a key only if currently mapped to a given value.
+     * This is equivalent to
+     * <pre>
+     *   if (map.containsKey(key) &amp;&amp; map.get(key).equals(oldValue)) {
+     *       map.put(key, newValue);
+     *       return true;
+     *   } else return false;</pre>
+     * except that the action is performed atomically.
+     *
+     * @param key key with which the specified value is associated
+     * @param oldValue value expected to be associated with the specified key
+     * @param newValue value to be associated with the specified key
+     * @return <tt>true</tt> if the value was replaced
+     * @throws UnsupportedOperationException if the <tt>put</tt> operation
+     *         is not supported by this map
+     * @throws ClassCastException if the class of a specified key or value
+     *         prevents it from being stored in this map
+     * @throws NullPointerException if a specified key or value is null,
+     *         and this map does not permit null keys or values
+     * @throws IllegalArgumentException if some property of a specified key
+     *         or value prevents it from being stored in this map
+     */
+    boolean replace(K key, V oldValue, V newValue);
+
+    /**
+     * Replaces the entry for a key only if currently mapped to some value.
+     * This is equivalent to
+     * <pre>
+     *   if (map.containsKey(key)) {
+     *       return map.put(key, value);
+     *   } else return null;</pre>
+     * except that the action is performed atomically.
+     *
+     * @param key key with which the specified value is associated
+     * @param value value to be associated with the specified key
+     * @return the previous value associated with the specified key, or
+     *         <tt>null</tt> if there was no mapping for the key.
+     *         (A <tt>null</tt> return can also indicate that the map
+     *         previously associated <tt>null</tt> with the key,
+     *         if the implementation supports null values.)
+     * @throws UnsupportedOperationException if the <tt>put</tt> operation
+     *         is not supported by this map
+     * @throws ClassCastException if the class of the specified key or value
+     *         prevents it from being stored in this map
+     * @throws NullPointerException if the specified key or value is null,
+     *         and this map does not permit null keys or values
+     * @throws IllegalArgumentException if some property of the specified key
+     *         or value prevents it from being stored in this map
+     */
+    V replace(K key, V value);
+}
diff --git a/external/jsr166/java/util/concurrent/ConcurrentNavigableMap.java b/external/jsr166/java/util/concurrent/ConcurrentNavigableMap.java
new file mode 100644
index 000000000..7d86afb70
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/ConcurrentNavigableMap.java
@@ -0,0 +1,148 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+import java.util.*;
+
+/**
+ * A {@link ConcurrentMap} supporting {@link NavigableMap} operations,
+ * and recursively so for its navigable sub-maps.
+ *
+ * <p>This interface is a member of the
+ * <a href="{@docRoot}/../technotes/guides/collections/index.html">
+ * Java Collections Framework</a>.
+ *
+ * @author Doug Lea
+ * @param <K> the type of keys maintained by this map
+ * @param <V> the type of mapped values
+ * @since 1.6
+ */
+public interface ConcurrentNavigableMap<K,V>
+    extends ConcurrentMap<K,V>, NavigableMap<K,V>
+{
+    /**
+     * @throws ClassCastException       {@inheritDoc}
+     * @throws NullPointerException     {@inheritDoc}
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    ConcurrentNavigableMap<K,V> subMap(K fromKey, boolean fromInclusive,
+                                       K toKey,   boolean toInclusive);
+
+    /**
+     * @throws ClassCastException       {@inheritDoc}
+     * @throws NullPointerException     {@inheritDoc}
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    ConcurrentNavigableMap<K,V> headMap(K toKey, boolean inclusive);
+
+
+    /**
+     * @throws ClassCastException       {@inheritDoc}
+     * @throws NullPointerException     {@inheritDoc}
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    ConcurrentNavigableMap<K,V> tailMap(K fromKey, boolean inclusive);
+
+    /**
+     * @throws ClassCastException       {@inheritDoc}
+     * @throws NullPointerException     {@inheritDoc}
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    ConcurrentNavigableMap<K,V> subMap(K fromKey, K toKey);
+
+    /**
+     * @throws ClassCastException       {@inheritDoc}
+     * @throws NullPointerException     {@inheritDoc}
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    ConcurrentNavigableMap<K,V> headMap(K toKey);
+
+    /**
+     * @throws ClassCastException       {@inheritDoc}
+     * @throws NullPointerException     {@inheritDoc}
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    ConcurrentNavigableMap<K,V> tailMap(K fromKey);
+
+    /**
+     * Returns a reverse order view of the mappings contained in this map.
+     * The descending map is backed by this map, so changes to the map are
+     * reflected in the descending map, and vice-versa.
+     *
+     * <p>The returned map has an ordering equivalent to
+     * <tt>{@link Collections#reverseOrder(Comparator) Collections.reverseOrder}(comparator())</tt>.
+     * The expression {@code m.descendingMap().descendingMap()} returns a
+     * view of {@code m} essentially equivalent to {@code m}.
+     *
+     * @return a reverse order view of this map
+     */
+    ConcurrentNavigableMap<K,V> descendingMap();
+
+    /**
+     * Returns a {@link NavigableSet} view of the keys contained in this map.
+     * The set's iterator returns the keys in ascending order.
+     * The set is backed by the map, so changes to the map are
+     * reflected in the set, and vice-versa.  The set supports element
+     * removal, which removes the corresponding mapping from the map,
+     * via the {@code Iterator.remove}, {@code Set.remove},
+     * {@code removeAll}, {@code retainAll}, and {@code clear}
+     * operations.  It does not support the {@code add} or {@code addAll}
+     * operations.
+     *
+     * <p>The view's {@code iterator} is a "weakly consistent" iterator
+     * that will never throw {@link ConcurrentModificationException},
+     * and guarantees to traverse elements as they existed upon
+     * construction of the iterator, and may (but is not guaranteed to)
+     * reflect any modifications subsequent to construction.
+     *
+     * @return a navigable set view of the keys in this map
+     */
+    public NavigableSet<K> navigableKeySet();
+
+    /**
+     * Returns a {@link NavigableSet} view of the keys contained in this map.
+     * The set's iterator returns the keys in ascending order.
+     * The set is backed by the map, so changes to the map are
+     * reflected in the set, and vice-versa.  The set supports element
+     * removal, which removes the corresponding mapping from the map,
+     * via the {@code Iterator.remove}, {@code Set.remove},
+     * {@code removeAll}, {@code retainAll}, and {@code clear}
+     * operations.  It does not support the {@code add} or {@code addAll}
+     * operations.
+     *
+     * <p>The view's {@code iterator} is a "weakly consistent" iterator
+     * that will never throw {@link ConcurrentModificationException},
+     * and guarantees to traverse elements as they existed upon
+     * construction of the iterator, and may (but is not guaranteed to)
+     * reflect any modifications subsequent to construction.
+     *
+     * <p>This method is equivalent to method {@code navigableKeySet}.
+     *
+     * @return a navigable set view of the keys in this map
+     */
+    NavigableSet<K> keySet();
+
+    /**
+     * Returns a reverse order {@link NavigableSet} view of the keys contained in this map.
+     * The set's iterator returns the keys in descending order.
+     * The set is backed by the map, so changes to the map are
+     * reflected in the set, and vice-versa.  The set supports element
+     * removal, which removes the corresponding mapping from the map,
+     * via the {@code Iterator.remove}, {@code Set.remove},
+     * {@code removeAll}, {@code retainAll}, and {@code clear}
+     * operations.  It does not support the {@code add} or {@code addAll}
+     * operations.
+     *
+     * <p>The view's {@code iterator} is a "weakly consistent" iterator
+     * that will never throw {@link ConcurrentModificationException},
+     * and guarantees to traverse elements as they existed upon
+     * construction of the iterator, and may (but is not guaranteed to)
+     * reflect any modifications subsequent to construction.
+     *
+     * @return a reverse order navigable set view of the keys in this map
+     */
+    public NavigableSet<K> descendingKeySet();
+}
diff --git a/external/jsr166/java/util/concurrent/ConcurrentSkipListMap.java b/external/jsr166/java/util/concurrent/ConcurrentSkipListMap.java
new file mode 100644
index 000000000..52cd17a52
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/ConcurrentSkipListMap.java
@@ -0,0 +1,3114 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+import java.util.*;
+import java.util.concurrent.atomic.*;
+
+/**
+ * A scalable concurrent {@link ConcurrentNavigableMap} implementation.
+ * The map is sorted according to the {@linkplain Comparable natural
+ * ordering} of its keys, or by a {@link Comparator} provided at map
+ * creation time, depending on which constructor is used.
+ *
+ * <p>This class implements a concurrent variant of <a
+ * href="http://www.cs.umd.edu/~pugh/">SkipLists</a> providing
+ * expected average <i>log(n)</i> time cost for the
+ * <tt>containsKey</tt>, <tt>get</tt>, <tt>put</tt> and
+ * <tt>remove</tt> operations and their variants.  Insertion, removal,
+ * update, and access operations safely execute concurrently by
+ * multiple threads.  Iterators are <i>weakly consistent</i>, returning
+ * elements reflecting the state of the map at some point at or since
+ * the creation of the iterator.  They do <em>not</em> throw {@link
+ * ConcurrentModificationException}, and may proceed concurrently with
+ * other operations. Ascending key ordered views and their iterators
+ * are faster than descending ones.
+ *
+ * <p>All <tt>Map.Entry</tt> pairs returned by methods in this class
+ * and its views represent snapshots of mappings at the time they were
+ * produced. They do <em>not</em> support the <tt>Entry.setValue</tt>
+ * method. (Note however that it is possible to change mappings in the
+ * associated map using <tt>put</tt>, <tt>putIfAbsent</tt>, or
+ * <tt>replace</tt>, depending on exactly which effect you need.)
+ *
+ * <p>Beware that, unlike in most collections, the <tt>size</tt>
+ * method is <em>not</em> a constant-time operation. Because of the
+ * asynchronous nature of these maps, determining the current number
+ * of elements requires a traversal of the elements.  Additionally,
+ * the bulk operations <tt>putAll</tt>, <tt>equals</tt>, and
+ * <tt>clear</tt> are <em>not</em> guaranteed to be performed
+ * atomically. For example, an iterator operating concurrently with a
+ * <tt>putAll</tt> operation might view only some of the added
+ * elements.
+ *
+ * <p>This class and its views and iterators implement all of the
+ * <em>optional</em> methods of the {@link Map} and {@link Iterator}
+ * interfaces. Like most other concurrent collections, this class does
+ * <em>not</em> permit the use of <tt>null</tt> keys or values because some
+ * null return values cannot be reliably distinguished from the absence of
+ * elements.
+ *
+ * <p>This class is a member of the
+ * <a href="{@docRoot}/../technotes/guides/collections/index.html">
+ * Java Collections Framework</a>.
+ *
+ * @author Doug Lea
+ * @param <K> the type of keys maintained by this map
+ * @param <V> the type of mapped values
+ * @since 1.6
+ */
+public class ConcurrentSkipListMap<K,V> extends AbstractMap<K,V>
+    implements ConcurrentNavigableMap<K,V>,
+               Cloneable,
+               java.io.Serializable {
+    /*
+     * This class implements a tree-like two-dimensionally linked skip
+     * list in which the index levels are represented in separate
+     * nodes from the base nodes holding data.  There are two reasons
+     * for taking this approach instead of the usual array-based
+     * structure: 1) Array based implementations seem to encounter
+     * more complexity and overhead 2) We can use cheaper algorithms
+     * for the heavily-traversed index lists than can be used for the
+     * base lists.  Here's a picture of some of the basics for a
+     * possible list with 2 levels of index:
+     *
+     * Head nodes          Index nodes
+     * +-+    right        +-+                      +-+
+     * |2|---------------->| |--------------------->| |->null
+     * +-+                 +-+                      +-+
+     *  | down              |                        |
+     *  v                   v                        v
+     * +-+            +-+  +-+       +-+            +-+       +-+
+     * |1|----------->| |->| |------>| |----------->| |------>| |->null
+     * +-+            +-+  +-+       +-+            +-+       +-+
+     *  v              |    |         |              |         |
+     * Nodes  next     v    v         v              v         v
+     * +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+
+     * | |->|A|->|B|->|C|->|D|->|E|->|F|->|G|->|H|->|I|->|J|->|K|->null
+     * +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+
+     *
+     * The base lists use a variant of the HM linked ordered set
+     * algorithm. See Tim Harris, "A pragmatic implementation of
+     * non-blocking linked lists"
+     * http://www.cl.cam.ac.uk/~tlh20/publications.html and Maged
+     * Michael "High Performance Dynamic Lock-Free Hash Tables and
+     * List-Based Sets"
+     * http://www.research.ibm.com/people/m/michael/pubs.htm.  The
+     * basic idea in these lists is to mark the "next" pointers of
+     * deleted nodes when deleting to avoid conflicts with concurrent
+     * insertions, and when traversing to keep track of triples
+     * (predecessor, node, successor) in order to detect when and how
+     * to unlink these deleted nodes.
+     *
+     * Rather than using mark-bits to mark list deletions (which can
+     * be slow and space-intensive using AtomicMarkedReference), nodes
+     * use direct CAS'able next pointers.  On deletion, instead of
+     * marking a pointer, they splice in another node that can be
+     * thought of as standing for a marked pointer (indicating this by
+     * using otherwise impossible field values).  Using plain nodes
+     * acts roughly like "boxed" implementations of marked pointers,
+     * but uses new nodes only when nodes are deleted, not for every
+     * link.  This requires less space and supports faster
+     * traversal. Even if marked references were better supported by
+     * JVMs, traversal using this technique might still be faster
+     * because any search need only read ahead one more node than
+     * otherwise required (to check for trailing marker) rather than
+     * unmasking mark bits or whatever on each read.
+     *
+     * This approach maintains the essential property needed in the HM
+     * algorithm of changing the next-pointer of a deleted node so
+     * that any other CAS of it will fail, but implements the idea by
+     * changing the pointer to point to a different node, not by
+     * marking it.  While it would be possible to further squeeze
+     * space by defining marker nodes not to have key/value fields, it
+     * isn't worth the extra type-testing overhead.  The deletion
+     * markers are rarely encountered during traversal and are
+     * normally quickly garbage collected. (Note that this technique
+     * would not work well in systems without garbage collection.)
+     *
+     * In addition to using deletion markers, the lists also use
+     * nullness of value fields to indicate deletion, in a style
+     * similar to typical lazy-deletion schemes.  If a node's value is
+     * null, then it is considered logically deleted and ignored even
+     * though it is still reachable. This maintains proper control of
+     * concurrent replace vs delete operations -- an attempted replace
+     * must fail if a delete beat it by nulling field, and a delete
+     * must return the last non-null value held in the field. (Note:
+     * Null, rather than some special marker, is used for value fields
+     * here because it just so happens to mesh with the Map API
+     * requirement that method get returns null if there is no
+     * mapping, which allows nodes to remain concurrently readable
+     * even when deleted. Using any other marker value here would be
+     * messy at best.)
+     *
+     * Here's the sequence of events for a deletion of node n with
+     * predecessor b and successor f, initially:
+     *
+     *        +------+       +------+      +------+
+     *   ...  |   b  |------>|   n  |----->|   f  | ...
+     *        +------+       +------+      +------+
+     *
+     * 1. CAS n's value field from non-null to null.
+     *    From this point on, no public operations encountering
+     *    the node consider this mapping to exist. However, other
+     *    ongoing insertions and deletions might still modify
+     *    n's next pointer.
+     *
+     * 2. CAS n's next pointer to point to a new marker node.
+     *    From this point on, no other nodes can be appended to n.
+     *    which avoids deletion errors in CAS-based linked lists.
+     *
+     *        +------+       +------+      +------+       +------+
+     *   ...  |   b  |------>|   n  |----->|marker|------>|   f  | ...
+     *        +------+       +------+      +------+       +------+
+     *
+     * 3. CAS b's next pointer over both n and its marker.
+     *    From this point on, no new traversals will encounter n,
+     *    and it can eventually be GCed.
+     *        +------+                                    +------+
+     *   ...  |   b  |----------------------------------->|   f  | ...
+     *        +------+                                    +------+
+     *
+     * A failure at step 1 leads to simple retry due to a lost race
+     * with another operation. Steps 2-3 can fail because some other
+     * thread noticed during a traversal a node with null value and
+     * helped out by marking and/or unlinking.  This helping-out
+     * ensures that no thread can become stuck waiting for progress of
+     * the deleting thread.  The use of marker nodes slightly
+     * complicates helping-out code because traversals must track
+     * consistent reads of up to four nodes (b, n, marker, f), not
+     * just (b, n, f), although the next field of a marker is
+     * immutable, and once a next field is CAS'ed to point to a
+     * marker, it never again changes, so this requires less care.
+     *
+     * Skip lists add indexing to this scheme, so that the base-level
+     * traversals start close to the locations being found, inserted
+     * or deleted -- usually base level traversals only traverse a few
+     * nodes. This doesn't change the basic algorithm except for the
+     * need to make sure base traversals start at predecessors (here,
+     * b) that are not (structurally) deleted, otherwise retrying
+     * after processing the deletion.
+     *
+     * Index levels are maintained as lists with volatile next fields,
+     * using CAS to link and unlink.  Races are allowed in index-list
+     * operations that can (rarely) fail to link in a new index node
+     * or delete one. (We can't do this of course for data nodes.)
+     * However, even when this happens, the index lists remain sorted,
+     * so correctly serve as indices.  This can impact performance,
+     * but since skip lists are probabilistic anyway, the net result
+     * is that under contention, the effective "p" value may be lower
+     * than its nominal value. And race windows are kept small enough
+     * that in practice these failures are rare, even under a lot of
+     * contention.
+     *
+     * The fact that retries (for both base and index lists) are
+     * relatively cheap due to indexing allows some minor
+     * simplifications of retry logic. Traversal restarts are
+     * performed after most "helping-out" CASes. This isn't always
+     * strictly necessary, but the implicit backoffs tend to help
+     * reduce other downstream failed CAS's enough to outweigh restart
+     * cost.  This worsens the worst case, but seems to improve even
+     * highly contended cases.
+     *
+     * Unlike most skip-list implementations, index insertion and
+     * deletion here require a separate traversal pass occuring after
+     * the base-level action, to add or remove index nodes.  This adds
+     * to single-threaded overhead, but improves contended
+     * multithreaded performance by narrowing interference windows,
+     * and allows deletion to ensure that all index nodes will be made
+     * unreachable upon return from a public remove operation, thus
+     * avoiding unwanted garbage retention. This is more important
+     * here than in some other data structures because we cannot null
+     * out node fields referencing user keys since they might still be
+     * read by other ongoing traversals.
+     *
+     * Indexing uses skip list parameters that maintain good search
+     * performance while using sparser-than-usual indices: The
+     * hardwired parameters k=1, p=0.5 (see method randomLevel) mean
+     * that about one-quarter of the nodes have indices. Of those that
+     * do, half have one level, a quarter have two, and so on (see
+     * Pugh's Skip List Cookbook, sec 3.4).  The expected total space
+     * requirement for a map is slightly less than for the current
+     * implementation of java.util.TreeMap.
+     *
+     * Changing the level of the index (i.e, the height of the
+     * tree-like structure) also uses CAS. The head index has initial
+     * level/height of one. Creation of an index with height greater
+     * than the current level adds a level to the head index by
+     * CAS'ing on a new top-most head. To maintain good performance
+     * after a lot of removals, deletion methods heuristically try to
+     * reduce the height if the topmost levels appear to be empty.
+     * This may encounter races in which it possible (but rare) to
+     * reduce and "lose" a level just as it is about to contain an
+     * index (that will then never be encountered). This does no
+     * structural harm, and in practice appears to be a better option
+     * than allowing unrestrained growth of levels.
+     *
+     * The code for all this is more verbose than you'd like. Most
+     * operations entail locating an element (or position to insert an
+     * element). The code to do this can't be nicely factored out
+     * because subsequent uses require a snapshot of predecessor
+     * and/or successor and/or value fields which can't be returned
+     * all at once, at least not without creating yet another object
+     * to hold them -- creating such little objects is an especially
+     * bad idea for basic internal search operations because it adds
+     * to GC overhead.  (This is one of the few times I've wished Java
+     * had macros.) Instead, some traversal code is interleaved within
+     * insertion and removal operations.  The control logic to handle
+     * all the retry conditions is sometimes twisty. Most search is
+     * broken into 2 parts. findPredecessor() searches index nodes
+     * only, returning a base-level predecessor of the key. findNode()
+     * finishes out the base-level search. Even with this factoring,
+     * there is a fair amount of near-duplication of code to handle
+     * variants.
+     *
+     * For explanation of algorithms sharing at least a couple of
+     * features with this one, see Mikhail Fomitchev's thesis
+     * (http://www.cs.yorku.ca/~mikhail/), Keir Fraser's thesis
+     * (http://www.cl.cam.ac.uk/users/kaf24/), and Hakan Sundell's
+     * thesis (http://www.cs.chalmers.se/~phs/).
+     *
+     * Given the use of tree-like index nodes, you might wonder why
+     * this doesn't use some kind of search tree instead, which would
+     * support somewhat faster search operations. The reason is that
+     * there are no known efficient lock-free insertion and deletion
+     * algorithms for search trees. The immutability of the "down"
+     * links of index nodes (as opposed to mutable "left" fields in
+     * true trees) makes this tractable using only CAS operations.
+     *
+     * Notation guide for local variables
+     * Node:         b, n, f    for  predecessor, node, successor
+     * Index:        q, r, d    for index node, right, down.
+     *               t          for another index node
+     * Head:         h
+     * Levels:       j
+     * Keys:         k, key
+     * Values:       v, value
+     * Comparisons:  c
+     */
+
+    private static final long serialVersionUID = -8627078645895051609L;
+
+    /**
+     * Generates the initial random seed for the cheaper per-instance
+     * random number generators used in randomLevel.
+     */
+    private static final Random seedGenerator = new Random();
+
+    /**
+     * Special value used to identify base-level header
+     */
+    private static final Object BASE_HEADER = new Object();
+
+    /**
+     * The topmost head index of the skiplist.
+     */
+    private transient volatile HeadIndex<K,V> head;
+
+    /**
+     * The comparator used to maintain order in this map, or null
+     * if using natural ordering.
+     * @serial
+     */
+    private final Comparator<? super K> comparator;
+
+    /**
+     * Seed for simple random number generator.  Not volatile since it
+     * doesn't matter too much if different threads don't see updates.
+     */
+    private transient int randomSeed;
+
+    /** Lazily initialized key set */
+    private transient KeySet keySet;
+    /** Lazily initialized entry set */
+    private transient EntrySet entrySet;
+    /** Lazily initialized values collection */
+    private transient Values values;
+    /** Lazily initialized descending key set */
+    private transient ConcurrentNavigableMap<K,V> descendingMap;
+
+    /**
+     * Initializes or resets state. Needed by constructors, clone,
+     * clear, readObject. and ConcurrentSkipListSet.clone.
+     * (Note that comparator must be separately initialized.)
+     */
+    final void initialize() {
+        keySet = null;
+        entrySet = null;
+        values = null;
+        descendingMap = null;
+        randomSeed = seedGenerator.nextInt() | 0x0100; // ensure nonzero
+        head = new HeadIndex<K,V>(new Node<K,V>(null, BASE_HEADER, null),
+                                  null, null, 1);
+    }
+
+    /** Updater for casHead */
+    private static final
+        AtomicReferenceFieldUpdater<ConcurrentSkipListMap, HeadIndex>
+        headUpdater = AtomicReferenceFieldUpdater.newUpdater
+        (ConcurrentSkipListMap.class, HeadIndex.class, "head");
+
+    /**
+     * compareAndSet head node
+     */
+    private boolean casHead(HeadIndex<K,V> cmp, HeadIndex<K,V> val) {
+        return headUpdater.compareAndSet(this, cmp, val);
+    }
+
+    /* ---------------- Nodes -------------- */
+
+    /**
+     * Nodes hold keys and values, and are singly linked in sorted
+     * order, possibly with some intervening marker nodes. The list is
+     * headed by a dummy node accessible as head.node. The value field
+     * is declared only as Object because it takes special non-V
+     * values for marker and header nodes.
+     */
+    static final class Node<K,V> {
+        final K key;
+        volatile Object value;
+        volatile Node<K,V> next;
+
+        /**
+         * Creates a new regular node.
+         */
+        Node(K key, Object value, Node<K,V> next) {
+            this.key = key;
+            this.value = value;
+            this.next = next;
+        }
+
+        /**
+         * Creates a new marker node. A marker is distinguished by
+         * having its value field point to itself.  Marker nodes also
+         * have null keys, a fact that is exploited in a few places,
+         * but this doesn't distinguish markers from the base-level
+         * header node (head.node), which also has a null key.
+         */
+        Node(Node<K,V> next) {
+            this.key = null;
+            this.value = this;
+            this.next = next;
+        }
+
+        /** Updater for casNext */
+        static final AtomicReferenceFieldUpdater<Node, Node>
+            nextUpdater = AtomicReferenceFieldUpdater.newUpdater
+            (Node.class, Node.class, "next");
+
+        /** Updater for casValue */
+        static final AtomicReferenceFieldUpdater<Node, Object>
+            valueUpdater = AtomicReferenceFieldUpdater.newUpdater
+            (Node.class, Object.class, "value");
+
+        /**
+         * compareAndSet value field
+         */
+        boolean casValue(Object cmp, Object val) {
+            return valueUpdater.compareAndSet(this, cmp, val);
+        }
+
+        /**
+         * compareAndSet next field
+         */
+        boolean casNext(Node<K,V> cmp, Node<K,V> val) {
+            return nextUpdater.compareAndSet(this, cmp, val);
+        }
+
+        /**
+         * Returns true if this node is a marker. This method isn't
+         * actually called in any current code checking for markers
+         * because callers will have already read value field and need
+         * to use that read (not another done here) and so directly
+         * test if value points to node.
+         * @param n a possibly null reference to a node
+         * @return true if this node is a marker node
+         */
+        boolean isMarker() {
+            return value == this;
+        }
+
+        /**
+         * Returns true if this node is the header of base-level list.
+         * @return true if this node is header node
+         */
+        boolean isBaseHeader() {
+            return value == BASE_HEADER;
+        }
+
+        /**
+         * Tries to append a deletion marker to this node.
+         * @param f the assumed current successor of this node
+         * @return true if successful
+         */
+        boolean appendMarker(Node<K,V> f) {
+            return casNext(f, new Node<K,V>(f));
+        }
+
+        /**
+         * Helps out a deletion by appending marker or unlinking from
+         * predecessor. This is called during traversals when value
+         * field seen to be null.
+         * @param b predecessor
+         * @param f successor
+         */
+        void helpDelete(Node<K,V> b, Node<K,V> f) {
+            /*
+             * Rechecking links and then doing only one of the
+             * help-out stages per call tends to minimize CAS
+             * interference among helping threads.
+             */
+            if (f == next && this == b.next) {
+                if (f == null || f.value != f) // not already marked
+                    appendMarker(f);
+                else
+                    b.casNext(this, f.next);
+            }
+        }
+
+        /**
+         * Returns value if this node contains a valid key-value pair,
+         * else null.
+         * @return this node's value if it isn't a marker or header or
+         * is deleted, else null.
+         */
+        V getValidValue() {
+            Object v = value;
+            if (v == this || v == BASE_HEADER)
+                return null;
+            return (V)v;
+        }
+
+        /**
+         * Creates and returns a new SimpleImmutableEntry holding current
+         * mapping if this node holds a valid value, else null.
+         * @return new entry or null
+         */
+        AbstractMap.SimpleImmutableEntry<K,V> createSnapshot() {
+            V v = getValidValue();
+            if (v == null)
+                return null;
+            return new AbstractMap.SimpleImmutableEntry<K,V>(key, v);
+        }
+    }
+
+    /* ---------------- Indexing -------------- */
+
+    /**
+     * Index nodes represent the levels of the skip list.  Note that
+     * even though both Nodes and Indexes have forward-pointing
+     * fields, they have different types and are handled in different
+     * ways, that can't nicely be captured by placing field in a
+     * shared abstract class.
+     */
+    static class Index<K,V> {
+        final Node<K,V> node;
+        final Index<K,V> down;
+        volatile Index<K,V> right;
+
+        /**
+         * Creates index node with given values.
+         */
+        Index(Node<K,V> node, Index<K,V> down, Index<K,V> right) {
+            this.node = node;
+            this.down = down;
+            this.right = right;
+        }
+
+        /** Updater for casRight */
+        static final AtomicReferenceFieldUpdater<Index, Index>
+            rightUpdater = AtomicReferenceFieldUpdater.newUpdater
+            (Index.class, Index.class, "right");
+
+        /**
+         * compareAndSet right field
+         */
+        final boolean casRight(Index<K,V> cmp, Index<K,V> val) {
+            return rightUpdater.compareAndSet(this, cmp, val);
+        }
+
+        /**
+         * Returns true if the node this indexes has been deleted.
+         * @return true if indexed node is known to be deleted
+         */
+        final boolean indexesDeletedNode() {
+            return node.value == null;
+        }
+
+        /**
+         * Tries to CAS newSucc as successor.  To minimize races with
+         * unlink that may lose this index node, if the node being
+         * indexed is known to be deleted, it doesn't try to link in.
+         * @param succ the expected current successor
+         * @param newSucc the new successor
+         * @return true if successful
+         */
+        final boolean link(Index<K,V> succ, Index<K,V> newSucc) {
+            Node<K,V> n = node;
+            newSucc.right = succ;
+            return n.value != null && casRight(succ, newSucc);
+        }
+
+        /**
+         * Tries to CAS right field to skip over apparent successor
+         * succ.  Fails (forcing a retraversal by caller) if this node
+         * is known to be deleted.
+         * @param succ the expected current successor
+         * @return true if successful
+         */
+        final boolean unlink(Index<K,V> succ) {
+            return !indexesDeletedNode() && casRight(succ, succ.right);
+        }
+    }
+
+    /* ---------------- Head nodes -------------- */
+
+    /**
+     * Nodes heading each level keep track of their level.
+     */
+    static final class HeadIndex<K,V> extends Index<K,V> {
+        final int level;
+        HeadIndex(Node<K,V> node, Index<K,V> down, Index<K,V> right, int level) {
+            super(node, down, right);
+            this.level = level;
+        }
+    }
+
+    /* ---------------- Comparison utilities -------------- */
+
+    /**
+     * Represents a key with a comparator as a Comparable.
+     *
+     * Because most sorted collections seem to use natural ordering on
+     * Comparables (Strings, Integers, etc), most internal methods are
+     * geared to use them. This is generally faster than checking
+     * per-comparison whether to use comparator or comparable because
+     * it doesn't require a (Comparable) cast for each comparison.
+     * (Optimizers can only sometimes remove such redundant checks
+     * themselves.) When Comparators are used,
+     * ComparableUsingComparators are created so that they act in the
+     * same way as natural orderings. This penalizes use of
+     * Comparators vs Comparables, which seems like the right
+     * tradeoff.
+     */
+    static final class ComparableUsingComparator<K> implements Comparable<K> {
+        final K actualKey;
+        final Comparator<? super K> cmp;
+        ComparableUsingComparator(K key, Comparator<? super K> cmp) {
+            this.actualKey = key;
+            this.cmp = cmp;
+        }
+        public int compareTo(K k2) {
+            return cmp.compare(actualKey, k2);
+        }
+    }
+
+    /**
+     * If using comparator, return a ComparableUsingComparator, else
+     * cast key as Comparable, which may cause ClassCastException,
+     * which is propagated back to caller.
+     */
+    private Comparable<? super K> comparable(Object key) throws ClassCastException {
+        if (key == null)
+            throw new NullPointerException();
+        if (comparator != null)
+            return new ComparableUsingComparator<K>((K)key, comparator);
+        else
+            return (Comparable<? super K>)key;
+    }
+
+    /**
+     * Compares using comparator or natural ordering. Used when the
+     * ComparableUsingComparator approach doesn't apply.
+     */
+    int compare(K k1, K k2) throws ClassCastException {
+        Comparator<? super K> cmp = comparator;
+        if (cmp != null)
+            return cmp.compare(k1, k2);
+        else
+            return ((Comparable<? super K>)k1).compareTo(k2);
+    }
+
+    /**
+     * Returns true if given key greater than or equal to least and
+     * strictly less than fence, bypassing either test if least or
+     * fence are null. Needed mainly in submap operations.
+     */
+    boolean inHalfOpenRange(K key, K least, K fence) {
+        if (key == null)
+            throw new NullPointerException();
+        return ((least == null || compare(key, least) >= 0) &&
+                (fence == null || compare(key, fence) <  0));
+    }
+
+    /**
+     * Returns true if given key greater than or equal to least and less
+     * or equal to fence. Needed mainly in submap operations.
+     */
+    boolean inOpenRange(K key, K least, K fence) {
+        if (key == null)
+            throw new NullPointerException();
+        return ((least == null || compare(key, least) >= 0) &&
+                (fence == null || compare(key, fence) <= 0));
+    }
+
+    /* ---------------- Traversal -------------- */
+
+    /**
+     * Returns a base-level node with key strictly less than given key,
+     * or the base-level header if there is no such node.  Also
+     * unlinks indexes to deleted nodes found along the way.  Callers
+     * rely on this side-effect of clearing indices to deleted nodes.
+     * @param key the key
+     * @return a predecessor of key
+     */
+    private Node<K,V> findPredecessor(Comparable<? super K> key) {
+        if (key == null)
+            throw new NullPointerException(); // don't postpone errors
+        for (;;) {
+            Index<K,V> q = head;
+            Index<K,V> r = q.right;
+            for (;;) {
+                if (r != null) {
+                    Node<K,V> n = r.node;
+                    K k = n.key;
+                    if (n.value == null) {
+                        if (!q.unlink(r))
+                            break;           // restart
+                        r = q.right;         // reread r
+                        continue;
+                    }
+                    if (key.compareTo(k) > 0) {
+                        q = r;
+                        r = r.right;
+                        continue;
+                    }
+                }
+                Index<K,V> d = q.down;
+                if (d != null) {
+                    q = d;
+                    r = d.right;
+                } else
+                    return q.node;
+            }
+        }
+    }
+
+    /**
+     * Returns node holding key or null if no such, clearing out any
+     * deleted nodes seen along the way.  Repeatedly traverses at
+     * base-level looking for key starting at predecessor returned
+     * from findPredecessor, processing base-level deletions as
+     * encountered. Some callers rely on this side-effect of clearing
+     * deleted nodes.
+     *
+     * Restarts occur, at traversal step centered on node n, if:
+     *
+     *   (1) After reading n's next field, n is no longer assumed
+     *       predecessor b's current successor, which means that
+     *       we don't have a consistent 3-node snapshot and so cannot
+     *       unlink any subsequent deleted nodes encountered.
+     *
+     *   (2) n's value field is null, indicating n is deleted, in
+     *       which case we help out an ongoing structural deletion
+     *       before retrying.  Even though there are cases where such
+     *       unlinking doesn't require restart, they aren't sorted out
+     *       here because doing so would not usually outweigh cost of
+     *       restarting.
+     *
+     *   (3) n is a marker or n's predecessor's value field is null,
+     *       indicating (among other possibilities) that
+     *       findPredecessor returned a deleted node. We can't unlink
+     *       the node because we don't know its predecessor, so rely
+     *       on another call to findPredecessor to notice and return
+     *       some earlier predecessor, which it will do. This check is
+     *       only strictly needed at beginning of loop, (and the
+     *       b.value check isn't strictly needed at all) but is done
+     *       each iteration to help avoid contention with other
+     *       threads by callers that will fail to be able to change
+     *       links, and so will retry anyway.
+     *
+     * The traversal loops in doPut, doRemove, and findNear all
+     * include the same three kinds of checks. And specialized
+     * versions appear in findFirst, and findLast and their
+     * variants. They can't easily share code because each uses the
+     * reads of fields held in locals occurring in the orders they
+     * were performed.
+     *
+     * @param key the key
+     * @return node holding key, or null if no such
+     */
+    private Node<K,V> findNode(Comparable<? super K> key) {
+        for (;;) {
+            Node<K,V> b = findPredecessor(key);
+            Node<K,V> n = b.next;
+            for (;;) {
+                if (n == null)
+                    return null;
+                Node<K,V> f = n.next;
+                if (n != b.next)                // inconsistent read
+                    break;
+                Object v = n.value;
+                if (v == null) {                // n is deleted
+                    n.helpDelete(b, f);
+                    break;
+                }
+                if (v == n || b.value == null)  // b is deleted
+                    break;
+                int c = key.compareTo(n.key);
+                if (c == 0)
+                    return n;
+                if (c < 0)
+                    return null;
+                b = n;
+                n = f;
+            }
+        }
+    }
+
+    /**
+     * Specialized variant of findNode to perform Map.get. Does a weak
+     * traversal, not bothering to fix any deleted index nodes,
+     * returning early if it happens to see key in index, and passing
+     * over any deleted base nodes, falling back to getUsingFindNode
+     * only if it would otherwise return value from an ongoing
+     * deletion. Also uses "bound" to eliminate need for some
+     * comparisons (see Pugh Cookbook). Also folds uses of null checks
+     * and node-skipping because markers have null keys.
+     * @param okey the key
+     * @return the value, or null if absent
+     */
+    private V doGet(Object okey) {
+        Comparable<? super K> key = comparable(okey);
+        Node<K,V> bound = null;
+        Index<K,V> q = head;
+        Index<K,V> r = q.right;
+        Node<K,V> n;
+        K k;
+        int c;
+        for (;;) {
+            Index<K,V> d;
+            // Traverse rights
+            if (r != null && (n = r.node) != bound && (k = n.key) != null) {
+                if ((c = key.compareTo(k)) > 0) {
+                    q = r;
+                    r = r.right;
+                    continue;
+                } else if (c == 0) {
+                    Object v = n.value;
+                    return (v != null)? (V)v : getUsingFindNode(key);
+                } else
+                    bound = n;
+            }
+
+            // Traverse down
+            if ((d = q.down) != null) {
+                q = d;
+                r = d.right;
+            } else
+                break;
+        }
+
+        // Traverse nexts
+        for (n = q.node.next;  n != null; n = n.next) {
+            if ((k = n.key) != null) {
+                if ((c = key.compareTo(k)) == 0) {
+                    Object v = n.value;
+                    return (v != null)? (V)v : getUsingFindNode(key);
+                } else if (c < 0)
+                    break;
+            }
+        }
+        return null;
+    }
+
+    /**
+     * Performs map.get via findNode.  Used as a backup if doGet
+     * encounters an in-progress deletion.
+     * @param key the key
+     * @return the value, or null if absent
+     */
+    private V getUsingFindNode(Comparable<? super K> key) {
+        /*
+         * Loop needed here and elsewhere in case value field goes
+         * null just as it is about to be returned, in which case we
+         * lost a race with a deletion, so must retry.
+         */
+        for (;;) {
+            Node<K,V> n = findNode(key);
+            if (n == null)
+                return null;
+            Object v = n.value;
+            if (v != null)
+                return (V)v;
+        }
+    }
+
+    /* ---------------- Insertion -------------- */
+
+    /**
+     * Main insertion method.  Adds element if not present, or
+     * replaces value if present and onlyIfAbsent is false.
+     * @param kkey the key
+     * @param value  the value that must be associated with key
+     * @param onlyIfAbsent if should not insert if already present
+     * @return the old value, or null if newly inserted
+     */
+    private V doPut(K kkey, V value, boolean onlyIfAbsent) {
+        Comparable<? super K> key = comparable(kkey);
+        for (;;) {
+            Node<K,V> b = findPredecessor(key);
+            Node<K,V> n = b.next;
+            for (;;) {
+                if (n != null) {
+                    Node<K,V> f = n.next;
+                    if (n != b.next)               // inconsistent read
+                        break;;
+                    Object v = n.value;
+                    if (v == null) {               // n is deleted
+                        n.helpDelete(b, f);
+                        break;
+                    }
+                    if (v == n || b.value == null) // b is deleted
+                        break;
+                    int c = key.compareTo(n.key);
+                    if (c > 0) {
+                        b = n;
+                        n = f;
+                        continue;
+                    }
+                    if (c == 0) {
+                        if (onlyIfAbsent || n.casValue(v, value))
+                            return (V)v;
+                        else
+                            break; // restart if lost race to replace value
+                    }
+                    // else c < 0; fall through
+                }
+
+                Node<K,V> z = new Node<K,V>(kkey, value, n);
+                if (!b.casNext(n, z))
+                    break;         // restart if lost race to append to b
+                int level = randomLevel();
+                if (level > 0)
+                    insertIndex(z, level);
+                return null;
+            }
+        }
+    }
+
+    /**
+     * Returns a random level for inserting a new node.
+     * Hardwired to k=1, p=0.5, max 31 (see above and
+     * Pugh's "Skip List Cookbook", sec 3.4).
+     *
+     * This uses the simplest of the generators described in George
+     * Marsaglia's "Xorshift RNGs" paper.  This is not a high-quality
+     * generator but is acceptable here.
+     */
+    private int randomLevel() {
+        int x = randomSeed;
+        x ^= x << 13;
+        x ^= x >>> 17;
+        randomSeed = x ^= x << 5;
+        if ((x & 0x8001) != 0) // test highest and lowest bits
+            return 0;
+        int level = 1;
+        while (((x >>>= 1) & 1) != 0) ++level;
+        return level;
+    }
+
+    /**
+     * Creates and adds index nodes for the given node.
+     * @param z the node
+     * @param level the level of the index
+     */
+    private void insertIndex(Node<K,V> z, int level) {
+        HeadIndex<K,V> h = head;
+        int max = h.level;
+
+        if (level <= max) {
+            Index<K,V> idx = null;
+            for (int i = 1; i <= level; ++i)
+                idx = new Index<K,V>(z, idx, null);
+            addIndex(idx, h, level);
+
+        } else { // Add a new level
+            /*
+             * To reduce interference by other threads checking for
+             * empty levels in tryReduceLevel, new levels are added
+             * with initialized right pointers. Which in turn requires
+             * keeping levels in an array to access them while
+             * creating new head index nodes from the opposite
+             * direction.
+             */
+            level = max + 1;
+            Index<K,V>[] idxs = (Index<K,V>[])new Index[level+1];
+            Index<K,V> idx = null;
+            for (int i = 1; i <= level; ++i)
+                idxs[i] = idx = new Index<K,V>(z, idx, null);
+
+            HeadIndex<K,V> oldh;
+            int k;
+            for (;;) {
+                oldh = head;
+                int oldLevel = oldh.level;
+                if (level <= oldLevel) { // lost race to add level
+                    k = level;
+                    break;
+                }
+                HeadIndex<K,V> newh = oldh;
+                Node<K,V> oldbase = oldh.node;
+                for (int j = oldLevel+1; j <= level; ++j)
+                    newh = new HeadIndex<K,V>(oldbase, newh, idxs[j], j);
+                if (casHead(oldh, newh)) {
+                    k = oldLevel;
+                    break;
+                }
+            }
+            addIndex(idxs[k], oldh, k);
+        }
+    }
+
+    /**
+     * Adds given index nodes from given level down to 1.
+     * @param idx the topmost index node being inserted
+     * @param h the value of head to use to insert. This must be
+     * snapshotted by callers to provide correct insertion level
+     * @param indexLevel the level of the index
+     */
+    private void addIndex(Index<K,V> idx, HeadIndex<K,V> h, int indexLevel) {
+        // Track next level to insert in case of retries
+        int insertionLevel = indexLevel;
+        Comparable<? super K> key = comparable(idx.node.key);
+        if (key == null) throw new NullPointerException();
+
+        // Similar to findPredecessor, but adding index nodes along
+        // path to key.
+        for (;;) {
+            int j = h.level;
+            Index<K,V> q = h;
+            Index<K,V> r = q.right;
+            Index<K,V> t = idx;
+            for (;;) {
+                if (r != null) {
+                    Node<K,V> n = r.node;
+                    // compare before deletion check avoids needing recheck
+                    int c = key.compareTo(n.key);
+                    if (n.value == null) {
+                        if (!q.unlink(r))
+                            break;
+                        r = q.right;
+                        continue;
+                    }
+                    if (c > 0) {
+                        q = r;
+                        r = r.right;
+                        continue;
+                    }
+                }
+
+                if (j == insertionLevel) {
+                    // Don't insert index if node already deleted
+                    if (t.indexesDeletedNode()) {
+                        findNode(key); // cleans up
+                        return;
+                    }
+                    if (!q.link(r, t))
+                        break; // restart
+                    if (--insertionLevel == 0) {
+                        // need final deletion check before return
+                        if (t.indexesDeletedNode())
+                            findNode(key);
+                        return;
+                    }
+                }
+
+                if (--j >= insertionLevel && j < indexLevel)
+                    t = t.down;
+                q = q.down;
+                r = q.right;
+            }
+        }
+    }
+
+    /* ---------------- Deletion -------------- */
+
+    /**
+     * Main deletion method. Locates node, nulls value, appends a
+     * deletion marker, unlinks predecessor, removes associated index
+     * nodes, and possibly reduces head index level.
+     *
+     * Index nodes are cleared out simply by calling findPredecessor.
+     * which unlinks indexes to deleted nodes found along path to key,
+     * which will include the indexes to this node.  This is done
+     * unconditionally. We can't check beforehand whether there are
+     * index nodes because it might be the case that some or all
+     * indexes hadn't been inserted yet for this node during initial
+     * search for it, and we'd like to ensure lack of garbage
+     * retention, so must call to be sure.
+     *
+     * @param okey the key
+     * @param value if non-null, the value that must be
+     * associated with key
+     * @return the node, or null if not found
+     */
+    final V doRemove(Object okey, Object value) {
+        Comparable<? super K> key = comparable(okey);
+        for (;;) {
+            Node<K,V> b = findPredecessor(key);
+            Node<K,V> n = b.next;
+            for (;;) {
+                if (n == null)
+                    return null;
+                Node<K,V> f = n.next;
+                if (n != b.next)                    // inconsistent read
+                    break;
+                Object v = n.value;
+                if (v == null) {                    // n is deleted
+                    n.helpDelete(b, f);
+                    break;
+                }
+                if (v == n || b.value == null)      // b is deleted
+                    break;
+                int c = key.compareTo(n.key);
+                if (c < 0)
+                    return null;
+                if (c > 0) {
+                    b = n;
+                    n = f;
+                    continue;
+                }
+                if (value != null && !value.equals(v))
+                    return null;
+                if (!n.casValue(v, null))
+                    break;
+                if (!n.appendMarker(f) || !b.casNext(n, f))
+                    findNode(key);                  // Retry via findNode
+                else {
+                    findPredecessor(key);           // Clean index
+                    if (head.right == null)
+                        tryReduceLevel();
+                }
+                return (V)v;
+            }
+        }
+    }
+
+    /**
+     * Possibly reduce head level if it has no nodes.  This method can
+     * (rarely) make mistakes, in which case levels can disappear even
+     * though they are about to contain index nodes. This impacts
+     * performance, not correctness.  To minimize mistakes as well as
+     * to reduce hysteresis, the level is reduced by one only if the
+     * topmost three levels look empty. Also, if the removed level
+     * looks non-empty after CAS, we try to change it back quick
+     * before anyone notices our mistake! (This trick works pretty
+     * well because this method will practically never make mistakes
+     * unless current thread stalls immediately before first CAS, in
+     * which case it is very unlikely to stall again immediately
+     * afterwards, so will recover.)
+     *
+     * We put up with all this rather than just let levels grow
+     * because otherwise, even a small map that has undergone a large
+     * number of insertions and removals will have a lot of levels,
+     * slowing down access more than would an occasional unwanted
+     * reduction.
+     */
+    private void tryReduceLevel() {
+        HeadIndex<K,V> h = head;
+        HeadIndex<K,V> d;
+        HeadIndex<K,V> e;
+        if (h.level > 3 &&
+            (d = (HeadIndex<K,V>)h.down) != null &&
+            (e = (HeadIndex<K,V>)d.down) != null &&
+            e.right == null &&
+            d.right == null &&
+            h.right == null &&
+            casHead(h, d) && // try to set
+            h.right != null) // recheck
+            casHead(d, h);   // try to backout
+    }
+
+    /* ---------------- Finding and removing first element -------------- */
+
+    /**
+     * Specialized variant of findNode to get first valid node.
+     * @return first node or null if empty
+     */
+    Node<K,V> findFirst() {
+        for (;;) {
+            Node<K,V> b = head.node;
+            Node<K,V> n = b.next;
+            if (n == null)
+                return null;
+            if (n.value != null)
+                return n;
+            n.helpDelete(b, n.next);
+        }
+    }
+
+    /**
+     * Removes first entry; returns its snapshot.
+     * @return null if empty, else snapshot of first entry
+     */
+    Map.Entry<K,V> doRemoveFirstEntry() {
+        for (;;) {
+            Node<K,V> b = head.node;
+            Node<K,V> n = b.next;
+            if (n == null)
+                return null;
+            Node<K,V> f = n.next;
+            if (n != b.next)
+                continue;
+            Object v = n.value;
+            if (v == null) {
+                n.helpDelete(b, f);
+                continue;
+            }
+            if (!n.casValue(v, null))
+                continue;
+            if (!n.appendMarker(f) || !b.casNext(n, f))
+                findFirst(); // retry
+            clearIndexToFirst();
+            return new AbstractMap.SimpleImmutableEntry<K,V>(n.key, (V)v);
+	}
+    }
+
+    /**
+     * Clears out index nodes associated with deleted first entry.
+     */
+    private void clearIndexToFirst() {
+        for (;;) {
+            Index<K,V> q = head;
+            for (;;) {
+                Index<K,V> r = q.right;
+                if (r != null && r.indexesDeletedNode() && !q.unlink(r))
+                    break;
+                if ((q = q.down) == null) {
+                    if (head.right == null)
+                        tryReduceLevel();
+                    return;
+                }
+            }
+        }
+    }
+
+
+    /* ---------------- Finding and removing last element -------------- */
+
+    /**
+     * Specialized version of find to get last valid node.
+     * @return last node or null if empty
+     */
+    Node<K,V> findLast() {
+        /*
+         * findPredecessor can't be used to traverse index level
+         * because this doesn't use comparisons.  So traversals of
+         * both levels are folded together.
+         */
+        Index<K,V> q = head;
+        for (;;) {
+            Index<K,V> d, r;
+            if ((r = q.right) != null) {
+                if (r.indexesDeletedNode()) {
+                    q.unlink(r);
+                    q = head; // restart
+                }
+                else
+                    q = r;
+            } else if ((d = q.down) != null) {
+                q = d;
+            } else {
+                Node<K,V> b = q.node;
+                Node<K,V> n = b.next;
+                for (;;) {
+                    if (n == null)
+                        return (b.isBaseHeader())? null : b;
+                    Node<K,V> f = n.next;            // inconsistent read
+                    if (n != b.next)
+                        break;
+                    Object v = n.value;
+                    if (v == null) {                 // n is deleted
+                        n.helpDelete(b, f);
+                        break;
+                    }
+                    if (v == n || b.value == null)   // b is deleted
+                        break;
+                    b = n;
+                    n = f;
+                }
+                q = head; // restart
+            }
+        }
+    }
+
+    /**
+     * Specialized variant of findPredecessor to get predecessor of last
+     * valid node.  Needed when removing the last entry.  It is possible
+     * that all successors of returned node will have been deleted upon
+     * return, in which case this method can be retried.
+     * @return likely predecessor of last node
+     */
+    private Node<K,V> findPredecessorOfLast() {
+        for (;;) {
+            Index<K,V> q = head;
+            for (;;) {
+                Index<K,V> d, r;
+                if ((r = q.right) != null) {
+                    if (r.indexesDeletedNode()) {
+                        q.unlink(r);
+                        break;    // must restart
+                    }
+                    // proceed as far across as possible without overshooting
+                    if (r.node.next != null) {
+                        q = r;
+                        continue;
+                    }
+                }
+                if ((d = q.down) != null)
+                    q = d;
+                else
+                    return q.node;
+            }
+        }
+    }
+
+    /**
+     * Removes last entry; returns its snapshot.
+     * Specialized variant of doRemove.
+     * @return null if empty, else snapshot of last entry
+     */
+    Map.Entry<K,V> doRemoveLastEntry() {
+        for (;;) {
+            Node<K,V> b = findPredecessorOfLast();
+            Node<K,V> n = b.next;
+            if (n == null) {
+                if (b.isBaseHeader())               // empty
+                    return null;
+                else
+                    continue; // all b's successors are deleted; retry
+            }
+            for (;;) {
+                Node<K,V> f = n.next;
+                if (n != b.next)                    // inconsistent read
+                    break;
+                Object v = n.value;
+                if (v == null) {                    // n is deleted
+                    n.helpDelete(b, f);
+                    break;
+                }
+                if (v == n || b.value == null)      // b is deleted
+                    break;
+                if (f != null) {
+                    b = n;
+                    n = f;
+                    continue;
+                }
+                if (!n.casValue(v, null))
+                    break;
+                K key = n.key;
+                Comparable<? super K> ck = comparable(key);
+                if (!n.appendMarker(f) || !b.casNext(n, f))
+                    findNode(ck);                  // Retry via findNode
+                else {
+                    findPredecessor(ck);           // Clean index
+                    if (head.right == null)
+                        tryReduceLevel();
+                }
+                return new AbstractMap.SimpleImmutableEntry<K,V>(key, (V)v);
+            }
+        }
+    }
+
+    /* ---------------- Relational operations -------------- */
+
+    // Control values OR'ed as arguments to findNear
+
+    private static final int EQ = 1;
+    private static final int LT = 2;
+    private static final int GT = 0; // Actually checked as !LT
+
+    /**
+     * Utility for ceiling, floor, lower, higher methods.
+     * @param kkey the key
+     * @param rel the relation -- OR'ed combination of EQ, LT, GT
+     * @return nearest node fitting relation, or null if no such
+     */
+    Node<K,V> findNear(K kkey, int rel) {
+        Comparable<? super K> key = comparable(kkey);
+        for (;;) {
+            Node<K,V> b = findPredecessor(key);
+            Node<K,V> n = b.next;
+            for (;;) {
+                if (n == null)
+                    return ((rel & LT) == 0 || b.isBaseHeader())? null : b;
+                Node<K,V> f = n.next;
+                if (n != b.next)                  // inconsistent read
+                    break;
+                Object v = n.value;
+                if (v == null) {                  // n is deleted
+                    n.helpDelete(b, f);
+                    break;
+                }
+                if (v == n || b.value == null)    // b is deleted
+                    break;
+                int c = key.compareTo(n.key);
+                if ((c == 0 && (rel & EQ) != 0) ||
+                    (c <  0 && (rel & LT) == 0))
+                    return n;
+                if ( c <= 0 && (rel & LT) != 0)
+                    return (b.isBaseHeader())? null : b;
+                b = n;
+                n = f;
+            }
+        }
+    }
+
+    /**
+     * Returns SimpleImmutableEntry for results of findNear.
+     * @param key the key
+     * @param rel the relation -- OR'ed combination of EQ, LT, GT
+     * @return Entry fitting relation, or null if no such
+     */
+    AbstractMap.SimpleImmutableEntry<K,V> getNear(K key, int rel) {
+        for (;;) {
+            Node<K,V> n = findNear(key, rel);
+            if (n == null)
+                return null;
+            AbstractMap.SimpleImmutableEntry<K,V> e = n.createSnapshot();
+            if (e != null)
+                return e;
+        }
+    }
+
+
+    /* ---------------- Constructors -------------- */
+
+    /**
+     * Constructs a new, empty map, sorted according to the
+     * {@linkplain Comparable natural ordering} of the keys.
+     */
+    public ConcurrentSkipListMap() {
+        this.comparator = null;
+        initialize();
+    }
+
+    /**
+     * Constructs a new, empty map, sorted according to the specified
+     * comparator.
+     *
+     * @param comparator the comparator that will be used to order this map.
+     *        If <tt>null</tt>, the {@linkplain Comparable natural
+     *        ordering} of the keys will be used.
+     */
+    public ConcurrentSkipListMap(Comparator<? super K> comparator) {
+        this.comparator = comparator;
+        initialize();
+    }
+
+    /**
+     * Constructs a new map containing the same mappings as the given map,
+     * sorted according to the {@linkplain Comparable natural ordering} of
+     * the keys.
+     *
+     * @param  m the map whose mappings are to be placed in this map
+     * @throws ClassCastException if the keys in <tt>m</tt> are not
+     *         {@link Comparable}, or are not mutually comparable
+     * @throws NullPointerException if the specified map or any of its keys
+     *         or values are null
+     */
+    public ConcurrentSkipListMap(Map<? extends K, ? extends V> m) {
+        this.comparator = null;
+        initialize();
+        putAll(m);
+    }
+
+    /**
+     * Constructs a new map containing the same mappings and using the
+     * same ordering as the specified sorted map.
+     *
+     * @param m the sorted map whose mappings are to be placed in this
+     *        map, and whose comparator is to be used to sort this map
+     * @throws NullPointerException if the specified sorted map or any of
+     *         its keys or values are null
+     */
+    public ConcurrentSkipListMap(SortedMap<K, ? extends V> m) {
+        this.comparator = m.comparator();
+        initialize();
+        buildFromSorted(m);
+    }
+
+    /**
+     * Returns a shallow copy of this <tt>ConcurrentSkipListMap</tt>
+     * instance. (The keys and values themselves are not cloned.)
+     *
+     * @return a shallow copy of this map
+     */
+    public ConcurrentSkipListMap<K,V> clone() {
+        ConcurrentSkipListMap<K,V> clone = null;
+        try {
+            clone = (ConcurrentSkipListMap<K,V>) super.clone();
+        } catch (CloneNotSupportedException e) {
+            throw new InternalError();
+        }
+
+        clone.initialize();
+        clone.buildFromSorted(this);
+        return clone;
+    }
+
+    /**
+     * Streamlined bulk insertion to initialize from elements of
+     * given sorted map.  Call only from constructor or clone
+     * method.
+     */
+    private void buildFromSorted(SortedMap<K, ? extends V> map) {
+        if (map == null)
+            throw new NullPointerException();
+
+        HeadIndex<K,V> h = head;
+        Node<K,V> basepred = h.node;
+
+        // Track the current rightmost node at each level. Uses an
+        // ArrayList to avoid committing to initial or maximum level.
+        ArrayList<Index<K,V>> preds = new ArrayList<Index<K,V>>();
+
+        // initialize
+        for (int i = 0; i <= h.level; ++i)
+            preds.add(null);
+        Index<K,V> q = h;
+        for (int i = h.level; i > 0; --i) {
+            preds.set(i, q);
+            q = q.down;
+        }
+
+        Iterator<? extends Map.Entry<? extends K, ? extends V>> it =
+            map.entrySet().iterator();
+        while (it.hasNext()) {
+            Map.Entry<? extends K, ? extends V> e = it.next();
+            int j = randomLevel();
+            if (j > h.level) j = h.level + 1;
+            K k = e.getKey();
+            V v = e.getValue();
+            if (k == null || v == null)
+                throw new NullPointerException();
+            Node<K,V> z = new Node<K,V>(k, v, null);
+            basepred.next = z;
+            basepred = z;
+            if (j > 0) {
+                Index<K,V> idx = null;
+                for (int i = 1; i <= j; ++i) {
+                    idx = new Index<K,V>(z, idx, null);
+                    if (i > h.level)
+                        h = new HeadIndex<K,V>(h.node, h, idx, i);
+
+                    if (i < preds.size()) {
+                        preds.get(i).right = idx;
+                        preds.set(i, idx);
+                    } else
+                        preds.add(idx);
+                }
+            }
+        }
+        head = h;
+    }
+
+    /* ---------------- Serialization -------------- */
+
+    /**
+     * Save the state of this map to a stream.
+     *
+     * @serialData The key (Object) and value (Object) for each
+     * key-value mapping represented by the map, followed by
+     * <tt>null</tt>. The key-value mappings are emitted in key-order
+     * (as determined by the Comparator, or by the keys' natural
+     * ordering if no Comparator).
+     */
+    private void writeObject(java.io.ObjectOutputStream s)
+        throws java.io.IOException {
+        // Write out the Comparator and any hidden stuff
+        s.defaultWriteObject();
+
+        // Write out keys and values (alternating)
+        for (Node<K,V> n = findFirst(); n != null; n = n.next) {
+            V v = n.getValidValue();
+            if (v != null) {
+                s.writeObject(n.key);
+                s.writeObject(v);
+            }
+        }
+        s.writeObject(null);
+    }
+
+    /**
+     * Reconstitute the map from a stream.
+     */
+    private void readObject(final java.io.ObjectInputStream s)
+        throws java.io.IOException, ClassNotFoundException {
+        // Read in the Comparator and any hidden stuff
+        s.defaultReadObject();
+        // Reset transients
+        initialize();
+
+        /*
+         * This is nearly identical to buildFromSorted, but is
+         * distinct because readObject calls can't be nicely adapted
+         * as the kind of iterator needed by buildFromSorted. (They
+         * can be, but doing so requires type cheats and/or creation
+         * of adaptor classes.) It is simpler to just adapt the code.
+         */
+
+        HeadIndex<K,V> h = head;
+        Node<K,V> basepred = h.node;
+        ArrayList<Index<K,V>> preds = new ArrayList<Index<K,V>>();
+        for (int i = 0; i <= h.level; ++i)
+            preds.add(null);
+        Index<K,V> q = h;
+        for (int i = h.level; i > 0; --i) {
+            preds.set(i, q);
+            q = q.down;
+        }
+
+        for (;;) {
+            Object k = s.readObject();
+            if (k == null)
+                break;
+            Object v = s.readObject();
+            if (v == null)
+                throw new NullPointerException();
+            K key = (K) k;
+            V val = (V) v;
+            int j = randomLevel();
+            if (j > h.level) j = h.level + 1;
+            Node<K,V> z = new Node<K,V>(key, val, null);
+            basepred.next = z;
+            basepred = z;
+            if (j > 0) {
+                Index<K,V> idx = null;
+                for (int i = 1; i <= j; ++i) {
+                    idx = new Index<K,V>(z, idx, null);
+                    if (i > h.level)
+                        h = new HeadIndex<K,V>(h.node, h, idx, i);
+
+                    if (i < preds.size()) {
+                        preds.get(i).right = idx;
+                        preds.set(i, idx);
+                    } else
+                        preds.add(idx);
+                }
+            }
+        }
+        head = h;
+    }
+
+    /* ------ Map API methods ------ */
+
+    /**
+     * Returns <tt>true</tt> if this map contains a mapping for the specified
+     * key.
+     *
+     * @param key key whose presence in this map is to be tested
+     * @return <tt>true</tt> if this map contains a mapping for the specified key
+     * @throws ClassCastException if the specified key cannot be compared
+     *         with the keys currently in the map
+     * @throws NullPointerException if the specified key is null
+     */
+    public boolean containsKey(Object key) {
+        return doGet(key) != null;
+    }
+
+    /**
+     * Returns the value to which the specified key is mapped,
+     * or {@code null} if this map contains no mapping for the key.
+     *
+     * <p>More formally, if this map contains a mapping from a key
+     * {@code k} to a value {@code v} such that {@code key} compares
+     * equal to {@code k} according to the map's ordering, then this
+     * method returns {@code v}; otherwise it returns {@code null}.
+     * (There can be at most one such mapping.)
+     *
+     * @throws ClassCastException if the specified key cannot be compared
+     *         with the keys currently in the map
+     * @throws NullPointerException if the specified key is null
+     */
+    public V get(Object key) {
+        return doGet(key);
+    }
+
+    /**
+     * Associates the specified value with the specified key in this map.
+     * If the map previously contained a mapping for the key, the old
+     * value is replaced.
+     *
+     * @param key key with which the specified value is to be associated
+     * @param value value to be associated with the specified key
+     * @return the previous value associated with the specified key, or
+     *         <tt>null</tt> if there was no mapping for the key
+     * @throws ClassCastException if the specified key cannot be compared
+     *         with the keys currently in the map
+     * @throws NullPointerException if the specified key or value is null
+     */
+    public V put(K key, V value) {
+        if (value == null)
+            throw new NullPointerException();
+        return doPut(key, value, false);
+    }
+
+    /**
+     * Removes the mapping for the specified key from this map if present.
+     *
+     * @param  key key for which mapping should be removed
+     * @return the previous value associated with the specified key, or
+     *         <tt>null</tt> if there was no mapping for the key
+     * @throws ClassCastException if the specified key cannot be compared
+     *         with the keys currently in the map
+     * @throws NullPointerException if the specified key is null
+     */
+    public V remove(Object key) {
+        return doRemove(key, null);
+    }
+
+    /**
+     * Returns <tt>true</tt> if this map maps one or more keys to the
+     * specified value.  This operation requires time linear in the
+     * map size.
+     *
+     * @param value value whose presence in this map is to be tested
+     * @return <tt>true</tt> if a mapping to <tt>value</tt> exists;
+     *         <tt>false</tt> otherwise
+     * @throws NullPointerException if the specified value is null
+     */
+    public boolean containsValue(Object value) {
+        if (value == null)
+            throw new NullPointerException();
+        for (Node<K,V> n = findFirst(); n != null; n = n.next) {
+            V v = n.getValidValue();
+            if (v != null && value.equals(v))
+                return true;
+        }
+        return false;
+    }
+
+    /**
+     * Returns the number of key-value mappings in this map.  If this map
+     * contains more than <tt>Integer.MAX_VALUE</tt> elements, it
+     * returns <tt>Integer.MAX_VALUE</tt>.
+     *
+     * <p>Beware that, unlike in most collections, this method is
+     * <em>NOT</em> a constant-time operation. Because of the
+     * asynchronous nature of these maps, determining the current
+     * number of elements requires traversing them all to count them.
+     * Additionally, it is possible for the size to change during
+     * execution of this method, in which case the returned result
+     * will be inaccurate. Thus, this method is typically not very
+     * useful in concurrent applications.
+     *
+     * @return the number of elements in this map
+     */
+    public int size() {
+        long count = 0;
+        for (Node<K,V> n = findFirst(); n != null; n = n.next) {
+            if (n.getValidValue() != null)
+                ++count;
+        }
+        return (count >= Integer.MAX_VALUE)? Integer.MAX_VALUE : (int)count;
+    }
+
+    /**
+     * Returns <tt>true</tt> if this map contains no key-value mappings.
+     * @return <tt>true</tt> if this map contains no key-value mappings
+     */
+    public boolean isEmpty() {
+        return findFirst() == null;
+    }
+
+    /**
+     * Removes all of the mappings from this map.
+     */
+    public void clear() {
+        initialize();
+    }
+
+    /* ---------------- View methods -------------- */
+
+    /*
+     * Note: Lazy initialization works for views because view classes
+     * are stateless/immutable so it doesn't matter wrt correctness if
+     * more than one is created (which will only rarely happen).  Even
+     * so, the following idiom conservatively ensures that the method
+     * returns the one it created if it does so, not one created by
+     * another racing thread.
+     */
+
+    /**
+     * Returns a {@link NavigableSet} view of the keys contained in this map.
+     * The set's iterator returns the keys in ascending order.
+     * The set is backed by the map, so changes to the map are
+     * reflected in the set, and vice-versa.  The set supports element
+     * removal, which removes the corresponding mapping from the map,
+     * via the {@code Iterator.remove}, {@code Set.remove},
+     * {@code removeAll}, {@code retainAll}, and {@code clear}
+     * operations.  It does not support the {@code add} or {@code addAll}
+     * operations.
+     *
+     * <p>The view's {@code iterator} is a "weakly consistent" iterator
+     * that will never throw {@link ConcurrentModificationException},
+     * and guarantees to traverse elements as they existed upon
+     * construction of the iterator, and may (but is not guaranteed to)
+     * reflect any modifications subsequent to construction.
+     *
+     * <p>This method is equivalent to method {@code navigableKeySet}.
+     *
+     * @return a navigable set view of the keys in this map
+     */
+     public NavigableSet<K> keySet() {
+        KeySet ks = keySet;
+        return (ks != null) ? ks : (keySet = new KeySet(this));
+    }
+
+    public NavigableSet<K> navigableKeySet() {
+        KeySet ks = keySet;
+        return (ks != null) ? ks : (keySet = new KeySet(this));
+    }
+
+    /**
+     * Returns a {@link Collection} view of the values contained in this map.
+     * The collection's iterator returns the values in ascending order
+     * of the corresponding keys.
+     * The collection is backed by the map, so changes to the map are
+     * reflected in the collection, and vice-versa.  The collection
+     * supports element removal, which removes the corresponding
+     * mapping from the map, via the <tt>Iterator.remove</tt>,
+     * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
+     * <tt>retainAll</tt> and <tt>clear</tt> operations.  It does not
+     * support the <tt>add</tt> or <tt>addAll</tt> operations.
+     *
+     * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator
+     * that will never throw {@link ConcurrentModificationException},
+     * and guarantees to traverse elements as they existed upon
+     * construction of the iterator, and may (but is not guaranteed to)
+     * reflect any modifications subsequent to construction.
+     */
+    public Collection<V> values() {
+        Values vs = values;
+        return (vs != null) ? vs : (values = new Values(this));
+    }
+
+    /**
+     * Returns a {@link Set} view of the mappings contained in this map.
+     * The set's iterator returns the entries in ascending key order.
+     * The set is backed by the map, so changes to the map are
+     * reflected in the set, and vice-versa.  The set supports element
+     * removal, which removes the corresponding mapping from the map,
+     * via the <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
+     * <tt>removeAll</tt>, <tt>retainAll</tt> and <tt>clear</tt>
+     * operations.  It does not support the <tt>add</tt> or
+     * <tt>addAll</tt> operations.
+     *
+     * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator
+     * that will never throw {@link ConcurrentModificationException},
+     * and guarantees to traverse elements as they existed upon
+     * construction of the iterator, and may (but is not guaranteed to)
+     * reflect any modifications subsequent to construction.
+     *
+     * <p>The <tt>Map.Entry</tt> elements returned by
+     * <tt>iterator.next()</tt> do <em>not</em> support the
+     * <tt>setValue</tt> operation.
+     *
+     * @return a set view of the mappings contained in this map,
+     *         sorted in ascending key order
+     */
+    public Set<Map.Entry<K,V>> entrySet() {
+        EntrySet es = entrySet;
+        return (es != null) ? es : (entrySet = new EntrySet(this));
+    }
+
+    public ConcurrentNavigableMap<K,V> descendingMap() {
+        ConcurrentNavigableMap<K,V> dm = descendingMap;
+        return (dm != null) ? dm : (descendingMap = new SubMap<K,V>
+                                    (this, null, false, null, false, true));
+    }
+
+    public NavigableSet<K> descendingKeySet() {
+        return descendingMap().navigableKeySet();
+    }
+
+    /* ---------------- AbstractMap Overrides -------------- */
+
+    /**
+     * Compares the specified object with this map for equality.
+     * Returns <tt>true</tt> if the given object is also a map and the
+     * two maps represent the same mappings.  More formally, two maps
+     * <tt>m1</tt> and <tt>m2</tt> represent the same mappings if
+     * <tt>m1.entrySet().equals(m2.entrySet())</tt>.  This
+     * operation may return misleading results if either map is
+     * concurrently modified during execution of this method.
+     *
+     * @param o object to be compared for equality with this map
+     * @return <tt>true</tt> if the specified object is equal to this map
+     */
+    public boolean equals(Object o) {
+	if (o == this)
+	    return true;
+	if (!(o instanceof Map))
+	    return false;
+	Map<?,?> m = (Map<?,?>) o;
+        try {
+	    for (Map.Entry<K,V> e : this.entrySet())
+		if (! e.getValue().equals(m.get(e.getKey())))
+                    return false;
+	    for (Map.Entry<?,?> e : m.entrySet()) {
+                Object k = e.getKey();
+                Object v = e.getValue();
+		if (k == null || v == null || !v.equals(get(k)))
+                    return false;
+            }
+            return true;
+        } catch (ClassCastException unused) {
+            return false;
+        } catch (NullPointerException unused) {
+            return false;
+        }
+    }
+
+    /* ------ ConcurrentMap API methods ------ */
+
+    /**
+     * {@inheritDoc}
+     *
+     * @return the previous value associated with the specified key,
+     *         or <tt>null</tt> if there was no mapping for the key
+     * @throws ClassCastException if the specified key cannot be compared
+     *         with the keys currently in the map
+     * @throws NullPointerException if the specified key or value is null
+     */
+    public V putIfAbsent(K key, V value) {
+        if (value == null)
+            throw new NullPointerException();
+        return doPut(key, value, true);
+    }
+
+    /**
+     * {@inheritDoc}
+     *
+     * @throws ClassCastException if the specified key cannot be compared
+     *         with the keys currently in the map
+     * @throws NullPointerException if the specified key is null
+     */
+    public boolean remove(Object key, Object value) {
+        if (key == null)
+            throw new NullPointerException();
+        if (value == null)
+            return false;
+        return doRemove(key, value) != null;
+    }
+
+    /**
+     * {@inheritDoc}
+     *
+     * @throws ClassCastException if the specified key cannot be compared
+     *         with the keys currently in the map
+     * @throws NullPointerException if any of the arguments are null
+     */
+    public boolean replace(K key, V oldValue, V newValue) {
+        if (oldValue == null || newValue == null)
+            throw new NullPointerException();
+        Comparable<? super K> k = comparable(key);
+        for (;;) {
+            Node<K,V> n = findNode(k);
+            if (n == null)
+                return false;
+            Object v = n.value;
+            if (v != null) {
+                if (!oldValue.equals(v))
+                    return false;
+                if (n.casValue(v, newValue))
+                    return true;
+            }
+        }
+    }
+
+    /**
+     * {@inheritDoc}
+     *
+     * @return the previous value associated with the specified key,
+     *         or <tt>null</tt> if there was no mapping for the key
+     * @throws ClassCastException if the specified key cannot be compared
+     *         with the keys currently in the map
+     * @throws NullPointerException if the specified key or value is null
+     */
+    public V replace(K key, V value) {
+        if (value == null)
+            throw new NullPointerException();
+        Comparable<? super K> k = comparable(key);
+        for (;;) {
+            Node<K,V> n = findNode(k);
+            if (n == null)
+                return null;
+            Object v = n.value;
+            if (v != null && n.casValue(v, value))
+                return (V)v;
+        }
+    }
+
+    /* ------ SortedMap API methods ------ */
+
+    public Comparator<? super K> comparator() {
+        return comparator;
+    }
+
+    /**
+     * @throws NoSuchElementException {@inheritDoc}
+     */
+    public K firstKey() {
+        Node<K,V> n = findFirst();
+        if (n == null)
+            throw new NoSuchElementException();
+        return n.key;
+    }
+
+    /**
+     * @throws NoSuchElementException {@inheritDoc}
+     */
+    public K lastKey() {
+        Node<K,V> n = findLast();
+        if (n == null)
+            throw new NoSuchElementException();
+        return n.key;
+    }
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if {@code fromKey} or {@code toKey} is null
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    public ConcurrentNavigableMap<K,V> subMap(K fromKey,
+                                              boolean fromInclusive,
+                                              K toKey,
+                                              boolean toInclusive) {
+        if (fromKey == null || toKey == null)
+            throw new NullPointerException();
+        return new SubMap<K,V>
+            (this, fromKey, fromInclusive, toKey, toInclusive, false);
+    }
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if {@code toKey} is null
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    public ConcurrentNavigableMap<K,V> headMap(K toKey,
+                                               boolean inclusive) {
+        if (toKey == null)
+            throw new NullPointerException();
+        return new SubMap<K,V>
+            (this, null, false, toKey, inclusive, false);
+    }
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if {@code fromKey} is null
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    public ConcurrentNavigableMap<K,V> tailMap(K fromKey,
+                                               boolean inclusive) {
+        if (fromKey == null)
+            throw new NullPointerException();
+        return new SubMap<K,V>
+            (this, fromKey, inclusive, null, false, false);
+    }
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if {@code fromKey} or {@code toKey} is null
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    public ConcurrentNavigableMap<K,V> subMap(K fromKey, K toKey) {
+        return subMap(fromKey, true, toKey, false);
+    }
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if {@code toKey} is null
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    public ConcurrentNavigableMap<K,V> headMap(K toKey) {
+        return headMap(toKey, false);
+    }
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if {@code fromKey} is null
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    public ConcurrentNavigableMap<K,V> tailMap(K fromKey) {
+        return tailMap(fromKey, true);
+    }
+
+    /* ---------------- Relational operations -------------- */
+
+    /**
+     * Returns a key-value mapping associated with the greatest key
+     * strictly less than the given key, or <tt>null</tt> if there is
+     * no such key. The returned entry does <em>not</em> support the
+     * <tt>Entry.setValue</tt> method.
+     *
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if the specified key is null
+     */
+    public Map.Entry<K,V> lowerEntry(K key) {
+        return getNear(key, LT);
+    }
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if the specified key is null
+     */
+    public K lowerKey(K key) {
+        Node<K,V> n = findNear(key, LT);
+        return (n == null)? null : n.key;
+    }
+
+    /**
+     * Returns a key-value mapping associated with the greatest key
+     * less than or equal to the given key, or <tt>null</tt> if there
+     * is no such key. The returned entry does <em>not</em> support
+     * the <tt>Entry.setValue</tt> method.
+     *
+     * @param key the key
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if the specified key is null
+     */
+    public Map.Entry<K,V> floorEntry(K key) {
+        return getNear(key, LT|EQ);
+    }
+
+    /**
+     * @param key the key
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if the specified key is null
+     */
+    public K floorKey(K key) {
+        Node<K,V> n = findNear(key, LT|EQ);
+        return (n == null)? null : n.key;
+    }
+
+    /**
+     * Returns a key-value mapping associated with the least key
+     * greater than or equal to the given key, or <tt>null</tt> if
+     * there is no such entry. The returned entry does <em>not</em>
+     * support the <tt>Entry.setValue</tt> method.
+     *
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if the specified key is null
+     */
+    public Map.Entry<K,V> ceilingEntry(K key) {
+        return getNear(key, GT|EQ);
+    }
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if the specified key is null
+     */
+    public K ceilingKey(K key) {
+        Node<K,V> n = findNear(key, GT|EQ);
+        return (n == null)? null : n.key;
+    }
+
+    /**
+     * Returns a key-value mapping associated with the least key
+     * strictly greater than the given key, or <tt>null</tt> if there
+     * is no such key. The returned entry does <em>not</em> support
+     * the <tt>Entry.setValue</tt> method.
+     *
+     * @param key the key
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if the specified key is null
+     */
+    public Map.Entry<K,V> higherEntry(K key) {
+        return getNear(key, GT);
+    }
+
+    /**
+     * @param key the key
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if the specified key is null
+     */
+    public K higherKey(K key) {
+        Node<K,V> n = findNear(key, GT);
+        return (n == null)? null : n.key;
+    }
+
+    /**
+     * Returns a key-value mapping associated with the least
+     * key in this map, or <tt>null</tt> if the map is empty.
+     * The returned entry does <em>not</em> support
+     * the <tt>Entry.setValue</tt> method.
+     */
+    public Map.Entry<K,V> firstEntry() {
+        for (;;) {
+            Node<K,V> n = findFirst();
+            if (n == null)
+                return null;
+            AbstractMap.SimpleImmutableEntry<K,V> e = n.createSnapshot();
+            if (e != null)
+                return e;
+        }
+    }
+
+    /**
+     * Returns a key-value mapping associated with the greatest
+     * key in this map, or <tt>null</tt> if the map is empty.
+     * The returned entry does <em>not</em> support
+     * the <tt>Entry.setValue</tt> method.
+     */
+    public Map.Entry<K,V> lastEntry() {
+        for (;;) {
+            Node<K,V> n = findLast();
+            if (n == null)
+                return null;
+            AbstractMap.SimpleImmutableEntry<K,V> e = n.createSnapshot();
+            if (e != null)
+                return e;
+        }
+    }
+
+    /**
+     * Removes and returns a key-value mapping associated with
+     * the least key in this map, or <tt>null</tt> if the map is empty.
+     * The returned entry does <em>not</em> support
+     * the <tt>Entry.setValue</tt> method.
+     */
+    public Map.Entry<K,V> pollFirstEntry() {
+        return doRemoveFirstEntry();
+    }
+
+    /**
+     * Removes and returns a key-value mapping associated with
+     * the greatest key in this map, or <tt>null</tt> if the map is empty.
+     * The returned entry does <em>not</em> support
+     * the <tt>Entry.setValue</tt> method.
+     */
+    public Map.Entry<K,V> pollLastEntry() {
+        return doRemoveLastEntry();
+    }
+
+
+    /* ---------------- Iterators -------------- */
+
+    /**
+     * Base of iterator classes:
+     */
+    abstract class Iter<T> implements Iterator<T> {
+        /** the last node returned by next() */
+        Node<K,V> lastReturned;
+        /** the next node to return from next(); */
+        Node<K,V> next;
+	/** Cache of next value field to maintain weak consistency */
+	V nextValue;
+
+        /** Initializes ascending iterator for entire range. */
+        Iter() {
+            for (;;) {
+		next = findFirst();
+                if (next == null)
+                    break;
+                Object x = next.value;
+                if (x != null && x != next) {
+		    nextValue = (V) x;
+                    break;
+		}
+            }
+        }
+
+        public final boolean hasNext() {
+            return next != null;
+        }
+
+        /** Advances next to higher entry. */
+        final void advance() {
+            if ((lastReturned = next) == null)
+                throw new NoSuchElementException();
+            for (;;) {
+		next = next.next;
+                if (next == null)
+                    break;
+                Object x = next.value;
+                if (x != null && x != next) {
+		    nextValue = (V) x;
+                    break;
+		}
+            }
+        }
+
+        public void remove() {
+            Node<K,V> l = lastReturned;
+            if (l == null)
+                throw new IllegalStateException();
+            // It would not be worth all of the overhead to directly
+            // unlink from here. Using remove is fast enough.
+            ConcurrentSkipListMap.this.remove(l.key);
+	    lastReturned = null;
+        }
+
+    }
+
+    final class ValueIterator extends Iter<V> {
+        public V next() {
+            V v = nextValue;
+            advance();
+            return v;
+        }
+    }
+
+    final class KeyIterator extends Iter<K> {
+        public K next() {
+            Node<K,V> n = next;
+            advance();
+            return n.key;
+        }
+    }
+
+    final class EntryIterator extends Iter<Map.Entry<K,V>> {
+        public Map.Entry<K,V> next() {
+            Node<K,V> n = next;
+            V v = nextValue;
+            advance();
+            return new AbstractMap.SimpleImmutableEntry<K,V>(n.key, v);
+        }
+    }
+
+    // Factory methods for iterators needed by ConcurrentSkipListSet etc
+
+    Iterator<K> keyIterator() {
+        return new KeyIterator();
+    }
+
+    Iterator<V> valueIterator() {
+        return new ValueIterator();
+    }
+
+    Iterator<Map.Entry<K,V>> entryIterator() {
+        return new EntryIterator();
+    }
+
+    /* ---------------- View Classes -------------- */
+
+    /*
+     * View classes are static, delegating to a ConcurrentNavigableMap
+     * to allow use by SubMaps, which outweighs the ugliness of
+     * needing type-tests for Iterator methods.
+     */
+
+    static final <E> List<E> toList(Collection<E> c) {
+	// Using size() here would be a pessimization.
+	List<E> list = new ArrayList<E>();
+	for (E e : c)
+	    list.add(e);
+	return list;
+    }
+
+    static final class KeySet<E> extends AbstractSet<E> implements NavigableSet<E> {
+        private final ConcurrentNavigableMap<E,Object> m;
+        KeySet(ConcurrentNavigableMap<E,Object> map) { m = map; }
+        public int size() { return m.size(); }
+        public boolean isEmpty() { return m.isEmpty(); }
+        public boolean contains(Object o) { return m.containsKey(o); }
+        public boolean remove(Object o) { return m.remove(o) != null; }
+        public void clear() { m.clear(); }
+        public E lower(E e) { return m.lowerKey(e); }
+        public E floor(E e) { return m.floorKey(e); }
+        public E ceiling(E e) { return m.ceilingKey(e); }
+        public E higher(E e) { return m.higherKey(e); }
+        public Comparator<? super E> comparator() { return m.comparator(); }
+        public E first() { return m.firstKey(); }
+        public E last() { return m.lastKey(); }
+        public E pollFirst() {
+            Map.Entry<E,Object> e = m.pollFirstEntry();
+            return e == null? null : e.getKey();
+        }
+        public E pollLast() {
+            Map.Entry<E,Object> e = m.pollLastEntry();
+            return e == null? null : e.getKey();
+        }
+        public Iterator<E> iterator() {
+            if (m instanceof ConcurrentSkipListMap)
+                return ((ConcurrentSkipListMap<E,Object>)m).keyIterator();
+            else
+                return ((ConcurrentSkipListMap.SubMap<E,Object>)m).keyIterator();
+        }
+        public boolean equals(Object o) {
+            if (o == this)
+                return true;
+            if (!(o instanceof Set))
+                return false;
+            Collection<?> c = (Collection<?>) o;
+            try {
+                return containsAll(c) && c.containsAll(this);
+            } catch (ClassCastException unused)   {
+                return false;
+            } catch (NullPointerException unused) {
+                return false;
+            }
+        }
+	public Object[] toArray()     { return toList(this).toArray();  }
+	public <T> T[] toArray(T[] a) { return toList(this).toArray(a); }
+        public Iterator<E> descendingIterator() {
+            return descendingSet().iterator();
+        }
+        public NavigableSet<E> subSet(E fromElement,
+                                      boolean fromInclusive,
+                                      E toElement,
+                                      boolean toInclusive) {
+            return new ConcurrentSkipListSet<E>
+                (m.subMap(fromElement, fromInclusive,
+                          toElement,   toInclusive));
+        }
+        public NavigableSet<E> headSet(E toElement, boolean inclusive) {
+            return new ConcurrentSkipListSet<E>(m.headMap(toElement, inclusive));
+        }
+        public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
+            return new ConcurrentSkipListSet<E>(m.tailMap(fromElement, inclusive));
+        }
+        public NavigableSet<E> subSet(E fromElement, E toElement) {
+            return subSet(fromElement, true, toElement, false);
+        }
+        public NavigableSet<E> headSet(E toElement) {
+            return headSet(toElement, false);
+        }
+        public NavigableSet<E> tailSet(E fromElement) {
+            return tailSet(fromElement, true);
+        }
+        public NavigableSet<E> descendingSet() {
+            return new ConcurrentSkipListSet(m.descendingMap());
+        }
+    }
+
+    static final class Values<E> extends AbstractCollection<E> {
+        private final ConcurrentNavigableMap<Object, E> m;
+        Values(ConcurrentNavigableMap<Object, E> map) {
+            m = map;
+        }
+        public Iterator<E> iterator() {
+            if (m instanceof ConcurrentSkipListMap)
+                return ((ConcurrentSkipListMap<Object,E>)m).valueIterator();
+            else
+                return ((SubMap<Object,E>)m).valueIterator();
+        }
+        public boolean isEmpty() {
+            return m.isEmpty();
+        }
+        public int size() {
+            return m.size();
+        }
+        public boolean contains(Object o) {
+            return m.containsValue(o);
+        }
+        public void clear() {
+            m.clear();
+        }
+	public Object[] toArray()     { return toList(this).toArray();  }
+	public <T> T[] toArray(T[] a) { return toList(this).toArray(a); }
+    }
+
+    static final class EntrySet<K1,V1> extends AbstractSet<Map.Entry<K1,V1>> {
+        private final ConcurrentNavigableMap<K1, V1> m;
+        EntrySet(ConcurrentNavigableMap<K1, V1> map) {
+            m = map;
+        }
+
+        public Iterator<Map.Entry<K1,V1>> iterator() {
+            if (m instanceof ConcurrentSkipListMap)
+                return ((ConcurrentSkipListMap<K1,V1>)m).entryIterator();
+            else
+                return ((SubMap<K1,V1>)m).entryIterator();
+        }
+
+        public boolean contains(Object o) {
+            if (!(o instanceof Map.Entry))
+                return false;
+            Map.Entry<K1,V1> e = (Map.Entry<K1,V1>)o;
+            V1 v = m.get(e.getKey());
+            return v != null && v.equals(e.getValue());
+        }
+        public boolean remove(Object o) {
+            if (!(o instanceof Map.Entry))
+                return false;
+            Map.Entry<K1,V1> e = (Map.Entry<K1,V1>)o;
+            return m.remove(e.getKey(),
+                            e.getValue());
+        }
+        public boolean isEmpty() {
+            return m.isEmpty();
+        }
+        public int size() {
+            return m.size();
+        }
+        public void clear() {
+            m.clear();
+        }
+        public boolean equals(Object o) {
+            if (o == this)
+                return true;
+            if (!(o instanceof Set))
+                return false;
+            Collection<?> c = (Collection<?>) o;
+            try {
+                return containsAll(c) && c.containsAll(this);
+            } catch (ClassCastException unused)   {
+                return false;
+            } catch (NullPointerException unused) {
+                return false;
+            }
+        }
+	public Object[] toArray()     { return toList(this).toArray();  }
+	public <T> T[] toArray(T[] a) { return toList(this).toArray(a); }
+    }
+
+    /**
+     * Submaps returned by {@link ConcurrentSkipListMap} submap operations
+     * represent a subrange of mappings of their underlying
+     * maps. Instances of this class support all methods of their
+     * underlying maps, differing in that mappings outside their range are
+     * ignored, and attempts to add mappings outside their ranges result
+     * in {@link IllegalArgumentException}.  Instances of this class are
+     * constructed only using the <tt>subMap</tt>, <tt>headMap</tt>, and
+     * <tt>tailMap</tt> methods of their underlying maps.
+     *
+     * @serial include
+     */
+    static final class SubMap<K,V> extends AbstractMap<K,V>
+        implements ConcurrentNavigableMap<K,V>, Cloneable,
+                   java.io.Serializable {
+        private static final long serialVersionUID = -7647078645895051609L;
+
+        /** Underlying map */
+        private final ConcurrentSkipListMap<K,V> m;
+        /** lower bound key, or null if from start */
+        private final K lo;
+        /** upper bound key, or null if to end */
+        private final K hi;
+        /** inclusion flag for lo */
+        private final boolean loInclusive;
+        /** inclusion flag for hi */
+        private final boolean hiInclusive;
+        /** direction */
+        private final boolean isDescending;
+
+        // Lazily initialized view holders
+        private transient KeySet<K> keySetView;
+        private transient Set<Map.Entry<K,V>> entrySetView;
+        private transient Collection<V> valuesView;
+
+        /**
+         * Creates a new submap, initializing all fields
+         */
+        SubMap(ConcurrentSkipListMap<K,V> map,
+               K fromKey, boolean fromInclusive,
+               K toKey, boolean toInclusive,
+               boolean isDescending) {
+            if (fromKey != null && toKey != null &&
+                map.compare(fromKey, toKey) > 0)
+                throw new IllegalArgumentException("inconsistent range");
+            this.m = map;
+            this.lo = fromKey;
+            this.hi = toKey;
+            this.loInclusive = fromInclusive;
+            this.hiInclusive = toInclusive;
+            this.isDescending = isDescending;
+        }
+
+        /* ----------------  Utilities -------------- */
+
+        private boolean tooLow(K key) {
+            if (lo != null) {
+                int c = m.compare(key, lo);
+                if (c < 0 || (c == 0 && !loInclusive))
+                    return true;
+            }
+            return false;
+        }
+
+        private boolean tooHigh(K key) {
+            if (hi != null) {
+                int c = m.compare(key, hi);
+                if (c > 0 || (c == 0 && !hiInclusive))
+                    return true;
+            }
+            return false;
+        }
+
+        private boolean inBounds(K key) {
+            return !tooLow(key) && !tooHigh(key);
+        }
+
+        private void checkKeyBounds(K key) throws IllegalArgumentException {
+            if (key == null)
+                throw new NullPointerException();
+            if (!inBounds(key))
+                throw new IllegalArgumentException("key out of range");
+        }
+
+        /**
+         * Returns true if node key is less than upper bound of range
+         */
+        private boolean isBeforeEnd(ConcurrentSkipListMap.Node<K,V> n) {
+            if (n == null)
+                return false;
+            if (hi == null)
+                return true;
+            K k = n.key;
+            if (k == null) // pass by markers and headers
+                return true;
+            int c = m.compare(k, hi);
+            if (c > 0 || (c == 0 && !hiInclusive))
+                return false;
+            return true;
+        }
+
+        /**
+         * Returns lowest node. This node might not be in range, so
+         * most usages need to check bounds
+         */
+        private ConcurrentSkipListMap.Node<K,V> loNode() {
+            if (lo == null)
+                return m.findFirst();
+            else if (loInclusive)
+                return m.findNear(lo, m.GT|m.EQ);
+            else
+                return m.findNear(lo, m.GT);
+        }
+
+        /**
+         * Returns highest node. This node might not be in range, so
+         * most usages need to check bounds
+         */
+        private ConcurrentSkipListMap.Node<K,V> hiNode() {
+            if (hi == null)
+                return m.findLast();
+            else if (hiInclusive)
+                return m.findNear(hi, m.LT|m.EQ);
+            else
+                return m.findNear(hi, m.LT);
+        }
+
+        /**
+         * Returns lowest absolute key (ignoring directonality)
+         */
+        private K lowestKey() {
+            ConcurrentSkipListMap.Node<K,V> n = loNode();
+            if (isBeforeEnd(n))
+                return n.key;
+            else
+                throw new NoSuchElementException();
+        }
+
+        /**
+         * Returns highest absolute key (ignoring directonality)
+         */
+        private K highestKey() {
+            ConcurrentSkipListMap.Node<K,V> n = hiNode();
+            if (n != null) {
+                K last = n.key;
+                if (inBounds(last))
+                    return last;
+            }
+            throw new NoSuchElementException();
+        }
+
+        private Map.Entry<K,V> lowestEntry() {
+            for (;;) {
+                ConcurrentSkipListMap.Node<K,V> n = loNode();
+                if (!isBeforeEnd(n))
+                    return null;
+                Map.Entry<K,V> e = n.createSnapshot();
+                if (e != null)
+                    return e;
+            }
+        }
+
+        private Map.Entry<K,V> highestEntry() {
+            for (;;) {
+                ConcurrentSkipListMap.Node<K,V> n = hiNode();
+                if (n == null || !inBounds(n.key))
+                    return null;
+                Map.Entry<K,V> e = n.createSnapshot();
+                if (e != null)
+                    return e;
+            }
+        }
+
+        private Map.Entry<K,V> removeLowest() {
+            for (;;) {
+                Node<K,V> n = loNode();
+                if (n == null)
+                    return null;
+                K k = n.key;
+                if (!inBounds(k))
+                    return null;
+                V v = m.doRemove(k, null);
+                if (v != null)
+                    return new AbstractMap.SimpleImmutableEntry<K,V>(k, v);
+            }
+        }
+
+        private Map.Entry<K,V> removeHighest() {
+            for (;;) {
+                Node<K,V> n = hiNode();
+                if (n == null)
+                    return null;
+                K k = n.key;
+                if (!inBounds(k))
+                    return null;
+                V v = m.doRemove(k, null);
+                if (v != null)
+                    return new AbstractMap.SimpleImmutableEntry<K,V>(k, v);
+            }
+        }
+
+        /**
+         * Submap version of ConcurrentSkipListMap.getNearEntry
+         */
+        private Map.Entry<K,V> getNearEntry(K key, int rel) {
+            if (isDescending) { // adjust relation for direction
+                if ((rel & m.LT) == 0)
+                    rel |= m.LT;
+                else
+                    rel &= ~m.LT;
+            }
+            if (tooLow(key))
+                return ((rel & m.LT) != 0)? null : lowestEntry();
+            if (tooHigh(key))
+                return ((rel & m.LT) != 0)? highestEntry() : null;
+            for (;;) {
+                Node<K,V> n = m.findNear(key, rel);
+                if (n == null || !inBounds(n.key))
+                    return null;
+                K k = n.key;
+                V v = n.getValidValue();
+                if (v != null)
+                    return new AbstractMap.SimpleImmutableEntry<K,V>(k, v);
+            }
+        }
+
+        // Almost the same as getNearEntry, except for keys
+        private K getNearKey(K key, int rel) {
+            if (isDescending) { // adjust relation for direction
+                if ((rel & m.LT) == 0)
+                    rel |= m.LT;
+                else
+                    rel &= ~m.LT;
+            }
+            if (tooLow(key)) {
+                if ((rel & m.LT) == 0) {
+                    ConcurrentSkipListMap.Node<K,V> n = loNode();
+                    if (isBeforeEnd(n))
+                        return n.key;
+                }
+                return null;
+            }
+            if (tooHigh(key)) {
+                if ((rel & m.LT) != 0) {
+                    ConcurrentSkipListMap.Node<K,V> n = hiNode();
+                    if (n != null) {
+                        K last = n.key;
+                        if (inBounds(last))
+                            return last;
+                    }
+                }
+                return null;
+            }
+            for (;;) {
+                Node<K,V> n = m.findNear(key, rel);
+                if (n == null || !inBounds(n.key))
+                    return null;
+                K k = n.key;
+                V v = n.getValidValue();
+                if (v != null)
+                    return k;
+            }
+        }
+
+        /* ----------------  Map API methods -------------- */
+
+        public boolean containsKey(Object key) {
+            if (key == null) throw new NullPointerException();
+            K k = (K)key;
+            return inBounds(k) && m.containsKey(k);
+        }
+
+        public V get(Object key) {
+            if (key == null) throw new NullPointerException();
+            K k = (K)key;
+            return ((!inBounds(k)) ? null : m.get(k));
+        }
+
+        public V put(K key, V value) {
+            checkKeyBounds(key);
+            return m.put(key, value);
+        }
+
+        public V remove(Object key) {
+            K k = (K)key;
+            return (!inBounds(k))? null : m.remove(k);
+        }
+
+        public int size() {
+            long count = 0;
+            for (ConcurrentSkipListMap.Node<K,V> n = loNode();
+                 isBeforeEnd(n);
+                 n = n.next) {
+                if (n.getValidValue() != null)
+                    ++count;
+            }
+            return count >= Integer.MAX_VALUE? Integer.MAX_VALUE : (int)count;
+        }
+
+        public boolean isEmpty() {
+            return !isBeforeEnd(loNode());
+        }
+
+        public boolean containsValue(Object value) {
+            if (value == null)
+                throw new NullPointerException();
+            for (ConcurrentSkipListMap.Node<K,V> n = loNode();
+                 isBeforeEnd(n);
+                 n = n.next) {
+                V v = n.getValidValue();
+                if (v != null && value.equals(v))
+                    return true;
+            }
+            return false;
+        }
+
+        public void clear() {
+            for (ConcurrentSkipListMap.Node<K,V> n = loNode();
+                 isBeforeEnd(n);
+                 n = n.next) {
+                if (n.getValidValue() != null)
+                    m.remove(n.key);
+            }
+        }
+
+        /* ----------------  ConcurrentMap API methods -------------- */
+
+        public V putIfAbsent(K key, V value) {
+            checkKeyBounds(key);
+            return m.putIfAbsent(key, value);
+        }
+
+        public boolean remove(Object key, Object value) {
+            K k = (K)key;
+            return inBounds(k) && m.remove(k, value);
+        }
+
+        public boolean replace(K key, V oldValue, V newValue) {
+            checkKeyBounds(key);
+            return m.replace(key, oldValue, newValue);
+        }
+
+        public V replace(K key, V value) {
+            checkKeyBounds(key);
+            return m.replace(key, value);
+        }
+
+        /* ----------------  SortedMap API methods -------------- */
+
+        public Comparator<? super K> comparator() {
+            Comparator<? super K> cmp = m.comparator();
+	    if (isDescending)
+		return Collections.reverseOrder(cmp);
+	    else
+		return cmp;
+        }
+
+        /**
+         * Utility to create submaps, where given bounds override
+         * unbounded(null) ones and/or are checked against bounded ones.
+         */
+        private SubMap<K,V> newSubMap(K fromKey,
+                                      boolean fromInclusive,
+                                      K toKey,
+                                      boolean toInclusive) {
+            if (isDescending) { // flip senses
+                K tk = fromKey;
+                fromKey = toKey;
+                toKey = tk;
+                boolean ti = fromInclusive;
+                fromInclusive = toInclusive;
+                toInclusive = ti;
+            }
+            if (lo != null) {
+                if (fromKey == null) {
+                    fromKey = lo;
+                    fromInclusive = loInclusive;
+                }
+                else {
+                    int c = m.compare(fromKey, lo);
+                    if (c < 0 || (c == 0 && !loInclusive && fromInclusive))
+                        throw new IllegalArgumentException("key out of range");
+                }
+            }
+            if (hi != null) {
+                if (toKey == null) {
+                    toKey = hi;
+                    toInclusive = hiInclusive;
+                }
+                else {
+                    int c = m.compare(toKey, hi);
+                    if (c > 0 || (c == 0 && !hiInclusive && toInclusive))
+                        throw new IllegalArgumentException("key out of range");
+                }
+            }
+            return new SubMap<K,V>(m, fromKey, fromInclusive,
+                                   toKey, toInclusive, isDescending);
+        }
+
+        public SubMap<K,V> subMap(K fromKey,
+                                  boolean fromInclusive,
+                                  K toKey,
+                                  boolean toInclusive) {
+            if (fromKey == null || toKey == null)
+                throw new NullPointerException();
+            return newSubMap(fromKey, fromInclusive, toKey, toInclusive);
+        }
+
+        public SubMap<K,V> headMap(K toKey,
+                                   boolean inclusive) {
+            if (toKey == null)
+                throw new NullPointerException();
+            return newSubMap(null, false, toKey, inclusive);
+        }
+
+        public SubMap<K,V> tailMap(K fromKey,
+                                   boolean inclusive) {
+            if (fromKey == null)
+                throw new NullPointerException();
+            return newSubMap(fromKey, inclusive, null, false);
+        }
+
+        public SubMap<K,V> subMap(K fromKey, K toKey) {
+            return subMap(fromKey, true, toKey, false);
+        }
+
+        public SubMap<K,V> headMap(K toKey) {
+            return headMap(toKey, false);
+        }
+
+        public SubMap<K,V> tailMap(K fromKey) {
+            return tailMap(fromKey, true);
+        }
+
+        public SubMap<K,V> descendingMap() {
+            return new SubMap<K,V>(m, lo, loInclusive,
+                                   hi, hiInclusive, !isDescending);
+        }
+
+        /* ----------------  Relational methods -------------- */
+
+        public Map.Entry<K,V> ceilingEntry(K key) {
+            return getNearEntry(key, (m.GT|m.EQ));
+        }
+
+        public K ceilingKey(K key) {
+            return getNearKey(key, (m.GT|m.EQ));
+        }
+
+        public Map.Entry<K,V> lowerEntry(K key) {
+            return getNearEntry(key, (m.LT));
+        }
+
+        public K lowerKey(K key) {
+            return getNearKey(key, (m.LT));
+        }
+
+        public Map.Entry<K,V> floorEntry(K key) {
+            return getNearEntry(key, (m.LT|m.EQ));
+        }
+
+        public K floorKey(K key) {
+            return getNearKey(key, (m.LT|m.EQ));
+        }
+
+        public Map.Entry<K,V> higherEntry(K key) {
+            return getNearEntry(key, (m.GT));
+        }
+
+        public K higherKey(K key) {
+            return getNearKey(key, (m.GT));
+        }
+
+        public K firstKey() {
+            return isDescending? highestKey() : lowestKey();
+        }
+
+        public K lastKey() {
+            return isDescending? lowestKey() : highestKey();
+        }
+
+        public Map.Entry<K,V> firstEntry() {
+            return isDescending? highestEntry() : lowestEntry();
+        }
+
+        public Map.Entry<K,V> lastEntry() {
+            return isDescending? lowestEntry() : highestEntry();
+        }
+
+        public Map.Entry<K,V> pollFirstEntry() {
+            return isDescending? removeHighest() : removeLowest();
+        }
+
+        public Map.Entry<K,V> pollLastEntry() {
+            return isDescending? removeLowest() : removeHighest();
+        }
+
+        /* ---------------- Submap Views -------------- */
+
+        public NavigableSet<K> keySet() {
+            KeySet<K> ks = keySetView;
+            return (ks != null) ? ks : (keySetView = new KeySet(this));
+        }
+
+        public NavigableSet<K> navigableKeySet() {
+            KeySet<K> ks = keySetView;
+            return (ks != null) ? ks : (keySetView = new KeySet(this));
+        }
+
+        public Collection<V> values() {
+            Collection<V> vs = valuesView;
+            return (vs != null) ? vs : (valuesView = new Values(this));
+        }
+
+        public Set<Map.Entry<K,V>> entrySet() {
+            Set<Map.Entry<K,V>> es = entrySetView;
+            return (es != null) ? es : (entrySetView = new EntrySet(this));
+        }
+
+        public NavigableSet<K> descendingKeySet() {
+            return descendingMap().navigableKeySet();
+        }
+
+        Iterator<K> keyIterator() {
+            return new SubMapKeyIterator();
+        }
+
+        Iterator<V> valueIterator() {
+            return new SubMapValueIterator();
+        }
+
+        Iterator<Map.Entry<K,V>> entryIterator() {
+            return new SubMapEntryIterator();
+        }
+
+        /**
+         * Variant of main Iter class to traverse through submaps.
+         */
+        abstract class SubMapIter<T> implements Iterator<T> {
+            /** the last node returned by next() */
+            Node<K,V> lastReturned;
+            /** the next node to return from next(); */
+            Node<K,V> next;
+            /** Cache of next value field to maintain weak consistency */
+            V nextValue;
+
+            SubMapIter() {
+                for (;;) {
+                    next = isDescending ? hiNode() : loNode();
+                    if (next == null)
+                        break;
+		    Object x = next.value;
+                    if (x != null && x != next) {
+			if (! inBounds(next.key))
+                            next = null;
+			else
+			    nextValue = (V) x;
+                        break;
+                    }
+                }
+            }
+
+            public final boolean hasNext() {
+                return next != null;
+            }
+
+            final void advance() {
+                if ((lastReturned = next) == null)
+                    throw new NoSuchElementException();
+                if (isDescending)
+                    descend();
+                else
+                    ascend();
+            }
+
+            private void ascend() {
+                for (;;) {
+                    next = next.next;
+                    if (next == null)
+                        break;
+		    Object x = next.value;
+                    if (x != null && x != next) {
+                        if (tooHigh(next.key))
+                            next = null;
+                        else
+			    nextValue = (V) x;
+                        break;
+                    }
+                }
+            }
+
+            private void descend() {
+                for (;;) {
+                    next = m.findNear(lastReturned.key, LT);
+                    if (next == null)
+                        break;
+		    Object x = next.value;
+                    if (x != null && x != next) {
+                        if (tooLow(next.key))
+                            next = null;
+			else
+                            nextValue = (V) x;
+                        break;
+                    }
+                }
+            }
+
+            public void remove() {
+                Node<K,V> l = lastReturned;
+                if (l == null)
+                    throw new IllegalStateException();
+                m.remove(l.key);
+		lastReturned = null;
+            }
+
+        }
+
+        final class SubMapValueIterator extends SubMapIter<V> {
+            public V next() {
+                V v = nextValue;
+                advance();
+                return v;
+            }
+        }
+
+        final class SubMapKeyIterator extends SubMapIter<K> {
+            public K next() {
+                Node<K,V> n = next;
+                advance();
+                return n.key;
+            }
+        }
+
+        final class SubMapEntryIterator extends SubMapIter<Map.Entry<K,V>> {
+            public Map.Entry<K,V> next() {
+                Node<K,V> n = next;
+                V v = nextValue;
+                advance();
+                return new AbstractMap.SimpleImmutableEntry<K,V>(n.key, v);
+            }
+        }
+    }
+}
diff --git a/external/jsr166/java/util/concurrent/ConcurrentSkipListSet.java b/external/jsr166/java/util/concurrent/ConcurrentSkipListSet.java
new file mode 100644
index 000000000..7da50d593
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/ConcurrentSkipListSet.java
@@ -0,0 +1,456 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+import java.util.*;
+import sun.misc.Unsafe;
+
+/**
+ * A scalable concurrent {@link NavigableSet} implementation based on
+ * a {@link ConcurrentSkipListMap}.  The elements of the set are kept
+ * sorted according to their {@linkplain Comparable natural ordering},
+ * or by a {@link Comparator} provided at set creation time, depending
+ * on which constructor is used.
+ *
+ * <p>This implementation provides expected average <i>log(n)</i> time
+ * cost for the <tt>contains</tt>, <tt>add</tt>, and <tt>remove</tt>
+ * operations and their variants.  Insertion, removal, and access
+ * operations safely execute concurrently by multiple threads.
+ * Iterators are <i>weakly consistent</i>, returning elements
+ * reflecting the state of the set at some point at or since the
+ * creation of the iterator.  They do <em>not</em> throw {@link
+ * ConcurrentModificationException}, and may proceed concurrently with
+ * other operations.  Ascending ordered views and their iterators are
+ * faster than descending ones.
+ *
+ * <p>Beware that, unlike in most collections, the <tt>size</tt>
+ * method is <em>not</em> a constant-time operation. Because of the
+ * asynchronous nature of these sets, determining the current number
+ * of elements requires a traversal of the elements. Additionally, the
+ * bulk operations <tt>addAll</tt>, <tt>removeAll</tt>,
+ * <tt>retainAll</tt>, and <tt>containsAll</tt> are <em>not</em>
+ * guaranteed to be performed atomically. For example, an iterator
+ * operating concurrently with an <tt>addAll</tt> operation might view
+ * only some of the added elements.
+ *
+ * <p>This class and its iterators implement all of the
+ * <em>optional</em> methods of the {@link Set} and {@link Iterator}
+ * interfaces. Like most other concurrent collection implementations,
+ * this class does not permit the use of <tt>null</tt> elements,
+ * because <tt>null</tt> arguments and return values cannot be reliably
+ * distinguished from the absence of elements.
+ *
+ * <p>This class is a member of the
+ * <a href="{@docRoot}/../technotes/guides/collections/index.html">
+ * Java Collections Framework</a>.
+ *
+ * @author Doug Lea
+ * @param <E> the type of elements maintained by this set
+ * @since 1.6
+ */
+public class ConcurrentSkipListSet<E>
+    extends AbstractSet<E>
+    implements NavigableSet<E>, Cloneable, java.io.Serializable {
+
+    private static final long serialVersionUID = -2479143111061671589L;
+
+    /**
+     * The underlying map. Uses Boolean.TRUE as value for each
+     * element.  This field is declared final for the sake of thread
+     * safety, which entails some ugliness in clone()
+     */
+    private final ConcurrentNavigableMap<E,Object> m;
+
+    /**
+     * Constructs a new, empty set that orders its elements according to
+     * their {@linkplain Comparable natural ordering}.
+     */
+    public ConcurrentSkipListSet() {
+        m = new ConcurrentSkipListMap<E,Object>();
+    }
+
+    /**
+     * Constructs a new, empty set that orders its elements according to
+     * the specified comparator.
+     *
+     * @param comparator the comparator that will be used to order this set.
+     *        If <tt>null</tt>, the {@linkplain Comparable natural
+     *        ordering} of the elements will be used.
+     */
+    public ConcurrentSkipListSet(Comparator<? super E> comparator) {
+        m = new ConcurrentSkipListMap<E,Object>(comparator);
+    }
+
+    /**
+     * Constructs a new set containing the elements in the specified
+     * collection, that orders its elements according to their
+     * {@linkplain Comparable natural ordering}.
+     *
+     * @param c The elements that will comprise the new set
+     * @throws ClassCastException if the elements in <tt>c</tt> are
+     *         not {@link Comparable}, or are not mutually comparable
+     * @throws NullPointerException if the specified collection or any
+     *         of its elements are null
+     */
+    public ConcurrentSkipListSet(Collection<? extends E> c) {
+        m = new ConcurrentSkipListMap<E,Object>();
+        addAll(c);
+    }
+
+    /**
+     * Constructs a new set containing the same elements and using the
+     * same ordering as the specified sorted set.
+     *
+     * @param s sorted set whose elements will comprise the new set
+     * @throws NullPointerException if the specified sorted set or any
+     *         of its elements are null
+     */
+    public ConcurrentSkipListSet(SortedSet<E> s) {
+        m = new ConcurrentSkipListMap<E,Object>(s.comparator());
+        addAll(s);
+    }
+
+    /**
+     * For use by submaps
+     */
+    ConcurrentSkipListSet(ConcurrentNavigableMap<E,Object> m) {
+        this.m = m;
+    }
+
+    /**
+     * Returns a shallow copy of this <tt>ConcurrentSkipListSet</tt>
+     * instance. (The elements themselves are not cloned.)
+     *
+     * @return a shallow copy of this set
+     */
+    public ConcurrentSkipListSet<E> clone() {
+        ConcurrentSkipListSet<E> clone = null;
+	try {
+	    clone = (ConcurrentSkipListSet<E>) super.clone();
+            clone.setMap(new ConcurrentSkipListMap(m));
+	} catch (CloneNotSupportedException e) {
+	    throw new InternalError();
+	}
+
+        return clone;
+    }
+
+    /* ---------------- Set operations -------------- */
+
+    /**
+     * Returns the number of elements in this set.  If this set
+     * contains more than <tt>Integer.MAX_VALUE</tt> elements, it
+     * returns <tt>Integer.MAX_VALUE</tt>.
+     *
+     * <p>Beware that, unlike in most collections, this method is
+     * <em>NOT</em> a constant-time operation. Because of the
+     * asynchronous nature of these sets, determining the current
+     * number of elements requires traversing them all to count them.
+     * Additionally, it is possible for the size to change during
+     * execution of this method, in which case the returned result
+     * will be inaccurate. Thus, this method is typically not very
+     * useful in concurrent applications.
+     *
+     * @return the number of elements in this set
+     */
+    public int size() {
+	return m.size();
+    }
+
+    /**
+     * Returns <tt>true</tt> if this set contains no elements.
+     * @return <tt>true</tt> if this set contains no elements
+     */
+    public boolean isEmpty() {
+	return m.isEmpty();
+    }
+
+    /**
+     * Returns <tt>true</tt> if this set contains the specified element.
+     * More formally, returns <tt>true</tt> if and only if this set
+     * contains an element <tt>e</tt> such that <tt>o.equals(e)</tt>.
+     *
+     * @param o object to be checked for containment in this set
+     * @return <tt>true</tt> if this set contains the specified element
+     * @throws ClassCastException if the specified element cannot be
+     *         compared with the elements currently in this set
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean contains(Object o) {
+	return m.containsKey(o);
+    }
+
+    /**
+     * Adds the specified element to this set if it is not already present.
+     * More formally, adds the specified element <tt>e</tt> to this set if
+     * the set contains no element <tt>e2</tt> such that <tt>e.equals(e2)</tt>.
+     * If this set already contains the element, the call leaves the set
+     * unchanged and returns <tt>false</tt>.
+     *
+     * @param e element to be added to this set
+     * @return <tt>true</tt> if this set did not already contain the
+     *         specified element
+     * @throws ClassCastException if <tt>e</tt> cannot be compared
+     *         with the elements currently in this set
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean add(E e) {
+	return m.putIfAbsent(e, Boolean.TRUE) == null;
+    }
+
+    /**
+     * Removes the specified element from this set if it is present.
+     * More formally, removes an element <tt>e</tt> such that
+     * <tt>o.equals(e)</tt>, if this set contains such an element.
+     * Returns <tt>true</tt> if this set contained the element (or
+     * equivalently, if this set changed as a result of the call).
+     * (This set will not contain the element once the call returns.)
+     *
+     * @param o object to be removed from this set, if present
+     * @return <tt>true</tt> if this set contained the specified element
+     * @throws ClassCastException if <tt>o</tt> cannot be compared
+     *         with the elements currently in this set
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean remove(Object o) {
+	return m.remove(o, Boolean.TRUE);
+    }
+
+    /**
+     * Removes all of the elements from this set.
+     */
+    public void clear() {
+	m.clear();
+    }
+
+    /**
+     * Returns an iterator over the elements in this set in ascending order.
+     *
+     * @return an iterator over the elements in this set in ascending order
+     */
+    public Iterator<E> iterator() {
+        return m.navigableKeySet().iterator();
+    }
+
+    /**
+     * Returns an iterator over the elements in this set in descending order.
+     *
+     * @return an iterator over the elements in this set in descending order
+     */
+    public Iterator<E> descendingIterator() {
+	return m.descendingKeySet().iterator();
+    }
+
+
+    /* ---------------- AbstractSet Overrides -------------- */
+
+    /**
+     * Compares the specified object with this set for equality.  Returns
+     * <tt>true</tt> if the specified object is also a set, the two sets
+     * have the same size, and every member of the specified set is
+     * contained in this set (or equivalently, every member of this set is
+     * contained in the specified set).  This definition ensures that the
+     * equals method works properly across different implementations of the
+     * set interface.
+     *
+     * @param o the object to be compared for equality with this set
+     * @return <tt>true</tt> if the specified object is equal to this set
+     */
+    public boolean equals(Object o) {
+        // Override AbstractSet version to avoid calling size()
+	if (o == this)
+	    return true;
+	if (!(o instanceof Set))
+	    return false;
+	Collection<?> c = (Collection<?>) o;
+        try {
+            return containsAll(c) && c.containsAll(this);
+        } catch (ClassCastException unused)   {
+            return false;
+        } catch (NullPointerException unused) {
+            return false;
+        }
+    }
+
+    /**
+     * Removes from this set all of its elements that are contained in
+     * the specified collection.  If the specified collection is also
+     * a set, this operation effectively modifies this set so that its
+     * value is the <i>asymmetric set difference</i> of the two sets.
+     *
+     * @param  c collection containing elements to be removed from this set
+     * @return <tt>true</tt> if this set changed as a result of the call
+     * @throws ClassCastException if the types of one or more elements in this
+     *         set are incompatible with the specified collection
+     * @throws NullPointerException if the specified collection or any
+     *         of its elements are null
+     */
+    public boolean removeAll(Collection<?> c) {
+        // Override AbstractSet version to avoid unnecessary call to size()
+        boolean modified = false;
+        for (Iterator<?> i = c.iterator(); i.hasNext(); )
+            if (remove(i.next()))
+                modified = true;
+        return modified;
+    }
+
+    /* ---------------- Relational operations -------------- */
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if the specified element is null
+     */
+    public E lower(E e) {
+        return m.lowerKey(e);
+    }
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if the specified element is null
+     */
+    public E floor(E e) {
+        return m.floorKey(e);
+    }
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if the specified element is null
+     */
+    public E ceiling(E e) {
+        return m.ceilingKey(e);
+    }
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if the specified element is null
+     */
+    public E higher(E e) {
+        return m.higherKey(e);
+    }
+
+    public E pollFirst() {
+        Map.Entry<E,Object> e = m.pollFirstEntry();
+        return e == null? null : e.getKey();
+    }
+
+    public E pollLast() {
+        Map.Entry<E,Object> e = m.pollLastEntry();
+        return e == null? null : e.getKey();
+    }
+
+
+    /* ---------------- SortedSet operations -------------- */
+
+
+    public Comparator<? super E> comparator() {
+        return m.comparator();
+    }
+
+    /**
+     * @throws NoSuchElementException {@inheritDoc}
+     */
+    public E first() {
+        return m.firstKey();
+    }
+
+    /**
+     * @throws NoSuchElementException {@inheritDoc}
+     */
+    public E last() {
+        return m.lastKey();
+    }
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if {@code fromElement} or
+     *         {@code toElement} is null
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    public NavigableSet<E> subSet(E fromElement,
+                                  boolean fromInclusive,
+                                  E toElement,
+                                  boolean toInclusive) {
+	return new ConcurrentSkipListSet<E>
+            (m.subMap(fromElement, fromInclusive,
+                      toElement,   toInclusive));
+    }
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if {@code toElement} is null
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    public NavigableSet<E> headSet(E toElement, boolean inclusive) {
+	return new ConcurrentSkipListSet<E>(m.headMap(toElement, inclusive));
+    }
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if {@code fromElement} is null
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
+	return new ConcurrentSkipListSet<E>(m.tailMap(fromElement, inclusive));
+    }
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if {@code fromElement} or
+     *         {@code toElement} is null
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    public NavigableSet<E> subSet(E fromElement, E toElement) {
+	return subSet(fromElement, true, toElement, false);
+    }
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if {@code toElement} is null
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    public NavigableSet<E> headSet(E toElement) {
+	return headSet(toElement, false);
+    }
+
+    /**
+     * @throws ClassCastException {@inheritDoc}
+     * @throws NullPointerException if {@code fromElement} is null
+     * @throws IllegalArgumentException {@inheritDoc}
+     */
+    public NavigableSet<E> tailSet(E fromElement) {
+	return tailSet(fromElement, true);
+    }
+
+    /**
+     * Returns a reverse order view of the elements contained in this set.
+     * The descending set is backed by this set, so changes to the set are
+     * reflected in the descending set, and vice-versa.
+     *
+     * <p>The returned set has an ordering equivalent to
+     * <tt>{@link Collections#reverseOrder(Comparator) Collections.reverseOrder}(comparator())</tt>.
+     * The expression {@code s.descendingSet().descendingSet()} returns a
+     * view of {@code s} essentially equivalent to {@code s}.
+     *
+     * @return a reverse order view of this set
+     */
+    public NavigableSet<E> descendingSet() {
+	return new ConcurrentSkipListSet(m.descendingMap());
+    }
+
+    // Support for resetting map in clone
+    private static final Unsafe unsafe = Unsafe.getUnsafe();
+    private static final long mapOffset;
+    static {
+        try {
+            mapOffset = unsafe.objectFieldOffset
+                (ConcurrentSkipListSet.class.getDeclaredField("m"));
+        } catch (Exception ex) { throw new Error(ex); }
+    }
+    private void setMap(ConcurrentNavigableMap<E,Object> map) {
+        unsafe.putObjectVolatile(this, mapOffset, map);
+    }
+
+}
diff --git a/external/jsr166/java/util/concurrent/CopyOnWriteArraySet.java b/external/jsr166/java/util/concurrent/CopyOnWriteArraySet.java
new file mode 100644
index 000000000..39c0e5868
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/CopyOnWriteArraySet.java
@@ -0,0 +1,364 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain. Use, modify, and
+ * redistribute this code in any way without acknowledgement.
+ */
+
+package java.util.concurrent;
+import java.util.*;
+
+/**
+ * A {@link java.util.Set} that uses an internal {@link CopyOnWriteArrayList}
+ * for all of its operations.  Thus, it shares the same basic properties:
+ * <ul>
+ *  <li>It is best suited for applications in which set sizes generally
+ *       stay small, read-only operations
+ *       vastly outnumber mutative operations, and you need
+ *       to prevent interference among threads during traversal.
+ *  <li>It is thread-safe.
+ *  <li>Mutative operations (<tt>add</tt>, <tt>set</tt>, <tt>remove</tt>, etc.)
+ *      are expensive since they usually entail copying the entire underlying
+ *      array.
+ *  <li>Iterators do not support the mutative <tt>remove</tt> operation.
+ *  <li>Traversal via iterators is fast and cannot encounter
+ *      interference from other threads. Iterators rely on
+ *      unchanging snapshots of the array at the time the iterators were
+ *      constructed.
+ * </ul>
+ *
+ * <p> <b>Sample Usage.</b> The following code sketch uses a
+ * copy-on-write set to maintain a set of Handler objects that
+ * perform some action upon state updates.
+ *
+ * <pre>
+ * class Handler { void handle(); ... }
+ *
+ * class X {
+ *    private final CopyOnWriteArraySet&lt;Handler&gt; handlers
+ *       = new CopyOnWriteArraySet&lt;Handler&gt;();
+ *    public void addHandler(Handler h) { handlers.add(h); }
+ *
+ *    private long internalState;
+ *    private synchronized void changeState() { internalState = ...; }
+ *
+ *    public void update() {
+ *       changeState();
+ *       for (Handler handler : handlers)
+ *          handler.handle();
+ *    }
+ * }
+ * </pre>
+ *
+ * <p>This class is a member of the
+ * <a href="{@docRoot}/../technotes/guides/collections/index.html">
+ * Java Collections Framework</a>.
+ *
+ * @see CopyOnWriteArrayList
+ * @since 1.5
+ * @author Doug Lea
+ * @param <E> the type of elements held in this collection
+ */
+public class CopyOnWriteArraySet<E> extends AbstractSet<E>
+        implements java.io.Serializable {
+    private static final long serialVersionUID = 5457747651344034263L;
+
+    private final CopyOnWriteArrayList<E> al;
+
+    /**
+     * Creates an empty set.
+     */
+    public CopyOnWriteArraySet() {
+        al = new CopyOnWriteArrayList<E>();
+    }
+
+    /**
+     * Creates a set containing all of the elements of the specified
+     * collection.
+     *
+     * @param c the collection of elements to initially contain
+     * @throws NullPointerException if the specified collection is null
+     */
+    public CopyOnWriteArraySet(Collection<? extends E> c) {
+        al = new CopyOnWriteArrayList<E>();
+        al.addAllAbsent(c);
+    }
+
+    /**
+     * Returns the number of elements in this set.
+     *
+     * @return the number of elements in this set
+     */
+    public int size() {
+	return al.size();
+    }
+
+    /**
+     * Returns <tt>true</tt> if this set contains no elements.
+     *
+     * @return <tt>true</tt> if this set contains no elements
+     */
+    public boolean isEmpty() {
+	return al.isEmpty();
+    }
+
+    /**
+     * Returns <tt>true</tt> if this set contains the specified element.
+     * More formally, returns <tt>true</tt> if and only if this set
+     * contains an element <tt>e</tt> such that
+     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
+     *
+     * @param o element whose presence in this set is to be tested
+     * @return <tt>true</tt> if this set contains the specified element
+     */
+    public boolean contains(Object o) {
+	return al.contains(o);
+    }
+
+    /**
+     * Returns an array containing all of the elements in this set.
+     * If this set makes any guarantees as to what order its elements
+     * are returned by its iterator, this method must return the
+     * elements in the same order.
+     *
+     * <p>The returned array will be "safe" in that no references to it
+     * are maintained by this set.  (In other words, this method must
+     * allocate a new array even if this set is backed by an array).
+     * The caller is thus free to modify the returned array.
+     *
+     * <p>This method acts as bridge between array-based and collection-based
+     * APIs.
+     *
+     * @return an array containing all the elements in this set
+     */
+    public Object[] toArray() {
+	return al.toArray();
+    }
+
+    /**
+     * Returns an array containing all of the elements in this set; the
+     * runtime type of the returned array is that of the specified array.
+     * If the set fits in the specified array, it is returned therein.
+     * Otherwise, a new array is allocated with the runtime type of the
+     * specified array and the size of this set.
+     *
+     * <p>If this set fits in the specified array with room to spare
+     * (i.e., the array has more elements than this set), the element in
+     * the array immediately following the end of the set is set to
+     * <tt>null</tt>.  (This is useful in determining the length of this
+     * set <i>only</i> if the caller knows that this set does not contain
+     * any null elements.)
+     *
+     * <p>If this set makes any guarantees as to what order its elements
+     * are returned by its iterator, this method must return the elements
+     * in the same order.
+     *
+     * <p>Like the {@link #toArray()} method, this method acts as bridge between
+     * array-based and collection-based APIs.  Further, this method allows
+     * precise control over the runtime type of the output array, and may,
+     * under certain circumstances, be used to save allocation costs.
+     *
+     * <p>Suppose <tt>x</tt> is a set known to contain only strings.
+     * The following code can be used to dump the set into a newly allocated
+     * array of <tt>String</tt>:
+     *
+     * <pre>
+     *     String[] y = x.toArray(new String[0]);</pre>
+     *
+     * Note that <tt>toArray(new Object[0])</tt> is identical in function to
+     * <tt>toArray()</tt>.
+     *
+     * @param a the array into which the elements of this set are to be
+     *        stored, if it is big enough; otherwise, a new array of the same
+     *        runtime type is allocated for this purpose.
+     * @return an array containing all the elements in this set
+     * @throws ArrayStoreException if the runtime type of the specified array
+     *         is not a supertype of the runtime type of every element in this
+     *         set
+     * @throws NullPointerException if the specified array is null
+     */
+    public <T> T[] toArray(T[] a) {
+	return al.toArray(a);
+    }
+
+    /**
+     * Removes all of the elements from this set.
+     * The set will be empty after this call returns.
+     */
+    public void clear() {
+        al.clear();
+    }
+
+    /**
+     * Removes the specified element from this set if it is present.
+     * More formally, removes an element <tt>e</tt> such that
+     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>,
+     * if this set contains such an element.  Returns <tt>true</tt> if
+     * this set contained the element (or equivalently, if this set
+     * changed as a result of the call).  (This set will not contain the
+     * element once the call returns.)
+     *
+     * @param o object to be removed from this set, if present
+     * @return <tt>true</tt> if this set contained the specified element
+     */
+    public boolean remove(Object o) {
+	return al.remove(o);
+    }
+
+    /**
+     * Adds the specified element to this set if it is not already present.
+     * More formally, adds the specified element <tt>e</tt> to this set if
+     * the set contains no element <tt>e2</tt> such that
+     * <tt>(e==null&nbsp;?&nbsp;e2==null&nbsp;:&nbsp;e.equals(e2))</tt>.
+     * If this set already contains the element, the call leaves the set
+     * unchanged and returns <tt>false</tt>.
+     *
+     * @param e element to be added to this set
+     * @return <tt>true</tt> if this set did not already contain the specified
+     *         element
+     */
+    public boolean add(E e) {
+	return al.addIfAbsent(e);
+    }
+
+    /**
+     * Returns <tt>true</tt> if this set contains all of the elements of the
+     * specified collection.  If the specified collection is also a set, this
+     * method returns <tt>true</tt> if it is a <i>subset</i> of this set.
+     *
+     * @param  c collection to be checked for containment in this set
+     * @return <tt>true</tt> if this set contains all of the elements of the
+     * 	       specified collection
+     * @throws NullPointerException if the specified collection is null
+     * @see #contains(Object)
+     */
+    public boolean containsAll(Collection<?> c) {
+	return al.containsAll(c);
+    }
+
+    /**
+     * Adds all of the elements in the specified collection to this set if
+     * they're not already present.  If the specified collection is also a
+     * set, the <tt>addAll</tt> operation effectively modifies this set so
+     * that its value is the <i>union</i> of the two sets.  The behavior of
+     * this operation is undefined if the specified collection is modified
+     * while the operation is in progress.
+     *
+     * @param  c collection containing elements to be added to this set
+     * @return <tt>true</tt> if this set changed as a result of the call
+     * @throws NullPointerException if the specified collection is null
+     * @see #add(Object)
+     */
+    public boolean addAll(Collection<? extends E> c) {
+	return al.addAllAbsent(c) > 0;
+    }
+
+    /**
+     * Removes from this set all of its elements that are contained in the
+     * specified collection.  If the specified collection is also a set,
+     * this operation effectively modifies this set so that its value is the
+     * <i>asymmetric set difference</i> of the two sets.
+     *
+     * @param  c collection containing elements to be removed from this set
+     * @return <tt>true</tt> if this set changed as a result of the call
+     * @throws ClassCastException if the class of an element of this set
+     *         is incompatible with the specified collection (optional)
+     * @throws NullPointerException if this set contains a null element and the
+     *         specified collection does not permit null elements (optional),
+     *         or if the specified collection is null
+     * @see #remove(Object)
+     */
+    public boolean removeAll(Collection<?> c) {
+	return al.removeAll(c);
+    }
+
+    /**
+     * Retains only the elements in this set that are contained in the
+     * specified collection.  In other words, removes from this set all of
+     * its elements that are not contained in the specified collection.  If
+     * the specified collection is also a set, this operation effectively
+     * modifies this set so that its value is the <i>intersection</i> of the
+     * two sets.
+     *
+     * @param  c collection containing elements to be retained in this set
+     * @return <tt>true</tt> if this set changed as a result of the call
+     * @throws ClassCastException if the class of an element of this set
+     *         is incompatible with the specified collection (optional)
+     * @throws NullPointerException if this set contains a null element and the
+     *         specified collection does not permit null elements (optional),
+     *         or if the specified collection is null
+     * @see #remove(Object)
+     */
+    public boolean retainAll(Collection<?> c) {
+	return al.retainAll(c);
+    }
+
+    /**
+     * Returns an iterator over the elements contained in this set
+     * in the order in which these elements were added.
+     *
+     * <p>The returned iterator provides a snapshot of the state of the set
+     * when the iterator was constructed. No synchronization is needed while
+     * traversing the iterator. The iterator does <em>NOT</em> support the
+     * <tt>remove</tt> method.
+     *
+     * @return an iterator over the elements in this set
+     */
+    public Iterator<E> iterator() {
+	return al.iterator();
+    }
+
+    /**
+     * Compares the specified object with this set for equality.
+     * Returns {@code true} if the specified object is the same object
+     * as this object, or if it is also a {@link Set} and the elements
+     * returned by an {@linkplain List#iterator() iterator} over the
+     * specified set are the same as the elements returned by an
+     * iterator over this set.  More formally, the two iterators are
+     * considered to return the same elements if they return the same
+     * number of elements and for every element {@code e1} returned by
+     * the iterator over the specified set, there is an element
+     * {@code e2} returned by the iterator over this set such that
+     * {@code (e1==null ? e2==null : e1.equals(e2))}.
+     *
+     * @param o object to be compared for equality with this set
+     * @return {@code true} if the specified object is equal to this set
+     */
+    public boolean equals(Object o) {
+        if (o == this)
+            return true;
+        if (!(o instanceof Set))
+            return false;
+        Set<?> set = (Set<?>)(o);
+	Iterator<?> it = set.iterator();
+
+        // Uses O(n^2) algorithm that is only appropriate
+        // for small sets, which CopyOnWriteArraySets should be.
+
+        //  Use a single snapshot of underlying array
+	Object[] elements = al.getArray();
+	int len = elements.length;
+        // Mark matched elements to avoid re-checking
+        boolean[] matched = new boolean[len];
+        int k = 0;
+        outer: while (it.hasNext()) {
+            if (++k > len)
+                return false;
+            Object x = it.next();
+            for (int i = 0; i < len; ++i) {
+                if (!matched[i] && eq(x, elements[i])) {
+                    matched[i] = true;
+		    continue outer;
+                }
+            }
+	    return false;
+        }
+        return k == len;
+    }
+
+    /**
+     * Test for equality, coping with nulls.
+     */
+    private static boolean eq(Object o1, Object o2) {
+        return (o1 == null ? o2 == null : o1.equals(o2));
+    }
+}
diff --git a/external/jsr166/java/util/concurrent/CountDownLatch.java b/external/jsr166/java/util/concurrent/CountDownLatch.java
new file mode 100644
index 000000000..016c1a7a5
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/CountDownLatch.java
@@ -0,0 +1,290 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+import java.util.concurrent.locks.*;
+import java.util.concurrent.atomic.*;
+
+/**
+ * A synchronization aid that allows one or more threads to wait until
+ * a set of operations being performed in other threads completes.
+ *
+ * <p>A {@code CountDownLatch} is initialized with a given <em>count</em>.
+ * The {@link #await await} methods block until the current count reaches
+ * zero due to invocations of the {@link #countDown} method, after which
+ * all waiting threads are released and any subsequent invocations of
+ * {@link #await await} return immediately.  This is a one-shot phenomenon
+ * -- the count cannot be reset.  If you need a version that resets the
+ * count, consider using a {@link CyclicBarrier}.
+ *
+ * <p>A {@code CountDownLatch} is a versatile synchronization tool
+ * and can be used for a number of purposes.  A
+ * {@code CountDownLatch} initialized with a count of one serves as a
+ * simple on/off latch, or gate: all threads invoking {@link #await await}
+ * wait at the gate until it is opened by a thread invoking {@link
+ * #countDown}.  A {@code CountDownLatch} initialized to <em>N</em>
+ * can be used to make one thread wait until <em>N</em> threads have
+ * completed some action, or some action has been completed N times.
+ *
+ * <p>A useful property of a {@code CountDownLatch} is that it
+ * doesn't require that threads calling {@code countDown} wait for
+ * the count to reach zero before proceeding, it simply prevents any
+ * thread from proceeding past an {@link #await await} until all
+ * threads could pass.
+ *
+ * <p><b>Sample usage:</b> Here is a pair of classes in which a group
+ * of worker threads use two countdown latches:
+ * <ul>
+ * <li>The first is a start signal that prevents any worker from proceeding
+ * until the driver is ready for them to proceed;
+ * <li>The second is a completion signal that allows the driver to wait
+ * until all workers have completed.
+ * </ul>
+ *
+ * <pre>
+ * class Driver { // ...
+ *   void main() throws InterruptedException {
+ *     CountDownLatch startSignal = new CountDownLatch(1);
+ *     CountDownLatch doneSignal = new CountDownLatch(N);
+ *
+ *     for (int i = 0; i < N; ++i) // create and start threads
+ *       new Thread(new Worker(startSignal, doneSignal)).start();
+ *
+ *     doSomethingElse();            // don't let run yet
+ *     startSignal.countDown();      // let all threads proceed
+ *     doSomethingElse();
+ *     doneSignal.await();           // wait for all to finish
+ *   }
+ * }
+ *
+ * class Worker implements Runnable {
+ *   private final CountDownLatch startSignal;
+ *   private final CountDownLatch doneSignal;
+ *   Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
+ *      this.startSignal = startSignal;
+ *      this.doneSignal = doneSignal;
+ *   }
+ *   public void run() {
+ *      try {
+ *        startSignal.await();
+ *        doWork();
+ *        doneSignal.countDown();
+ *      } catch (InterruptedException ex) {} // return;
+ *   }
+ *
+ *   void doWork() { ... }
+ * }
+ *
+ * </pre>
+ *
+ * <p>Another typical usage would be to divide a problem into N parts,
+ * describe each part with a Runnable that executes that portion and
+ * counts down on the latch, and queue all the Runnables to an
+ * Executor.  When all sub-parts are complete, the coordinating thread
+ * will be able to pass through await. (When threads must repeatedly
+ * count down in this way, instead use a {@link CyclicBarrier}.)
+ *
+ * <pre>
+ * class Driver2 { // ...
+ *   void main() throws InterruptedException {
+ *     CountDownLatch doneSignal = new CountDownLatch(N);
+ *     Executor e = ...
+ *
+ *     for (int i = 0; i < N; ++i) // create and start threads
+ *       e.execute(new WorkerRunnable(doneSignal, i));
+ *
+ *     doneSignal.await();           // wait for all to finish
+ *   }
+ * }
+ *
+ * class WorkerRunnable implements Runnable {
+ *   private final CountDownLatch doneSignal;
+ *   private final int i;
+ *   WorkerRunnable(CountDownLatch doneSignal, int i) {
+ *      this.doneSignal = doneSignal;
+ *      this.i = i;
+ *   }
+ *   public void run() {
+ *      try {
+ *        doWork(i);
+ *        doneSignal.countDown();
+ *      } catch (InterruptedException ex) {} // return;
+ *   }
+ *
+ *   void doWork() { ... }
+ * }
+ *
+ * </pre>
+ *
+ * <p>Memory consistency effects: Actions in a thread prior to calling
+ * {@code countDown()}
+ * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
+ * actions following a successful return from a corresponding
+ * {@code await()} in another thread.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ */
+public class CountDownLatch {
+    /**
+     * Synchronization control For CountDownLatch.
+     * Uses AQS state to represent count.
+     */
+    private static final class Sync extends AbstractQueuedSynchronizer {
+        private static final long serialVersionUID = 4982264981922014374L;
+
+        Sync(int count) {
+            setState(count);
+        }
+
+        int getCount() {
+            return getState();
+        }
+
+        public int tryAcquireShared(int acquires) {
+            return getState() == 0? 1 : -1;
+        }
+
+        public boolean tryReleaseShared(int releases) {
+            // Decrement count; signal when transition to zero
+            for (;;) {
+                int c = getState();
+                if (c == 0)
+                    return false;
+                int nextc = c-1;
+                if (compareAndSetState(c, nextc))
+                    return nextc == 0;
+            }
+        }
+    }
+
+    private final Sync sync;
+
+    /**
+     * Constructs a {@code CountDownLatch} initialized with the given count.
+     *
+     * @param count the number of times {@link #countDown} must be invoked
+     *        before threads can pass through {@link #await}
+     * @throws IllegalArgumentException if {@code count} is negative
+     */
+    public CountDownLatch(int count) {
+        if (count < 0) throw new IllegalArgumentException("count < 0");
+        this.sync = new Sync(count);
+    }
+
+    /**
+     * Causes the current thread to wait until the latch has counted down to
+     * zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
+     *
+     * <p>If the current count is zero then this method returns immediately.
+     *
+     * <p>If the current count is greater than zero then the current
+     * thread becomes disabled for thread scheduling purposes and lies
+     * dormant until one of two things happen:
+     * <ul>
+     * <li>The count reaches zero due to invocations of the
+     * {@link #countDown} method; or
+     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
+     * the current thread.
+     * </ul>
+     *
+     * <p>If the current thread:
+     * <ul>
+     * <li>has its interrupted status set on entry to this method; or
+     * <li>is {@linkplain Thread#interrupt interrupted} while waiting,
+     * </ul>
+     * then {@link InterruptedException} is thrown and the current thread's
+     * interrupted status is cleared.
+     *
+     * @throws InterruptedException if the current thread is interrupted
+     *         while waiting
+     */
+    public void await() throws InterruptedException {
+        sync.acquireSharedInterruptibly(1);
+    }
+
+    /**
+     * Causes the current thread to wait until the latch has counted down to
+     * zero, unless the thread is {@linkplain Thread#interrupt interrupted},
+     * or the specified waiting time elapses.
+     *
+     * <p>If the current count is zero then this method returns immediately
+     * with the value {@code true}.
+     *
+     * <p>If the current count is greater than zero then the current
+     * thread becomes disabled for thread scheduling purposes and lies
+     * dormant until one of three things happen:
+     * <ul>
+     * <li>The count reaches zero due to invocations of the
+     * {@link #countDown} method; or
+     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
+     * the current thread; or
+     * <li>The specified waiting time elapses.
+     * </ul>
+     *
+     * <p>If the count reaches zero then the method returns with the
+     * value {@code true}.
+     *
+     * <p>If the current thread:
+     * <ul>
+     * <li>has its interrupted status set on entry to this method; or
+     * <li>is {@linkplain Thread#interrupt interrupted} while waiting,
+     * </ul>
+     * then {@link InterruptedException} is thrown and the current thread's
+     * interrupted status is cleared.
+     *
+     * <p>If the specified waiting time elapses then the value {@code false}
+     * is returned.  If the time is less than or equal to zero, the method
+     * will not wait at all.
+     *
+     * @param timeout the maximum time to wait
+     * @param unit the time unit of the {@code timeout} argument
+     * @return {@code true} if the count reached zero and {@code false}
+     *         if the waiting time elapsed before the count reached zero
+     * @throws InterruptedException if the current thread is interrupted
+     *         while waiting
+     */
+    public boolean await(long timeout, TimeUnit unit)
+        throws InterruptedException {
+        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
+    }
+
+    /**
+     * Decrements the count of the latch, releasing all waiting threads if
+     * the count reaches zero.
+     *
+     * <p>If the current count is greater than zero then it is decremented.
+     * If the new count is zero then all waiting threads are re-enabled for
+     * thread scheduling purposes.
+     *
+     * <p>If the current count equals zero then nothing happens.
+     */
+    public void countDown() {
+        sync.releaseShared(1);
+    }
+
+    /**
+     * Returns the current count.
+     *
+     * <p>This method is typically used for debugging and testing purposes.
+     *
+     * @return the current count
+     */
+    public long getCount() {
+        return sync.getCount();
+    }
+
+    /**
+     * Returns a string identifying this latch, as well as its state.
+     * The state, in brackets, includes the String {@code "Count ="}
+     * followed by the current count.
+     *
+     * @return a string identifying this latch, as well as its state
+     */
+    public String toString() {
+        return super.toString() + "[Count = " + sync.getCount() + "]";
+    }
+}
diff --git a/external/jsr166/java/util/concurrent/CyclicBarrier.java b/external/jsr166/java/util/concurrent/CyclicBarrier.java
new file mode 100644
index 000000000..e72577537
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/CyclicBarrier.java
@@ -0,0 +1,454 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+import java.util.concurrent.locks.*;
+
+/**
+ * A synchronization aid that allows a set of threads to all wait for
+ * each other to reach a common barrier point.  CyclicBarriers are
+ * useful in programs involving a fixed sized party of threads that
+ * must occasionally wait for each other. The barrier is called
+ * <em>cyclic</em> because it can be re-used after the waiting threads
+ * are released.
+ *
+ * <p>A <tt>CyclicBarrier</tt> supports an optional {@link Runnable} command
+ * that is run once per barrier point, after the last thread in the party
+ * arrives, but before any threads are released.
+ * This <em>barrier action</em> is useful
+ * for updating shared-state before any of the parties continue.
+ *
+ * <p><b>Sample usage:</b> Here is an example of
+ *  using a barrier in a parallel decomposition design:
+ * <pre>
+ * class Solver {
+ *   final int N;
+ *   final float[][] data;
+ *   final CyclicBarrier barrier;
+ *
+ *   class Worker implements Runnable {
+ *     int myRow;
+ *     Worker(int row) { myRow = row; }
+ *     public void run() {
+ *       while (!done()) {
+ *         processRow(myRow);
+ *
+ *         try {
+ *           barrier.await();
+ *         } catch (InterruptedException ex) {
+ *           return;
+ *         } catch (BrokenBarrierException ex) {
+ *           return;
+ *         }
+ *       }
+ *     }
+ *   }
+ *
+ *   public Solver(float[][] matrix) {
+ *     data = matrix;
+ *     N = matrix.length;
+ *     barrier = new CyclicBarrier(N,
+ *                                 new Runnable() {
+ *                                   public void run() {
+ *                                     mergeRows(...);
+ *                                   }
+ *                                 });
+ *     for (int i = 0; i < N; ++i)
+ *       new Thread(new Worker(i)).start();
+ *
+ *     waitUntilDone();
+ *   }
+ * }
+ * </pre>
+ * Here, each worker thread processes a row of the matrix then waits at the
+ * barrier until all rows have been processed. When all rows are processed
+ * the supplied {@link Runnable} barrier action is executed and merges the
+ * rows. If the merger
+ * determines that a solution has been found then <tt>done()</tt> will return
+ * <tt>true</tt> and each worker will terminate.
+ *
+ * <p>If the barrier action does not rely on the parties being suspended when
+ * it is executed, then any of the threads in the party could execute that
+ * action when it is released. To facilitate this, each invocation of
+ * {@link #await} returns the arrival index of that thread at the barrier.
+ * You can then choose which thread should execute the barrier action, for
+ * example:
+ * <pre>  if (barrier.await() == 0) {
+ *     // log the completion of this iteration
+ *   }</pre>
+ *
+ * <p>The <tt>CyclicBarrier</tt> uses an all-or-none breakage model
+ * for failed synchronization attempts: If a thread leaves a barrier
+ * point prematurely because of interruption, failure, or timeout, all
+ * other threads waiting at that barrier point will also leave
+ * abnormally via {@link BrokenBarrierException} (or
+ * {@link InterruptedException} if they too were interrupted at about
+ * the same time).
+ *
+ * <p>Memory consistency effects: Actions in a thread prior to calling
+ * {@code await()}
+ * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
+ * actions that are part of the barrier action, which in turn
+ * <i>happen-before</i> actions following a successful return from the
+ * corresponding {@code await()} in other threads.
+ *
+ * @since 1.5
+ * @see CountDownLatch
+ *
+ * @author Doug Lea
+ */
+public class CyclicBarrier {
+    /**
+     * Each use of the barrier is represented as a generation instance.
+     * The generation changes whenever the barrier is tripped, or
+     * is reset. There can be many generations associated with threads
+     * using the barrier - due to the non-deterministic way the lock
+     * may be allocated to waiting threads - but only one of these
+     * can be active at a time (the one to which <tt>count</tt> applies)
+     * and all the rest are either broken or tripped.
+     * There need not be an active generation if there has been a break
+     * but no subsequent reset.
+     */
+    private static class Generation {
+        boolean broken = false;
+    }
+
+    /** The lock for guarding barrier entry */
+    private final ReentrantLock lock = new ReentrantLock();
+    /** Condition to wait on until tripped */
+    private final Condition trip = lock.newCondition();
+    /** The number of parties */
+    private final int parties;
+    /* The command to run when tripped */
+    private final Runnable barrierCommand;
+    /** The current generation */
+    private Generation generation = new Generation();
+
+    /**
+     * Number of parties still waiting. Counts down from parties to 0
+     * on each generation.  It is reset to parties on each new
+     * generation or when broken.
+     */
+    private int count;
+
+    /**
+     * Updates state on barrier trip and wakes up everyone.
+     * Called only while holding lock.
+     */
+    private void nextGeneration() {
+        // signal completion of last generation
+        trip.signalAll();
+        // set up next generation
+        count = parties;
+        generation = new Generation();
+    }
+
+    /**
+     * Sets current barrier generation as broken and wakes up everyone.
+     * Called only while holding lock.
+     */
+    private void breakBarrier() {
+        generation.broken = true;
+	count = parties;
+        trip.signalAll();
+    }
+
+    /**
+     * Main barrier code, covering the various policies.
+     */
+    private int dowait(boolean timed, long nanos)
+        throws InterruptedException, BrokenBarrierException,
+               TimeoutException {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            final Generation g = generation;
+
+            if (g.broken)
+                throw new BrokenBarrierException();
+
+            if (Thread.interrupted()) {
+                breakBarrier();
+                throw new InterruptedException();
+            }
+
+           int index = --count;
+           if (index == 0) {  // tripped
+               boolean ranAction = false;
+               try {
+		   final Runnable command = barrierCommand;
+                   if (command != null)
+                       command.run();
+                   ranAction = true;
+                   nextGeneration();
+                   return 0;
+               } finally {
+                   if (!ranAction)
+                       breakBarrier();
+               }
+           }
+
+            // loop until tripped, broken, interrupted, or timed out
+            for (;;) {
+                try {
+                    if (!timed)
+                        trip.await();
+                    else if (nanos > 0L)
+                        nanos = trip.awaitNanos(nanos);
+                } catch (InterruptedException ie) {
+                    if (g == generation && ! g.broken) {
+                        breakBarrier();
+			throw ie;
+		    } else {
+			// We're about to finish waiting even if we had not
+			// been interrupted, so this interrupt is deemed to
+			// "belong" to subsequent execution.
+			Thread.currentThread().interrupt();
+		    }
+                }
+
+                if (g.broken)
+                    throw new BrokenBarrierException();
+
+                if (g != generation)
+                    return index;
+
+                if (timed && nanos <= 0L) {
+                    breakBarrier();
+                    throw new TimeoutException();
+                }
+            }
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Creates a new <tt>CyclicBarrier</tt> that will trip when the
+     * given number of parties (threads) are waiting upon it, and which
+     * will execute the given barrier action when the barrier is tripped,
+     * performed by the last thread entering the barrier.
+     *
+     * @param parties the number of threads that must invoke {@link #await}
+     *        before the barrier is tripped
+     * @param barrierAction the command to execute when the barrier is
+     *        tripped, or {@code null} if there is no action
+     * @throws IllegalArgumentException if {@code parties} is less than 1
+     */
+    public CyclicBarrier(int parties, Runnable barrierAction) {
+        if (parties <= 0) throw new IllegalArgumentException();
+        this.parties = parties;
+        this.count = parties;
+        this.barrierCommand = barrierAction;
+    }
+
+    /**
+     * Creates a new <tt>CyclicBarrier</tt> that will trip when the
+     * given number of parties (threads) are waiting upon it, and
+     * does not perform a predefined action when the barrier is tripped.
+     *
+     * @param parties the number of threads that must invoke {@link #await}
+     *        before the barrier is tripped
+     * @throws IllegalArgumentException if {@code parties} is less than 1
+     */
+    public CyclicBarrier(int parties) {
+        this(parties, null);
+    }
+
+    /**
+     * Returns the number of parties required to trip this barrier.
+     *
+     * @return the number of parties required to trip this barrier
+     */
+    public int getParties() {
+        return parties;
+    }
+
+    /**
+     * Waits until all {@linkplain #getParties parties} have invoked
+     * <tt>await</tt> on this barrier.
+     *
+     * <p>If the current thread is not the last to arrive then it is
+     * disabled for thread scheduling purposes and lies dormant until
+     * one of the following things happens:
+     * <ul>
+     * <li>The last thread arrives; or
+     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
+     * the current thread; or
+     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
+     * one of the other waiting threads; or
+     * <li>Some other thread times out while waiting for barrier; or
+     * <li>Some other thread invokes {@link #reset} on this barrier.
+     * </ul>
+     *
+     * <p>If the current thread:
+     * <ul>
+     * <li>has its interrupted status set on entry to this method; or
+     * <li>is {@linkplain Thread#interrupt interrupted} while waiting
+     * </ul>
+     * then {@link InterruptedException} is thrown and the current thread's
+     * interrupted status is cleared.
+     *
+     * <p>If the barrier is {@link #reset} while any thread is waiting,
+     * or if the barrier {@linkplain #isBroken is broken} when
+     * <tt>await</tt> is invoked, or while any thread is waiting, then
+     * {@link BrokenBarrierException} is thrown.
+     *
+     * <p>If any thread is {@linkplain Thread#interrupt interrupted} while waiting,
+     * then all other waiting threads will throw
+     * {@link BrokenBarrierException} and the barrier is placed in the broken
+     * state.
+     *
+     * <p>If the current thread is the last thread to arrive, and a
+     * non-null barrier action was supplied in the constructor, then the
+     * current thread runs the action before allowing the other threads to
+     * continue.
+     * If an exception occurs during the barrier action then that exception
+     * will be propagated in the current thread and the barrier is placed in
+     * the broken state.
+     *
+     * @return the arrival index of the current thread, where index
+     *         <tt>{@link #getParties()} - 1</tt> indicates the first
+     *         to arrive and zero indicates the last to arrive
+     * @throws InterruptedException if the current thread was interrupted
+     *         while waiting
+     * @throws BrokenBarrierException if <em>another</em> thread was
+     *         interrupted or timed out while the current thread was
+     *         waiting, or the barrier was reset, or the barrier was
+     *         broken when {@code await} was called, or the barrier
+     *         action (if present) failed due an exception.
+     */
+    public int await() throws InterruptedException, BrokenBarrierException {
+        try {
+            return dowait(false, 0L);
+        } catch (TimeoutException toe) {
+            throw new Error(toe); // cannot happen;
+        }
+    }
+
+    /**
+     * Waits until all {@linkplain #getParties parties} have invoked
+     * <tt>await</tt> on this barrier, or the specified waiting time elapses.
+     *
+     * <p>If the current thread is not the last to arrive then it is
+     * disabled for thread scheduling purposes and lies dormant until
+     * one of the following things happens:
+     * <ul>
+     * <li>The last thread arrives; or
+     * <li>The specified timeout elapses; or
+     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
+     * the current thread; or
+     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
+     * one of the other waiting threads; or
+     * <li>Some other thread times out while waiting for barrier; or
+     * <li>Some other thread invokes {@link #reset} on this barrier.
+     * </ul>
+     *
+     * <p>If the current thread:
+     * <ul>
+     * <li>has its interrupted status set on entry to this method; or
+     * <li>is {@linkplain Thread#interrupt interrupted} while waiting
+     * </ul>
+     * then {@link InterruptedException} is thrown and the current thread's
+     * interrupted status is cleared.
+     *
+     * <p>If the specified waiting time elapses then {@link TimeoutException}
+     * is thrown. If the time is less than or equal to zero, the
+     * method will not wait at all.
+     *
+     * <p>If the barrier is {@link #reset} while any thread is waiting,
+     * or if the barrier {@linkplain #isBroken is broken} when
+     * <tt>await</tt> is invoked, or while any thread is waiting, then
+     * {@link BrokenBarrierException} is thrown.
+     *
+     * <p>If any thread is {@linkplain Thread#interrupt interrupted} while
+     * waiting, then all other waiting threads will throw {@link
+     * BrokenBarrierException} and the barrier is placed in the broken
+     * state.
+     *
+     * <p>If the current thread is the last thread to arrive, and a
+     * non-null barrier action was supplied in the constructor, then the
+     * current thread runs the action before allowing the other threads to
+     * continue.
+     * If an exception occurs during the barrier action then that exception
+     * will be propagated in the current thread and the barrier is placed in
+     * the broken state.
+     *
+     * @param timeout the time to wait for the barrier
+     * @param unit the time unit of the timeout parameter
+     * @return the arrival index of the current thread, where index
+     *         <tt>{@link #getParties()} - 1</tt> indicates the first
+     *         to arrive and zero indicates the last to arrive
+     * @throws InterruptedException if the current thread was interrupted
+     *         while waiting
+     * @throws TimeoutException if the specified timeout elapses
+     * @throws BrokenBarrierException if <em>another</em> thread was
+     *         interrupted or timed out while the current thread was
+     *         waiting, or the barrier was reset, or the barrier was broken
+     *         when {@code await} was called, or the barrier action (if
+     *         present) failed due an exception
+     */
+    public int await(long timeout, TimeUnit unit)
+        throws InterruptedException,
+               BrokenBarrierException,
+               TimeoutException {
+        return dowait(true, unit.toNanos(timeout));
+    }
+
+    /**
+     * Queries if this barrier is in a broken state.
+     *
+     * @return {@code true} if one or more parties broke out of this
+     *         barrier due to interruption or timeout since
+     *         construction or the last reset, or a barrier action
+     *         failed due to an exception; {@code false} otherwise.
+     */
+    public boolean isBroken() {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            return generation.broken;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Resets the barrier to its initial state.  If any parties are
+     * currently waiting at the barrier, they will return with a
+     * {@link BrokenBarrierException}. Note that resets <em>after</em>
+     * a breakage has occurred for other reasons can be complicated to
+     * carry out; threads need to re-synchronize in some other way,
+     * and choose one to perform the reset.  It may be preferable to
+     * instead create a new barrier for subsequent use.
+     */
+    public void reset() {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            breakBarrier();   // break the current generation
+            nextGeneration(); // start a new generation
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Returns the number of parties currently waiting at the barrier.
+     * This method is primarily useful for debugging and assertions.
+     *
+     * @return the number of parties currently blocked in {@link #await}
+     */
+    public int getNumberWaiting() {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            return parties - count;
+        } finally {
+            lock.unlock();
+        }
+    }
+}
diff --git a/external/jsr166/java/util/concurrent/DelayQueue.java b/external/jsr166/java/util/concurrent/DelayQueue.java
new file mode 100644
index 000000000..8b839878c
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/DelayQueue.java
@@ -0,0 +1,487 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+
+package java.util.concurrent;
+import java.util.concurrent.locks.*;
+import java.util.*;
+
+/**
+ * An unbounded {@linkplain BlockingQueue blocking queue} of
+ * <tt>Delayed</tt> elements, in which an element can only be taken
+ * when its delay has expired.  The <em>head</em> of the queue is that
+ * <tt>Delayed</tt> element whose delay expired furthest in the
+ * past.  If no delay has expired there is no head and <tt>poll</tt>
+ * will return <tt>null</tt>. Expiration occurs when an element's
+ * <tt>getDelay(TimeUnit.NANOSECONDS)</tt> method returns a value less
+ * than or equal to zero.  Even though unexpired elements cannot be
+ * removed using <tt>take</tt> or <tt>poll</tt>, they are otherwise
+ * treated as normal elements. For example, the <tt>size</tt> method
+ * returns the count of both expired and unexpired elements.
+ * This queue does not permit null elements.
+ *
+ * <p>This class and its iterator implement all of the
+ * <em>optional</em> methods of the {@link Collection} and {@link
+ * Iterator} interfaces.
+ *
+ * <p>This class is a member of the
+ * <a href="{@docRoot}/../technotes/guides/collections/index.html">
+ * Java Collections Framework</a>.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ * @param <E> the type of elements held in this collection
+ */
+
+public class DelayQueue<E extends Delayed> extends AbstractQueue<E>
+    implements BlockingQueue<E> {
+
+    private transient final ReentrantLock lock = new ReentrantLock();
+    private transient final Condition available = lock.newCondition();
+    private final PriorityQueue<E> q = new PriorityQueue<E>();
+
+    /**
+     * Creates a new <tt>DelayQueue</tt> that is initially empty.
+     */
+    public DelayQueue() {}
+
+    /**
+     * Creates a <tt>DelayQueue</tt> initially containing the elements of the
+     * given collection of {@link Delayed} instances.
+     *
+     * @param c the collection of elements to initially contain
+     * @throws NullPointerException if the specified collection or any
+     *         of its elements are null
+     */
+    public DelayQueue(Collection<? extends E> c) {
+        this.addAll(c);
+    }
+
+    /**
+     * Inserts the specified element into this delay queue.
+     *
+     * @param e the element to add
+     * @return <tt>true</tt> (as specified by {@link Collection#add})
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean add(E e) {
+        return offer(e);
+    }
+
+    /**
+     * Inserts the specified element into this delay queue.
+     *
+     * @param e the element to add
+     * @return <tt>true</tt>
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean offer(E e) {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            E first = q.peek();
+            q.offer(e);
+            if (first == null || e.compareTo(first) < 0)
+                available.signalAll();
+            return true;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Inserts the specified element into this delay queue. As the queue is
+     * unbounded this method will never block.
+     *
+     * @param e the element to add
+     * @throws NullPointerException {@inheritDoc}
+     */
+    public void put(E e) {
+        offer(e);
+    }
+
+    /**
+     * Inserts the specified element into this delay queue. As the queue is
+     * unbounded this method will never block.
+     *
+     * @param e the element to add
+     * @param timeout This parameter is ignored as the method never blocks
+     * @param unit This parameter is ignored as the method never blocks
+     * @return <tt>true</tt>
+     * @throws NullPointerException {@inheritDoc}
+     */
+    public boolean offer(E e, long timeout, TimeUnit unit) {
+        return offer(e);
+    }
+
+    /**
+     * Retrieves and removes the head of this queue, or returns <tt>null</tt>
+     * if this queue has no elements with an expired delay.
+     *
+     * @return the head of this queue, or <tt>null</tt> if this
+     *         queue has no elements with an expired delay
+     */
+    public E poll() {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            E first = q.peek();
+            if (first == null || first.getDelay(TimeUnit.NANOSECONDS) > 0)
+                return null;
+            else {
+                E x = q.poll();
+                assert x != null;
+                if (q.size() != 0)
+                    available.signalAll();
+                return x;
+            }
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Retrieves and removes the head of this queue, waiting if necessary
+     * until an element with an expired delay is available on this queue.
+     *
+     * @return the head of this queue
+     * @throws InterruptedException {@inheritDoc}
+     */
+    public E take() throws InterruptedException {
+        final ReentrantLock lock = this.lock;
+        lock.lockInterruptibly();
+        try {
+            for (;;) {
+                E first = q.peek();
+                if (first == null) {
+                    available.await();
+                } else {
+                    long delay =  first.getDelay(TimeUnit.NANOSECONDS);
+                    if (delay > 0) {
+                        long tl = available.awaitNanos(delay);
+                    } else {
+                        E x = q.poll();
+                        assert x != null;
+                        if (q.size() != 0)
+                            available.signalAll(); // wake up other takers
+                        return x;
+
+                    }
+                }
+            }
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Retrieves and removes the head of this queue, waiting if necessary
+     * until an element with an expired delay is available on this queue,
+     * or the specified wait time expires.
+     *
+     * @return the head of this queue, or <tt>null</tt> if the
+     *         specified waiting time elapses before an element with
+     *         an expired delay becomes available
+     * @throws InterruptedException {@inheritDoc}
+     */
+    public E poll(long timeout, TimeUnit unit) throws InterruptedException {
+        long nanos = unit.toNanos(timeout);
+        final ReentrantLock lock = this.lock;
+        lock.lockInterruptibly();
+        try {
+            for (;;) {
+                E first = q.peek();
+                if (first == null) {
+                    if (nanos <= 0)
+                        return null;
+                    else
+                        nanos = available.awaitNanos(nanos);
+                } else {
+                    long delay = first.getDelay(TimeUnit.NANOSECONDS);
+                    if (delay > 0) {
+                        if (nanos <= 0)
+                            return null;
+                        if (delay > nanos)
+                            delay = nanos;
+                        long timeLeft = available.awaitNanos(delay);
+                        nanos -= delay - timeLeft;
+                    } else {
+                        E x = q.poll();
+                        assert x != null;
+                        if (q.size() != 0)
+                            available.signalAll();
+                        return x;
+                    }
+                }
+            }
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Retrieves, but does not remove, the head of this queue, or
+     * returns <tt>null</tt> if this queue is empty.  Unlike
+     * <tt>poll</tt>, if no expired elements are available in the queue,
+     * this method returns the element that will expire next,
+     * if one exists.
+     *
+     * @return the head of this queue, or <tt>null</tt> if this
+     *         queue is empty.
+     */
+    public E peek() {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            return q.peek();
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    public int size() {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            return q.size();
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * @throws UnsupportedOperationException {@inheritDoc}
+     * @throws ClassCastException            {@inheritDoc}
+     * @throws NullPointerException          {@inheritDoc}
+     * @throws IllegalArgumentException      {@inheritDoc}
+     */
+    public int drainTo(Collection<? super E> c) {
+        if (c == null)
+            throw new NullPointerException();
+        if (c == this)
+            throw new IllegalArgumentException();
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            int n = 0;
+            for (;;) {
+                E first = q.peek();
+                if (first == null || first.getDelay(TimeUnit.NANOSECONDS) > 0)
+                    break;
+                c.add(q.poll());
+                ++n;
+            }
+            if (n > 0)
+                available.signalAll();
+            return n;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * @throws UnsupportedOperationException {@inheritDoc}
+     * @throws ClassCastException            {@inheritDoc}
+     * @throws NullPointerException          {@inheritDoc}
+     * @throws IllegalArgumentException      {@inheritDoc}
+     */
+    public int drainTo(Collection<? super E> c, int maxElements) {
+        if (c == null)
+            throw new NullPointerException();
+        if (c == this)
+            throw new IllegalArgumentException();
+        if (maxElements <= 0)
+            return 0;
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            int n = 0;
+            while (n < maxElements) {
+                E first = q.peek();
+                if (first == null || first.getDelay(TimeUnit.NANOSECONDS) > 0)
+                    break;
+                c.add(q.poll());
+                ++n;
+            }
+            if (n > 0)
+                available.signalAll();
+            return n;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Atomically removes all of the elements from this delay queue.
+     * The queue will be empty after this call returns.
+     * Elements with an unexpired delay are not waited for; they are
+     * simply discarded from the queue.
+     */
+    public void clear() {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            q.clear();
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Always returns <tt>Integer.MAX_VALUE</tt> because
+     * a <tt>DelayQueue</tt> is not capacity constrained.
+     *
+     * @return <tt>Integer.MAX_VALUE</tt>
+     */
+    public int remainingCapacity() {
+        return Integer.MAX_VALUE;
+    }
+
+    /**
+     * Returns an array containing all of the elements in this queue.
+     * The returned array elements are in no particular order.
+     *
+     * <p>The returned array will be "safe" in that no references to it are
+     * maintained by this queue.  (In other words, this method must allocate
+     * a new array).  The caller is thus free to modify the returned array.
+     *
+     * <p>This method acts as bridge between array-based and collection-based
+     * APIs.
+     *
+     * @return an array containing all of the elements in this queue
+     */
+    public Object[] toArray() {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            return q.toArray();
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Returns an array containing all of the elements in this queue; the
+     * runtime type of the returned array is that of the specified array.
+     * The returned array elements are in no particular order.
+     * If the queue fits in the specified array, it is returned therein.
+     * Otherwise, a new array is allocated with the runtime type of the
+     * specified array and the size of this queue.
+     *
+     * <p>If this queue fits in the specified array with room to spare
+     * (i.e., the array has more elements than this queue), the element in
+     * the array immediately following the end of the queue is set to
+     * <tt>null</tt>.
+     *
+     * <p>Like the {@link #toArray()} method, this method acts as bridge between
+     * array-based and collection-based APIs.  Further, this method allows
+     * precise control over the runtime type of the output array, and may,
+     * under certain circumstances, be used to save allocation costs.
+     *
+     * <p>The following code can be used to dump a delay queue into a newly
+     * allocated array of <tt>Delayed</tt>:
+     *
+     * <pre>
+     *     Delayed[] a = q.toArray(new Delayed[0]);</pre>
+     *
+     * Note that <tt>toArray(new Object[0])</tt> is identical in function to
+     * <tt>toArray()</tt>.
+     *
+     * @param a the array into which the elements of the queue are to
+     *          be stored, if it is big enough; otherwise, a new array of the
+     *          same runtime type is allocated for this purpose
+     * @return an array containing all of the elements in this queue
+     * @throws ArrayStoreException if the runtime type of the specified array
+     *         is not a supertype of the runtime type of every element in
+     *         this queue
+     * @throws NullPointerException if the specified array is null
+     */
+    public <T> T[] toArray(T[] a) {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            return q.toArray(a);
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Removes a single instance of the specified element from this
+     * queue, if it is present, whether or not it has expired.
+     */
+    public boolean remove(Object o) {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            return q.remove(o);
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Returns an iterator over all the elements (both expired and
+     * unexpired) in this queue. The iterator does not return the
+     * elements in any particular order.  The returned
+     * <tt>Iterator</tt> is a "weakly consistent" iterator that will
+     * never throw {@link ConcurrentModificationException}, and
+     * guarantees to traverse elements as they existed upon
+     * construction of the iterator, and may (but is not guaranteed
+     * to) reflect any modifications subsequent to construction.
+     *
+     * @return an iterator over the elements in this queue
+     */
+    public Iterator<E> iterator() {
+        return new Itr(toArray());
+    }
+
+    /**
+     * Snapshot iterator that works off copy of underlying q array.
+     */
+    private class Itr implements Iterator<E> {
+        final Object[] array; // Array of all elements
+	int cursor;           // index of next element to return;
+	int lastRet;          // index of last element, or -1 if no such
+
+        Itr(Object[] array) {
+            lastRet = -1;
+            this.array = array;
+        }
+
+        public boolean hasNext() {
+            return cursor < array.length;
+        }
+
+        public E next() {
+            if (cursor >= array.length)
+                throw new NoSuchElementException();
+            lastRet = cursor;
+            return (E)array[cursor++];
+        }
+
+        public void remove() {
+            if (lastRet < 0)
+		throw new IllegalStateException();
+            Object x = array[lastRet];
+            lastRet = -1;
+            // Traverse underlying queue to find == element,
+            // not just a .equals element.
+            lock.lock();
+            try {
+                for (Iterator it = q.iterator(); it.hasNext(); ) {
+                    if (it.next() == x) {
+                        it.remove();
+                        return;
+                    }
+                }
+            } finally {
+                lock.unlock();
+            }
+        }
+    }
+
+}
diff --git a/external/jsr166/java/util/concurrent/Delayed.java b/external/jsr166/java/util/concurrent/Delayed.java
new file mode 100644
index 000000000..b1ff4eee5
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/Delayed.java
@@ -0,0 +1,33 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+
+import java.util.*;
+
+/**
+ * A mix-in style interface for marking objects that should be
+ * acted upon after a given delay.
+ *
+ * <p>An implementation of this interface must define a
+ * <tt>compareTo</tt> method that provides an ordering consistent with
+ * its <tt>getDelay</tt> method.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ */
+public interface Delayed extends Comparable<Delayed> {
+
+    /**
+     * Returns the remaining delay associated with this object, in the
+     * given time unit.
+     *
+     * @param unit the time unit
+     * @return the remaining delay; zero or negative values indicate
+     * that the delay has already elapsed
+     */
+    long getDelay(TimeUnit unit);
+}
diff --git a/external/jsr166/java/util/concurrent/Exchanger.java b/external/jsr166/java/util/concurrent/Exchanger.java
new file mode 100644
index 000000000..fb917f432
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/Exchanger.java
@@ -0,0 +1,656 @@
+/*
+ * Written by Doug Lea, Bill Scherer, and Michael Scott with
+ * assistance from members of JCP JSR-166 Expert Group and released to
+ * the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+import java.util.concurrent.atomic.*;
+import java.util.concurrent.locks.LockSupport;
+
+/**
+ * A synchronization point at which threads can pair and swap elements
+ * within pairs.  Each thread presents some object on entry to the
+ * {@link #exchange exchange} method, matches with a partner thread,
+ * and receives its partner's object on return.  An Exchanger may be
+ * viewed as a bidirectional form of a {@link SynchronousQueue}.
+ * Exchangers may be useful in applications such as genetic algorithms
+ * and pipeline designs.
+ *
+ * <p><b>Sample Usage:</b>
+ * Here are the highlights of a class that uses an {@code Exchanger}
+ * to swap buffers between threads so that the thread filling the
+ * buffer gets a freshly emptied one when it needs it, handing off the
+ * filled one to the thread emptying the buffer.
+ * <pre>{@code
+ * class FillAndEmpty {
+ *   Exchanger<DataBuffer> exchanger = new Exchanger<DataBuffer>();
+ *   DataBuffer initialEmptyBuffer = ... a made-up type
+ *   DataBuffer initialFullBuffer = ...
+ *
+ *   class FillingLoop implements Runnable {
+ *     public void run() {
+ *       DataBuffer currentBuffer = initialEmptyBuffer;
+ *       try {
+ *         while (currentBuffer != null) {
+ *           addToBuffer(currentBuffer);
+ *           if (currentBuffer.isFull())
+ *             currentBuffer = exchanger.exchange(currentBuffer);
+ *         }
+ *       } catch (InterruptedException ex) { ... handle ... }
+ *     }
+ *   }
+ *
+ *   class EmptyingLoop implements Runnable {
+ *     public void run() {
+ *       DataBuffer currentBuffer = initialFullBuffer;
+ *       try {
+ *         while (currentBuffer != null) {
+ *           takeFromBuffer(currentBuffer);
+ *           if (currentBuffer.isEmpty())
+ *             currentBuffer = exchanger.exchange(currentBuffer);
+ *         }
+ *       } catch (InterruptedException ex) { ... handle ...}
+ *     }
+ *   }
+ *
+ *   void start() {
+ *     new Thread(new FillingLoop()).start();
+ *     new Thread(new EmptyingLoop()).start();
+ *   }
+ * }
+ * }</pre>
+ *
+ * <p>Memory consistency effects: For each pair of threads that
+ * successfully exchange objects via an {@code Exchanger}, actions
+ * prior to the {@code exchange()} in each thread
+ * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
+ * those subsequent to a return from the corresponding {@code exchange()}
+ * in the other thread.
+ *
+ * @since 1.5
+ * @author Doug Lea and Bill Scherer and Michael Scott
+ * @param <V> The type of objects that may be exchanged
+ */
+public class Exchanger<V> {
+    /*
+     * Algorithm Description:
+     *
+     * The basic idea is to maintain a "slot", which is a reference to
+     * a Node containing both an Item to offer and a "hole" waiting to
+     * get filled in.  If an incoming "occupying" thread sees that the
+     * slot is null, it CAS'es (compareAndSets) a Node there and waits
+     * for another to invoke exchange.  That second "fulfilling" thread
+     * sees that the slot is non-null, and so CASes it back to null,
+     * also exchanging items by CASing the hole, plus waking up the
+     * occupying thread if it is blocked.  In each case CAS'es may
+     * fail because a slot at first appears non-null but is null upon
+     * CAS, or vice-versa.  So threads may need to retry these
+     * actions.
+     *
+     * This simple approach works great when there are only a few
+     * threads using an Exchanger, but performance rapidly
+     * deteriorates due to CAS contention on the single slot when
+     * there are lots of threads using an exchanger.  So instead we use
+     * an "arena"; basically a kind of hash table with a dynamically
+     * varying number of slots, any one of which can be used by
+     * threads performing an exchange.  Incoming threads pick slots
+     * based on a hash of their Thread ids.  If an incoming thread
+     * fails to CAS in its chosen slot, it picks an alternative slot
+     * instead.  And similarly from there.  If a thread successfully
+     * CASes into a slot but no other thread arrives, it tries
+     * another, heading toward the zero slot, which always exists even
+     * if the table shrinks.  The particular mechanics controlling this
+     * are as follows:
+     *
+     * Waiting: Slot zero is special in that it is the only slot that
+     * exists when there is no contention.  A thread occupying slot
+     * zero will block if no thread fulfills it after a short spin.
+     * In other cases, occupying threads eventually give up and try
+     * another slot.  Waiting threads spin for a while (a period that
+     * should be a little less than a typical context-switch time)
+     * before either blocking (if slot zero) or giving up (if other
+     * slots) and restarting.  There is no reason for threads to block
+     * unless there are unlikely to be any other threads present.
+     * Occupants are mainly avoiding memory contention so sit there
+     * quietly polling for a shorter period than it would take to
+     * block and then unblock them.  Non-slot-zero waits that elapse
+     * because of lack of other threads waste around one extra
+     * context-switch time per try, which is still on average much
+     * faster than alternative approaches.
+     *
+     * Sizing: Usually, using only a few slots suffices to reduce
+     * contention.  Especially with small numbers of threads, using
+     * too many slots can lead to just as poor performance as using
+     * too few of them, and there's not much room for error.  The
+     * variable "max" maintains the number of slots actually in
+     * use.  It is increased when a thread sees too many CAS
+     * failures.  (This is analogous to resizing a regular hash table
+     * based on a target load factor, except here, growth steps are
+     * just one-by-one rather than proportional.)  Growth requires
+     * contention failures in each of three tried slots.  Requiring
+     * multiple failures for expansion copes with the fact that some
+     * failed CASes are not due to contention but instead to simple
+     * races between two threads or thread pre-emptions occurring
+     * between reading and CASing.  Also, very transient peak
+     * contention can be much higher than the average sustainable
+     * levels.  The max limit is decreased on average 50% of the times
+     * that a non-slot-zero wait elapses without being fulfilled.
+     * Threads experiencing elapsed waits move closer to zero, so
+     * eventually find existing (or future) threads even if the table
+     * has been shrunk due to inactivity.  The chosen mechanics and
+     * thresholds for growing and shrinking are intrinsically
+     * entangled with indexing and hashing inside the exchange code,
+     * and can't be nicely abstracted out.
+     *
+     * Hashing: Each thread picks its initial slot to use in accord
+     * with a simple hashcode.  The sequence is the same on each
+     * encounter by any given thread, but effectively random across
+     * threads.  Using arenas encounters the classic cost vs quality
+     * tradeoffs of all hash tables.  Here, we use a one-step FNV-1a
+     * hash code based on the current thread's Thread.getId(), along
+     * with a cheap approximation to a mod operation to select an
+     * index.  The downside of optimizing index selection in this way
+     * is that the code is hardwired to use a maximum table size of
+     * 32.  But this value more than suffices for known platforms and
+     * applications.
+     *
+     * Probing: On sensed contention of a selected slot, we probe
+     * sequentially through the table, analogously to linear probing
+     * after collision in a hash table.  (We move circularly, in
+     * reverse order, to mesh best with table growth and shrinkage
+     * rules.)  Except that to minimize the effects of false-alarms
+     * and cache thrashing, we try the first selected slot twice
+     * before moving.
+     *
+     * Padding: Even with contention management, slots are heavily
+     * contended, so use cache-padding to avoid poor memory
+     * performance.  Because of this, slots are lazily constructed
+     * only when used, to avoid wasting this space unnecessarily.
+     * While isolation of locations is not much of an issue at first
+     * in an application, as time goes on and garbage-collectors
+     * perform compaction, slots are very likely to be moved adjacent
+     * to each other, which can cause much thrashing of cache lines on
+     * MPs unless padding is employed.
+     *
+     * This is an improvement of the algorithm described in the paper
+     * "A Scalable Elimination-based Exchange Channel" by William
+     * Scherer, Doug Lea, and Michael Scott in Proceedings of SCOOL05
+     * workshop.  Available at: http://hdl.handle.net/1802/2104
+     */
+
+    /** The number of CPUs, for sizing and spin control */
+    private static final int NCPU = Runtime.getRuntime().availableProcessors();
+
+    /**
+     * The capacity of the arena.  Set to a value that provides more
+     * than enough space to handle contention.  On small machines
+     * most slots won't be used, but it is still not wasted because
+     * the extra space provides some machine-level address padding
+     * to minimize interference with heavily CAS'ed Slot locations.
+     * And on very large machines, performance eventually becomes
+     * bounded by memory bandwidth, not numbers of threads/CPUs.
+     * This constant cannot be changed without also modifying
+     * indexing and hashing algorithms.
+     */
+    private static final int CAPACITY = 32;
+
+    /**
+     * The value of "max" that will hold all threads without
+     * contention.  When this value is less than CAPACITY, some
+     * otherwise wasted expansion can be avoided.
+     */
+    private static final int FULL =
+        Math.max(0, Math.min(CAPACITY, NCPU / 2) - 1);
+
+    /**
+     * The number of times to spin (doing nothing except polling a
+     * memory location) before blocking or giving up while waiting to
+     * be fulfilled.  Should be zero on uniprocessors.  On
+     * multiprocessors, this value should be large enough so that two
+     * threads exchanging items as fast as possible block only when
+     * one of them is stalled (due to GC or preemption), but not much
+     * longer, to avoid wasting CPU resources.  Seen differently, this
+     * value is a little over half the number of cycles of an average
+     * context switch time on most systems.  The value here is
+     * approximately the average of those across a range of tested
+     * systems.
+     */
+    private static final int SPINS = (NCPU == 1) ? 0 : 2000;
+
+    /**
+     * The number of times to spin before blocking in timed waits.
+     * Timed waits spin more slowly because checking the time takes
+     * time.  The best value relies mainly on the relative rate of
+     * System.nanoTime vs memory accesses.  The value is empirically
+     * derived to work well across a variety of systems.
+     */
+    private static final int TIMED_SPINS = SPINS / 20;
+
+    /**
+     * Sentinel item representing cancellation of a wait due to
+     * interruption, timeout, or elapsed spin-waits.  This value is
+     * placed in holes on cancellation, and used as a return value
+     * from waiting methods to indicate failure to set or get hole.
+     */
+    private static final Object CANCEL = new Object();
+
+    /**
+     * Value representing null arguments/returns from public
+     * methods.  This disambiguates from internal requirement that
+     * holes start out as null to mean they are not yet set.
+     */
+    private static final Object NULL_ITEM = new Object();
+
+    /**
+     * Nodes hold partially exchanged data.  This class
+     * opportunistically subclasses AtomicReference to represent the
+     * hole.  So get() returns hole, and compareAndSet CAS'es value
+     * into hole.  This class cannot be parameterized as "V" because
+     * of the use of non-V CANCEL sentinels.
+     */
+    private static final class Node extends AtomicReference<Object> {
+        /** The element offered by the Thread creating this node. */
+        public final Object item;
+
+        /** The Thread waiting to be signalled; null until waiting. */
+        public volatile Thread waiter;
+
+        /**
+         * Creates node with given item and empty hole.
+         * @param item the item
+         */
+        public Node(Object item) {
+            this.item = item;
+        }
+    }
+
+    /**
+     * A Slot is an AtomicReference with heuristic padding to lessen
+     * cache effects of this heavily CAS'ed location.  While the
+     * padding adds noticeable space, all slots are created only on
+     * demand, and there will be more than one of them only when it
+     * would improve throughput more than enough to outweigh using
+     * extra space.
+     */
+    private static final class Slot extends AtomicReference<Object> {
+        // Improve likelihood of isolation on <= 64 byte cache lines
+        long q0, q1, q2, q3, q4, q5, q6, q7, q8, q9, qa, qb, qc, qd, qe;
+    }
+
+    /**
+     * Slot array.  Elements are lazily initialized when needed.
+     * Declared volatile to enable double-checked lazy construction.
+     */
+    private volatile Slot[] arena = new Slot[CAPACITY];
+
+    /**
+     * The maximum slot index being used.  The value sometimes
+     * increases when a thread experiences too many CAS contentions,
+     * and sometimes decreases when a spin-wait elapses.  Changes
+     * are performed only via compareAndSet, to avoid stale values
+     * when a thread happens to stall right before setting.
+     */
+    private final AtomicInteger max = new AtomicInteger();
+
+    /**
+     * Main exchange function, handling the different policy variants.
+     * Uses Object, not "V" as argument and return value to simplify
+     * handling of sentinel values.  Callers from public methods decode
+     * and cast accordingly.
+     *
+     * @param item the (non-null) item to exchange
+     * @param timed true if the wait is timed
+     * @param nanos if timed, the maximum wait time
+     * @return the other thread's item, or CANCEL if interrupted or timed out
+     */
+    private Object doExchange(Object item, boolean timed, long nanos) {
+        Node me = new Node(item);                 // Create in case occupying
+        int index = hashIndex();                  // Index of current slot
+        int fails = 0;                            // Number of CAS failures
+
+        for (;;) {
+            Object y;                             // Contents of current slot
+            Slot slot = arena[index];
+            if (slot == null)                     // Lazily initialize slots
+                createSlot(index);                // Continue loop to reread
+            else if ((y = slot.get()) != null &&  // Try to fulfill
+                     slot.compareAndSet(y, null)) {
+                Node you = (Node)y;               // Transfer item
+                if (you.compareAndSet(null, item)) {
+                    LockSupport.unpark(you.waiter);
+                    return you.item;
+                }                                 // Else cancelled; continue
+            }
+            else if (y == null &&                 // Try to occupy
+                     slot.compareAndSet(null, me)) {
+                if (index == 0)                   // Blocking wait for slot 0
+                    return timed? awaitNanos(me, slot, nanos): await(me, slot);
+                Object v = spinWait(me, slot);    // Spin wait for non-0
+                if (v != CANCEL)
+                    return v;
+                me = new Node(item);              // Throw away cancelled node
+                int m = max.get();
+                if (m > (index >>>= 1))           // Decrease index
+                    max.compareAndSet(m, m - 1);  // Maybe shrink table
+            }
+            else if (++fails > 1) {               // Allow 2 fails on 1st slot
+                int m = max.get();
+                if (fails > 3 && m < FULL && max.compareAndSet(m, m + 1))
+                    index = m + 1;                // Grow on 3rd failed slot
+                else if (--index < 0)
+                    index = m;                    // Circularly traverse
+            }
+        }
+    }
+
+    /**
+     * Returns a hash index for the current thread.  Uses a one-step
+     * FNV-1a hash code (http://www.isthe.com/chongo/tech/comp/fnv/)
+     * based on the current thread's Thread.getId().  These hash codes
+     * have more uniform distribution properties with respect to small
+     * moduli (here 1-31) than do other simple hashing functions.
+     *
+     * <p>To return an index between 0 and max, we use a cheap
+     * approximation to a mod operation, that also corrects for bias
+     * due to non-power-of-2 remaindering (see {@link
+     * java.util.Random#nextInt}).  Bits of the hashcode are masked
+     * with "nbits", the ceiling power of two of table size (looked up
+     * in a table packed into three ints).  If too large, this is
+     * retried after rotating the hash by nbits bits, while forcing new
+     * top bit to 0, which guarantees eventual termination (although
+     * with a non-random-bias).  This requires an average of less than
+     * 2 tries for all table sizes, and has a maximum 2% difference
+     * from perfectly uniform slot probabilities when applied to all
+     * possible hash codes for sizes less than 32.
+     *
+     * @return a per-thread-random index, 0 <= index < max
+     */
+    private final int hashIndex() {
+        long id = Thread.currentThread().getId();
+        int hash = (((int)(id ^ (id >>> 32))) ^ 0x811c9dc5) * 0x01000193;
+
+        int m = max.get();
+        int nbits = (((0xfffffc00  >> m) & 4) | // Compute ceil(log2(m+1))
+                     ((0x000001f8 >>> m) & 2) | // The constants hold
+                     ((0xffff00f2 >>> m) & 1)); // a lookup table
+        int index;
+        while ((index = hash & ((1 << nbits) - 1)) > m)       // May retry on
+            hash = (hash >>> nbits) | (hash << (33 - nbits)); // non-power-2 m
+        return index;
+    }
+
+    /**
+     * Creates a new slot at given index.  Called only when the slot
+     * appears to be null.  Relies on double-check using builtin
+     * locks, since they rarely contend.  This in turn relies on the
+     * arena array being declared volatile.
+     *
+     * @param index the index to add slot at
+     */
+    private void createSlot(int index) {
+        // Create slot outside of lock to narrow sync region
+        Slot newSlot = new Slot();
+        Slot[] a = arena;
+        synchronized (a) {
+            if (a[index] == null)
+                a[index] = newSlot;
+        }
+    }
+
+    /**
+     * Tries to cancel a wait for the given node waiting in the given
+     * slot, if so, helping clear the node from its slot to avoid
+     * garbage retention.
+     *
+     * @param node the waiting node
+     * @param the slot it is waiting in
+     * @return true if successfully cancelled
+     */
+    private static boolean tryCancel(Node node, Slot slot) {
+        if (!node.compareAndSet(null, CANCEL))
+            return false;
+        if (slot.get() == node) // pre-check to minimize contention
+            slot.compareAndSet(node, null);
+        return true;
+    }
+
+    // Three forms of waiting. Each just different enough not to merge
+    // code with others.
+
+    /**
+     * Spin-waits for hole for a non-0 slot.  Fails if spin elapses
+     * before hole filled.  Does not check interrupt, relying on check
+     * in public exchange method to abort if interrupted on entry.
+     *
+     * @param node the waiting node
+     * @return on success, the hole; on failure, CANCEL
+     */
+    private static Object spinWait(Node node, Slot slot) {
+        int spins = SPINS;
+        for (;;) {
+            Object v = node.get();
+            if (v != null)
+                return v;
+            else if (spins > 0)
+                --spins;
+            else
+                tryCancel(node, slot);
+        }
+    }
+
+    /**
+     * Waits for (by spinning and/or blocking) and gets the hole
+     * filled in by another thread.  Fails if interrupted before
+     * hole filled.
+     *
+     * When a node/thread is about to block, it sets its waiter field
+     * and then rechecks state at least one more time before actually
+     * parking, thus covering race vs fulfiller noticing that waiter
+     * is non-null so should be woken.
+     *
+     * Thread interruption status is checked only surrounding calls to
+     * park.  The caller is assumed to have checked interrupt status
+     * on entry.
+     *
+     * @param node the waiting node
+     * @return on success, the hole; on failure, CANCEL
+     */
+    private static Object await(Node node, Slot slot) {
+        Thread w = Thread.currentThread();
+        int spins = SPINS;
+        for (;;) {
+            Object v = node.get();
+            if (v != null)
+                return v;
+            else if (spins > 0)                 // Spin-wait phase
+                --spins;
+            else if (node.waiter == null)       // Set up to block next
+                node.waiter = w;
+            else if (w.isInterrupted())         // Abort on interrupt
+                tryCancel(node, slot);
+            else                                // Block
+                LockSupport.park(node);
+        }
+    }
+
+    /**
+     * Waits for (at index 0) and gets the hole filled in by another
+     * thread.  Fails if timed out or interrupted before hole filled.
+     * Same basic logic as untimed version, but a bit messier.
+     *
+     * @param node the waiting node
+     * @param nanos the wait time
+     * @return on success, the hole; on failure, CANCEL
+     */
+    private Object awaitNanos(Node node, Slot slot, long nanos) {
+        int spins = TIMED_SPINS;
+        long lastTime = 0;
+        Thread w = null;
+        for (;;) {
+            Object v = node.get();
+            if (v != null)
+                return v;
+            long now = System.nanoTime();
+            if (w == null)
+                w = Thread.currentThread();
+            else
+                nanos -= now - lastTime;
+            lastTime = now;
+            if (nanos > 0) {
+                if (spins > 0)
+                    --spins;
+                else if (node.waiter == null)
+                    node.waiter = w;
+                else if (w.isInterrupted())
+                    tryCancel(node, slot);
+                else
+                    LockSupport.parkNanos(node, nanos);
+            }
+            else if (tryCancel(node, slot) && !w.isInterrupted())
+                return scanOnTimeout(node);
+        }
+    }
+
+    /**
+     * Sweeps through arena checking for any waiting threads.  Called
+     * only upon return from timeout while waiting in slot 0.  When a
+     * thread gives up on a timed wait, it is possible that a
+     * previously-entered thread is still waiting in some other
+     * slot.  So we scan to check for any.  This is almost always
+     * overkill, but decreases the likelihood of timeouts when there
+     * are other threads present to far less than that in lock-based
+     * exchangers in which earlier-arriving threads may still be
+     * waiting on entry locks.
+     *
+     * @param node the waiting node
+     * @return another thread's item, or CANCEL
+     */
+    private Object scanOnTimeout(Node node) {
+        Object y;
+        for (int j = arena.length - 1; j >= 0; --j) {
+            Slot slot = arena[j];
+            if (slot != null) {
+                while ((y = slot.get()) != null) {
+                    if (slot.compareAndSet(y, null)) {
+                        Node you = (Node)y;
+                        if (you.compareAndSet(null, node.item)) {
+                            LockSupport.unpark(you.waiter);
+                            return you.item;
+                        }
+                    }
+                }
+            }
+        }
+        return CANCEL;
+    }
+
+    /**
+     * Creates a new Exchanger.
+     */
+    public Exchanger() {
+    }
+
+    /**
+     * Waits for another thread to arrive at this exchange point (unless
+     * the current thread is {@linkplain Thread#interrupt interrupted}),
+     * and then transfers the given object to it, receiving its object
+     * in return.
+     *
+     * <p>If another thread is already waiting at the exchange point then
+     * it is resumed for thread scheduling purposes and receives the object
+     * passed in by the current thread.  The current thread returns immediately,
+     * receiving the object passed to the exchange by that other thread.
+     *
+     * <p>If no other thread is already waiting at the exchange then the
+     * current thread is disabled for thread scheduling purposes and lies
+     * dormant until one of two things happens:
+     * <ul>
+     * <li>Some other thread enters the exchange; or
+     * <li>Some other thread {@linkplain Thread#interrupt interrupts} the current
+     * thread.
+     * </ul>
+     * <p>If the current thread:
+     * <ul>
+     * <li>has its interrupted status set on entry to this method; or
+     * <li>is {@linkplain Thread#interrupt interrupted} while waiting
+     * for the exchange,
+     * </ul>
+     * then {@link InterruptedException} is thrown and the current thread's
+     * interrupted status is cleared.
+     *
+     * @param x the object to exchange
+     * @return the object provided by the other thread
+     * @throws InterruptedException if the current thread was
+     *         interrupted while waiting
+     */
+    public V exchange(V x) throws InterruptedException {
+        if (!Thread.interrupted()) {
+            Object v = doExchange(x == null? NULL_ITEM : x, false, 0);
+            if (v == NULL_ITEM)
+                return null;
+            if (v != CANCEL)
+                return (V)v;
+            Thread.interrupted(); // Clear interrupt status on IE throw
+        }
+        throw new InterruptedException();
+    }
+
+    /**
+     * Waits for another thread to arrive at this exchange point (unless
+     * the current thread is {@linkplain Thread#interrupt interrupted} or
+     * the specified waiting time elapses), and then transfers the given
+     * object to it, receiving its object in return.
+     *
+     * <p>If another thread is already waiting at the exchange point then
+     * it is resumed for thread scheduling purposes and receives the object
+     * passed in by the current thread.  The current thread returns immediately,
+     * receiving the object passed to the exchange by that other thread.
+     *
+     * <p>If no other thread is already waiting at the exchange then the
+     * current thread is disabled for thread scheduling purposes and lies
+     * dormant until one of three things happens:
+     * <ul>
+     * <li>Some other thread enters the exchange; or
+     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
+     * the current thread; or
+     * <li>The specified waiting time elapses.
+     * </ul>
+     * <p>If the current thread:
+     * <ul>
+     * <li>has its interrupted status set on entry to this method; or
+     * <li>is {@linkplain Thread#interrupt interrupted} while waiting
+     * for the exchange,
+     * </ul>
+     * then {@link InterruptedException} is thrown and the current thread's
+     * interrupted status is cleared.
+     *
+     * <p>If the specified waiting time elapses then {@link
+     * TimeoutException} is thrown.  If the time is less than or equal
+     * to zero, the method will not wait at all.
+     *
+     * @param x the object to exchange
+     * @param timeout the maximum time to wait
+     * @param unit the time unit of the <tt>timeout</tt> argument
+     * @return the object provided by the other thread
+     * @throws InterruptedException if the current thread was
+     *         interrupted while waiting
+     * @throws TimeoutException if the specified waiting time elapses
+     *         before another thread enters the exchange
+     */
+    public V exchange(V x, long timeout, TimeUnit unit)
+        throws InterruptedException, TimeoutException {
+        if (!Thread.interrupted()) {
+            Object v = doExchange(x == null? NULL_ITEM : x,
+                                  true, unit.toNanos(timeout));
+            if (v == NULL_ITEM)
+                return null;
+            if (v != CANCEL)
+                return (V)v;
+            if (!Thread.interrupted())
+                throw new TimeoutException();
+        }
+        throw new InterruptedException();
+    }
+}
diff --git a/external/jsr166/java/util/concurrent/ExecutionException.java b/external/jsr166/java/util/concurrent/ExecutionException.java
new file mode 100644
index 000000000..bc561e58e
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/ExecutionException.java
@@ -0,0 +1,65 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+
+/**
+ * Exception thrown when attempting to retrieve the result of a task
+ * that aborted by throwing an exception. This exception can be
+ * inspected using the {@link #getCause()} method.
+ *
+ * @see Future
+ * @since 1.5
+ * @author Doug Lea
+ */
+public class ExecutionException extends Exception {
+    private static final long serialVersionUID = 7830266012832686185L;
+
+    /**
+     * Constructs an <tt>ExecutionException</tt> with no detail message.
+     * The cause is not initialized, and may subsequently be
+     * initialized by a call to {@link #initCause(Throwable) initCause}.
+     */
+    protected ExecutionException() { }
+
+    /**
+     * Constructs an <tt>ExecutionException</tt> with the specified detail
+     * message. The cause is not initialized, and may subsequently be
+     * initialized by a call to {@link #initCause(Throwable) initCause}.
+     *
+     * @param message the detail message
+     */
+    protected ExecutionException(String message) {
+        super(message);
+    }
+
+    /**
+     * Constructs an <tt>ExecutionException</tt> with the specified detail
+     * message and cause.
+     *
+     * @param  message the detail message
+     * @param  cause the cause (which is saved for later retrieval by the
+     *         {@link #getCause()} method)
+     */
+    public ExecutionException(String message, Throwable cause) {
+        super(message, cause);
+    }
+
+    /**
+     * Constructs an <tt>ExecutionException</tt> with the specified cause.
+     * The detail message is set to:
+     * <pre>
+     *  (cause == null ? null : cause.toString())</pre>
+     * (which typically contains the class and detail message of
+     * <tt>cause</tt>).
+     *
+     * @param  cause the cause (which is saved for later retrieval by the
+     *         {@link #getCause()} method)
+     */
+    public ExecutionException(Throwable cause) {
+        super(cause);
+    }
+}
diff --git a/external/jsr166/java/util/concurrent/Executor.java b/external/jsr166/java/util/concurrent/Executor.java
new file mode 100644
index 000000000..a61e92152
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/Executor.java
@@ -0,0 +1,112 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+
+/**
+ * An object that executes submitted {@link Runnable} tasks. This
+ * interface provides a way of decoupling task submission from the
+ * mechanics of how each task will be run, including details of thread
+ * use, scheduling, etc.  An <tt>Executor</tt> is normally used
+ * instead of explicitly creating threads. For example, rather than
+ * invoking <tt>new Thread(new(RunnableTask())).start()</tt> for each
+ * of a set of tasks, you might use:
+ *
+ * <pre>
+ * Executor executor = <em>anExecutor</em>;
+ * executor.execute(new RunnableTask1());
+ * executor.execute(new RunnableTask2());
+ * ...
+ * </pre>
+ *
+ * However, the <tt>Executor</tt> interface does not strictly
+ * require that execution be asynchronous. In the simplest case, an
+ * executor can run the submitted task immediately in the caller's
+ * thread:
+ *
+ * <pre>
+ * class DirectExecutor implements Executor {
+ *     public void execute(Runnable r) {
+ *         r.run();
+ *     }
+ * }</pre>
+ *
+ * More typically, tasks are executed in some thread other
+ * than the caller's thread.  The executor below spawns a new thread
+ * for each task.
+ *
+ * <pre>
+ * class ThreadPerTaskExecutor implements Executor {
+ *     public void execute(Runnable r) {
+ *         new Thread(r).start();
+ *     }
+ * }</pre>
+ *
+ * Many <tt>Executor</tt> implementations impose some sort of
+ * limitation on how and when tasks are scheduled.  The executor below
+ * serializes the submission of tasks to a second executor,
+ * illustrating a composite executor.
+ *
+ * <pre>
+ * class SerialExecutor implements Executor {
+ *     final Queue&lt;Runnable&gt; tasks = new ArrayDeque&lt;Runnable&gt;();
+ *     final Executor executor;
+ *     Runnable active;
+ *
+ *     SerialExecutor(Executor executor) {
+ *         this.executor = executor;
+ *     }
+ *
+ *     public synchronized void execute(final Runnable r) {
+ *         tasks.offer(new Runnable() {
+ *             public void run() {
+ *                 try {
+ *                     r.run();
+ *                 } finally {
+ *                     scheduleNext();
+ *                 }
+ *             }
+ *         });
+ *         if (active == null) {
+ *             scheduleNext();
+ *         }
+ *     }
+ *
+ *     protected synchronized void scheduleNext() {
+ *         if ((active = tasks.poll()) != null) {
+ *             executor.execute(active);
+ *         }
+ *     }
+ * }</pre>
+ *
+ * The <tt>Executor</tt> implementations provided in this package
+ * implement {@link ExecutorService}, which is a more extensive
+ * interface.  The {@link ThreadPoolExecutor} class provides an
+ * extensible thread pool implementation. The {@link Executors} class
+ * provides convenient factory methods for these Executors.
+ *
+ * <p>Memory consistency effects: Actions in a thread prior to
+ * submitting a {@code Runnable} object to an {@code Executor}
+ * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
+ * its execution begins, perhaps in another thread.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ */
+public interface Executor {
+
+    /**
+     * Executes the given command at some time in the future.  The command
+     * may execute in a new thread, in a pooled thread, or in the calling
+     * thread, at the discretion of the <tt>Executor</tt> implementation.
+     *
+     * @param command the runnable task
+     * @throws RejectedExecutionException if this task cannot be
+     * accepted for execution.
+     * @throws NullPointerException if command is null
+     */
+    void execute(Runnable command);
+}
diff --git a/external/jsr166/java/util/concurrent/ExecutorCompletionService.java b/external/jsr166/java/util/concurrent/ExecutorCompletionService.java
new file mode 100644
index 000000000..9b7a0e027
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/ExecutorCompletionService.java
@@ -0,0 +1,174 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+
+/**
+ * A {@link CompletionService} that uses a supplied {@link Executor}
+ * to execute tasks.  This class arranges that submitted tasks are,
+ * upon completion, placed on a queue accessible using <tt>take</tt>.
+ * The class is lightweight enough to be suitable for transient use
+ * when processing groups of tasks.
+ *
+ * <p>
+ *
+ * <b>Usage Examples.</b>
+ *
+ * Suppose you have a set of solvers for a certain problem, each
+ * returning a value of some type <tt>Result</tt>, and would like to
+ * run them concurrently, processing the results of each of them that
+ * return a non-null value, in some method <tt>use(Result r)</tt>. You
+ * could write this as:
+ *
+ * <pre>
+ *   void solve(Executor e,
+ *              Collection&lt;Callable&lt;Result&gt;&gt; solvers)
+ *     throws InterruptedException, ExecutionException {
+ *       CompletionService&lt;Result&gt; ecs
+ *           = new ExecutorCompletionService&lt;Result&gt;(e);
+ *       for (Callable&lt;Result&gt; s : solvers)
+ *           ecs.submit(s);
+ *       int n = solvers.size();
+ *       for (int i = 0; i &lt; n; ++i) {
+ *           Result r = ecs.take().get();
+ *           if (r != null)
+ *               use(r);
+ *       }
+ *   }
+ * </pre>
+ *
+ * Suppose instead that you would like to use the first non-null result
+ * of the set of tasks, ignoring any that encounter exceptions,
+ * and cancelling all other tasks when the first one is ready:
+ *
+ * <pre>
+ *   void solve(Executor e,
+ *              Collection&lt;Callable&lt;Result&gt;&gt; solvers)
+ *     throws InterruptedException {
+ *       CompletionService&lt;Result&gt; ecs
+ *           = new ExecutorCompletionService&lt;Result&gt;(e);
+ *       int n = solvers.size();
+ *       List&lt;Future&lt;Result&gt;&gt; futures
+ *           = new ArrayList&lt;Future&lt;Result&gt;&gt;(n);
+ *       Result result = null;
+ *       try {
+ *           for (Callable&lt;Result&gt; s : solvers)
+ *               futures.add(ecs.submit(s));
+ *           for (int i = 0; i &lt; n; ++i) {
+ *               try {
+ *                   Result r = ecs.take().get();
+ *                   if (r != null) {
+ *                       result = r;
+ *                       break;
+ *                   }
+ *               } catch (ExecutionException ignore) {}
+ *           }
+ *       }
+ *       finally {
+ *           for (Future&lt;Result&gt; f : futures)
+ *               f.cancel(true);
+ *       }
+ *
+ *       if (result != null)
+ *           use(result);
+ *   }
+ * </pre>
+ */
+public class ExecutorCompletionService<V> implements CompletionService<V> {
+    private final Executor executor;
+    private final AbstractExecutorService aes;
+    private final BlockingQueue<Future<V>> completionQueue;
+
+    /**
+     * FutureTask extension to enqueue upon completion
+     */
+    private class QueueingFuture extends FutureTask<Void> {
+        QueueingFuture(RunnableFuture<V> task) {
+            super(task, null);
+            this.task = task;
+        }
+        protected void done() { completionQueue.add(task); }
+        private final Future<V> task;
+    }
+
+    private RunnableFuture<V> newTaskFor(Callable<V> task) {
+        if (aes == null)
+            return new FutureTask<V>(task);
+        else
+            return aes.newTaskFor(task);
+    }
+
+    private RunnableFuture<V> newTaskFor(Runnable task, V result) {
+        if (aes == null)
+            return new FutureTask<V>(task, result);
+        else
+            return aes.newTaskFor(task, result);
+    }
+
+    /**
+     * Creates an ExecutorCompletionService using the supplied
+     * executor for base task execution and a
+     * {@link LinkedBlockingQueue} as a completion queue.
+     *
+     * @param executor the executor to use
+     * @throws NullPointerException if executor is <tt>null</tt>
+     */
+    public ExecutorCompletionService(Executor executor) {
+        if (executor == null)
+            throw new NullPointerException();
+        this.executor = executor;
+        this.aes = (executor instanceof AbstractExecutorService) ?
+            (AbstractExecutorService) executor : null;
+        this.completionQueue = new LinkedBlockingQueue<Future<V>>();
+    }
+
+    /**
+     * Creates an ExecutorCompletionService using the supplied
+     * executor for base task execution and the supplied queue as its
+     * completion queue.
+     *
+     * @param executor the executor to use
+     * @param completionQueue the queue to use as the completion queue
+     * normally one dedicated for use by this service
+     * @throws NullPointerException if executor or completionQueue are <tt>null</tt>
+     */
+    public ExecutorCompletionService(Executor executor,
+                                     BlockingQueue<Future<V>> completionQueue) {
+        if (executor == null || completionQueue == null)
+            throw new NullPointerException();
+        this.executor = executor;
+        this.aes = (executor instanceof AbstractExecutorService) ?
+            (AbstractExecutorService) executor : null;
+        this.completionQueue = completionQueue;
+    }
+
+    public Future<V> submit(Callable<V> task) {
+        if (task == null) throw new NullPointerException();
+        RunnableFuture<V> f = newTaskFor(task);
+        executor.execute(new QueueingFuture(f));
+        return f;
+    }
+
+    public Future<V> submit(Runnable task, V result) {
+        if (task == null) throw new NullPointerException();
+        RunnableFuture<V> f = newTaskFor(task, result);
+        executor.execute(new QueueingFuture(f));
+        return f;
+    }
+
+    public Future<V> take() throws InterruptedException {
+        return completionQueue.take();
+    }
+
+    public Future<V> poll() {
+        return completionQueue.poll();
+    }
+
+    public Future<V> poll(long timeout, TimeUnit unit) throws InterruptedException {
+        return completionQueue.poll(timeout, unit);
+    }
+
+}
diff --git a/external/jsr166/java/util/concurrent/ExecutorService.java b/external/jsr166/java/util/concurrent/ExecutorService.java
new file mode 100644
index 000000000..777319265
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/ExecutorService.java
@@ -0,0 +1,306 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+import java.util.List;
+import java.util.Collection;
+import java.security.PrivilegedAction;
+import java.security.PrivilegedExceptionAction;
+
+/**
+ * An {@link Executor} that provides methods to manage termination and
+ * methods that can produce a {@link Future} for tracking progress of
+ * one or more asynchronous tasks.
+ *
+ * <p>
+ * An <tt>ExecutorService</tt> can be shut down, which will cause it
+ * to stop accepting new tasks.  After being shut down, the executor
+ * will eventually terminate, at which point no tasks are actively
+ * executing, no tasks are awaiting execution, and no new tasks can be
+ * submitted.  An unused <tt>ExecutorService</tt> should be shut down
+ * to allow reclamation of its resources.
+ *
+ * <p> Method <tt>submit</tt> extends base method {@link
+ * Executor#execute} by creating and returning a {@link Future} that
+ * can be used to cancel execution and/or wait for completion.
+ * Methods <tt>invokeAny</tt> and <tt>invokeAll</tt> perform the most
+ * commonly useful forms of bulk execution, executing a collection of
+ * tasks and then waiting for at least one, or all, to
+ * complete. (Class {@link ExecutorCompletionService} can be used to
+ * write customized variants of these methods.)
+ *
+ * <p>The {@link Executors} class provides factory methods for the
+ * executor services provided in this package.
+ *
+ * <h3>Usage Example</h3>
+ *
+ * Here is a sketch of a network service in which threads in a thread
+ * pool service incoming requests. It uses the preconfigured {@link
+ * Executors#newFixedThreadPool} factory method:
+ *
+ * <pre>
+ * class NetworkService {
+ *   private final ServerSocket serverSocket;
+ *   private final ExecutorService pool;
+ *
+ *   public NetworkService(int port, int poolSize)
+ *       throws IOException {
+ *     serverSocket = new ServerSocket(port);
+ *     pool = Executors.newFixedThreadPool(poolSize);
+ *   }
+ *
+ *   public void serve() {
+ *     try {
+ *       for (;;) {
+ *         pool.execute(new Handler(serverSocket.accept()));
+ *       }
+ *     } catch (IOException ex) {
+ *       pool.shutdown();
+ *     }
+ *   }
+ * }
+ *
+ * class Handler implements Runnable {
+ *   private final Socket socket;
+ *   Handler(Socket socket) { this.socket = socket; }
+ *   public void run() {
+ *     // read and service request
+ *   }
+ * }
+ * </pre>
+ *
+ * <p>Memory consistency effects: Actions in a thread prior to the
+ * submission of a {@code Runnable} or {@code Callable} task to an
+ * {@code ExecutorService}
+ * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
+ * any actions taken by that task, which in turn <i>happen-before</i> the
+ * result is retrieved via {@code Future.get()}.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ */
+public interface ExecutorService extends Executor {
+
+    /**
+     * Initiates an orderly shutdown in which previously submitted
+     * tasks are executed, but no new tasks will be accepted.
+     * Invocation has no additional effect if already shut down.
+     *
+     * @throws SecurityException if a security manager exists and
+     *         shutting down this ExecutorService may manipulate
+     *         threads that the caller is not permitted to modify
+     *         because it does not hold {@link
+     *         java.lang.RuntimePermission}<tt>("modifyThread")</tt>,
+     *         or the security manager's <tt>checkAccess</tt> method
+     *         denies access.
+     */
+    void shutdown();
+
+    /**
+     * Attempts to stop all actively executing tasks, halts the
+     * processing of waiting tasks, and returns a list of the tasks that were
+     * awaiting execution.
+     *
+     * <p>There are no guarantees beyond best-effort attempts to stop
+     * processing actively executing tasks.  For example, typical
+     * implementations will cancel via {@link Thread#interrupt}, so any
+     * task that fails to respond to interrupts may never terminate.
+     *
+     * @return list of tasks that never commenced execution
+     * @throws SecurityException if a security manager exists and
+     *         shutting down this ExecutorService may manipulate
+     *         threads that the caller is not permitted to modify
+     *         because it does not hold {@link
+     *         java.lang.RuntimePermission}<tt>("modifyThread")</tt>,
+     *         or the security manager's <tt>checkAccess</tt> method
+     *         denies access.
+     */
+    List<Runnable> shutdownNow();
+
+    /**
+     * Returns <tt>true</tt> if this executor has been shut down.
+     *
+     * @return <tt>true</tt> if this executor has been shut down
+     */
+    boolean isShutdown();
+
+    /**
+     * Returns <tt>true</tt> if all tasks have completed following shut down.
+     * Note that <tt>isTerminated</tt> is never <tt>true</tt> unless
+     * either <tt>shutdown</tt> or <tt>shutdownNow</tt> was called first.
+     *
+     * @return <tt>true</tt> if all tasks have completed following shut down
+     */
+    boolean isTerminated();
+
+    /**
+     * Blocks until all tasks have completed execution after a shutdown
+     * request, or the timeout occurs, or the current thread is
+     * interrupted, whichever happens first.
+     *
+     * @param timeout the maximum time to wait
+     * @param unit the time unit of the timeout argument
+     * @return <tt>true</tt> if this executor terminated and
+     *         <tt>false</tt> if the timeout elapsed before termination
+     * @throws InterruptedException if interrupted while waiting
+     */
+    boolean awaitTermination(long timeout, TimeUnit unit)
+        throws InterruptedException;
+
+
+    /**
+     * Submits a value-returning task for execution and returns a
+     * Future representing the pending results of the task. The
+     * Future's <tt>get</tt> method will return the task's result upon
+     * successful completion.
+     *
+     * <p>
+     * If you would like to immediately block waiting
+     * for a task, you can use constructions of the form
+     * <tt>result = exec.submit(aCallable).get();</tt>
+     *
+     * <p> Note: The {@link Executors} class includes a set of methods
+     * that can convert some other common closure-like objects,
+     * for example, {@link java.security.PrivilegedAction} to
+     * {@link Callable} form so they can be submitted.
+     *
+     * @param task the task to submit
+     * @return a Future representing pending completion of the task
+     * @throws RejectedExecutionException if the task cannot be
+     *         scheduled for execution
+     * @throws NullPointerException if the task is null
+     */
+    <T> Future<T> submit(Callable<T> task);
+
+    /**
+     * Submits a Runnable task for execution and returns a Future
+     * representing that task. The Future's <tt>get</tt> method will
+     * return the given result upon successful completion.
+     *
+     * @param task the task to submit
+     * @param result the result to return
+     * @return a Future representing pending completion of the task
+     * @throws RejectedExecutionException if the task cannot be
+     *         scheduled for execution
+     * @throws NullPointerException if the task is null
+     */
+    <T> Future<T> submit(Runnable task, T result);
+
+    /**
+     * Submits a Runnable task for execution and returns a Future
+     * representing that task. The Future's <tt>get</tt> method will
+     * return <tt>null</tt> upon <em>successful</em> completion.
+     *
+     * @param task the task to submit
+     * @return a Future representing pending completion of the task
+     * @throws RejectedExecutionException if the task cannot be
+     *         scheduled for execution
+     * @throws NullPointerException if the task is null
+     */
+    Future<?> submit(Runnable task);
+
+    /**
+     * Executes the given tasks, returning a list of Futures holding
+     * their status and results when all complete.
+     * {@link Future#isDone} is <tt>true</tt> for each
+     * element of the returned list.
+     * Note that a <em>completed</em> task could have
+     * terminated either normally or by throwing an exception.
+     * The results of this method are undefined if the given
+     * collection is modified while this operation is in progress.
+     *
+     * @param tasks the collection of tasks
+     * @return A list of Futures representing the tasks, in the same
+     *         sequential order as produced by the iterator for the
+     *         given task list, each of which has completed.
+     * @throws InterruptedException if interrupted while waiting, in
+     *         which case unfinished tasks are cancelled.
+     * @throws NullPointerException if tasks or any of its elements are <tt>null</tt>
+     * @throws RejectedExecutionException if any task cannot be
+     *         scheduled for execution
+     */
+
+    <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
+        throws InterruptedException;
+
+    /**
+     * Executes the given tasks, returning a list of Futures holding
+     * their status and results
+     * when all complete or the timeout expires, whichever happens first.
+     * {@link Future#isDone} is <tt>true</tt> for each
+     * element of the returned list.
+     * Upon return, tasks that have not completed are cancelled.
+     * Note that a <em>completed</em> task could have
+     * terminated either normally or by throwing an exception.
+     * The results of this method are undefined if the given
+     * collection is modified while this operation is in progress.
+     *
+     * @param tasks the collection of tasks
+     * @param timeout the maximum time to wait
+     * @param unit the time unit of the timeout argument
+     * @return a list of Futures representing the tasks, in the same
+     *         sequential order as produced by the iterator for the
+     *         given task list. If the operation did not time out,
+     *         each task will have completed. If it did time out, some
+     *         of these tasks will not have completed.
+     * @throws InterruptedException if interrupted while waiting, in
+     *         which case unfinished tasks are cancelled
+     * @throws NullPointerException if tasks, any of its elements, or
+     *         unit are <tt>null</tt>
+     * @throws RejectedExecutionException if any task cannot be scheduled
+     *         for execution
+     */
+    <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,
+                                  long timeout, TimeUnit unit)
+        throws InterruptedException;
+
+    /**
+     * Executes the given tasks, returning the result
+     * of one that has completed successfully (i.e., without throwing
+     * an exception), if any do. Upon normal or exceptional return,
+     * tasks that have not completed are cancelled.
+     * The results of this method are undefined if the given
+     * collection is modified while this operation is in progress.
+     *
+     * @param tasks the collection of tasks
+     * @return the result returned by one of the tasks
+     * @throws InterruptedException if interrupted while waiting
+     * @throws NullPointerException if tasks or any of its elements
+     *         are <tt>null</tt>
+     * @throws IllegalArgumentException if tasks is empty
+     * @throws ExecutionException if no task successfully completes
+     * @throws RejectedExecutionException if tasks cannot be scheduled
+     *         for execution
+     */
+    <T> T invokeAny(Collection<? extends Callable<T>> tasks)
+        throws InterruptedException, ExecutionException;
+
+    /**
+     * Executes the given tasks, returning the result
+     * of one that has completed successfully (i.e., without throwing
+     * an exception), if any do before the given timeout elapses.
+     * Upon normal or exceptional return, tasks that have not
+     * completed are cancelled.
+     * The results of this method are undefined if the given
+     * collection is modified while this operation is in progress.
+     *
+     * @param tasks the collection of tasks
+     * @param timeout the maximum time to wait
+     * @param unit the time unit of the timeout argument
+     * @return the result returned by one of the tasks.
+     * @throws InterruptedException if interrupted while waiting
+     * @throws NullPointerException if tasks, any of its elements, or
+     *         unit are <tt>null</tt>
+     * @throws TimeoutException if the given timeout elapses before
+     *         any task successfully completes
+     * @throws ExecutionException if no task successfully completes
+     * @throws RejectedExecutionException if tasks cannot be scheduled
+     *         for execution
+     */
+    <T> T invokeAny(Collection<? extends Callable<T>> tasks,
+                    long timeout, TimeUnit unit)
+        throws InterruptedException, ExecutionException, TimeoutException;
+}
diff --git a/external/jsr166/java/util/concurrent/Executors.java b/external/jsr166/java/util/concurrent/Executors.java
new file mode 100644
index 000000000..f44e66160
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/Executors.java
@@ -0,0 +1,666 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+import java.util.*;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.security.AccessControlContext;
+import java.security.AccessController;
+import java.security.PrivilegedAction;
+import java.security.PrivilegedExceptionAction;
+import java.security.AccessControlException;
+
+/**
+ * Factory and utility methods for {@link Executor}, {@link
+ * ExecutorService}, {@link ScheduledExecutorService}, {@link
+ * ThreadFactory}, and {@link Callable} classes defined in this
+ * package. This class supports the following kinds of methods:
+ *
+ * <ul>
+ *   <li> Methods that create and return an {@link ExecutorService}
+ *        set up with commonly useful configuration settings.
+ *   <li> Methods that create and return a {@link ScheduledExecutorService}
+ *        set up with commonly useful configuration settings.
+ *   <li> Methods that create and return a "wrapped" ExecutorService, that
+ *        disables reconfiguration by making implementation-specific methods
+ *        inaccessible.
+ *   <li> Methods that create and return a {@link ThreadFactory}
+ *        that sets newly created threads to a known state.
+ *   <li> Methods that create and return a {@link Callable}
+ *        out of other closure-like forms, so they can be used
+ *        in execution methods requiring <tt>Callable</tt>.
+ * </ul>
+ *
+ * @since 1.5
+ * @author Doug Lea
+ */
+public class Executors {
+
+    /**
+     * Creates a thread pool that reuses a fixed number of threads
+     * operating off a shared unbounded queue.  At any point, at most
+     * <tt>nThreads</tt> threads will be active processing tasks.
+     * If additional tasks are submitted when all threads are active,
+     * they will wait in the queue until a thread is available.
+     * If any thread terminates due to a failure during execution
+     * prior to shutdown, a new one will take its place if needed to
+     * execute subsequent tasks.  The threads in the pool will exist
+     * until it is explicitly {@link ExecutorService#shutdown shutdown}.
+     *
+     * @param nThreads the number of threads in the pool
+     * @return the newly created thread pool
+     * @throws IllegalArgumentException if <tt>nThreads &lt;= 0</tt>
+     */
+    public static ExecutorService newFixedThreadPool(int nThreads) {
+        return new ThreadPoolExecutor(nThreads, nThreads,
+                                      0L, TimeUnit.MILLISECONDS,
+                                      new LinkedBlockingQueue<Runnable>());
+    }
+
+    /**
+     * Creates a thread pool that reuses a fixed number of threads
+     * operating off a shared unbounded queue, using the provided
+     * ThreadFactory to create new threads when needed.  At any point,
+     * at most <tt>nThreads</tt> threads will be active processing
+     * tasks.  If additional tasks are submitted when all threads are
+     * active, they will wait in the queue until a thread is
+     * available.  If any thread terminates due to a failure during
+     * execution prior to shutdown, a new one will take its place if
+     * needed to execute subsequent tasks.  The threads in the pool will
+     * exist until it is explicitly {@link ExecutorService#shutdown
+     * shutdown}.
+     *
+     * @param nThreads the number of threads in the pool
+     * @param threadFactory the factory to use when creating new threads
+     * @return the newly created thread pool
+     * @throws NullPointerException if threadFactory is null
+     * @throws IllegalArgumentException if <tt>nThreads &lt;= 0</tt>
+     */
+    public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) {
+        return new ThreadPoolExecutor(nThreads, nThreads,
+                                      0L, TimeUnit.MILLISECONDS,
+                                      new LinkedBlockingQueue<Runnable>(),
+                                      threadFactory);
+    }
+
+    /**
+     * Creates an Executor that uses a single worker thread operating
+     * off an unbounded queue. (Note however that if this single
+     * thread terminates due to a failure during execution prior to
+     * shutdown, a new one will take its place if needed to execute
+     * subsequent tasks.)  Tasks are guaranteed to execute
+     * sequentially, and no more than one task will be active at any
+     * given time. Unlike the otherwise equivalent
+     * <tt>newFixedThreadPool(1)</tt> the returned executor is
+     * guaranteed not to be reconfigurable to use additional threads.
+     *
+     * @return the newly created single-threaded Executor
+     */
+    public static ExecutorService newSingleThreadExecutor() {
+        return new FinalizableDelegatedExecutorService
+            (new ThreadPoolExecutor(1, 1,
+                                    0L, TimeUnit.MILLISECONDS,
+                                    new LinkedBlockingQueue<Runnable>()));
+    }
+
+    /**
+     * Creates an Executor that uses a single worker thread operating
+     * off an unbounded queue, and uses the provided ThreadFactory to
+     * create a new thread when needed. Unlike the otherwise
+     * equivalent <tt>newFixedThreadPool(1, threadFactory)</tt> the
+     * returned executor is guaranteed not to be reconfigurable to use
+     * additional threads.
+     *
+     * @param threadFactory the factory to use when creating new
+     * threads
+     *
+     * @return the newly created single-threaded Executor
+     * @throws NullPointerException if threadFactory is null
+     */
+    public static ExecutorService newSingleThreadExecutor(ThreadFactory threadFactory) {
+        return new FinalizableDelegatedExecutorService
+            (new ThreadPoolExecutor(1, 1,
+                                    0L, TimeUnit.MILLISECONDS,
+                                    new LinkedBlockingQueue<Runnable>(),
+                                    threadFactory));
+    }
+
+    /**
+     * Creates a thread pool that creates new threads as needed, but
+     * will reuse previously constructed threads when they are
+     * available.  These pools will typically improve the performance
+     * of programs that execute many short-lived asynchronous tasks.
+     * Calls to <tt>execute</tt> will reuse previously constructed
+     * threads if available. If no existing thread is available, a new
+     * thread will be created and added to the pool. Threads that have
+     * not been used for sixty seconds are terminated and removed from
+     * the cache. Thus, a pool that remains idle for long enough will
+     * not consume any resources. Note that pools with similar
+     * properties but different details (for example, timeout parameters)
+     * may be created using {@link ThreadPoolExecutor} constructors.
+     *
+     * @return the newly created thread pool
+     */
+    public static ExecutorService newCachedThreadPool() {
+        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
+                                      60L, TimeUnit.SECONDS,
+                                      new SynchronousQueue<Runnable>());
+    }
+
+    /**
+     * Creates a thread pool that creates new threads as needed, but
+     * will reuse previously constructed threads when they are
+     * available, and uses the provided
+     * ThreadFactory to create new threads when needed.
+     * @param threadFactory the factory to use when creating new threads
+     * @return the newly created thread pool
+     * @throws NullPointerException if threadFactory is null
+     */
+    public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory) {
+        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
+                                      60L, TimeUnit.SECONDS,
+                                      new SynchronousQueue<Runnable>(),
+                                      threadFactory);
+    }
+
+    /**
+     * Creates a single-threaded executor that can schedule commands
+     * to run after a given delay, or to execute periodically.
+     * (Note however that if this single
+     * thread terminates due to a failure during execution prior to
+     * shutdown, a new one will take its place if needed to execute
+     * subsequent tasks.)  Tasks are guaranteed to execute
+     * sequentially, and no more than one task will be active at any
+     * given time. Unlike the otherwise equivalent
+     * <tt>newScheduledThreadPool(1)</tt> the returned executor is
+     * guaranteed not to be reconfigurable to use additional threads.
+     * @return the newly created scheduled executor
+     */
+    public static ScheduledExecutorService newSingleThreadScheduledExecutor() {
+        return new DelegatedScheduledExecutorService
+            (new ScheduledThreadPoolExecutor(1));
+    }
+
+    /**
+     * Creates a single-threaded executor that can schedule commands
+     * to run after a given delay, or to execute periodically.  (Note
+     * however that if this single thread terminates due to a failure
+     * during execution prior to shutdown, a new one will take its
+     * place if needed to execute subsequent tasks.)  Tasks are
+     * guaranteed to execute sequentially, and no more than one task
+     * will be active at any given time. Unlike the otherwise
+     * equivalent <tt>newScheduledThreadPool(1, threadFactory)</tt>
+     * the returned executor is guaranteed not to be reconfigurable to
+     * use additional threads.
+     * @param threadFactory the factory to use when creating new
+     * threads
+     * @return a newly created scheduled executor
+     * @throws NullPointerException if threadFactory is null
+     */
+    public static ScheduledExecutorService newSingleThreadScheduledExecutor(ThreadFactory threadFactory) {
+        return new DelegatedScheduledExecutorService
+            (new ScheduledThreadPoolExecutor(1, threadFactory));
+    }
+
+    /**
+     * Creates a thread pool that can schedule commands to run after a
+     * given delay, or to execute periodically.
+     * @param corePoolSize the number of threads to keep in the pool,
+     * even if they are idle.
+     * @return a newly created scheduled thread pool
+     * @throws IllegalArgumentException if <tt>corePoolSize &lt; 0</tt>
+     */
+    public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
+        return new ScheduledThreadPoolExecutor(corePoolSize);
+    }
+
+    /**
+     * Creates a thread pool that can schedule commands to run after a
+     * given delay, or to execute periodically.
+     * @param corePoolSize the number of threads to keep in the pool,
+     * even if they are idle.
+     * @param threadFactory the factory to use when the executor
+     * creates a new thread.
+     * @return a newly created scheduled thread pool
+     * @throws IllegalArgumentException if <tt>corePoolSize &lt; 0</tt>
+     * @throws NullPointerException if threadFactory is null
+     */
+    public static ScheduledExecutorService newScheduledThreadPool(
+            int corePoolSize, ThreadFactory threadFactory) {
+        return new ScheduledThreadPoolExecutor(corePoolSize, threadFactory);
+    }
+
+
+    /**
+     * Returns an object that delegates all defined {@link
+     * ExecutorService} methods to the given executor, but not any
+     * other methods that might otherwise be accessible using
+     * casts. This provides a way to safely "freeze" configuration and
+     * disallow tuning of a given concrete implementation.
+     * @param executor the underlying implementation
+     * @return an <tt>ExecutorService</tt> instance
+     * @throws NullPointerException if executor null
+     */
+    public static ExecutorService unconfigurableExecutorService(ExecutorService executor) {
+        if (executor == null)
+            throw new NullPointerException();
+        return new DelegatedExecutorService(executor);
+    }
+
+    /**
+     * Returns an object that delegates all defined {@link
+     * ScheduledExecutorService} methods to the given executor, but
+     * not any other methods that might otherwise be accessible using
+     * casts. This provides a way to safely "freeze" configuration and
+     * disallow tuning of a given concrete implementation.
+     * @param executor the underlying implementation
+     * @return a <tt>ScheduledExecutorService</tt> instance
+     * @throws NullPointerException if executor null
+     */
+    public static ScheduledExecutorService unconfigurableScheduledExecutorService(ScheduledExecutorService executor) {
+        if (executor == null)
+            throw new NullPointerException();
+        return new DelegatedScheduledExecutorService(executor);
+    }
+
+    /**
+     * Returns a default thread factory used to create new threads.
+     * This factory creates all new threads used by an Executor in the
+     * same {@link ThreadGroup}. If there is a {@link
+     * java.lang.SecurityManager}, it uses the group of {@link
+     * System#getSecurityManager}, else the group of the thread
+     * invoking this <tt>defaultThreadFactory</tt> method. Each new
+     * thread is created as a non-daemon thread with priority set to
+     * the smaller of <tt>Thread.NORM_PRIORITY</tt> and the maximum
+     * priority permitted in the thread group.  New threads have names
+     * accessible via {@link Thread#getName} of
+     * <em>pool-N-thread-M</em>, where <em>N</em> is the sequence
+     * number of this factory, and <em>M</em> is the sequence number
+     * of the thread created by this factory.
+     * @return a thread factory
+     */
+    public static ThreadFactory defaultThreadFactory() {
+        return new DefaultThreadFactory();
+    }
+
+    /**
+     * Returns a thread factory used to create new threads that
+     * have the same permissions as the current thread.
+     * This factory creates threads with the same settings as {@link
+     * Executors#defaultThreadFactory}, additionally setting the
+     * AccessControlContext and contextClassLoader of new threads to
+     * be the same as the thread invoking this
+     * <tt>privilegedThreadFactory</tt> method.  A new
+     * <tt>privilegedThreadFactory</tt> can be created within an
+     * {@link AccessController#doPrivileged} action setting the
+     * current thread's access control context to create threads with
+     * the selected permission settings holding within that action.
+     *
+     * <p> Note that while tasks running within such threads will have
+     * the same access control and class loader settings as the
+     * current thread, they need not have the same {@link
+     * java.lang.ThreadLocal} or {@link
+     * java.lang.InheritableThreadLocal} values. If necessary,
+     * particular values of thread locals can be set or reset before
+     * any task runs in {@link ThreadPoolExecutor} subclasses using
+     * {@link ThreadPoolExecutor#beforeExecute}. Also, if it is
+     * necessary to initialize worker threads to have the same
+     * InheritableThreadLocal settings as some other designated
+     * thread, you can create a custom ThreadFactory in which that
+     * thread waits for and services requests to create others that
+     * will inherit its values.
+     *
+     * @return a thread factory
+     * @throws AccessControlException if the current access control
+     * context does not have permission to both get and set context
+     * class loader.
+     */
+    public static ThreadFactory privilegedThreadFactory() {
+        return new PrivilegedThreadFactory();
+    }
+
+    /**
+     * Returns a {@link Callable} object that, when
+     * called, runs the given task and returns the given result.  This
+     * can be useful when applying methods requiring a
+     * <tt>Callable</tt> to an otherwise resultless action.
+     * @param task the task to run
+     * @param result the result to return
+     * @return a callable object
+     * @throws NullPointerException if task null
+     */
+    public static <T> Callable<T> callable(Runnable task, T result) {
+        if (task == null)
+            throw new NullPointerException();
+        return new RunnableAdapter<T>(task, result);
+    }
+
+    /**
+     * Returns a {@link Callable} object that, when
+     * called, runs the given task and returns <tt>null</tt>.
+     * @param task the task to run
+     * @return a callable object
+     * @throws NullPointerException if task null
+     */
+    public static Callable<Object> callable(Runnable task) {
+        if (task == null)
+            throw new NullPointerException();
+        return new RunnableAdapter<Object>(task, null);
+    }
+
+    /**
+     * Returns a {@link Callable} object that, when
+     * called, runs the given privileged action and returns its result.
+     * @param action the privileged action to run
+     * @return a callable object
+     * @throws NullPointerException if action null
+     */
+    public static Callable<Object> callable(final PrivilegedAction<?> action) {
+        if (action == null)
+            throw new NullPointerException();
+        return new Callable<Object>() {
+	    public Object call() { return action.run(); }};
+    }
+
+    /**
+     * Returns a {@link Callable} object that, when
+     * called, runs the given privileged exception action and returns
+     * its result.
+     * @param action the privileged exception action to run
+     * @return a callable object
+     * @throws NullPointerException if action null
+     */
+    public static Callable<Object> callable(final PrivilegedExceptionAction<?> action) {
+        if (action == null)
+            throw new NullPointerException();
+	return new Callable<Object>() {
+	    public Object call() throws Exception { return action.run(); }};
+    }
+
+    /**
+     * Returns a {@link Callable} object that will, when
+     * called, execute the given <tt>callable</tt> under the current
+     * access control context. This method should normally be
+     * invoked within an {@link AccessController#doPrivileged} action
+     * to create callables that will, if possible, execute under the
+     * selected permission settings holding within that action; or if
+     * not possible, throw an associated {@link
+     * AccessControlException}.
+     * @param callable the underlying task
+     * @return a callable object
+     * @throws NullPointerException if callable null
+     *
+     */
+    public static <T> Callable<T> privilegedCallable(Callable<T> callable) {
+        if (callable == null)
+            throw new NullPointerException();
+        return new PrivilegedCallable<T>(callable);
+    }
+
+    /**
+     * Returns a {@link Callable} object that will, when
+     * called, execute the given <tt>callable</tt> under the current
+     * access control context, with the current context class loader
+     * as the context class loader. This method should normally be
+     * invoked within an {@link AccessController#doPrivileged} action
+     * to create callables that will, if possible, execute under the
+     * selected permission settings holding within that action; or if
+     * not possible, throw an associated {@link
+     * AccessControlException}.
+     * @param callable the underlying task
+     *
+     * @return a callable object
+     * @throws NullPointerException if callable null
+     * @throws AccessControlException if the current access control
+     * context does not have permission to both set and get context
+     * class loader.
+     */
+    public static <T> Callable<T> privilegedCallableUsingCurrentClassLoader(Callable<T> callable) {
+        if (callable == null)
+            throw new NullPointerException();
+        return new PrivilegedCallableUsingCurrentClassLoader<T>(callable);
+    }
+
+    // Non-public classes supporting the public methods
+
+    /**
+     * A callable that runs given task and returns given result
+     */
+    static final class RunnableAdapter<T> implements Callable<T> {
+        final Runnable task;
+        final T result;
+        RunnableAdapter(Runnable  task, T result) {
+            this.task = task;
+            this.result = result;
+        }
+        public T call() {
+            task.run();
+            return result;
+        }
+    }
+
+    /**
+     * A callable that runs under established access control settings
+     */
+    static final class PrivilegedCallable<T> implements Callable<T> {
+        private final AccessControlContext acc;
+        private final Callable<T> task;
+        private T result;
+        private Exception exception;
+        PrivilegedCallable(Callable<T> task) {
+            this.task = task;
+            this.acc = AccessController.getContext();
+        }
+
+        public T call() throws Exception {
+            AccessController.doPrivileged(new PrivilegedAction<T>() {
+                    public T run() {
+                        try {
+                            result = task.call();
+                        } catch (Exception ex) {
+                            exception = ex;
+                        }
+                        return null;
+                    }
+                }, acc);
+            if (exception != null)
+                throw exception;
+            else
+                return result;
+        }
+    }
+
+    /**
+     * A callable that runs under established access control settings and
+     * current ClassLoader
+     */
+    static final class PrivilegedCallableUsingCurrentClassLoader<T> implements Callable<T> {
+        private final ClassLoader ccl;
+        private final AccessControlContext acc;
+        private final Callable<T> task;
+        private T result;
+        private Exception exception;
+        PrivilegedCallableUsingCurrentClassLoader(Callable<T> task) {
+            this.task = task;
+            this.ccl = Thread.currentThread().getContextClassLoader();
+            this.acc = AccessController.getContext();
+            acc.checkPermission(new RuntimePermission("getContextClassLoader"));
+            acc.checkPermission(new RuntimePermission("setContextClassLoader"));
+        }
+
+        public T call() throws Exception {
+            AccessController.doPrivileged(new PrivilegedAction<T>() {
+                    public T run() {
+                        ClassLoader savedcl = null;
+                        Thread t = Thread.currentThread();
+                        try {
+                            ClassLoader cl = t.getContextClassLoader();
+                            if (ccl != cl) {
+                                t.setContextClassLoader(ccl);
+                                savedcl = cl;
+                            }
+                            result = task.call();
+                        } catch (Exception ex) {
+                            exception = ex;
+                        } finally {
+                            if (savedcl != null)
+                                t.setContextClassLoader(savedcl);
+                        }
+                        return null;
+                    }
+                }, acc);
+            if (exception != null)
+                throw exception;
+            else
+                return result;
+        }
+    }
+
+    /**
+     * The default thread factory
+     */
+    static class DefaultThreadFactory implements ThreadFactory {
+        static final AtomicInteger poolNumber = new AtomicInteger(1);
+        final ThreadGroup group;
+        final AtomicInteger threadNumber = new AtomicInteger(1);
+        final String namePrefix;
+
+        DefaultThreadFactory() {
+            SecurityManager s = System.getSecurityManager();
+            group = (s != null)? s.getThreadGroup() :
+                                 Thread.currentThread().getThreadGroup();
+            namePrefix = "pool-" +
+                          poolNumber.getAndIncrement() +
+                         "-thread-";
+        }
+
+        public Thread newThread(Runnable r) {
+            Thread t = new Thread(group, r,
+                                  namePrefix + threadNumber.getAndIncrement(),
+                                  0);
+            if (t.isDaemon())
+                t.setDaemon(false);
+            if (t.getPriority() != Thread.NORM_PRIORITY)
+                t.setPriority(Thread.NORM_PRIORITY);
+            return t;
+        }
+    }
+
+    /**
+     *  Thread factory capturing access control and class loader
+     */
+    static class PrivilegedThreadFactory extends DefaultThreadFactory {
+        private final ClassLoader ccl;
+        private final AccessControlContext acc;
+
+        PrivilegedThreadFactory() {
+            super();
+            this.ccl = Thread.currentThread().getContextClassLoader();
+            this.acc = AccessController.getContext();
+            acc.checkPermission(new RuntimePermission("setContextClassLoader"));
+        }
+
+        public Thread newThread(final Runnable r) {
+            return super.newThread(new Runnable() {
+                public void run() {
+                    AccessController.doPrivileged(new PrivilegedAction<Object>() {
+                        public Object run() {
+                            Thread.currentThread().setContextClassLoader(ccl);
+                            r.run();
+                            return null;
+                        }
+                    }, acc);
+                }
+            });
+        }
+
+    }
+
+    /**
+     * A wrapper class that exposes only the ExecutorService methods
+     * of an ExecutorService implementation.
+     */
+    static class DelegatedExecutorService extends AbstractExecutorService {
+        private final ExecutorService e;
+        DelegatedExecutorService(ExecutorService executor) { e = executor; }
+        public void execute(Runnable command) { e.execute(command); }
+        public void shutdown() { e.shutdown(); }
+        public List<Runnable> shutdownNow() { return e.shutdownNow(); }
+        public boolean isShutdown() { return e.isShutdown(); }
+        public boolean isTerminated() { return e.isTerminated(); }
+        public boolean awaitTermination(long timeout, TimeUnit unit)
+            throws InterruptedException {
+            return e.awaitTermination(timeout, unit);
+        }
+        public Future<?> submit(Runnable task) {
+            return e.submit(task);
+        }
+        public <T> Future<T> submit(Callable<T> task) {
+            return e.submit(task);
+        }
+        public <T> Future<T> submit(Runnable task, T result) {
+            return e.submit(task, result);
+        }
+        public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
+            throws InterruptedException {
+            return e.invokeAll(tasks);
+        }
+        public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,
+                                             long timeout, TimeUnit unit)
+            throws InterruptedException {
+            return e.invokeAll(tasks, timeout, unit);
+        }
+        public <T> T invokeAny(Collection<? extends Callable<T>> tasks)
+            throws InterruptedException, ExecutionException {
+            return e.invokeAny(tasks);
+        }
+        public <T> T invokeAny(Collection<? extends Callable<T>> tasks,
+                               long timeout, TimeUnit unit)
+            throws InterruptedException, ExecutionException, TimeoutException {
+            return e.invokeAny(tasks, timeout, unit);
+        }
+    }
+
+    static class FinalizableDelegatedExecutorService
+	extends DelegatedExecutorService {
+	FinalizableDelegatedExecutorService(ExecutorService executor) {
+	    super(executor);
+	}
+	protected void finalize()  {
+	    super.shutdown();
+	}
+    }
+
+    /**
+     * A wrapper class that exposes only the ScheduledExecutorService
+     * methods of a ScheduledExecutorService implementation.
+     */
+    static class DelegatedScheduledExecutorService
+            extends DelegatedExecutorService
+            implements ScheduledExecutorService {
+        private final ScheduledExecutorService e;
+        DelegatedScheduledExecutorService(ScheduledExecutorService executor) {
+            super(executor);
+            e = executor;
+        }
+        public ScheduledFuture<?> schedule(Runnable command, long delay,  TimeUnit unit) {
+            return e.schedule(command, delay, unit);
+        }
+        public <V> ScheduledFuture<V> schedule(Callable<V> callable, long delay, TimeUnit unit) {
+            return e.schedule(callable, delay, unit);
+        }
+        public ScheduledFuture<?> scheduleAtFixedRate(Runnable command, long initialDelay,  long period, TimeUnit unit) {
+            return e.scheduleAtFixedRate(command, initialDelay, period, unit);
+        }
+        public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command, long initialDelay,  long delay, TimeUnit unit) {
+            return e.scheduleWithFixedDelay(command, initialDelay, delay, unit);
+        }
+    }
+
+
+    /** Cannot instantiate. */
+    private Executors() {}
+}
diff --git a/external/jsr166/java/util/concurrent/Future.java b/external/jsr166/java/util/concurrent/Future.java
new file mode 100644
index 000000000..0459ee453
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/Future.java
@@ -0,0 +1,142 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+
+/**
+ * A <tt>Future</tt> represents the result of an asynchronous
+ * computation.  Methods are provided to check if the computation is
+ * complete, to wait for its completion, and to retrieve the result of
+ * the computation.  The result can only be retrieved using method
+ * <tt>get</tt> when the computation has completed, blocking if
+ * necessary until it is ready.  Cancellation is performed by the
+ * <tt>cancel</tt> method.  Additional methods are provided to
+ * determine if the task completed normally or was cancelled. Once a
+ * computation has completed, the computation cannot be cancelled.
+ * If you would like to use a <tt>Future</tt> for the sake
+ * of cancellability but not provide a usable result, you can
+ * declare types of the form <tt>Future&lt;?&gt;</tt> and
+ * return <tt>null</tt> as a result of the underlying task.
+ *
+ * <p>
+ * <b>Sample Usage</b> (Note that the following classes are all
+ * made-up.) <p>
+ * <pre>
+ * interface ArchiveSearcher { String search(String target); }
+ * class App {
+ *   ExecutorService executor = ...
+ *   ArchiveSearcher searcher = ...
+ *   void showSearch(final String target)
+ *       throws InterruptedException {
+ *     Future&lt;String&gt; future
+ *       = executor.submit(new Callable&lt;String&gt;() {
+ *         public String call() {
+ *             return searcher.search(target);
+ *         }});
+ *     displayOtherThings(); // do other things while searching
+ *     try {
+ *       displayText(future.get()); // use future
+ *     } catch (ExecutionException ex) { cleanup(); return; }
+ *   }
+ * }
+ * </pre>
+ *
+ * The {@link FutureTask} class is an implementation of <tt>Future</tt> that
+ * implements <tt>Runnable</tt>, and so may be executed by an <tt>Executor</tt>.
+ * For example, the above construction with <tt>submit</tt> could be replaced by:
+ * <pre>
+ *     FutureTask&lt;String&gt; future =
+ *       new FutureTask&lt;String&gt;(new Callable&lt;String&gt;() {
+ *         public String call() {
+ *           return searcher.search(target);
+ *       }});
+ *     executor.execute(future);
+ * </pre>
+ *
+ * <p>Memory consistency effects: Actions taken by the asynchronous computation
+ * <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a>
+ * actions following the corresponding {@code Future.get()} in another thread.
+ *
+ * @see FutureTask
+ * @see Executor
+ * @since 1.5
+ * @author Doug Lea
+ * @param <V> The result type returned by this Future's <tt>get</tt> method
+ */
+public interface Future<V> {
+
+    /**
+     * Attempts to cancel execution of this task.  This attempt will
+     * fail if the task has already completed, has already been cancelled,
+     * or could not be cancelled for some other reason. If successful,
+     * and this task has not started when <tt>cancel</tt> is called,
+     * this task should never run.  If the task has already started,
+     * then the <tt>mayInterruptIfRunning</tt> parameter determines
+     * whether the thread executing this task should be interrupted in
+     * an attempt to stop the task.
+     *
+     * <p>After this method returns, subsequent calls to {@link #isDone} will
+     * always return <tt>true</tt>.  Subsequent calls to {@link #isCancelled}
+     * will always return <tt>true</tt> if this method returned <tt>true</tt>.
+     *
+     * @param mayInterruptIfRunning <tt>true</tt> if the thread executing this
+     * task should be interrupted; otherwise, in-progress tasks are allowed
+     * to complete
+     * @return <tt>false</tt> if the task could not be cancelled,
+     * typically because it has already completed normally;
+     * <tt>true</tt> otherwise
+     */
+    boolean cancel(boolean mayInterruptIfRunning);
+
+    /**
+     * Returns <tt>true</tt> if this task was cancelled before it completed
+     * normally.
+     *
+     * @return <tt>true</tt> if this task was cancelled before it completed
+     */
+    boolean isCancelled();
+
+    /**
+     * Returns <tt>true</tt> if this task completed.
+     *
+     * Completion may be due to normal termination, an exception, or
+     * cancellation -- in all of these cases, this method will return
+     * <tt>true</tt>.
+     *
+     * @return <tt>true</tt> if this task completed
+     */
+    boolean isDone();
+
+    /**
+     * Waits if necessary for the computation to complete, and then
+     * retrieves its result.
+     *
+     * @return the computed result
+     * @throws CancellationException if the computation was cancelled
+     * @throws ExecutionException if the computation threw an
+     * exception
+     * @throws InterruptedException if the current thread was interrupted
+     * while waiting
+     */
+    V get() throws InterruptedException, ExecutionException;
+
+    /**
+     * Waits if necessary for at most the given time for the computation
+     * to complete, and then retrieves its result, if available.
+     *
+     * @param timeout the maximum time to wait
+     * @param unit the time unit of the timeout argument
+     * @return the computed result
+     * @throws CancellationException if the computation was cancelled
+     * @throws ExecutionException if the computation threw an
+     * exception
+     * @throws InterruptedException if the current thread was interrupted
+     * while waiting
+     * @throws TimeoutException if the wait timed out
+     */
+    V get(long timeout, TimeUnit unit)
+        throws InterruptedException, ExecutionException, TimeoutException;
+}
diff --git a/external/jsr166/java/util/concurrent/FutureTask.java b/external/jsr166/java/util/concurrent/FutureTask.java
new file mode 100644
index 000000000..bbe49d7ae
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/FutureTask.java
@@ -0,0 +1,325 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+import java.util.concurrent.locks.*;
+
+/**
+ * A cancellable asynchronous computation.  This class provides a base
+ * implementation of {@link Future}, with methods to start and cancel
+ * a computation, query to see if the computation is complete, and
+ * retrieve the result of the computation.  The result can only be
+ * retrieved when the computation has completed; the <tt>get</tt>
+ * method will block if the computation has not yet completed.  Once
+ * the computation has completed, the computation cannot be restarted
+ * or cancelled.
+ *
+ * <p>A <tt>FutureTask</tt> can be used to wrap a {@link Callable} or
+ * {@link java.lang.Runnable} object.  Because <tt>FutureTask</tt>
+ * implements <tt>Runnable</tt>, a <tt>FutureTask</tt> can be
+ * submitted to an {@link Executor} for execution.
+ *
+ * <p>In addition to serving as a standalone class, this class provides
+ * <tt>protected</tt> functionality that may be useful when creating
+ * customized task classes.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ * @param <V> The result type returned by this FutureTask's <tt>get</tt> method
+ */
+public class FutureTask<V> implements RunnableFuture<V> {
+    /** Synchronization control for FutureTask */
+    private final Sync sync;
+
+    /**
+     * Creates a <tt>FutureTask</tt> that will upon running, execute the
+     * given <tt>Callable</tt>.
+     *
+     * @param  callable the callable task
+     * @throws NullPointerException if callable is null
+     */
+    public FutureTask(Callable<V> callable) {
+        if (callable == null)
+            throw new NullPointerException();
+        sync = new Sync(callable);
+    }
+
+    /**
+     * Creates a <tt>FutureTask</tt> that will upon running, execute the
+     * given <tt>Runnable</tt>, and arrange that <tt>get</tt> will return the
+     * given result on successful completion.
+     *
+     * @param  runnable the runnable task
+     * @param result the result to return on successful completion. If
+     * you don't need a particular result, consider using
+     * constructions of the form:
+     * <tt>Future&lt;?&gt; f = new FutureTask&lt;Object&gt;(runnable, null)</tt>
+     * @throws NullPointerException if runnable is null
+     */
+    public FutureTask(Runnable runnable, V result) {
+        sync = new Sync(Executors.callable(runnable, result));
+    }
+
+    public boolean isCancelled() {
+        return sync.innerIsCancelled();
+    }
+
+    public boolean isDone() {
+        return sync.innerIsDone();
+    }
+
+    public boolean cancel(boolean mayInterruptIfRunning) {
+        return sync.innerCancel(mayInterruptIfRunning);
+    }
+
+    /**
+     * @throws CancellationException {@inheritDoc}
+     */
+    public V get() throws InterruptedException, ExecutionException {
+        return sync.innerGet();
+    }
+
+    /**
+     * @throws CancellationException {@inheritDoc}
+     */
+    public V get(long timeout, TimeUnit unit)
+        throws InterruptedException, ExecutionException, TimeoutException {
+        return sync.innerGet(unit.toNanos(timeout));
+    }
+
+    /**
+     * Protected method invoked when this task transitions to state
+     * <tt>isDone</tt> (whether normally or via cancellation). The
+     * default implementation does nothing.  Subclasses may override
+     * this method to invoke completion callbacks or perform
+     * bookkeeping. Note that you can query status inside the
+     * implementation of this method to determine whether this task
+     * has been cancelled.
+     */
+    protected void done() { }
+
+    /**
+     * Sets the result of this Future to the given value unless
+     * this future has already been set or has been cancelled.
+     * This method is invoked internally by the <tt>run</tt> method
+     * upon successful completion of the computation.
+     * @param v the value
+     */
+    protected void set(V v) {
+        sync.innerSet(v);
+    }
+
+    /**
+     * Causes this future to report an <tt>ExecutionException</tt>
+     * with the given throwable as its cause, unless this Future has
+     * already been set or has been cancelled.
+     * This method is invoked internally by the <tt>run</tt> method
+     * upon failure of the computation.
+     * @param t the cause of failure
+     */
+    protected void setException(Throwable t) {
+        sync.innerSetException(t);
+    }
+
+    // The following (duplicated) doc comment can be removed once
+    //
+    // 6270645: Javadoc comments should be inherited from most derived
+    //          superinterface or superclass
+    // is fixed.
+    /**
+     * Sets this Future to the result of its computation
+     * unless it has been cancelled.
+     */
+    public void run() {
+        sync.innerRun();
+    }
+
+    /**
+     * Executes the computation without setting its result, and then
+     * resets this Future to initial state, failing to do so if the
+     * computation encounters an exception or is cancelled.  This is
+     * designed for use with tasks that intrinsically execute more
+     * than once.
+     * @return true if successfully run and reset
+     */
+    protected boolean runAndReset() {
+        return sync.innerRunAndReset();
+    }
+
+    /**
+     * Synchronization control for FutureTask. Note that this must be
+     * a non-static inner class in order to invoke the protected
+     * <tt>done</tt> method. For clarity, all inner class support
+     * methods are same as outer, prefixed with "inner".
+     *
+     * Uses AQS sync state to represent run status
+     */
+    private final class Sync extends AbstractQueuedSynchronizer {
+        private static final long serialVersionUID = -7828117401763700385L;
+
+        /** State value representing that task is running */
+        private static final int RUNNING   = 1;
+        /** State value representing that task ran */
+        private static final int RAN       = 2;
+        /** State value representing that task was cancelled */
+        private static final int CANCELLED = 4;
+
+        /** The underlying callable */
+        private final Callable<V> callable;
+        /** The result to return from get() */
+        private V result;
+        /** The exception to throw from get() */
+        private Throwable exception;
+
+        /**
+         * The thread running task. When nulled after set/cancel, this
+         * indicates that the results are accessible.  Must be
+         * volatile, to ensure visibility upon completion.
+         */
+        private volatile Thread runner;
+
+        Sync(Callable<V> callable) {
+            this.callable = callable;
+        }
+
+        private boolean ranOrCancelled(int state) {
+            return (state & (RAN | CANCELLED)) != 0;
+        }
+
+        /**
+         * Implements AQS base acquire to succeed if ran or cancelled
+         */
+        protected int tryAcquireShared(int ignore) {
+            return innerIsDone()? 1 : -1;
+        }
+
+        /**
+         * Implements AQS base release to always signal after setting
+         * final done status by nulling runner thread.
+         */
+        protected boolean tryReleaseShared(int ignore) {
+            runner = null;
+            return true;
+        }
+
+        boolean innerIsCancelled() {
+            return getState() == CANCELLED;
+        }
+
+        boolean innerIsDone() {
+            return ranOrCancelled(getState()) && runner == null;
+        }
+
+        V innerGet() throws InterruptedException, ExecutionException {
+            acquireSharedInterruptibly(0);
+            if (getState() == CANCELLED)
+                throw new CancellationException();
+            if (exception != null)
+                throw new ExecutionException(exception);
+            return result;
+        }
+
+        V innerGet(long nanosTimeout) throws InterruptedException, ExecutionException, TimeoutException {
+            if (!tryAcquireSharedNanos(0, nanosTimeout))
+                throw new TimeoutException();
+            if (getState() == CANCELLED)
+                throw new CancellationException();
+            if (exception != null)
+                throw new ExecutionException(exception);
+            return result;
+        }
+
+        void innerSet(V v) {
+	    for (;;) {
+		int s = getState();
+		if (s == RAN)
+		    return;
+                if (s == CANCELLED) {
+		    // aggressively release to set runner to null,
+		    // in case we are racing with a cancel request
+		    // that will try to interrupt runner
+                    releaseShared(0);
+                    return;
+                }
+		if (compareAndSetState(s, RAN)) {
+                    result = v;
+                    releaseShared(0);
+                    done();
+		    return;
+                }
+            }
+        }
+
+        void innerSetException(Throwable t) {
+	    for (;;) {
+		int s = getState();
+		if (s == RAN)
+		    return;
+                if (s == CANCELLED) {
+		    // aggressively release to set runner to null,
+		    // in case we are racing with a cancel request
+		    // that will try to interrupt runner
+                    releaseShared(0);
+                    return;
+                }
+		if (compareAndSetState(s, RAN)) {
+                    exception = t;
+                    result = null;
+                    releaseShared(0);
+                    done();
+		    return;
+                }
+	    }
+        }
+
+        boolean innerCancel(boolean mayInterruptIfRunning) {
+	    for (;;) {
+		int s = getState();
+		if (ranOrCancelled(s))
+		    return false;
+		if (compareAndSetState(s, CANCELLED))
+		    break;
+	    }
+            if (mayInterruptIfRunning) {
+                Thread r = runner;
+                if (r != null) 
+                    r.interrupt();
+            }
+            releaseShared(0);
+            done();
+            return true;
+        }
+
+        void innerRun() {
+            if (!compareAndSetState(0, RUNNING))
+                return;
+            try {
+                runner = Thread.currentThread();
+                if (getState() == RUNNING) // recheck after setting thread
+                    innerSet(callable.call());
+                else
+                    releaseShared(0); // cancel
+            } catch (Throwable ex) {
+                innerSetException(ex);
+            }
+        }
+
+        boolean innerRunAndReset() {
+            if (!compareAndSetState(0, RUNNING))
+                return false;
+            try {
+                runner = Thread.currentThread();
+                if (getState() == RUNNING)
+                    callable.call(); // don't set result
+                runner = null;
+                return compareAndSetState(RUNNING, 0);
+            } catch (Throwable ex) {
+                innerSetException(ex);
+                return false;
+            }
+        }
+    }
+}
diff --git a/external/jsr166/java/util/concurrent/LinkedBlockingDeque.java b/external/jsr166/java/util/concurrent/LinkedBlockingDeque.java
new file mode 100644
index 000000000..91acccfbe
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/LinkedBlockingDeque.java
@@ -0,0 +1,1021 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+import java.util.*;
+import java.util.concurrent.locks.*;
+
+/**
+ * An optionally-bounded {@linkplain BlockingDeque blocking deque} based on
+ * linked nodes.
+ *
+ * <p> The optional capacity bound constructor argument serves as a
+ * way to prevent excessive expansion. The capacity, if unspecified,
+ * is equal to {@link Integer#MAX_VALUE}.  Linked nodes are
+ * dynamically created upon each insertion unless this would bring the
+ * deque above capacity.
+ *
+ * <p>Most operations run in constant time (ignoring time spent
+ * blocking).  Exceptions include {@link #remove(Object) remove},
+ * {@link #removeFirstOccurrence removeFirstOccurrence}, {@link
+ * #removeLastOccurrence removeLastOccurrence}, {@link #contains
+ * contains}, {@link #iterator iterator.remove()}, and the bulk
+ * operations, all of which run in linear time.
+ *
+ * <p>This class and its iterator implement all of the
+ * <em>optional</em> methods of the {@link Collection} and {@link
+ * Iterator} interfaces.
+ *
+ * <p>This class is a member of the
+ * <a href="{@docRoot}/../technotes/guides/collections/index.html">
+ * Java Collections Framework</a>.
+ *
+ * @since 1.6
+ * @author  Doug Lea
+ * @param <E> the type of elements held in this collection
+ */
+public class LinkedBlockingDeque<E>
+    extends AbstractQueue<E>
+    implements BlockingDeque<E>,  java.io.Serializable {
+
+    /*
+     * Implemented as a simple doubly-linked list protected by a
+     * single lock and using conditions to manage blocking.
+     */
+
+    /*
+     * We have "diamond" multiple interface/abstract class inheritance
+     * here, and that introduces ambiguities. Often we want the
+     * BlockingDeque javadoc combined with the AbstractQueue
+     * implementation, so a lot of method specs are duplicated here.
+     */
+
+    private static final long serialVersionUID = -387911632671998426L;
+
+    /** Doubly-linked list node class */
+    static final class Node<E> {
+	E item;
+        Node<E> prev;
+        Node<E> next;
+        Node(E x, Node<E> p, Node<E> n) {
+            item = x;
+            prev = p;
+            next = n;
+        }
+    }
+
+    /** Pointer to first node */
+    private transient Node<E> first;
+    /** Pointer to last node */
+    private transient Node<E> last;
+    /** Number of items in the deque */
+    private transient int count;
+    /** Maximum number of items in the deque */
+    private final int capacity;
+    /** Main lock guarding all access */
+    private final ReentrantLock lock = new ReentrantLock();
+    /** Condition for waiting takes */
+    private final Condition notEmpty = lock.newCondition();
+    /** Condition for waiting puts */
+    private final Condition notFull = lock.newCondition();
+
+    /**
+     * Creates a <tt>LinkedBlockingDeque</tt> with a capacity of
+     * {@link Integer#MAX_VALUE}.
+     */
+    public LinkedBlockingDeque() {
+        this(Integer.MAX_VALUE);
+    }
+
+    /**
+     * Creates a <tt>LinkedBlockingDeque</tt> with the given (fixed) capacity.
+     *
+     * @param capacity the capacity of this deque
+     * @throws IllegalArgumentException if <tt>capacity</tt> is less than 1
+     */
+    public LinkedBlockingDeque(int capacity) {
+        if (capacity <= 0) throw new IllegalArgumentException();
+        this.capacity = capacity;
+    }
+
+    /**
+     * Creates a <tt>LinkedBlockingDeque</tt> with a capacity of
+     * {@link Integer#MAX_VALUE}, initially containing the elements of
+     * the given collection, added in traversal order of the
+     * collection's iterator.
+     *
+     * @param c the collection of elements to initially contain
+     * @throws NullPointerException if the specified collection or any
+     *         of its elements are null
+     */
+    public LinkedBlockingDeque(Collection<? extends E> c) {
+        this(Integer.MAX_VALUE);
+        for (E e : c)
+            add(e);
+    }
+
+
+    // Basic linking and unlinking operations, called only while holding lock
+
+    /**
+     * Links e as first element, or returns false if full.
+     */
+    private boolean linkFirst(E e) {
+        if (count >= capacity)
+            return false;
+        ++count;
+        Node<E> f = first;
+        Node<E> x = new Node<E>(e, null, f);
+        first = x;
+        if (last == null)
+            last = x;
+        else
+            f.prev = x;
+        notEmpty.signal();
+        return true;
+    }
+
+    /**
+     * Links e as last element, or returns false if full.
+     */
+    private boolean linkLast(E e) {
+        if (count >= capacity)
+            return false;
+        ++count;
+        Node<E> l = last;
+        Node<E> x = new Node<E>(e, l, null);
+        last = x;
+        if (first == null)
+            first = x;
+        else
+            l.next = x;
+        notEmpty.signal();
+        return true;
+    }
+
+    /**
+     * Removes and returns first element, or null if empty.
+     */
+    private E unlinkFirst() {
+        Node<E> f = first;
+        if (f == null)
+            return null;
+        Node<E> n = f.next;
+        first = n;
+        if (n == null)
+            last = null;
+        else
+            n.prev = null;
+        --count;
+        notFull.signal();
+        return f.item;
+    }
+
+    /**
+     * Removes and returns last element, or null if empty.
+     */
+    private E unlinkLast() {
+        Node<E> l = last;
+        if (l == null)
+            return null;
+        Node<E> p = l.prev;
+        last = p;
+        if (p == null)
+            first = null;
+        else
+            p.next = null;
+        --count;
+        notFull.signal();
+        return l.item;
+    }
+
+    /**
+     * Unlink e
+     */
+    private void unlink(Node<E> x) {
+        Node<E> p = x.prev;
+        Node<E> n = x.next;
+        if (p == null) {
+            if (n == null)
+                first = last = null;
+            else {
+                n.prev = null;
+                first = n;
+            }
+        } else if (n == null) {
+            p.next = null;
+            last = p;
+        } else {
+            p.next = n;
+            n.prev = p;
+        }
+        --count;
+        notFull.signalAll();
+    }
+
+    // BlockingDeque methods
+
+    /**
+     * @throws IllegalStateException {@inheritDoc}
+     * @throws NullPointerException  {@inheritDoc}
+     */
+    public void addFirst(E e) {
+        if (!offerFirst(e))
+            throw new IllegalStateException("Deque full");
+    }
+
+    /**
+     * @throws IllegalStateException {@inheritDoc}
+     * @throws NullPointerException  {@inheritDoc}
+     */
+    public void addLast(E e) {
+        if (!offerLast(e))
+            throw new IllegalStateException("Deque full");
+    }
+
+    /**
+     * @throws NullPointerException {@inheritDoc}
+     */
+    public boolean offerFirst(E e) {
+        if (e == null) throw new NullPointerException();
+        lock.lock();
+        try {
+            return linkFirst(e);
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * @throws NullPointerException {@inheritDoc}
+     */
+    public boolean offerLast(E e) {
+        if (e == null) throw new NullPointerException();
+        lock.lock();
+        try {
+            return linkLast(e);
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * @throws NullPointerException {@inheritDoc}
+     * @throws InterruptedException {@inheritDoc}
+     */
+    public void putFirst(E e) throws InterruptedException {
+        if (e == null) throw new NullPointerException();
+        lock.lock();
+        try {
+            while (!linkFirst(e))
+                notFull.await();
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * @throws NullPointerException {@inheritDoc}
+     * @throws InterruptedException {@inheritDoc}
+     */
+    public void putLast(E e) throws InterruptedException {
+        if (e == null) throw new NullPointerException();
+        lock.lock();
+        try {
+            while (!linkLast(e))
+                notFull.await();
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * @throws NullPointerException {@inheritDoc}
+     * @throws InterruptedException {@inheritDoc}
+     */
+    public boolean offerFirst(E e, long timeout, TimeUnit unit)
+        throws InterruptedException {
+        if (e == null) throw new NullPointerException();
+	long nanos = unit.toNanos(timeout);
+        lock.lockInterruptibly();
+        try {
+            for (;;) {
+                if (linkFirst(e))
+                    return true;
+                if (nanos <= 0)
+                    return false;
+                nanos = notFull.awaitNanos(nanos);
+            }
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * @throws NullPointerException {@inheritDoc}
+     * @throws InterruptedException {@inheritDoc}
+     */
+    public boolean offerLast(E e, long timeout, TimeUnit unit)
+        throws InterruptedException {
+        if (e == null) throw new NullPointerException();
+	long nanos = unit.toNanos(timeout);
+        lock.lockInterruptibly();
+        try {
+            for (;;) {
+                if (linkLast(e))
+                    return true;
+                if (nanos <= 0)
+                    return false;
+                nanos = notFull.awaitNanos(nanos);
+            }
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * @throws NoSuchElementException {@inheritDoc}
+     */
+    public E removeFirst() {
+        E x = pollFirst();
+        if (x == null) throw new NoSuchElementException();
+        return x;
+    }
+
+    /**
+     * @throws NoSuchElementException {@inheritDoc}
+     */
+    public E removeLast() {
+        E x = pollLast();
+        if (x == null) throw new NoSuchElementException();
+        return x;
+    }
+
+    public E pollFirst() {
+        lock.lock();
+        try {
+            return unlinkFirst();
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    public E pollLast() {
+        lock.lock();
+        try {
+            return unlinkLast();
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    public E takeFirst() throws InterruptedException {
+        lock.lock();
+        try {
+            E x;
+            while ( (x = unlinkFirst()) == null)
+                notEmpty.await();
+            return x;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    public E takeLast() throws InterruptedException {
+        lock.lock();
+        try {
+            E x;
+            while ( (x = unlinkLast()) == null)
+                notEmpty.await();
+            return x;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    public E pollFirst(long timeout, TimeUnit unit)
+        throws InterruptedException {
+	long nanos = unit.toNanos(timeout);
+        lock.lockInterruptibly();
+        try {
+            for (;;) {
+                E x = unlinkFirst();
+                if (x != null)
+                    return x;
+                if (nanos <= 0)
+                    return null;
+                nanos = notEmpty.awaitNanos(nanos);
+            }
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    public E pollLast(long timeout, TimeUnit unit)
+        throws InterruptedException {
+	long nanos = unit.toNanos(timeout);
+        lock.lockInterruptibly();
+        try {
+            for (;;) {
+                E x = unlinkLast();
+                if (x != null)
+                    return x;
+                if (nanos <= 0)
+                    return null;
+                nanos = notEmpty.awaitNanos(nanos);
+            }
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * @throws NoSuchElementException {@inheritDoc}
+     */
+    public E getFirst() {
+        E x = peekFirst();
+        if (x == null) throw new NoSuchElementException();
+        return x;
+    }
+
+    /**
+     * @throws NoSuchElementException {@inheritDoc}
+     */
+    public E getLast() {
+        E x = peekLast();
+        if (x == null) throw new NoSuchElementException();
+        return x;
+    }
+
+    public E peekFirst() {
+        lock.lock();
+        try {
+            return (first == null) ? null : first.item;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    public E peekLast() {
+        lock.lock();
+        try {
+            return (last == null) ? null : last.item;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    public boolean removeFirstOccurrence(Object o) {
+        if (o == null) return false;
+        lock.lock();
+        try {
+            for (Node<E> p = first; p != null; p = p.next) {
+                if (o.equals(p.item)) {
+                    unlink(p);
+                    return true;
+                }
+            }
+            return false;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    public boolean removeLastOccurrence(Object o) {
+        if (o == null) return false;
+        lock.lock();
+        try {
+            for (Node<E> p = last; p != null; p = p.prev) {
+                if (o.equals(p.item)) {
+                    unlink(p);
+                    return true;
+                }
+            }
+            return false;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    // BlockingQueue methods
+
+    /**
+     * Inserts the specified element at the end of this deque unless it would
+     * violate capacity restrictions.  When using a capacity-restricted deque,
+     * it is generally preferable to use method {@link #offer(Object) offer}.
+     *
+     * <p>This method is equivalent to {@link #addLast}.
+     *
+     * @throws IllegalStateException if the element cannot be added at this
+     *         time due to capacity restrictions
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean add(E e) {
+	addLast(e);
+	return true;
+    }
+
+    /**
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean offer(E e) {
+	return offerLast(e);
+    }
+
+    /**
+     * @throws NullPointerException {@inheritDoc}
+     * @throws InterruptedException {@inheritDoc}
+     */
+    public void put(E e) throws InterruptedException {
+	putLast(e);
+    }
+
+    /**
+     * @throws NullPointerException {@inheritDoc}
+     * @throws InterruptedException {@inheritDoc}
+     */
+    public boolean offer(E e, long timeout, TimeUnit unit)
+        throws InterruptedException {
+	return offerLast(e, timeout, unit);
+    }
+
+    /**
+     * Retrieves and removes the head of the queue represented by this deque.
+     * This method differs from {@link #poll poll} only in that it throws an
+     * exception if this deque is empty.
+     *
+     * <p>This method is equivalent to {@link #removeFirst() removeFirst}.
+     *
+     * @return the head of the queue represented by this deque
+     * @throws NoSuchElementException if this deque is empty
+     */
+    public E remove() {
+	return removeFirst();
+    }
+
+    public E poll() {
+	return pollFirst();
+    }
+
+    public E take() throws InterruptedException {
+	return takeFirst();
+    }
+
+    public E poll(long timeout, TimeUnit unit) throws InterruptedException {
+	return pollFirst(timeout, unit);
+    }
+
+    /**
+     * Retrieves, but does not remove, the head of the queue represented by
+     * this deque.  This method differs from {@link #peek peek} only in that
+     * it throws an exception if this deque is empty.
+     *
+     * <p>This method is equivalent to {@link #getFirst() getFirst}.
+     *
+     * @return the head of the queue represented by this deque
+     * @throws NoSuchElementException if this deque is empty
+     */
+    public E element() {
+	return getFirst();
+    }
+
+    public E peek() {
+	return peekFirst();
+    }
+
+    /**
+     * Returns the number of additional elements that this deque can ideally
+     * (in the absence of memory or resource constraints) accept without
+     * blocking. This is always equal to the initial capacity of this deque
+     * less the current <tt>size</tt> of this deque.
+     *
+     * <p>Note that you <em>cannot</em> always tell if an attempt to insert
+     * an element will succeed by inspecting <tt>remainingCapacity</tt>
+     * because it may be the case that another thread is about to
+     * insert or remove an element.
+     */
+    public int remainingCapacity() {
+        lock.lock();
+        try {
+            return capacity - count;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * @throws UnsupportedOperationException {@inheritDoc}
+     * @throws ClassCastException            {@inheritDoc}
+     * @throws NullPointerException          {@inheritDoc}
+     * @throws IllegalArgumentException      {@inheritDoc}
+     */
+    public int drainTo(Collection<? super E> c) {
+        if (c == null)
+            throw new NullPointerException();
+        if (c == this)
+            throw new IllegalArgumentException();
+        lock.lock();
+        try {
+            for (Node<E> p = first; p != null; p = p.next)
+                c.add(p.item);
+            int n = count;
+            count = 0;
+            first = last = null;
+            notFull.signalAll();
+            return n;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * @throws UnsupportedOperationException {@inheritDoc}
+     * @throws ClassCastException            {@inheritDoc}
+     * @throws NullPointerException          {@inheritDoc}
+     * @throws IllegalArgumentException      {@inheritDoc}
+     */
+    public int drainTo(Collection<? super E> c, int maxElements) {
+        if (c == null)
+            throw new NullPointerException();
+        if (c == this)
+            throw new IllegalArgumentException();
+        lock.lock();
+        try {
+            int n = 0;
+            while (n < maxElements && first != null) {
+                c.add(first.item);
+                first.prev = null;
+                first = first.next;
+                --count;
+                ++n;
+            }
+            if (first == null)
+                last = null;
+            notFull.signalAll();
+            return n;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    // Stack methods
+
+    /**
+     * @throws IllegalStateException {@inheritDoc}
+     * @throws NullPointerException  {@inheritDoc}
+     */
+    public void push(E e) {
+	addFirst(e);
+    }
+
+    /**
+     * @throws NoSuchElementException {@inheritDoc}
+     */
+    public E pop() {
+	return removeFirst();
+    }
+
+    // Collection methods
+
+    /**
+     * Removes the first occurrence of the specified element from this deque.
+     * If the deque does not contain the element, it is unchanged.
+     * More formally, removes the first element <tt>e</tt> such that
+     * <tt>o.equals(e)</tt> (if such an element exists).
+     * Returns <tt>true</tt> if this deque contained the specified element
+     * (or equivalently, if this deque changed as a result of the call).
+     *
+     * <p>This method is equivalent to
+     * {@link #removeFirstOccurrence(Object) removeFirstOccurrence}.
+     *
+     * @param o element to be removed from this deque, if present
+     * @return <tt>true</tt> if this deque changed as a result of the call
+     */
+    public boolean remove(Object o) {
+	return removeFirstOccurrence(o);
+    }
+
+    /**
+     * Returns the number of elements in this deque.
+     *
+     * @return the number of elements in this deque
+     */
+    public int size() {
+        lock.lock();
+        try {
+            return count;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Returns <tt>true</tt> if this deque contains the specified element.
+     * More formally, returns <tt>true</tt> if and only if this deque contains
+     * at least one element <tt>e</tt> such that <tt>o.equals(e)</tt>.
+     *
+     * @param o object to be checked for containment in this deque
+     * @return <tt>true</tt> if this deque contains the specified element
+     */
+    public boolean contains(Object o) {
+        if (o == null) return false;
+        lock.lock();
+        try {
+            for (Node<E> p = first; p != null; p = p.next)
+                if (o.equals(p.item))
+                    return true;
+            return false;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Variant of removeFirstOccurrence needed by iterator.remove.
+     * Searches for the node, not its contents.
+     */
+    boolean removeNode(Node<E> e) {
+        lock.lock();
+        try {
+            for (Node<E> p = first; p != null; p = p.next) {
+                if (p == e) {
+                    unlink(p);
+                    return true;
+                }
+            }
+            return false;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Returns an array containing all of the elements in this deque, in
+     * proper sequence (from first to last element).
+     *
+     * <p>The returned array will be "safe" in that no references to it are
+     * maintained by this deque.  (In other words, this method must allocate
+     * a new array).  The caller is thus free to modify the returned array.
+     *
+     * <p>This method acts as bridge between array-based and collection-based
+     * APIs.
+     *
+     * @return an array containing all of the elements in this deque
+     */
+    public Object[] toArray() {
+        lock.lock();
+        try {
+            Object[] a = new Object[count];
+            int k = 0;
+            for (Node<E> p = first; p != null; p = p.next)
+                a[k++] = p.item;
+            return a;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Returns an array containing all of the elements in this deque, in
+     * proper sequence; the runtime type of the returned array is that of
+     * the specified array.  If the deque fits in the specified array, it
+     * is returned therein.  Otherwise, a new array is allocated with the
+     * runtime type of the specified array and the size of this deque.
+     *
+     * <p>If this deque fits in the specified array with room to spare
+     * (i.e., the array has more elements than this deque), the element in
+     * the array immediately following the end of the deque is set to
+     * <tt>null</tt>.
+     *
+     * <p>Like the {@link #toArray()} method, this method acts as bridge between
+     * array-based and collection-based APIs.  Further, this method allows
+     * precise control over the runtime type of the output array, and may,
+     * under certain circumstances, be used to save allocation costs.
+     *
+     * <p>Suppose <tt>x</tt> is a deque known to contain only strings.
+     * The following code can be used to dump the deque into a newly
+     * allocated array of <tt>String</tt>:
+     *
+     * <pre>
+     *     String[] y = x.toArray(new String[0]);</pre>
+     *
+     * Note that <tt>toArray(new Object[0])</tt> is identical in function to
+     * <tt>toArray()</tt>.
+     *
+     * @param a the array into which the elements of the deque are to
+     *          be stored, if it is big enough; otherwise, a new array of the
+     *          same runtime type is allocated for this purpose
+     * @return an array containing all of the elements in this deque
+     * @throws ArrayStoreException if the runtime type of the specified array
+     *         is not a supertype of the runtime type of every element in
+     *         this deque
+     * @throws NullPointerException if the specified array is null
+     */
+    public <T> T[] toArray(T[] a) {
+        lock.lock();
+        try {
+            if (a.length < count)
+                a = (T[])java.lang.reflect.Array.newInstance(
+                    a.getClass().getComponentType(),
+                    count
+                    );
+
+            int k = 0;
+            for (Node<E> p = first; p != null; p = p.next)
+                a[k++] = (T)p.item;
+            if (a.length > k)
+                a[k] = null;
+            return a;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    public String toString() {
+        lock.lock();
+        try {
+            return super.toString();
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Atomically removes all of the elements from this deque.
+     * The deque will be empty after this call returns.
+     */
+    public void clear() {
+        lock.lock();
+        try {
+            first = last = null;
+            count = 0;
+            notFull.signalAll();
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Returns an iterator over the elements in this deque in proper sequence.
+     * The elements will be returned in order from first (head) to last (tail).
+     * The returned <tt>Iterator</tt> is a "weakly consistent" iterator that
+     * will never throw {@link ConcurrentModificationException},
+     * and guarantees to traverse elements as they existed upon
+     * construction of the iterator, and may (but is not guaranteed to)
+     * reflect any modifications subsequent to construction.
+     *
+     * @return an iterator over the elements in this deque in proper sequence
+     */
+    public Iterator<E> iterator() {
+        return new Itr();
+    }
+
+    /**
+     * Returns an iterator over the elements in this deque in reverse
+     * sequential order.  The elements will be returned in order from
+     * last (tail) to first (head).
+     * The returned <tt>Iterator</tt> is a "weakly consistent" iterator that
+     * will never throw {@link ConcurrentModificationException},
+     * and guarantees to traverse elements as they existed upon
+     * construction of the iterator, and may (but is not guaranteed to)
+     * reflect any modifications subsequent to construction.
+     */
+    public Iterator<E> descendingIterator() {
+        return new DescendingItr();
+    }
+
+    /**
+     * Base class for Iterators for LinkedBlockingDeque
+     */
+    private abstract class AbstractItr implements Iterator<E> {
+        /**
+         * The next node to return in next
+         */
+         Node<E> next;
+
+        /**
+         * nextItem holds on to item fields because once we claim that
+         * an element exists in hasNext(), we must return item read
+         * under lock (in advance()) even if it was in the process of
+         * being removed when hasNext() was called.
+         */
+        E nextItem;
+
+        /**
+         * Node returned by most recent call to next. Needed by remove.
+         * Reset to null if this element is deleted by a call to remove.
+         */
+        private Node<E> lastRet;
+
+        AbstractItr() {
+            advance(); // set to initial position
+        }
+
+        /**
+         * Advances next, or if not yet initialized, sets to first node.
+         * Implemented to move forward vs backward in the two subclasses.
+         */
+        abstract void advance();
+
+        public boolean hasNext() {
+            return next != null;
+        }
+
+        public E next() {
+            if (next == null)
+                throw new NoSuchElementException();
+            lastRet = next;
+            E x = nextItem;
+            advance();
+            return x;
+        }
+
+        public void remove() {
+            Node<E> n = lastRet;
+            if (n == null)
+                throw new IllegalStateException();
+            lastRet = null;
+            // Note: removeNode rescans looking for this node to make
+            // sure it was not already removed. Otherwise, trying to
+            // re-remove could corrupt list.
+            removeNode(n);
+        }
+    }
+
+    /** Forward iterator */
+    private class Itr extends AbstractItr {
+        void advance() {
+            final ReentrantLock lock = LinkedBlockingDeque.this.lock;
+            lock.lock();
+            try {
+                next = (next == null)? first : next.next;
+                nextItem = (next == null)? null : next.item;
+            } finally {
+                lock.unlock();
+            }
+        }
+    }
+
+    /**
+     * Descending iterator for LinkedBlockingDeque
+     */
+    private class DescendingItr extends AbstractItr {
+        void advance() {
+            final ReentrantLock lock = LinkedBlockingDeque.this.lock;
+            lock.lock();
+            try {
+                next = (next == null)? last : next.prev;
+                nextItem = (next == null)? null : next.item;
+            } finally {
+                lock.unlock();
+            }
+        }
+    }
+
+    /**
+     * Save the state of this deque to a stream (that is, serialize it).
+     *
+     * @serialData The capacity (int), followed by elements (each an
+     * <tt>Object</tt>) in the proper order, followed by a null
+     * @param s the stream
+     */
+    private void writeObject(java.io.ObjectOutputStream s)
+        throws java.io.IOException {
+        lock.lock();
+        try {
+            // Write out capacity and any hidden stuff
+            s.defaultWriteObject();
+            // Write out all elements in the proper order.
+            for (Node<E> p = first; p != null; p = p.next)
+                s.writeObject(p.item);
+            // Use trailing null as sentinel
+            s.writeObject(null);
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Reconstitute this deque from a stream (that is,
+     * deserialize it).
+     * @param s the stream
+     */
+    private void readObject(java.io.ObjectInputStream s)
+        throws java.io.IOException, ClassNotFoundException {
+        s.defaultReadObject();
+        count = 0;
+        first = null;
+        last = null;
+        // Read in all elements and place in queue
+        for (;;) {
+            E item = (E)s.readObject();
+            if (item == null)
+                break;
+            add(item);
+        }
+    }
+
+}
diff --git a/external/jsr166/java/util/concurrent/LinkedBlockingQueue.java b/external/jsr166/java/util/concurrent/LinkedBlockingQueue.java
new file mode 100644
index 000000000..3dedee56d
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/LinkedBlockingQueue.java
@@ -0,0 +1,807 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+import java.util.concurrent.atomic.*;
+import java.util.concurrent.locks.*;
+import java.util.*;
+
+/**
+ * An optionally-bounded {@linkplain BlockingQueue blocking queue} based on
+ * linked nodes.
+ * This queue orders elements FIFO (first-in-first-out).
+ * The <em>head</em> of the queue is that element that has been on the
+ * queue the longest time.
+ * The <em>tail</em> of the queue is that element that has been on the
+ * queue the shortest time. New elements
+ * are inserted at the tail of the queue, and the queue retrieval
+ * operations obtain elements at the head of the queue.
+ * Linked queues typically have higher throughput than array-based queues but
+ * less predictable performance in most concurrent applications.
+ *
+ * <p> The optional capacity bound constructor argument serves as a
+ * way to prevent excessive queue expansion. The capacity, if unspecified,
+ * is equal to {@link Integer#MAX_VALUE}.  Linked nodes are
+ * dynamically created upon each insertion unless this would bring the
+ * queue above capacity.
+ *
+ * <p>This class and its iterator implement all of the
+ * <em>optional</em> methods of the {@link Collection} and {@link
+ * Iterator} interfaces.
+ *
+ * <p>This class is a member of the
+ * <a href="{@docRoot}/../technotes/guides/collections/index.html">
+ * Java Collections Framework</a>.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ * @param <E> the type of elements held in this collection
+ *
+ */
+public class LinkedBlockingQueue<E> extends AbstractQueue<E>
+        implements BlockingQueue<E>, java.io.Serializable {
+    private static final long serialVersionUID = -6903933977591709194L;
+
+    /*
+     * A variant of the "two lock queue" algorithm.  The putLock gates
+     * entry to put (and offer), and has an associated condition for
+     * waiting puts.  Similarly for the takeLock.  The "count" field
+     * that they both rely on is maintained as an atomic to avoid
+     * needing to get both locks in most cases. Also, to minimize need
+     * for puts to get takeLock and vice-versa, cascading notifies are
+     * used. When a put notices that it has enabled at least one take,
+     * it signals taker. That taker in turn signals others if more
+     * items have been entered since the signal. And symmetrically for
+     * takes signalling puts. Operations such as remove(Object) and
+     * iterators acquire both locks.
+     */
+
+    /**
+     * Linked list node class
+     */
+    static class Node<E> {
+        /** The item, volatile to ensure barrier separating write and read */
+        volatile E item;
+        Node<E> next;
+        Node(E x) { item = x; }
+    }
+
+    /** The capacity bound, or Integer.MAX_VALUE if none */
+    private final int capacity;
+
+    /** Current number of elements */
+    private final AtomicInteger count = new AtomicInteger(0);
+
+    /** Head of linked list */
+    private transient Node<E> head;
+
+    /** Tail of linked list */
+    private transient Node<E> last;
+
+    /** Lock held by take, poll, etc */
+    private final ReentrantLock takeLock = new ReentrantLock();
+
+    /** Wait queue for waiting takes */
+    private final Condition notEmpty = takeLock.newCondition();
+
+    /** Lock held by put, offer, etc */
+    private final ReentrantLock putLock = new ReentrantLock();
+
+    /** Wait queue for waiting puts */
+    private final Condition notFull = putLock.newCondition();
+
+    /**
+     * Signals a waiting take. Called only from put/offer (which do not
+     * otherwise ordinarily lock takeLock.)
+     */
+    private void signalNotEmpty() {
+        final ReentrantLock takeLock = this.takeLock;
+        takeLock.lock();
+        try {
+            notEmpty.signal();
+        } finally {
+            takeLock.unlock();
+        }
+    }
+
+    /**
+     * Signals a waiting put. Called only from take/poll.
+     */
+    private void signalNotFull() {
+        final ReentrantLock putLock = this.putLock;
+        putLock.lock();
+        try {
+            notFull.signal();
+        } finally {
+            putLock.unlock();
+        }
+    }
+
+    /**
+     * Creates a node and links it at end of queue.
+     * @param x the item
+     */
+    private void insert(E x) {
+        last = last.next = new Node<E>(x);
+    }
+
+    /**
+     * Removes a node from head of queue,
+     * @return the node
+     */
+    private E extract() {
+        Node<E> first = head.next;
+        head = first;
+        E x = first.item;
+        first.item = null;
+        return x;
+    }
+
+    /**
+     * Lock to prevent both puts and takes.
+     */
+    private void fullyLock() {
+        putLock.lock();
+        takeLock.lock();
+    }
+
+    /**
+     * Unlock to allow both puts and takes.
+     */
+    private void fullyUnlock() {
+        takeLock.unlock();
+        putLock.unlock();
+    }
+
+
+    /**
+     * Creates a <tt>LinkedBlockingQueue</tt> with a capacity of
+     * {@link Integer#MAX_VALUE}.
+     */
+    public LinkedBlockingQueue() {
+        this(Integer.MAX_VALUE);
+    }
+
+    /**
+     * Creates a <tt>LinkedBlockingQueue</tt> with the given (fixed) capacity.
+     *
+     * @param capacity the capacity of this queue
+     * @throws IllegalArgumentException if <tt>capacity</tt> is not greater
+     *         than zero
+     */
+    public LinkedBlockingQueue(int capacity) {
+        if (capacity <= 0) throw new IllegalArgumentException();
+        this.capacity = capacity;
+        last = head = new Node<E>(null);
+    }
+
+    /**
+     * Creates a <tt>LinkedBlockingQueue</tt> with a capacity of
+     * {@link Integer#MAX_VALUE}, initially containing the elements of the
+     * given collection,
+     * added in traversal order of the collection's iterator.
+     *
+     * @param c the collection of elements to initially contain
+     * @throws NullPointerException if the specified collection or any
+     *         of its elements are null
+     */
+    public LinkedBlockingQueue(Collection<? extends E> c) {
+        this(Integer.MAX_VALUE);
+        for (E e : c)
+            add(e);
+    }
+
+
+    // this doc comment is overridden to remove the reference to collections
+    // greater in size than Integer.MAX_VALUE
+    /**
+     * Returns the number of elements in this queue.
+     *
+     * @return the number of elements in this queue
+     */
+    public int size() {
+        return count.get();
+    }
+
+    // this doc comment is a modified copy of the inherited doc comment,
+    // without the reference to unlimited queues.
+    /**
+     * Returns the number of additional elements that this queue can ideally
+     * (in the absence of memory or resource constraints) accept without
+     * blocking. This is always equal to the initial capacity of this queue
+     * less the current <tt>size</tt> of this queue.
+     *
+     * <p>Note that you <em>cannot</em> always tell if an attempt to insert
+     * an element will succeed by inspecting <tt>remainingCapacity</tt>
+     * because it may be the case that another thread is about to
+     * insert or remove an element.
+     */
+    public int remainingCapacity() {
+        return capacity - count.get();
+    }
+
+    /**
+     * Inserts the specified element at the tail of this queue, waiting if
+     * necessary for space to become available.
+     *
+     * @throws InterruptedException {@inheritDoc}
+     * @throws NullPointerException {@inheritDoc}
+     */
+    public void put(E e) throws InterruptedException {
+        if (e == null) throw new NullPointerException();
+        // Note: convention in all put/take/etc is to preset
+        // local var holding count  negative to indicate failure unless set.
+        int c = -1;
+        final ReentrantLock putLock = this.putLock;
+        final AtomicInteger count = this.count;
+        putLock.lockInterruptibly();
+        try {
+            /*
+             * Note that count is used in wait guard even though it is
+             * not protected by lock. This works because count can
+             * only decrease at this point (all other puts are shut
+             * out by lock), and we (or some other waiting put) are
+             * signalled if it ever changes from
+             * capacity. Similarly for all other uses of count in
+             * other wait guards.
+             */
+            try {
+                while (count.get() == capacity)
+                    notFull.await();
+            } catch (InterruptedException ie) {
+                notFull.signal(); // propagate to a non-interrupted thread
+                throw ie;
+            }
+            insert(e);
+            c = count.getAndIncrement();
+            if (c + 1 < capacity)
+                notFull.signal();
+        } finally {
+            putLock.unlock();
+        }
+        if (c == 0)
+            signalNotEmpty();
+    }
+
+    /**
+     * Inserts the specified element at the tail of this queue, waiting if
+     * necessary up to the specified wait time for space to become available.
+     *
+     * @return <tt>true</tt> if successful, or <tt>false</tt> if
+     *         the specified waiting time elapses before space is available.
+     * @throws InterruptedException {@inheritDoc}
+     * @throws NullPointerException {@inheritDoc}
+     */
+    public boolean offer(E e, long timeout, TimeUnit unit)
+        throws InterruptedException {
+
+        if (e == null) throw new NullPointerException();
+        long nanos = unit.toNanos(timeout);
+        int c = -1;
+        final ReentrantLock putLock = this.putLock;
+        final AtomicInteger count = this.count;
+        putLock.lockInterruptibly();
+        try {
+            for (;;) {
+                if (count.get() < capacity) {
+                    insert(e);
+                    c = count.getAndIncrement();
+                    if (c + 1 < capacity)
+                        notFull.signal();
+                    break;
+                }
+                if (nanos <= 0)
+                    return false;
+                try {
+                    nanos = notFull.awaitNanos(nanos);
+                } catch (InterruptedException ie) {
+                    notFull.signal(); // propagate to a non-interrupted thread
+                    throw ie;
+                }
+            }
+        } finally {
+            putLock.unlock();
+        }
+        if (c == 0)
+            signalNotEmpty();
+        return true;
+    }
+
+    /**
+     * Inserts the specified element at the tail of this queue if it is
+     * possible to do so immediately without exceeding the queue's capacity,
+     * returning <tt>true</tt> upon success and <tt>false</tt> if this queue
+     * is full.
+     * When using a capacity-restricted queue, this method is generally
+     * preferable to method {@link BlockingQueue#add add}, which can fail to
+     * insert an element only by throwing an exception.
+     *
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean offer(E e) {
+        if (e == null) throw new NullPointerException();
+        final AtomicInteger count = this.count;
+        if (count.get() == capacity)
+            return false;
+        int c = -1;
+        final ReentrantLock putLock = this.putLock;
+        putLock.lock();
+        try {
+            if (count.get() < capacity) {
+                insert(e);
+                c = count.getAndIncrement();
+                if (c + 1 < capacity)
+                    notFull.signal();
+            }
+        } finally {
+            putLock.unlock();
+        }
+        if (c == 0)
+            signalNotEmpty();
+        return c >= 0;
+    }
+
+
+    public E take() throws InterruptedException {
+        E x;
+        int c = -1;
+        final AtomicInteger count = this.count;
+        final ReentrantLock takeLock = this.takeLock;
+        takeLock.lockInterruptibly();
+        try {
+            try {
+                while (count.get() == 0)
+                    notEmpty.await();
+            } catch (InterruptedException ie) {
+                notEmpty.signal(); // propagate to a non-interrupted thread
+                throw ie;
+            }
+
+            x = extract();
+            c = count.getAndDecrement();
+            if (c > 1)
+                notEmpty.signal();
+        } finally {
+            takeLock.unlock();
+        }
+        if (c == capacity)
+            signalNotFull();
+        return x;
+    }
+
+    public E poll(long timeout, TimeUnit unit) throws InterruptedException {
+        E x = null;
+        int c = -1;
+        long nanos = unit.toNanos(timeout);
+        final AtomicInteger count = this.count;
+        final ReentrantLock takeLock = this.takeLock;
+        takeLock.lockInterruptibly();
+        try {
+            for (;;) {
+                if (count.get() > 0) {
+                    x = extract();
+                    c = count.getAndDecrement();
+                    if (c > 1)
+                        notEmpty.signal();
+                    break;
+                }
+                if (nanos <= 0)
+                    return null;
+                try {
+                    nanos = notEmpty.awaitNanos(nanos);
+                } catch (InterruptedException ie) {
+                    notEmpty.signal(); // propagate to a non-interrupted thread
+                    throw ie;
+                }
+            }
+        } finally {
+            takeLock.unlock();
+        }
+        if (c == capacity)
+            signalNotFull();
+        return x;
+    }
+
+    public E poll() {
+        final AtomicInteger count = this.count;
+        if (count.get() == 0)
+            return null;
+        E x = null;
+        int c = -1;
+        final ReentrantLock takeLock = this.takeLock;
+        takeLock.lock();
+        try {
+            if (count.get() > 0) {
+                x = extract();
+                c = count.getAndDecrement();
+                if (c > 1)
+                    notEmpty.signal();
+            }
+        } finally {
+            takeLock.unlock();
+        }
+        if (c == capacity)
+            signalNotFull();
+        return x;
+    }
+
+
+    public E peek() {
+        if (count.get() == 0)
+            return null;
+        final ReentrantLock takeLock = this.takeLock;
+        takeLock.lock();
+        try {
+            Node<E> first = head.next;
+            if (first == null)
+                return null;
+            else
+                return first.item;
+        } finally {
+            takeLock.unlock();
+        }
+    }
+
+    /**
+     * Removes a single instance of the specified element from this queue,
+     * if it is present.  More formally, removes an element <tt>e</tt> such
+     * that <tt>o.equals(e)</tt>, if this queue contains one or more such
+     * elements.
+     * Returns <tt>true</tt> if this queue contained the specified element
+     * (or equivalently, if this queue changed as a result of the call).
+     *
+     * @param o element to be removed from this queue, if present
+     * @return <tt>true</tt> if this queue changed as a result of the call
+     */
+    public boolean remove(Object o) {
+        if (o == null) return false;
+        boolean removed = false;
+        fullyLock();
+        try {
+            Node<E> trail = head;
+            Node<E> p = head.next;
+            while (p != null) {
+                if (o.equals(p.item)) {
+                    removed = true;
+                    break;
+                }
+                trail = p;
+                p = p.next;
+            }
+            if (removed) {
+                p.item = null;
+                trail.next = p.next;
+                if (last == p)
+                    last = trail;
+                if (count.getAndDecrement() == capacity)
+                    notFull.signalAll();
+            }
+        } finally {
+            fullyUnlock();
+        }
+        return removed;
+    }
+
+    /**
+     * Returns an array containing all of the elements in this queue, in
+     * proper sequence.
+     *
+     * <p>The returned array will be "safe" in that no references to it are
+     * maintained by this queue.  (In other words, this method must allocate
+     * a new array).  The caller is thus free to modify the returned array.
+     *
+     * <p>This method acts as bridge between array-based and collection-based
+     * APIs.
+     *
+     * @return an array containing all of the elements in this queue
+     */
+    public Object[] toArray() {
+        fullyLock();
+        try {
+            int size = count.get();
+            Object[] a = new Object[size];
+            int k = 0;
+            for (Node<E> p = head.next; p != null; p = p.next)
+                a[k++] = p.item;
+            return a;
+        } finally {
+            fullyUnlock();
+        }
+    }
+
+    /**
+     * Returns an array containing all of the elements in this queue, in
+     * proper sequence; the runtime type of the returned array is that of
+     * the specified array.  If the queue fits in the specified array, it
+     * is returned therein.  Otherwise, a new array is allocated with the
+     * runtime type of the specified array and the size of this queue.
+     *
+     * <p>If this queue fits in the specified array with room to spare
+     * (i.e., the array has more elements than this queue), the element in
+     * the array immediately following the end of the queue is set to
+     * <tt>null</tt>.
+     *
+     * <p>Like the {@link #toArray()} method, this method acts as bridge between
+     * array-based and collection-based APIs.  Further, this method allows
+     * precise control over the runtime type of the output array, and may,
+     * under certain circumstances, be used to save allocation costs.
+     *
+     * <p>Suppose <tt>x</tt> is a queue known to contain only strings.
+     * The following code can be used to dump the queue into a newly
+     * allocated array of <tt>String</tt>:
+     *
+     * <pre>
+     *     String[] y = x.toArray(new String[0]);</pre>
+     *
+     * Note that <tt>toArray(new Object[0])</tt> is identical in function to
+     * <tt>toArray()</tt>.
+     *
+     * @param a the array into which the elements of the queue are to
+     *          be stored, if it is big enough; otherwise, a new array of the
+     *          same runtime type is allocated for this purpose
+     * @return an array containing all of the elements in this queue
+     * @throws ArrayStoreException if the runtime type of the specified array
+     *         is not a supertype of the runtime type of every element in
+     *         this queue
+     * @throws NullPointerException if the specified array is null
+     */
+    public <T> T[] toArray(T[] a) {
+        fullyLock();
+        try {
+            int size = count.get();
+            if (a.length < size)
+                a = (T[])java.lang.reflect.Array.newInstance
+                    (a.getClass().getComponentType(), size);
+
+            int k = 0;
+            for (Node p = head.next; p != null; p = p.next)
+                a[k++] = (T)p.item;
+            if (a.length > k)
+                a[k] = null;
+            return a;
+        } finally {
+            fullyUnlock();
+        }
+    }
+
+    public String toString() {
+        fullyLock();
+        try {
+            return super.toString();
+        } finally {
+            fullyUnlock();
+        }
+    }
+
+    /**
+     * Atomically removes all of the elements from this queue.
+     * The queue will be empty after this call returns.
+     */
+    public void clear() {
+        fullyLock();
+        try {
+            head.next = null;
+	    assert head.item == null;
+	    last = head;
+            if (count.getAndSet(0) == capacity)
+                notFull.signalAll();
+        } finally {
+            fullyUnlock();
+        }
+    }
+
+    /**
+     * @throws UnsupportedOperationException {@inheritDoc}
+     * @throws ClassCastException            {@inheritDoc}
+     * @throws NullPointerException          {@inheritDoc}
+     * @throws IllegalArgumentException      {@inheritDoc}
+     */
+    public int drainTo(Collection<? super E> c) {
+        if (c == null)
+            throw new NullPointerException();
+        if (c == this)
+            throw new IllegalArgumentException();
+        Node<E> first;
+        fullyLock();
+        try {
+            first = head.next;
+            head.next = null;
+	    assert head.item == null;
+	    last = head;
+            if (count.getAndSet(0) == capacity)
+                notFull.signalAll();
+        } finally {
+            fullyUnlock();
+        }
+        // Transfer the elements outside of locks
+        int n = 0;
+        for (Node<E> p = first; p != null; p = p.next) {
+            c.add(p.item);
+            p.item = null;
+            ++n;
+        }
+        return n;
+    }
+
+    /**
+     * @throws UnsupportedOperationException {@inheritDoc}
+     * @throws ClassCastException            {@inheritDoc}
+     * @throws NullPointerException          {@inheritDoc}
+     * @throws IllegalArgumentException      {@inheritDoc}
+     */
+    public int drainTo(Collection<? super E> c, int maxElements) {
+        if (c == null)
+            throw new NullPointerException();
+        if (c == this)
+            throw new IllegalArgumentException();
+        fullyLock();
+        try {
+            int n = 0;
+            Node<E> p = head.next;
+            while (p != null && n < maxElements) {
+                c.add(p.item);
+                p.item = null;
+                p = p.next;
+                ++n;
+            }
+            if (n != 0) {
+                head.next = p;
+		assert head.item == null;
+		if (p == null)
+		    last = head;
+                if (count.getAndAdd(-n) == capacity)
+                    notFull.signalAll();
+            }
+            return n;
+        } finally {
+            fullyUnlock();
+        }
+    }
+
+    /**
+     * Returns an iterator over the elements in this queue in proper sequence.
+     * The returned <tt>Iterator</tt> is a "weakly consistent" iterator that
+     * will never throw {@link ConcurrentModificationException},
+     * and guarantees to traverse elements as they existed upon
+     * construction of the iterator, and may (but is not guaranteed to)
+     * reflect any modifications subsequent to construction.
+     *
+     * @return an iterator over the elements in this queue in proper sequence
+     */
+    public Iterator<E> iterator() {
+      return new Itr();
+    }
+
+    private class Itr implements Iterator<E> {
+        /*
+         * Basic weak-consistent iterator.  At all times hold the next
+         * item to hand out so that if hasNext() reports true, we will
+         * still have it to return even if lost race with a take etc.
+         */
+        private Node<E> current;
+        private Node<E> lastRet;
+        private E currentElement;
+
+        Itr() {
+            final ReentrantLock putLock = LinkedBlockingQueue.this.putLock;
+            final ReentrantLock takeLock = LinkedBlockingQueue.this.takeLock;
+            putLock.lock();
+            takeLock.lock();
+            try {
+                current = head.next;
+                if (current != null)
+                    currentElement = current.item;
+            } finally {
+                takeLock.unlock();
+                putLock.unlock();
+            }
+        }
+
+        public boolean hasNext() {
+            return current != null;
+        }
+
+        public E next() {
+            final ReentrantLock putLock = LinkedBlockingQueue.this.putLock;
+            final ReentrantLock takeLock = LinkedBlockingQueue.this.takeLock;
+            putLock.lock();
+            takeLock.lock();
+            try {
+                if (current == null)
+                    throw new NoSuchElementException();
+                E x = currentElement;
+                lastRet = current;
+                current = current.next;
+                if (current != null)
+                    currentElement = current.item;
+                return x;
+            } finally {
+                takeLock.unlock();
+                putLock.unlock();
+            }
+        }
+
+        public void remove() {
+            if (lastRet == null)
+                throw new IllegalStateException();
+            final ReentrantLock putLock = LinkedBlockingQueue.this.putLock;
+            final ReentrantLock takeLock = LinkedBlockingQueue.this.takeLock;
+            putLock.lock();
+            takeLock.lock();
+            try {
+                Node<E> node = lastRet;
+                lastRet = null;
+                Node<E> trail = head;
+                Node<E> p = head.next;
+                while (p != null && p != node) {
+                    trail = p;
+                    p = p.next;
+                }
+                if (p == node) {
+                    p.item = null;
+                    trail.next = p.next;
+                    if (last == p)
+                        last = trail;
+                    int c = count.getAndDecrement();
+                    if (c == capacity)
+                        notFull.signalAll();
+                }
+            } finally {
+                takeLock.unlock();
+                putLock.unlock();
+            }
+        }
+    }
+
+    /**
+     * Save the state to a stream (that is, serialize it).
+     *
+     * @serialData The capacity is emitted (int), followed by all of
+     * its elements (each an <tt>Object</tt>) in the proper order,
+     * followed by a null
+     * @param s the stream
+     */
+    private void writeObject(java.io.ObjectOutputStream s)
+        throws java.io.IOException {
+
+        fullyLock();
+        try {
+            // Write out any hidden stuff, plus capacity
+            s.defaultWriteObject();
+
+            // Write out all elements in the proper order.
+            for (Node<E> p = head.next; p != null; p = p.next)
+                s.writeObject(p.item);
+
+            // Use trailing null as sentinel
+            s.writeObject(null);
+        } finally {
+            fullyUnlock();
+        }
+    }
+
+    /**
+     * Reconstitute this queue instance from a stream (that is,
+     * deserialize it).
+     * @param s the stream
+     */
+    private void readObject(java.io.ObjectInputStream s)
+        throws java.io.IOException, ClassNotFoundException {
+        // Read in capacity, and any hidden stuff
+        s.defaultReadObject();
+
+        count.set(0);
+        last = head = new Node<E>(null);
+
+        // Read in all elements and place in queue
+        for (;;) {
+            E item = (E)s.readObject();
+            if (item == null)
+                break;
+            add(item);
+        }
+    }
+}
diff --git a/external/jsr166/java/util/concurrent/PriorityBlockingQueue.java b/external/jsr166/java/util/concurrent/PriorityBlockingQueue.java
new file mode 100644
index 000000000..91ae61b63
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/PriorityBlockingQueue.java
@@ -0,0 +1,563 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+
+import java.util.concurrent.locks.*;
+import java.util.*;
+
+/**
+ * An unbounded {@linkplain BlockingQueue blocking queue} that uses
+ * the same ordering rules as class {@link PriorityQueue} and supplies
+ * blocking retrieval operations.  While this queue is logically
+ * unbounded, attempted additions may fail due to resource exhaustion
+ * (causing <tt>OutOfMemoryError</tt>). This class does not permit
+ * <tt>null</tt> elements.  A priority queue relying on {@linkplain
+ * Comparable natural ordering} also does not permit insertion of
+ * non-comparable objects (doing so results in
+ * <tt>ClassCastException</tt>).
+ *
+ * <p>This class and its iterator implement all of the
+ * <em>optional</em> methods of the {@link Collection} and {@link
+ * Iterator} interfaces.  The Iterator provided in method {@link
+ * #iterator()} is <em>not</em> guaranteed to traverse the elements of
+ * the PriorityBlockingQueue in any particular order. If you need
+ * ordered traversal, consider using
+ * <tt>Arrays.sort(pq.toArray())</tt>.  Also, method <tt>drainTo</tt>
+ * can be used to <em>remove</em> some or all elements in priority
+ * order and place them in another collection.
+ *
+ * <p>Operations on this class make no guarantees about the ordering
+ * of elements with equal priority. If you need to enforce an
+ * ordering, you can define custom classes or comparators that use a
+ * secondary key to break ties in primary priority values.  For
+ * example, here is a class that applies first-in-first-out
+ * tie-breaking to comparable elements. To use it, you would insert a
+ * <tt>new FIFOEntry(anEntry)</tt> instead of a plain entry object.
+ *
+ * <pre>
+ * class FIFOEntry&lt;E extends Comparable&lt;? super E&gt;&gt;
+ *     implements Comparable&lt;FIFOEntry&lt;E&gt;&gt; {
+ *   final static AtomicLong seq = new AtomicLong();
+ *   final long seqNum;
+ *   final E entry;
+ *   public FIFOEntry(E entry) {
+ *     seqNum = seq.getAndIncrement();
+ *     this.entry = entry;
+ *   }
+ *   public E getEntry() { return entry; }
+ *   public int compareTo(FIFOEntry&lt;E&gt; other) {
+ *     int res = entry.compareTo(other.entry);
+ *     if (res == 0 &amp;&amp; other.entry != this.entry)
+ *       res = (seqNum &lt; other.seqNum ? -1 : 1);
+ *     return res;
+ *   }
+ * }</pre>
+ *
+ * <p>This class is a member of the
+ * <a href="{@docRoot}/../technotes/guides/collections/index.html">
+ * Java Collections Framework</a>.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ * @param <E> the type of elements held in this collection
+ */
+public class PriorityBlockingQueue<E> extends AbstractQueue<E>
+    implements BlockingQueue<E>, java.io.Serializable {
+    private static final long serialVersionUID = 5595510919245408276L;
+
+    private final PriorityQueue<E> q;
+    private final ReentrantLock lock = new ReentrantLock(true);
+    private final Condition notEmpty = lock.newCondition();
+
+    /**
+     * Creates a <tt>PriorityBlockingQueue</tt> with the default
+     * initial capacity (11) that orders its elements according to
+     * their {@linkplain Comparable natural ordering}.
+     */
+    public PriorityBlockingQueue() {
+        q = new PriorityQueue<E>();
+    }
+
+    /**
+     * Creates a <tt>PriorityBlockingQueue</tt> with the specified
+     * initial capacity that orders its elements according to their
+     * {@linkplain Comparable natural ordering}.
+     *
+     * @param initialCapacity the initial capacity for this priority queue
+     * @throws IllegalArgumentException if <tt>initialCapacity</tt> is less
+     *         than 1
+     */
+    public PriorityBlockingQueue(int initialCapacity) {
+        q = new PriorityQueue<E>(initialCapacity, null);
+    }
+
+    /**
+     * Creates a <tt>PriorityBlockingQueue</tt> with the specified initial
+     * capacity that orders its elements according to the specified
+     * comparator.
+     *
+     * @param initialCapacity the initial capacity for this priority queue
+     * @param  comparator the comparator that will be used to order this
+     *         priority queue.  If {@code null}, the {@linkplain Comparable
+     *         natural ordering} of the elements will be used.
+     * @throws IllegalArgumentException if <tt>initialCapacity</tt> is less
+     *         than 1
+     */
+    public PriorityBlockingQueue(int initialCapacity,
+                                 Comparator<? super E> comparator) {
+        q = new PriorityQueue<E>(initialCapacity, comparator);
+    }
+
+    /**
+     * Creates a <tt>PriorityBlockingQueue</tt> containing the elements
+     * in the specified collection.  If the specified collection is a
+     * {@link SortedSet} or a {@link PriorityQueue},  this
+     * priority queue will be ordered according to the same ordering.
+     * Otherwise, this priority queue will be ordered according to the
+     * {@linkplain Comparable natural ordering} of its elements.
+     *
+     * @param  c the collection whose elements are to be placed
+     *         into this priority queue
+     * @throws ClassCastException if elements of the specified collection
+     *         cannot be compared to one another according to the priority
+     *         queue's ordering
+     * @throws NullPointerException if the specified collection or any
+     *         of its elements are null
+     */
+    public PriorityBlockingQueue(Collection<? extends E> c) {
+        q = new PriorityQueue<E>(c);
+    }
+
+    /**
+     * Inserts the specified element into this priority queue.
+     *
+     * @param e the element to add
+     * @return <tt>true</tt> (as specified by {@link Collection#add})
+     * @throws ClassCastException if the specified element cannot be compared
+     *         with elements currently in the priority queue according to the
+     *         priority queue's ordering
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean add(E e) {
+        return offer(e);
+    }
+
+    /**
+     * Inserts the specified element into this priority queue.
+     *
+     * @param e the element to add
+     * @return <tt>true</tt> (as specified by {@link Queue#offer})
+     * @throws ClassCastException if the specified element cannot be compared
+     *         with elements currently in the priority queue according to the
+     *         priority queue's ordering
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean offer(E e) {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            boolean ok = q.offer(e);
+            assert ok;
+            notEmpty.signal();
+            return true;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Inserts the specified element into this priority queue. As the queue is
+     * unbounded this method will never block.
+     *
+     * @param e the element to add
+     * @throws ClassCastException if the specified element cannot be compared
+     *         with elements currently in the priority queue according to the
+     *         priority queue's ordering
+     * @throws NullPointerException if the specified element is null
+     */
+    public void put(E e) {
+        offer(e); // never need to block
+    }
+
+    /**
+     * Inserts the specified element into this priority queue. As the queue is
+     * unbounded this method will never block.
+     *
+     * @param e the element to add
+     * @param timeout This parameter is ignored as the method never blocks
+     * @param unit This parameter is ignored as the method never blocks
+     * @return <tt>true</tt>
+     * @throws ClassCastException if the specified element cannot be compared
+     *         with elements currently in the priority queue according to the
+     *         priority queue's ordering
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean offer(E e, long timeout, TimeUnit unit) {
+        return offer(e); // never need to block
+    }
+
+    public E poll() {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            return q.poll();
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    public E take() throws InterruptedException {
+        final ReentrantLock lock = this.lock;
+        lock.lockInterruptibly();
+        try {
+            try {
+                while (q.size() == 0)
+                    notEmpty.await();
+            } catch (InterruptedException ie) {
+                notEmpty.signal(); // propagate to non-interrupted thread
+                throw ie;
+            }
+            E x = q.poll();
+            assert x != null;
+            return x;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    public E poll(long timeout, TimeUnit unit) throws InterruptedException {
+        long nanos = unit.toNanos(timeout);
+        final ReentrantLock lock = this.lock;
+        lock.lockInterruptibly();
+        try {
+            for (;;) {
+                E x = q.poll();
+                if (x != null)
+                    return x;
+                if (nanos <= 0)
+                    return null;
+                try {
+                    nanos = notEmpty.awaitNanos(nanos);
+                } catch (InterruptedException ie) {
+                    notEmpty.signal(); // propagate to non-interrupted thread
+                    throw ie;
+                }
+            }
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    public E peek() {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            return q.peek();
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Returns the comparator used to order the elements in this queue,
+     * or <tt>null</tt> if this queue uses the {@linkplain Comparable
+     * natural ordering} of its elements.
+     *
+     * @return the comparator used to order the elements in this queue,
+     *         or <tt>null</tt> if this queue uses the natural
+     *         ordering of its elements
+     */
+    public Comparator<? super E> comparator() {
+        return q.comparator();
+    }
+
+    public int size() {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            return q.size();
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Always returns <tt>Integer.MAX_VALUE</tt> because
+     * a <tt>PriorityBlockingQueue</tt> is not capacity constrained.
+     * @return <tt>Integer.MAX_VALUE</tt>
+     */
+    public int remainingCapacity() {
+        return Integer.MAX_VALUE;
+    }
+
+    /**
+     * Removes a single instance of the specified element from this queue,
+     * if it is present.  More formally, removes an element {@code e} such
+     * that {@code o.equals(e)}, if this queue contains one or more such
+     * elements.  Returns {@code true} if and only if this queue contained
+     * the specified element (or equivalently, if this queue changed as a
+     * result of the call).
+     *
+     * @param o element to be removed from this queue, if present
+     * @return <tt>true</tt> if this queue changed as a result of the call
+     */
+    public boolean remove(Object o) {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            return q.remove(o);
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Returns {@code true} if this queue contains the specified element.
+     * More formally, returns {@code true} if and only if this queue contains
+     * at least one element {@code e} such that {@code o.equals(e)}.
+     *
+     * @param o object to be checked for containment in this queue
+     * @return <tt>true</tt> if this queue contains the specified element
+     */
+    public boolean contains(Object o) {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            return q.contains(o);
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Returns an array containing all of the elements in this queue.
+     * The returned array elements are in no particular order.
+     *
+     * <p>The returned array will be "safe" in that no references to it are
+     * maintained by this queue.  (In other words, this method must allocate
+     * a new array).  The caller is thus free to modify the returned array.
+     *
+     * <p>This method acts as bridge between array-based and collection-based
+     * APIs.
+     *
+     * @return an array containing all of the elements in this queue
+     */
+    public Object[] toArray() {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            return q.toArray();
+        } finally {
+            lock.unlock();
+        }
+    }
+
+
+    public String toString() {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            return q.toString();
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * @throws UnsupportedOperationException {@inheritDoc}
+     * @throws ClassCastException            {@inheritDoc}
+     * @throws NullPointerException          {@inheritDoc}
+     * @throws IllegalArgumentException      {@inheritDoc}
+     */
+    public int drainTo(Collection<? super E> c) {
+        if (c == null)
+            throw new NullPointerException();
+        if (c == this)
+            throw new IllegalArgumentException();
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            int n = 0;
+            E e;
+            while ( (e = q.poll()) != null) {
+                c.add(e);
+                ++n;
+            }
+            return n;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * @throws UnsupportedOperationException {@inheritDoc}
+     * @throws ClassCastException            {@inheritDoc}
+     * @throws NullPointerException          {@inheritDoc}
+     * @throws IllegalArgumentException      {@inheritDoc}
+     */
+    public int drainTo(Collection<? super E> c, int maxElements) {
+        if (c == null)
+            throw new NullPointerException();
+        if (c == this)
+            throw new IllegalArgumentException();
+        if (maxElements <= 0)
+            return 0;
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            int n = 0;
+            E e;
+            while (n < maxElements && (e = q.poll()) != null) {
+                c.add(e);
+                ++n;
+            }
+            return n;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Atomically removes all of the elements from this queue.
+     * The queue will be empty after this call returns.
+     */
+    public void clear() {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            q.clear();
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Returns an array containing all of the elements in this queue; the
+     * runtime type of the returned array is that of the specified array.
+     * The returned array elements are in no particular order.
+     * If the queue fits in the specified array, it is returned therein.
+     * Otherwise, a new array is allocated with the runtime type of the
+     * specified array and the size of this queue.
+     *
+     * <p>If this queue fits in the specified array with room to spare
+     * (i.e., the array has more elements than this queue), the element in
+     * the array immediately following the end of the queue is set to
+     * <tt>null</tt>.
+     *
+     * <p>Like the {@link #toArray()} method, this method acts as bridge between
+     * array-based and collection-based APIs.  Further, this method allows
+     * precise control over the runtime type of the output array, and may,
+     * under certain circumstances, be used to save allocation costs.
+     *
+     * <p>Suppose <tt>x</tt> is a queue known to contain only strings.
+     * The following code can be used to dump the queue into a newly
+     * allocated array of <tt>String</tt>:
+     *
+     * <pre>
+     *     String[] y = x.toArray(new String[0]);</pre>
+     *
+     * Note that <tt>toArray(new Object[0])</tt> is identical in function to
+     * <tt>toArray()</tt>.
+     *
+     * @param a the array into which the elements of the queue are to
+     *          be stored, if it is big enough; otherwise, a new array of the
+     *          same runtime type is allocated for this purpose
+     * @return an array containing all of the elements in this queue
+     * @throws ArrayStoreException if the runtime type of the specified array
+     *         is not a supertype of the runtime type of every element in
+     *         this queue
+     * @throws NullPointerException if the specified array is null
+     */
+    public <T> T[] toArray(T[] a) {
+        final ReentrantLock lock = this.lock;
+        lock.lock();
+        try {
+            return q.toArray(a);
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Returns an iterator over the elements in this queue. The
+     * iterator does not return the elements in any particular order.
+     * The returned <tt>Iterator</tt> is a "weakly consistent"
+     * iterator that will never throw {@link
+     * ConcurrentModificationException}, and guarantees to traverse
+     * elements as they existed upon construction of the iterator, and
+     * may (but is not guaranteed to) reflect any modifications
+     * subsequent to construction.
+     *
+     * @return an iterator over the elements in this queue
+     */
+    public Iterator<E> iterator() {
+        return new Itr(toArray());
+    }
+
+    /**
+     * Snapshot iterator that works off copy of underlying q array.
+     */
+    private class Itr implements Iterator<E> {
+        final Object[] array; // Array of all elements
+	int cursor;           // index of next element to return;
+	int lastRet;          // index of last element, or -1 if no such
+
+        Itr(Object[] array) {
+            lastRet = -1;
+            this.array = array;
+        }
+
+        public boolean hasNext() {
+            return cursor < array.length;
+        }
+
+        public E next() {
+            if (cursor >= array.length)
+                throw new NoSuchElementException();
+            lastRet = cursor;
+            return (E)array[cursor++];
+        }
+
+        public void remove() {
+            if (lastRet < 0)
+		throw new IllegalStateException();
+            Object x = array[lastRet];
+            lastRet = -1;
+            // Traverse underlying queue to find == element,
+            // not just a .equals element.
+            lock.lock();
+            try {
+                for (Iterator it = q.iterator(); it.hasNext(); ) {
+                    if (it.next() == x) {
+                        it.remove();
+                        return;
+                    }
+                }
+            } finally {
+                lock.unlock();
+            }
+        }
+    }
+
+    /**
+     * Saves the state to a stream (that is, serializes it).  This
+     * merely wraps default serialization within lock.  The
+     * serialization strategy for items is left to underlying
+     * Queue. Note that locking is not needed on deserialization, so
+     * readObject is not defined, just relying on default.
+     */
+    private void writeObject(java.io.ObjectOutputStream s)
+        throws java.io.IOException {
+        lock.lock();
+        try {
+            s.defaultWriteObject();
+        } finally {
+            lock.unlock();
+        }
+    }
+
+}
diff --git a/external/jsr166/java/util/concurrent/RejectedExecutionException.java b/external/jsr166/java/util/concurrent/RejectedExecutionException.java
new file mode 100644
index 000000000..30b043d66
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/RejectedExecutionException.java
@@ -0,0 +1,62 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+
+/**
+ * Exception thrown by an {@link Executor} when a task cannot be
+ * accepted for execution.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ */
+public class RejectedExecutionException extends RuntimeException {
+    private static final long serialVersionUID = -375805702767069545L;
+
+    /**
+     * Constructs a <tt>RejectedExecutionException</tt> with no detail message.
+     * The cause is not initialized, and may subsequently be
+     * initialized by a call to {@link #initCause(Throwable) initCause}.
+     */
+    public RejectedExecutionException() { }
+
+    /**
+     * Constructs a <tt>RejectedExecutionException</tt> with the
+     * specified detail message. The cause is not initialized, and may
+     * subsequently be initialized by a call to {@link
+     * #initCause(Throwable) initCause}.
+     *
+     * @param message the detail message
+     */
+    public RejectedExecutionException(String message) {
+        super(message);
+    }
+
+    /**
+     * Constructs a <tt>RejectedExecutionException</tt> with the
+     * specified detail message and cause.
+     *
+     * @param  message the detail message
+     * @param  cause the cause (which is saved for later retrieval by the
+     *         {@link #getCause()} method)
+     */
+    public RejectedExecutionException(String message, Throwable cause) {
+        super(message, cause);
+    }
+
+    /**
+     * Constructs a <tt>RejectedExecutionException</tt> with the
+     * specified cause.  The detail message is set to: <pre> (cause ==
+     * null ? null : cause.toString())</pre> (which typically contains
+     * the class and detail message of <tt>cause</tt>).
+     *
+     * @param  cause the cause (which is saved for later retrieval by the
+     *         {@link #getCause()} method)
+     */
+    public RejectedExecutionException(Throwable cause) {
+        super(cause);
+    }
+}
diff --git a/external/jsr166/java/util/concurrent/RejectedExecutionHandler.java b/external/jsr166/java/util/concurrent/RejectedExecutionHandler.java
new file mode 100644
index 000000000..4b4bbeab1
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/RejectedExecutionHandler.java
@@ -0,0 +1,33 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+
+/**
+ * A handler for tasks that cannot be executed by a {@link
+ * ThreadPoolExecutor}.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ */
+public interface RejectedExecutionHandler {
+
+    /**
+     * Method that may be invoked by a {@link ThreadPoolExecutor} when
+     * <tt>execute</tt> cannot accept a task. This may occur when no
+     * more threads or queue slots are available because their bounds
+     * would be exceeded, or upon shutdown of the Executor.
+     *
+     * In the absence other alternatives, the method may throw an
+     * unchecked {@link RejectedExecutionException}, which will be
+     * propagated to the caller of <tt>execute</tt>.
+     *
+     * @param r the runnable task requested to be executed
+     * @param executor the executor attempting to execute this task
+     * @throws RejectedExecutionException if there is no remedy
+     */
+    void rejectedExecution(Runnable r, ThreadPoolExecutor executor);
+}
diff --git a/external/jsr166/java/util/concurrent/RunnableFuture.java b/external/jsr166/java/util/concurrent/RunnableFuture.java
new file mode 100644
index 000000000..d74211d13
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/RunnableFuture.java
@@ -0,0 +1,25 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+
+/**
+ * A {@link Future} that is {@link Runnable}. Successful execution of
+ * the <tt>run</tt> method causes completion of the <tt>Future</tt>
+ * and allows access to its results.
+ * @see FutureTask
+ * @see Executor
+ * @since 1.6
+ * @author Doug Lea
+ * @param <V> The result type returned by this Future's <tt>get</tt> method
+ */
+public interface RunnableFuture<V> extends Runnable, Future<V> {
+    /**
+     * Sets this Future to the result of its computation
+     * unless it has been cancelled.
+     */
+    void run();
+}
diff --git a/external/jsr166/java/util/concurrent/RunnableScheduledFuture.java b/external/jsr166/java/util/concurrent/RunnableScheduledFuture.java
new file mode 100644
index 000000000..0e8cc328c
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/RunnableScheduledFuture.java
@@ -0,0 +1,29 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+
+/**
+ * A {@link ScheduledFuture} that is {@link Runnable}. Successful
+ * execution of the <tt>run</tt> method causes completion of the
+ * <tt>Future</tt> and allows access to its results.
+ * @see FutureTask
+ * @see Executor
+ * @since 1.6
+ * @author Doug Lea
+ * @param <V> The result type returned by this Future's <tt>get</tt> method
+ */
+public interface RunnableScheduledFuture<V> extends RunnableFuture<V>, ScheduledFuture<V> {
+
+    /**
+     * Returns true if this is a periodic task. A periodic task may
+     * re-run according to some schedule. A non-periodic task can be
+     * run only once.
+     *
+     * @return true if this task is periodic
+     */
+    boolean isPeriodic();
+}
diff --git a/external/jsr166/java/util/concurrent/ScheduledExecutorService.java b/external/jsr166/java/util/concurrent/ScheduledExecutorService.java
new file mode 100644
index 000000000..976537ef5
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/ScheduledExecutorService.java
@@ -0,0 +1,159 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+import java.util.concurrent.atomic.*;
+import java.util.*;
+
+/**
+ * An {@link ExecutorService} that can schedule commands to run after a given
+ * delay, or to execute periodically.
+ *
+ * <p> The <tt>schedule</tt> methods create tasks with various delays
+ * and return a task object that can be used to cancel or check
+ * execution. The <tt>scheduleAtFixedRate</tt> and
+ * <tt>scheduleWithFixedDelay</tt> methods create and execute tasks
+ * that run periodically until cancelled.
+ *
+ * <p> Commands submitted using the {@link Executor#execute} and
+ * {@link ExecutorService} <tt>submit</tt> methods are scheduled with
+ * a requested delay of zero. Zero and negative delays (but not
+ * periods) are also allowed in <tt>schedule</tt> methods, and are
+ * treated as requests for immediate execution.
+ *
+ * <p>All <tt>schedule</tt> methods accept <em>relative</em> delays and
+ * periods as arguments, not absolute times or dates. It is a simple
+ * matter to transform an absolute time represented as a {@link
+ * java.util.Date} to the required form. For example, to schedule at
+ * a certain future <tt>date</tt>, you can use: <tt>schedule(task,
+ * date.getTime() - System.currentTimeMillis(),
+ * TimeUnit.MILLISECONDS)</tt>. Beware however that expiration of a
+ * relative delay need not coincide with the current <tt>Date</tt> at
+ * which the task is enabled due to network time synchronization
+ * protocols, clock drift, or other factors.
+ *
+ * The {@link Executors} class provides convenient factory methods for
+ * the ScheduledExecutorService implementations provided in this package.
+ *
+ * <h3>Usage Example</h3>
+ *
+ * Here is a class with a method that sets up a ScheduledExecutorService
+ * to beep every ten seconds for an hour:
+ *
+ * <pre>
+ * import static java.util.concurrent.TimeUnit.*;
+ * class BeeperControl {
+ *    private final ScheduledExecutorService scheduler =
+ *       Executors.newScheduledThreadPool(1);
+ *
+ *    public void beepForAnHour() {
+ *        final Runnable beeper = new Runnable() {
+ *                public void run() { System.out.println("beep"); }
+ *            };
+ *        final ScheduledFuture&lt;?&gt; beeperHandle =
+ *            scheduler.scheduleAtFixedRate(beeper, 10, 10, SECONDS);
+ *        scheduler.schedule(new Runnable() {
+ *                public void run() { beeperHandle.cancel(true); }
+ *            }, 60 * 60, SECONDS);
+ *    }
+ * }
+ * </pre>
+ *
+ * @since 1.5
+ * @author Doug Lea
+ */
+public interface ScheduledExecutorService extends ExecutorService {
+
+    /**
+     * Creates and executes a one-shot action that becomes enabled
+     * after the given delay.
+     *
+     * @param command the task to execute
+     * @param delay the time from now to delay execution
+     * @param unit the time unit of the delay parameter
+     * @return a ScheduledFuture representing pending completion of
+     *         the task and whose <tt>get()</tt> method will return
+     *         <tt>null</tt> upon completion
+     * @throws RejectedExecutionException if the task cannot be
+     *         scheduled for execution
+     * @throws NullPointerException if command is null
+     */
+    public ScheduledFuture<?> schedule(Runnable command,
+				       long delay, TimeUnit unit);
+
+    /**
+     * Creates and executes a ScheduledFuture that becomes enabled after the
+     * given delay.
+     *
+     * @param callable the function to execute
+     * @param delay the time from now to delay execution
+     * @param unit the time unit of the delay parameter
+     * @return a ScheduledFuture that can be used to extract result or cancel
+     * @throws RejectedExecutionException if the task cannot be
+     *         scheduled for execution
+     * @throws NullPointerException if callable is null
+     */
+    public <V> ScheduledFuture<V> schedule(Callable<V> callable,
+					   long delay, TimeUnit unit);
+
+    /**
+     * Creates and executes a periodic action that becomes enabled first
+     * after the given initial delay, and subsequently with the given
+     * period; that is executions will commence after
+     * <tt>initialDelay</tt> then <tt>initialDelay+period</tt>, then
+     * <tt>initialDelay + 2 * period</tt>, and so on.
+     * If any execution of the task
+     * encounters an exception, subsequent executions are suppressed.
+     * Otherwise, the task will only terminate via cancellation or
+     * termination of the executor.  If any execution of this task
+     * takes longer than its period, then subsequent executions
+     * may start late, but will not concurrently execute.
+     *
+     * @param command the task to execute
+     * @param initialDelay the time to delay first execution
+     * @param period the period between successive executions
+     * @param unit the time unit of the initialDelay and period parameters
+     * @return a ScheduledFuture representing pending completion of
+     *         the task, and whose <tt>get()</tt> method will throw an
+     *         exception upon cancellation
+     * @throws RejectedExecutionException if the task cannot be
+     *         scheduled for execution
+     * @throws NullPointerException if command is null
+     * @throws IllegalArgumentException if period less than or equal to zero
+     */
+    public ScheduledFuture<?> scheduleAtFixedRate(Runnable command,
+						  long initialDelay,
+						  long period,
+						  TimeUnit unit);
+
+    /**
+     * Creates and executes a periodic action that becomes enabled first
+     * after the given initial delay, and subsequently with the
+     * given delay between the termination of one execution and the
+     * commencement of the next.  If any execution of the task
+     * encounters an exception, subsequent executions are suppressed.
+     * Otherwise, the task will only terminate via cancellation or
+     * termination of the executor.
+     *
+     * @param command the task to execute
+     * @param initialDelay the time to delay first execution
+     * @param delay the delay between the termination of one
+     * execution and the commencement of the next
+     * @param unit the time unit of the initialDelay and delay parameters
+     * @return a ScheduledFuture representing pending completion of
+     *         the task, and whose <tt>get()</tt> method will throw an
+     *         exception upon cancellation
+     * @throws RejectedExecutionException if the task cannot be
+     *         scheduled for execution
+     * @throws NullPointerException if command is null
+     * @throws IllegalArgumentException if delay less than or equal to zero
+     */
+    public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command,
+						     long initialDelay,
+						     long delay,
+						     TimeUnit unit);
+
+}
diff --git a/external/jsr166/java/util/concurrent/ScheduledFuture.java b/external/jsr166/java/util/concurrent/ScheduledFuture.java
new file mode 100644
index 000000000..239d681f6
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/ScheduledFuture.java
@@ -0,0 +1,19 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+
+/**
+ * A delayed result-bearing action that can be cancelled.
+ * Usually a scheduled future is the result of scheduling
+ * a task with a {@link ScheduledExecutorService}.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ * @param <V> The result type returned by this Future
+ */
+public interface ScheduledFuture<V> extends Delayed, Future<V> {
+}
diff --git a/external/jsr166/java/util/concurrent/ScheduledThreadPoolExecutor.java b/external/jsr166/java/util/concurrent/ScheduledThreadPoolExecutor.java
new file mode 100644
index 000000000..66dd25eaa
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/ScheduledThreadPoolExecutor.java
@@ -0,0 +1,623 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+import java.util.concurrent.atomic.*;
+import java.util.*;
+
+/**
+ * A {@link ThreadPoolExecutor} that can additionally schedule
+ * commands to run after a given delay, or to execute
+ * periodically. This class is preferable to {@link java.util.Timer}
+ * when multiple worker threads are needed, or when the additional
+ * flexibility or capabilities of {@link ThreadPoolExecutor} (which
+ * this class extends) are required.
+ *
+ * <p> Delayed tasks execute no sooner than they are enabled, but
+ * without any real-time guarantees about when, after they are
+ * enabled, they will commence. Tasks scheduled for exactly the same
+ * execution time are enabled in first-in-first-out (FIFO) order of
+ * submission.
+ *
+ * <p>While this class inherits from {@link ThreadPoolExecutor}, a few
+ * of the inherited tuning methods are not useful for it. In
+ * particular, because it acts as a fixed-sized pool using
+ * <tt>corePoolSize</tt> threads and an unbounded queue, adjustments
+ * to <tt>maximumPoolSize</tt> have no useful effect.
+ *
+ * <p><b>Extension notes:</b> This class overrides {@link
+ * AbstractExecutorService} <tt>submit</tt> methods to generate
+ * internal objects to control per-task delays and scheduling. To
+ * preserve functionality, any further overrides of these methods in
+ * subclasses must invoke superclass versions, which effectively
+ * disables additional task customization. However, this class
+ * provides alternative protected extension method
+ * <tt>decorateTask</tt> (one version each for <tt>Runnable</tt> and
+ * <tt>Callable</tt>) that can be used to customize the concrete task
+ * types used to execute commands entered via <tt>execute</tt>,
+ * <tt>submit</tt>, <tt>schedule</tt>, <tt>scheduleAtFixedRate</tt>,
+ * and <tt>scheduleWithFixedDelay</tt>.  By default, a
+ * <tt>ScheduledThreadPoolExecutor</tt> uses a task type extending
+ * {@link FutureTask}. However, this may be modified or replaced using
+ * subclasses of the form:
+ *
+ * <pre>
+ * public class CustomScheduledExecutor extends ScheduledThreadPoolExecutor {
+ *
+ *   static class CustomTask&lt;V&gt; implements RunnableScheduledFuture&lt;V&gt; { ... }
+ *
+ *   protected &lt;V&gt; RunnableScheduledFuture&lt;V&gt; decorateTask(
+ *                Runnable r, RunnableScheduledFuture&lt;V&gt; task) {
+ *       return new CustomTask&lt;V&gt;(r, task);
+ *   }
+ *
+ *   protected &lt;V&gt; RunnableScheduledFuture&lt;V&gt; decorateTask(
+ *                Callable&lt;V&gt; c, RunnableScheduledFuture&lt;V&gt; task) {
+ *       return new CustomTask&lt;V&gt;(c, task);
+ *   }
+ *   // ... add constructors, etc.
+ * }
+ * </pre>
+ * @since 1.5
+ * @author Doug Lea
+ */
+public class ScheduledThreadPoolExecutor
+        extends ThreadPoolExecutor
+        implements ScheduledExecutorService {
+
+    /**
+     * False if should cancel/suppress periodic tasks on shutdown.
+     */
+    private volatile boolean continueExistingPeriodicTasksAfterShutdown;
+
+    /**
+     * False if should cancel non-periodic tasks on shutdown.
+     */
+    private volatile boolean executeExistingDelayedTasksAfterShutdown = true;
+
+    /**
+     * Sequence number to break scheduling ties, and in turn to
+     * guarantee FIFO order among tied entries.
+     */
+    private static final AtomicLong sequencer = new AtomicLong(0);
+
+    /** Base of nanosecond timings, to avoid wrapping */
+    private static final long NANO_ORIGIN = System.nanoTime();
+
+    /**
+     * Returns nanosecond time offset by origin
+     */
+    final long now() {
+        return System.nanoTime() - NANO_ORIGIN;
+    }
+
+    private class ScheduledFutureTask<V>
+            extends FutureTask<V> implements RunnableScheduledFuture<V> {
+
+        /** Sequence number to break ties FIFO */
+        private final long sequenceNumber;
+        /** The time the task is enabled to execute in nanoTime units */
+        private long time;
+        /**
+         * Period in nanoseconds for repeating tasks.  A positive
+         * value indicates fixed-rate execution.  A negative value
+         * indicates fixed-delay execution.  A value of 0 indicates a
+         * non-repeating task.
+         */
+        private final long period;
+
+        /**
+         * Creates a one-shot action with given nanoTime-based trigger time.
+         */
+        ScheduledFutureTask(Runnable r, V result, long ns) {
+            super(r, result);
+            this.time = ns;
+            this.period = 0;
+            this.sequenceNumber = sequencer.getAndIncrement();
+        }
+
+        /**
+         * Creates a periodic action with given nano time and period.
+         */
+        ScheduledFutureTask(Runnable r, V result, long ns, long period) {
+            super(r, result);
+            this.time = ns;
+            this.period = period;
+            this.sequenceNumber = sequencer.getAndIncrement();
+        }
+
+        /**
+         * Creates a one-shot action with given nanoTime-based trigger.
+         */
+        ScheduledFutureTask(Callable<V> callable, long ns) {
+            super(callable);
+            this.time = ns;
+            this.period = 0;
+            this.sequenceNumber = sequencer.getAndIncrement();
+        }
+
+        public long getDelay(TimeUnit unit) {
+            long d = unit.convert(time - now(), TimeUnit.NANOSECONDS);
+            return d;
+        }
+
+        public int compareTo(Delayed other) {
+            if (other == this) // compare zero ONLY if same object
+                return 0;
+            if (other instanceof ScheduledFutureTask) {
+                ScheduledFutureTask<?> x = (ScheduledFutureTask<?>)other;
+                long diff = time - x.time;
+                if (diff < 0)
+                    return -1;
+                else if (diff > 0)
+                    return 1;
+                else if (sequenceNumber < x.sequenceNumber)
+                    return -1;
+                else
+                    return 1;
+            }
+            long d = (getDelay(TimeUnit.NANOSECONDS) -
+                      other.getDelay(TimeUnit.NANOSECONDS));
+            return (d == 0)? 0 : ((d < 0)? -1 : 1);
+        }
+
+        /**
+         * Returns true if this is a periodic (not a one-shot) action.
+         *
+         * @return true if periodic
+         */
+        public boolean isPeriodic() {
+            return period != 0;
+        }
+
+        /**
+         * Runs a periodic task.
+         */
+        private void runPeriodic() {
+            boolean ok = ScheduledFutureTask.super.runAndReset();
+            boolean down = isShutdown();
+            // Reschedule if not cancelled and not shutdown or policy allows
+            if (ok && (!down ||
+                       (getContinueExistingPeriodicTasksAfterShutdownPolicy() &&
+                        !isTerminating()))) {
+                long p = period;
+                if (p > 0)
+                    time += p;
+                else
+                    time = now() - p;
+                ScheduledThreadPoolExecutor.super.getQueue().add(this);
+            }
+            // This might have been the final executed delayed
+            // task.  Wake up threads to check.
+            else if (down)
+                interruptIdleWorkers();
+        }
+
+        /**
+         * Overrides FutureTask version so as to reset/requeue if periodic.
+         */
+        public void run() {
+            if (isPeriodic())
+                runPeriodic();
+            else
+                ScheduledFutureTask.super.run();
+        }
+    }
+
+    /**
+     * Specialized variant of ThreadPoolExecutor.execute for delayed tasks.
+     */
+    private void delayedExecute(Runnable command) {
+        if (isShutdown()) {
+            reject(command);
+            return;
+        }
+        // Prestart a thread if necessary. We cannot prestart it
+        // running the task because the task (probably) shouldn't be
+        // run yet, so thread will just idle until delay elapses.
+        if (getPoolSize() < getCorePoolSize())
+            prestartCoreThread();
+
+        super.getQueue().add(command);
+    }
+
+    /**
+     * Cancels and clears the queue of all tasks that should not be run
+     * due to shutdown policy.
+     */
+    private void cancelUnwantedTasks() {
+        boolean keepDelayed = getExecuteExistingDelayedTasksAfterShutdownPolicy();
+        boolean keepPeriodic = getContinueExistingPeriodicTasksAfterShutdownPolicy();
+        if (!keepDelayed && !keepPeriodic)
+            super.getQueue().clear();
+        else if (keepDelayed || keepPeriodic) {
+            Object[] entries = super.getQueue().toArray();
+            for (int i = 0; i < entries.length; ++i) {
+                Object e = entries[i];
+                if (e instanceof RunnableScheduledFuture) {
+                    RunnableScheduledFuture<?> t = (RunnableScheduledFuture<?>)e;
+                    if (t.isPeriodic()? !keepPeriodic : !keepDelayed)
+                        t.cancel(false);
+                }
+            }
+            entries = null;
+            purge();
+        }
+    }
+
+    public boolean remove(Runnable task) {
+        if (!(task instanceof RunnableScheduledFuture))
+            return false;
+        return getQueue().remove(task);
+    }
+
+    /**
+     * Modifies or replaces the task used to execute a runnable.
+     * This method can be used to override the concrete
+     * class used for managing internal tasks.
+     * The default implementation simply returns the given task.
+     *
+     * @param runnable the submitted Runnable
+     * @param task the task created to execute the runnable
+     * @return a task that can execute the runnable
+     * @since 1.6
+     */
+    protected <V> RunnableScheduledFuture<V> decorateTask(
+        Runnable runnable, RunnableScheduledFuture<V> task) {
+        return task;
+    }
+
+    /**
+     * Modifies or replaces the task used to execute a callable.
+     * This method can be used to override the concrete
+     * class used for managing internal tasks.
+     * The default implementation simply returns the given task.
+     *
+     * @param callable the submitted Callable
+     * @param task the task created to execute the callable
+     * @return a task that can execute the callable
+     * @since 1.6
+     */
+    protected <V> RunnableScheduledFuture<V> decorateTask(
+        Callable<V> callable, RunnableScheduledFuture<V> task) {
+        return task;
+    }
+
+    /**
+     * Creates a new ScheduledThreadPoolExecutor with the given core
+     * pool size.
+     *
+     * @param corePoolSize the number of threads to keep in the pool,
+     * even if they are idle
+     * @throws IllegalArgumentException if <tt>corePoolSize &lt; 0</tt>
+     */
+    public ScheduledThreadPoolExecutor(int corePoolSize) {
+        super(corePoolSize, Integer.MAX_VALUE, 0, TimeUnit.NANOSECONDS,
+              new DelayedWorkQueue());
+    }
+
+    /**
+     * Creates a new ScheduledThreadPoolExecutor with the given
+     * initial parameters.
+     *
+     * @param corePoolSize the number of threads to keep in the pool,
+     * even if they are idle
+     * @param threadFactory the factory to use when the executor
+     * creates a new thread
+     * @throws IllegalArgumentException if <tt>corePoolSize &lt; 0</tt>
+     * @throws NullPointerException if threadFactory is null
+     */
+    public ScheduledThreadPoolExecutor(int corePoolSize,
+                             ThreadFactory threadFactory) {
+        super(corePoolSize, Integer.MAX_VALUE, 0, TimeUnit.NANOSECONDS,
+              new DelayedWorkQueue(), threadFactory);
+    }
+
+    /**
+     * Creates a new ScheduledThreadPoolExecutor with the given
+     * initial parameters.
+     *
+     * @param corePoolSize the number of threads to keep in the pool,
+     * even if they are idle
+     * @param handler the handler to use when execution is blocked
+     * because the thread bounds and queue capacities are reached
+     * @throws IllegalArgumentException if <tt>corePoolSize &lt; 0</tt>
+     * @throws NullPointerException if handler is null
+     */
+    public ScheduledThreadPoolExecutor(int corePoolSize,
+                              RejectedExecutionHandler handler) {
+        super(corePoolSize, Integer.MAX_VALUE, 0, TimeUnit.NANOSECONDS,
+              new DelayedWorkQueue(), handler);
+    }
+
+    /**
+     * Creates a new ScheduledThreadPoolExecutor with the given
+     * initial parameters.
+     *
+     * @param corePoolSize the number of threads to keep in the pool,
+     * even if they are idle
+     * @param threadFactory the factory to use when the executor
+     * creates a new thread
+     * @param handler the handler to use when execution is blocked
+     * because the thread bounds and queue capacities are reached.
+     * @throws IllegalArgumentException if <tt>corePoolSize &lt; 0</tt>
+     * @throws NullPointerException if threadFactory or handler is null
+     */
+    public ScheduledThreadPoolExecutor(int corePoolSize,
+                              ThreadFactory threadFactory,
+                              RejectedExecutionHandler handler) {
+        super(corePoolSize, Integer.MAX_VALUE, 0, TimeUnit.NANOSECONDS,
+              new DelayedWorkQueue(), threadFactory, handler);
+    }
+
+    public ScheduledFuture<?> schedule(Runnable command,
+                                       long delay,
+                                       TimeUnit unit) {
+        if (command == null || unit == null)
+            throw new NullPointerException();
+        long triggerTime = now() + unit.toNanos(delay);
+        RunnableScheduledFuture<?> t = decorateTask(command,
+            new ScheduledFutureTask<Boolean>(command, null, triggerTime));
+        delayedExecute(t);
+        return t;
+    }
+
+    public <V> ScheduledFuture<V> schedule(Callable<V> callable,
+                                           long delay,
+                                           TimeUnit unit) {
+        if (callable == null || unit == null)
+            throw new NullPointerException();
+        if (delay < 0) delay = 0;
+        long triggerTime = now() + unit.toNanos(delay);
+        RunnableScheduledFuture<V> t = decorateTask(callable,
+            new ScheduledFutureTask<V>(callable, triggerTime));
+        delayedExecute(t);
+        return t;
+    }
+
+    public ScheduledFuture<?> scheduleAtFixedRate(Runnable command,
+                                                  long initialDelay,
+                                                  long period,
+                                                  TimeUnit unit) {
+        if (command == null || unit == null)
+            throw new NullPointerException();
+        if (period <= 0)
+            throw new IllegalArgumentException();
+        if (initialDelay < 0) initialDelay = 0;
+        long triggerTime = now() + unit.toNanos(initialDelay);
+        RunnableScheduledFuture<?> t = decorateTask(command,
+            new ScheduledFutureTask<Object>(command,
+                                            null,
+                                            triggerTime,
+                                            unit.toNanos(period)));
+        delayedExecute(t);
+        return t;
+    }
+
+    public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command,
+                                                     long initialDelay,
+                                                     long delay,
+                                                     TimeUnit unit) {
+        if (command == null || unit == null)
+            throw new NullPointerException();
+        if (delay <= 0)
+            throw new IllegalArgumentException();
+        if (initialDelay < 0) initialDelay = 0;
+        long triggerTime = now() + unit.toNanos(initialDelay);
+        RunnableScheduledFuture<?> t = decorateTask(command,
+            new ScheduledFutureTask<Boolean>(command,
+                                             null,
+                                             triggerTime,
+                                             unit.toNanos(-delay)));
+        delayedExecute(t);
+        return t;
+    }
+
+
+    /**
+     * Executes command with zero required delay. This has effect
+     * equivalent to <tt>schedule(command, 0, anyUnit)</tt>.  Note
+     * that inspections of the queue and of the list returned by
+     * <tt>shutdownNow</tt> will access the zero-delayed
+     * {@link ScheduledFuture}, not the <tt>command</tt> itself.
+     *
+     * @param command the task to execute
+     * @throws RejectedExecutionException at discretion of
+     * <tt>RejectedExecutionHandler</tt>, if task cannot be accepted
+     * for execution because the executor has been shut down.
+     * @throws NullPointerException if command is null
+     */
+    public void execute(Runnable command) {
+        if (command == null)
+            throw new NullPointerException();
+        schedule(command, 0, TimeUnit.NANOSECONDS);
+    }
+
+    // Override AbstractExecutorService methods
+
+    public Future<?> submit(Runnable task) {
+        return schedule(task, 0, TimeUnit.NANOSECONDS);
+    }
+
+    public <T> Future<T> submit(Runnable task, T result) {
+        return schedule(Executors.callable(task, result),
+                        0, TimeUnit.NANOSECONDS);
+    }
+
+    public <T> Future<T> submit(Callable<T> task) {
+        return schedule(task, 0, TimeUnit.NANOSECONDS);
+    }
+
+    /**
+     * Sets the policy on whether to continue executing existing periodic
+     * tasks even when this executor has been <tt>shutdown</tt>. In
+     * this case, these tasks will only terminate upon
+     * <tt>shutdownNow</tt>, or after setting the policy to
+     * <tt>false</tt> when already shutdown. This value is by default
+     * false.
+     *
+     * @param value if true, continue after shutdown, else don't.
+     * @see #getContinueExistingPeriodicTasksAfterShutdownPolicy
+     */
+    public void setContinueExistingPeriodicTasksAfterShutdownPolicy(boolean value) {
+        continueExistingPeriodicTasksAfterShutdown = value;
+        if (!value && isShutdown())
+            cancelUnwantedTasks();
+    }
+
+    /**
+     * Gets the policy on whether to continue executing existing
+     * periodic tasks even when this executor has been
+     * <tt>shutdown</tt>. In this case, these tasks will only
+     * terminate upon <tt>shutdownNow</tt> or after setting the policy
+     * to <tt>false</tt> when already shutdown. This value is by
+     * default false.
+     *
+     * @return true if will continue after shutdown
+     * @see #setContinueExistingPeriodicTasksAfterShutdownPolicy
+     */
+    public boolean getContinueExistingPeriodicTasksAfterShutdownPolicy() {
+        return continueExistingPeriodicTasksAfterShutdown;
+    }
+
+    /**
+     * Sets the policy on whether to execute existing delayed
+     * tasks even when this executor has been <tt>shutdown</tt>. In
+     * this case, these tasks will only terminate upon
+     * <tt>shutdownNow</tt>, or after setting the policy to
+     * <tt>false</tt> when already shutdown. This value is by default
+     * true.
+     *
+     * @param value if true, execute after shutdown, else don't.
+     * @see #getExecuteExistingDelayedTasksAfterShutdownPolicy
+     */
+    public void setExecuteExistingDelayedTasksAfterShutdownPolicy(boolean value) {
+        executeExistingDelayedTasksAfterShutdown = value;
+        if (!value && isShutdown())
+            cancelUnwantedTasks();
+    }
+
+    /**
+     * Gets the policy on whether to execute existing delayed
+     * tasks even when this executor has been <tt>shutdown</tt>. In
+     * this case, these tasks will only terminate upon
+     * <tt>shutdownNow</tt>, or after setting the policy to
+     * <tt>false</tt> when already shutdown. This value is by default
+     * true.
+     *
+     * @return true if will execute after shutdown
+     * @see #setExecuteExistingDelayedTasksAfterShutdownPolicy
+     */
+    public boolean getExecuteExistingDelayedTasksAfterShutdownPolicy() {
+        return executeExistingDelayedTasksAfterShutdown;
+    }
+
+
+    /**
+     * Initiates an orderly shutdown in which previously submitted
+     * tasks are executed, but no new tasks will be accepted. If the
+     * <tt>ExecuteExistingDelayedTasksAfterShutdownPolicy</tt> has
+     * been set <tt>false</tt>, existing delayed tasks whose delays
+     * have not yet elapsed are cancelled. And unless the
+     * <tt>ContinueExistingPeriodicTasksAfterShutdownPolicy</tt> has
+     * been set <tt>true</tt>, future executions of existing periodic
+     * tasks will be cancelled.
+     */
+    public void shutdown() {
+        cancelUnwantedTasks();
+        super.shutdown();
+    }
+
+    /**
+     * Attempts to stop all actively executing tasks, halts the
+     * processing of waiting tasks, and returns a list of the tasks
+     * that were awaiting execution.
+     *
+     * <p>There are no guarantees beyond best-effort attempts to stop
+     * processing actively executing tasks.  This implementation
+     * cancels tasks via {@link Thread#interrupt}, so any task that
+     * fails to respond to interrupts may never terminate.
+     *
+     * @return list of tasks that never commenced execution.  Each
+     * element of this list is a {@link ScheduledFuture},
+     * including those tasks submitted using <tt>execute</tt>, which
+     * are for scheduling purposes used as the basis of a zero-delay
+     * <tt>ScheduledFuture</tt>.
+     * @throws SecurityException {@inheritDoc}
+     */
+    public List<Runnable> shutdownNow() {
+        return super.shutdownNow();
+    }
+
+    /**
+     * Returns the task queue used by this executor.  Each element of
+     * this queue is a {@link ScheduledFuture}, including those
+     * tasks submitted using <tt>execute</tt> which are for scheduling
+     * purposes used as the basis of a zero-delay
+     * <tt>ScheduledFuture</tt>. Iteration over this queue is
+     * <em>not</em> guaranteed to traverse tasks in the order in
+     * which they will execute.
+     *
+     * @return the task queue
+     */
+    public BlockingQueue<Runnable> getQueue() {
+        return super.getQueue();
+    }
+
+    /**
+     * An annoying wrapper class to convince javac to use a
+     * DelayQueue<RunnableScheduledFuture> as a BlockingQueue<Runnable>
+     */
+    private static class DelayedWorkQueue
+        extends AbstractCollection<Runnable>
+        implements BlockingQueue<Runnable> {
+
+        private final DelayQueue<RunnableScheduledFuture> dq = new DelayQueue<RunnableScheduledFuture>();
+        public Runnable poll() { return dq.poll(); }
+        public Runnable peek() { return dq.peek(); }
+        public Runnable take() throws InterruptedException { return dq.take(); }
+        public Runnable poll(long timeout, TimeUnit unit) throws InterruptedException {
+            return dq.poll(timeout, unit);
+        }
+
+        public boolean add(Runnable x) {
+	    return dq.add((RunnableScheduledFuture)x);
+	}
+        public boolean offer(Runnable x) {
+	    return dq.offer((RunnableScheduledFuture)x);
+	}
+        public void put(Runnable x) {
+            dq.put((RunnableScheduledFuture)x);
+        }
+        public boolean offer(Runnable x, long timeout, TimeUnit unit) {
+            return dq.offer((RunnableScheduledFuture)x, timeout, unit);
+        }
+
+        public Runnable remove() { return dq.remove(); }
+        public Runnable element() { return dq.element(); }
+        public void clear() { dq.clear(); }
+        public int drainTo(Collection<? super Runnable> c) { return dq.drainTo(c); }
+        public int drainTo(Collection<? super Runnable> c, int maxElements) {
+            return dq.drainTo(c, maxElements);
+        }
+
+        public int remainingCapacity() { return dq.remainingCapacity(); }
+        public boolean remove(Object x) { return dq.remove(x); }
+        public boolean contains(Object x) { return dq.contains(x); }
+        public int size() { return dq.size(); }
+        public boolean isEmpty() { return dq.isEmpty(); }
+        public Object[] toArray() { return dq.toArray(); }
+        public <T> T[] toArray(T[] array) { return dq.toArray(array); }
+        public Iterator<Runnable> iterator() {
+            return new Iterator<Runnable>() {
+                private Iterator<RunnableScheduledFuture> it = dq.iterator();
+                public boolean hasNext() { return it.hasNext(); }
+                public Runnable next() { return it.next(); }
+                public void remove() { it.remove(); }
+            };
+        }
+    }
+}
diff --git a/external/jsr166/java/util/concurrent/Semaphore.java b/external/jsr166/java/util/concurrent/Semaphore.java
new file mode 100644
index 000000000..94e9746ae
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/Semaphore.java
@@ -0,0 +1,681 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+import java.util.*;
+import java.util.concurrent.locks.*;
+import java.util.concurrent.atomic.*;
+
+/**
+ * A counting semaphore.  Conceptually, a semaphore maintains a set of
+ * permits.  Each {@link #acquire} blocks if necessary until a permit is
+ * available, and then takes it.  Each {@link #release} adds a permit,
+ * potentially releasing a blocking acquirer.
+ * However, no actual permit objects are used; the {@code Semaphore} just
+ * keeps a count of the number available and acts accordingly.
+ *
+ * <p>Semaphores are often used to restrict the number of threads than can
+ * access some (physical or logical) resource. For example, here is
+ * a class that uses a semaphore to control access to a pool of items:
+ * <pre>
+ * class Pool {
+ *   private static final int MAX_AVAILABLE = 100;
+ *   private final Semaphore available = new Semaphore(MAX_AVAILABLE, true);
+ *
+ *   public Object getItem() throws InterruptedException {
+ *     available.acquire();
+ *     return getNextAvailableItem();
+ *   }
+ *
+ *   public void putItem(Object x) {
+ *     if (markAsUnused(x))
+ *       available.release();
+ *   }
+ *
+ *   // Not a particularly efficient data structure; just for demo
+ *
+ *   protected Object[] items = ... whatever kinds of items being managed
+ *   protected boolean[] used = new boolean[MAX_AVAILABLE];
+ *
+ *   protected synchronized Object getNextAvailableItem() {
+ *     for (int i = 0; i < MAX_AVAILABLE; ++i) {
+ *       if (!used[i]) {
+ *          used[i] = true;
+ *          return items[i];
+ *       }
+ *     }
+ *     return null; // not reached
+ *   }
+ *
+ *   protected synchronized boolean markAsUnused(Object item) {
+ *     for (int i = 0; i < MAX_AVAILABLE; ++i) {
+ *       if (item == items[i]) {
+ *          if (used[i]) {
+ *            used[i] = false;
+ *            return true;
+ *          } else
+ *            return false;
+ *       }
+ *     }
+ *     return false;
+ *   }
+ *
+ * }
+ * </pre>
+ *
+ * <p>Before obtaining an item each thread must acquire a permit from
+ * the semaphore, guaranteeing that an item is available for use. When
+ * the thread has finished with the item it is returned back to the
+ * pool and a permit is returned to the semaphore, allowing another
+ * thread to acquire that item.  Note that no synchronization lock is
+ * held when {@link #acquire} is called as that would prevent an item
+ * from being returned to the pool.  The semaphore encapsulates the
+ * synchronization needed to restrict access to the pool, separately
+ * from any synchronization needed to maintain the consistency of the
+ * pool itself.
+ *
+ * <p>A semaphore initialized to one, and which is used such that it
+ * only has at most one permit available, can serve as a mutual
+ * exclusion lock.  This is more commonly known as a <em>binary
+ * semaphore</em>, because it only has two states: one permit
+ * available, or zero permits available.  When used in this way, the
+ * binary semaphore has the property (unlike many {@link Lock}
+ * implementations), that the &quot;lock&quot; can be released by a
+ * thread other than the owner (as semaphores have no notion of
+ * ownership).  This can be useful in some specialized contexts, such
+ * as deadlock recovery.
+ *
+ * <p> The constructor for this class optionally accepts a
+ * <em>fairness</em> parameter. When set false, this class makes no
+ * guarantees about the order in which threads acquire permits. In
+ * particular, <em>barging</em> is permitted, that is, a thread
+ * invoking {@link #acquire} can be allocated a permit ahead of a
+ * thread that has been waiting - logically the new thread places itself at
+ * the head of the queue of waiting threads. When fairness is set true, the
+ * semaphore guarantees that threads invoking any of the {@link
+ * #acquire() acquire} methods are selected to obtain permits in the order in
+ * which their invocation of those methods was processed
+ * (first-in-first-out; FIFO). Note that FIFO ordering necessarily
+ * applies to specific internal points of execution within these
+ * methods.  So, it is possible for one thread to invoke
+ * {@code acquire} before another, but reach the ordering point after
+ * the other, and similarly upon return from the method.
+ * Also note that the untimed {@link #tryAcquire() tryAcquire} methods do not
+ * honor the fairness setting, but will take any permits that are
+ * available.
+ *
+ * <p>Generally, semaphores used to control resource access should be
+ * initialized as fair, to ensure that no thread is starved out from
+ * accessing a resource. When using semaphores for other kinds of
+ * synchronization control, the throughput advantages of non-fair
+ * ordering often outweigh fairness considerations.
+ *
+ * <p>This class also provides convenience methods to {@link
+ * #acquire(int) acquire} and {@link #release(int) release} multiple
+ * permits at a time.  Beware of the increased risk of indefinite
+ * postponement when these methods are used without fairness set true.
+ *
+ * <p>Memory consistency effects: Actions in a thread prior to calling
+ * a "release" method such as {@code release()}
+ * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
+ * actions following a successful "acquire" method such as {@code acquire()}
+ * in another thread.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ *
+ */
+
+public class Semaphore implements java.io.Serializable {
+    private static final long serialVersionUID = -3222578661600680210L;
+    /** All mechanics via AbstractQueuedSynchronizer subclass */
+    private final Sync sync;
+
+    /**
+     * Synchronization implementation for semaphore.  Uses AQS state
+     * to represent permits. Subclassed into fair and nonfair
+     * versions.
+     */
+    abstract static class Sync extends AbstractQueuedSynchronizer {
+        private static final long serialVersionUID = 1192457210091910933L;
+
+        Sync(int permits) {
+            setState(permits);
+        }
+
+        final int getPermits() {
+            return getState();
+        }
+
+        final int nonfairTryAcquireShared(int acquires) {
+            for (;;) {
+                int available = getState();
+                int remaining = available - acquires;
+                if (remaining < 0 ||
+                    compareAndSetState(available, remaining))
+                    return remaining;
+            }
+        }
+
+        protected final boolean tryReleaseShared(int releases) {
+            for (;;) {
+                int p = getState();
+                if (compareAndSetState(p, p + releases))
+                    return true;
+            }
+        }
+
+        final void reducePermits(int reductions) {
+            for (;;) {
+                int current = getState();
+                int next = current - reductions;
+                if (compareAndSetState(current, next))
+                    return;
+            }
+        }
+
+        final int drainPermits() {
+            for (;;) {
+                int current = getState();
+                if (current == 0 || compareAndSetState(current, 0))
+                    return current;
+            }
+        }
+    }
+
+    /**
+     * NonFair version
+     */
+    final static class NonfairSync extends Sync {
+        private static final long serialVersionUID = -2694183684443567898L;
+
+        NonfairSync(int permits) {
+            super(permits);
+        }
+
+        protected int tryAcquireShared(int acquires) {
+            return nonfairTryAcquireShared(acquires);
+        }
+    }
+
+    /**
+     * Fair version
+     */
+    final static class FairSync extends Sync {
+        private static final long serialVersionUID = 2014338818796000944L;
+
+        FairSync(int permits) {
+            super(permits);
+        }
+
+        protected int tryAcquireShared(int acquires) {
+            Thread current = Thread.currentThread();
+            for (;;) {
+                Thread first = getFirstQueuedThread();
+                if (first != null && first != current)
+                    return -1;
+                int available = getState();
+                int remaining = available - acquires;
+                if (remaining < 0 ||
+                    compareAndSetState(available, remaining))
+                    return remaining;
+            }
+        }
+    }
+
+    /**
+     * Creates a {@code Semaphore} with the given number of
+     * permits and nonfair fairness setting.
+     *
+     * @param permits the initial number of permits available.
+     *        This value may be negative, in which case releases
+     *        must occur before any acquires will be granted.
+     */
+    public Semaphore(int permits) {
+        sync = new NonfairSync(permits);
+    }
+
+    /**
+     * Creates a {@code Semaphore} with the given number of
+     * permits and the given fairness setting.
+     *
+     * @param permits the initial number of permits available.
+     *        This value may be negative, in which case releases
+     *        must occur before any acquires will be granted.
+     * @param fair {@code true} if this semaphore will guarantee
+     *        first-in first-out granting of permits under contention,
+     *        else {@code false}
+     */
+    public Semaphore(int permits, boolean fair) {
+        sync = (fair)? new FairSync(permits) : new NonfairSync(permits);
+    }
+
+    /**
+     * Acquires a permit from this semaphore, blocking until one is
+     * available, or the thread is {@linkplain Thread#interrupt interrupted}.
+     *
+     * <p>Acquires a permit, if one is available and returns immediately,
+     * reducing the number of available permits by one.
+     *
+     * <p>If no permit is available then the current thread becomes
+     * disabled for thread scheduling purposes and lies dormant until
+     * one of two things happens:
+     * <ul>
+     * <li>Some other thread invokes the {@link #release} method for this
+     * semaphore and the current thread is next to be assigned a permit; or
+     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
+     * the current thread.
+     * </ul>
+     *
+     * <p>If the current thread:
+     * <ul>
+     * <li>has its interrupted status set on entry to this method; or
+     * <li>is {@linkplain Thread#interrupt interrupted} while waiting
+     * for a permit,
+     * </ul>
+     * then {@link InterruptedException} is thrown and the current thread's
+     * interrupted status is cleared.
+     *
+     * @throws InterruptedException if the current thread is interrupted
+     */
+    public void acquire() throws InterruptedException {
+        sync.acquireSharedInterruptibly(1);
+    }
+
+    /**
+     * Acquires a permit from this semaphore, blocking until one is
+     * available.
+     *
+     * <p>Acquires a permit, if one is available and returns immediately,
+     * reducing the number of available permits by one.
+     *
+     * <p>If no permit is available then the current thread becomes
+     * disabled for thread scheduling purposes and lies dormant until
+     * some other thread invokes the {@link #release} method for this
+     * semaphore and the current thread is next to be assigned a permit.
+     *
+     * <p>If the current thread is {@linkplain Thread#interrupt interrupted}
+     * while waiting for a permit then it will continue to wait, but the
+     * time at which the thread is assigned a permit may change compared to
+     * the time it would have received the permit had no interruption
+     * occurred.  When the thread does return from this method its interrupt
+     * status will be set.
+     */
+    public void acquireUninterruptibly() {
+        sync.acquireShared(1);
+    }
+
+    /**
+     * Acquires a permit from this semaphore, only if one is available at the
+     * time of invocation.
+     *
+     * <p>Acquires a permit, if one is available and returns immediately,
+     * with the value {@code true},
+     * reducing the number of available permits by one.
+     *
+     * <p>If no permit is available then this method will return
+     * immediately with the value {@code false}.
+     *
+     * <p>Even when this semaphore has been set to use a
+     * fair ordering policy, a call to {@code tryAcquire()} <em>will</em>
+     * immediately acquire a permit if one is available, whether or not
+     * other threads are currently waiting.
+     * This &quot;barging&quot; behavior can be useful in certain
+     * circumstances, even though it breaks fairness. If you want to honor
+     * the fairness setting, then use
+     * {@link #tryAcquire(long, TimeUnit) tryAcquire(0, TimeUnit.SECONDS) }
+     * which is almost equivalent (it also detects interruption).
+     *
+     * @return {@code true} if a permit was acquired and {@code false}
+     *         otherwise
+     */
+    public boolean tryAcquire() {
+        return sync.nonfairTryAcquireShared(1) >= 0;
+    }
+
+    /**
+     * Acquires a permit from this semaphore, if one becomes available
+     * within the given waiting time and the current thread has not
+     * been {@linkplain Thread#interrupt interrupted}.
+     *
+     * <p>Acquires a permit, if one is available and returns immediately,
+     * with the value {@code true},
+     * reducing the number of available permits by one.
+     *
+     * <p>If no permit is available then the current thread becomes
+     * disabled for thread scheduling purposes and lies dormant until
+     * one of three things happens:
+     * <ul>
+     * <li>Some other thread invokes the {@link #release} method for this
+     * semaphore and the current thread is next to be assigned a permit; or
+     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
+     * the current thread; or
+     * <li>The specified waiting time elapses.
+     * </ul>
+     *
+     * <p>If a permit is acquired then the value {@code true} is returned.
+     *
+     * <p>If the current thread:
+     * <ul>
+     * <li>has its interrupted status set on entry to this method; or
+     * <li>is {@linkplain Thread#interrupt interrupted} while waiting
+     * to acquire a permit,
+     * </ul>
+     * then {@link InterruptedException} is thrown and the current thread's
+     * interrupted status is cleared.
+     *
+     * <p>If the specified waiting time elapses then the value {@code false}
+     * is returned.  If the time is less than or equal to zero, the method
+     * will not wait at all.
+     *
+     * @param timeout the maximum time to wait for a permit
+     * @param unit the time unit of the {@code timeout} argument
+     * @return {@code true} if a permit was acquired and {@code false}
+     *         if the waiting time elapsed before a permit was acquired
+     * @throws InterruptedException if the current thread is interrupted
+     */
+    public boolean tryAcquire(long timeout, TimeUnit unit)
+        throws InterruptedException {
+        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
+    }
+
+    /**
+     * Releases a permit, returning it to the semaphore.
+     *
+     * <p>Releases a permit, increasing the number of available permits by
+     * one.  If any threads are trying to acquire a permit, then one is
+     * selected and given the permit that was just released.  That thread
+     * is (re)enabled for thread scheduling purposes.
+     *
+     * <p>There is no requirement that a thread that releases a permit must
+     * have acquired that permit by calling {@link #acquire}.
+     * Correct usage of a semaphore is established by programming convention
+     * in the application.
+     */
+    public void release() {
+        sync.releaseShared(1);
+    }
+
+    /**
+     * Acquires the given number of permits from this semaphore,
+     * blocking until all are available,
+     * or the thread is {@linkplain Thread#interrupt interrupted}.
+     *
+     * <p>Acquires the given number of permits, if they are available,
+     * and returns immediately, reducing the number of available permits
+     * by the given amount.
+     *
+     * <p>If insufficient permits are available then the current thread becomes
+     * disabled for thread scheduling purposes and lies dormant until
+     * one of two things happens:
+     * <ul>
+     * <li>Some other thread invokes one of the {@link #release() release}
+     * methods for this semaphore, the current thread is next to be assigned
+     * permits and the number of available permits satisfies this request; or
+     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
+     * the current thread.
+     * </ul>
+     *
+     * <p>If the current thread:
+     * <ul>
+     * <li>has its interrupted status set on entry to this method; or
+     * <li>is {@linkplain Thread#interrupt interrupted} while waiting
+     * for a permit,
+     * </ul>
+     * then {@link InterruptedException} is thrown and the current thread's
+     * interrupted status is cleared.
+     * Any permits that were to be assigned to this thread are instead
+     * assigned to other threads trying to acquire permits, as if
+     * permits had been made available by a call to {@link #release()}.
+     *
+     * @param permits the number of permits to acquire
+     * @throws InterruptedException if the current thread is interrupted
+     * @throws IllegalArgumentException if {@code permits} is negative
+     */
+    public void acquire(int permits) throws InterruptedException {
+        if (permits < 0) throw new IllegalArgumentException();
+        sync.acquireSharedInterruptibly(permits);
+    }
+
+    /**
+     * Acquires the given number of permits from this semaphore,
+     * blocking until all are available.
+     *
+     * <p>Acquires the given number of permits, if they are available,
+     * and returns immediately, reducing the number of available permits
+     * by the given amount.
+     *
+     * <p>If insufficient permits are available then the current thread becomes
+     * disabled for thread scheduling purposes and lies dormant until
+     * some other thread invokes one of the {@link #release() release}
+     * methods for this semaphore, the current thread is next to be assigned
+     * permits and the number of available permits satisfies this request.
+     *
+     * <p>If the current thread is {@linkplain Thread#interrupt interrupted}
+     * while waiting for permits then it will continue to wait and its
+     * position in the queue is not affected.  When the thread does return
+     * from this method its interrupt status will be set.
+     *
+     * @param permits the number of permits to acquire
+     * @throws IllegalArgumentException if {@code permits} is negative
+     *
+     */
+    public void acquireUninterruptibly(int permits) {
+        if (permits < 0) throw new IllegalArgumentException();
+        sync.acquireShared(permits);
+    }
+
+    /**
+     * Acquires the given number of permits from this semaphore, only
+     * if all are available at the time of invocation.
+     *
+     * <p>Acquires the given number of permits, if they are available, and
+     * returns immediately, with the value {@code true},
+     * reducing the number of available permits by the given amount.
+     *
+     * <p>If insufficient permits are available then this method will return
+     * immediately with the value {@code false} and the number of available
+     * permits is unchanged.
+     *
+     * <p>Even when this semaphore has been set to use a fair ordering
+     * policy, a call to {@code tryAcquire} <em>will</em>
+     * immediately acquire a permit if one is available, whether or
+     * not other threads are currently waiting.  This
+     * &quot;barging&quot; behavior can be useful in certain
+     * circumstances, even though it breaks fairness. If you want to
+     * honor the fairness setting, then use {@link #tryAcquire(int,
+     * long, TimeUnit) tryAcquire(permits, 0, TimeUnit.SECONDS) }
+     * which is almost equivalent (it also detects interruption).
+     *
+     * @param permits the number of permits to acquire
+     * @return {@code true} if the permits were acquired and
+     *         {@code false} otherwise
+     * @throws IllegalArgumentException if {@code permits} is negative
+     */
+    public boolean tryAcquire(int permits) {
+        if (permits < 0) throw new IllegalArgumentException();
+        return sync.nonfairTryAcquireShared(permits) >= 0;
+    }
+
+    /**
+     * Acquires the given number of permits from this semaphore, if all
+     * become available within the given waiting time and the current
+     * thread has not been {@linkplain Thread#interrupt interrupted}.
+     *
+     * <p>Acquires the given number of permits, if they are available and
+     * returns immediately, with the value {@code true},
+     * reducing the number of available permits by the given amount.
+     *
+     * <p>If insufficient permits are available then
+     * the current thread becomes disabled for thread scheduling
+     * purposes and lies dormant until one of three things happens:
+     * <ul>
+     * <li>Some other thread invokes one of the {@link #release() release}
+     * methods for this semaphore, the current thread is next to be assigned
+     * permits and the number of available permits satisfies this request; or
+     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
+     * the current thread; or
+     * <li>The specified waiting time elapses.
+     * </ul>
+     *
+     * <p>If the permits are acquired then the value {@code true} is returned.
+     *
+     * <p>If the current thread:
+     * <ul>
+     * <li>has its interrupted status set on entry to this method; or
+     * <li>is {@linkplain Thread#interrupt interrupted} while waiting
+     * to acquire the permits,
+     * </ul>
+     * then {@link InterruptedException} is thrown and the current thread's
+     * interrupted status is cleared.
+     * Any permits that were to be assigned to this thread, are instead
+     * assigned to other threads trying to acquire permits, as if
+     * the permits had been made available by a call to {@link #release()}.
+     *
+     * <p>If the specified waiting time elapses then the value {@code false}
+     * is returned.  If the time is less than or equal to zero, the method
+     * will not wait at all.  Any permits that were to be assigned to this
+     * thread, are instead assigned to other threads trying to acquire
+     * permits, as if the permits had been made available by a call to
+     * {@link #release()}.
+     *
+     * @param permits the number of permits to acquire
+     * @param timeout the maximum time to wait for the permits
+     * @param unit the time unit of the {@code timeout} argument
+     * @return {@code true} if all permits were acquired and {@code false}
+     *         if the waiting time elapsed before all permits were acquired
+     * @throws InterruptedException if the current thread is interrupted
+     * @throws IllegalArgumentException if {@code permits} is negative
+     */
+    public boolean tryAcquire(int permits, long timeout, TimeUnit unit)
+        throws InterruptedException {
+        if (permits < 0) throw new IllegalArgumentException();
+        return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout));
+    }
+
+    /**
+     * Releases the given number of permits, returning them to the semaphore.
+     *
+     * <p>Releases the given number of permits, increasing the number of
+     * available permits by that amount.
+     * If any threads are trying to acquire permits, then one
+     * is selected and given the permits that were just released.
+     * If the number of available permits satisfies that thread's request
+     * then that thread is (re)enabled for thread scheduling purposes;
+     * otherwise the thread will wait until sufficient permits are available.
+     * If there are still permits available
+     * after this thread's request has been satisfied, then those permits
+     * are assigned in turn to other threads trying to acquire permits.
+     *
+     * <p>There is no requirement that a thread that releases a permit must
+     * have acquired that permit by calling {@link Semaphore#acquire acquire}.
+     * Correct usage of a semaphore is established by programming convention
+     * in the application.
+     *
+     * @param permits the number of permits to release
+     * @throws IllegalArgumentException if {@code permits} is negative
+     */
+    public void release(int permits) {
+        if (permits < 0) throw new IllegalArgumentException();
+        sync.releaseShared(permits);
+    }
+
+    /**
+     * Returns the current number of permits available in this semaphore.
+     *
+     * <p>This method is typically used for debugging and testing purposes.
+     *
+     * @return the number of permits available in this semaphore
+     */
+    public int availablePermits() {
+        return sync.getPermits();
+    }
+
+    /**
+     * Acquires and returns all permits that are immediately available.
+     *
+     * @return the number of permits acquired
+     */
+    public int drainPermits() {
+        return sync.drainPermits();
+    }
+
+    /**
+     * Shrinks the number of available permits by the indicated
+     * reduction. This method can be useful in subclasses that use
+     * semaphores to track resources that become unavailable. This
+     * method differs from {@code acquire} in that it does not block
+     * waiting for permits to become available.
+     *
+     * @param reduction the number of permits to remove
+     * @throws IllegalArgumentException if {@code reduction} is negative
+     */
+    protected void reducePermits(int reduction) {
+	if (reduction < 0) throw new IllegalArgumentException();
+        sync.reducePermits(reduction);
+    }
+
+    /**
+     * Returns {@code true} if this semaphore has fairness set true.
+     *
+     * @return {@code true} if this semaphore has fairness set true
+     */
+    public boolean isFair() {
+        return sync instanceof FairSync;
+    }
+
+    /**
+     * Queries whether any threads are waiting to acquire. Note that
+     * because cancellations may occur at any time, a {@code true}
+     * return does not guarantee that any other thread will ever
+     * acquire.  This method is designed primarily for use in
+     * monitoring of the system state.
+     *
+     * @return {@code true} if there may be other threads waiting to
+     *         acquire the lock
+     */
+    public final boolean hasQueuedThreads() {
+        return sync.hasQueuedThreads();
+    }
+
+    /**
+     * Returns an estimate of the number of threads waiting to acquire.
+     * The value is only an estimate because the number of threads may
+     * change dynamically while this method traverses internal data
+     * structures.  This method is designed for use in monitoring of the
+     * system state, not for synchronization control.
+     *
+     * @return the estimated number of threads waiting for this lock
+     */
+    public final int getQueueLength() {
+        return sync.getQueueLength();
+    }
+
+    /**
+     * Returns a collection containing threads that may be waiting to acquire.
+     * Because the actual set of threads may change dynamically while
+     * constructing this result, the returned collection is only a best-effort
+     * estimate.  The elements of the returned collection are in no particular
+     * order.  This method is designed to facilitate construction of
+     * subclasses that provide more extensive monitoring facilities.
+     *
+     * @return the collection of threads
+     */
+    protected Collection<Thread> getQueuedThreads() {
+        return sync.getQueuedThreads();
+    }
+
+    /**
+     * Returns a string identifying this semaphore, as well as its state.
+     * The state, in brackets, includes the String {@code "Permits ="}
+     * followed by the number of permits.
+     *
+     * @return a string identifying this semaphore, as well as its state
+     */
+    public String toString() {
+        return super.toString() + "[Permits = " + sync.getPermits() + "]";
+    }
+}
diff --git a/external/jsr166/java/util/concurrent/SynchronousQueue.java b/external/jsr166/java/util/concurrent/SynchronousQueue.java
new file mode 100644
index 000000000..e47e0401c
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/SynchronousQueue.java
@@ -0,0 +1,1127 @@
+/*
+ * Written by Doug Lea, Bill Scherer, and Michael Scott with
+ * assistance from members of JCP JSR-166 Expert Group and released to
+ * the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+import java.util.concurrent.locks.*;
+import java.util.concurrent.atomic.*;
+import java.util.*;
+
+/**
+ * A {@linkplain BlockingQueue blocking queue} in which each insert
+ * operation must wait for a corresponding remove operation by another
+ * thread, and vice versa.  A synchronous queue does not have any
+ * internal capacity, not even a capacity of one.  You cannot
+ * <tt>peek</tt> at a synchronous queue because an element is only
+ * present when you try to remove it; you cannot insert an element
+ * (using any method) unless another thread is trying to remove it;
+ * you cannot iterate as there is nothing to iterate.  The
+ * <em>head</em> of the queue is the element that the first queued
+ * inserting thread is trying to add to the queue; if there is no such
+ * queued thread then no element is available for removal and
+ * <tt>poll()</tt> will return <tt>null</tt>.  For purposes of other
+ * <tt>Collection</tt> methods (for example <tt>contains</tt>), a
+ * <tt>SynchronousQueue</tt> acts as an empty collection.  This queue
+ * does not permit <tt>null</tt> elements.
+ *
+ * <p>Synchronous queues are similar to rendezvous channels used in
+ * CSP and Ada. They are well suited for handoff designs, in which an
+ * object running in one thread must sync up with an object running
+ * in another thread in order to hand it some information, event, or
+ * task.
+ *
+ * <p> This class supports an optional fairness policy for ordering
+ * waiting producer and consumer threads.  By default, this ordering
+ * is not guaranteed. However, a queue constructed with fairness set
+ * to <tt>true</tt> grants threads access in FIFO order.
+ *
+ * <p>This class and its iterator implement all of the
+ * <em>optional</em> methods of the {@link Collection} and {@link
+ * Iterator} interfaces.
+ *
+ * <p>This class is a member of the
+ * <a href="{@docRoot}/../technotes/guides/collections/index.html">
+ * Java Collections Framework</a>.
+ *
+ * @since 1.5
+ * @author Doug Lea and Bill Scherer and Michael Scott
+ * @param <E> the type of elements held in this collection
+ */
+public class SynchronousQueue<E> extends AbstractQueue<E>
+    implements BlockingQueue<E>, java.io.Serializable {
+    private static final long serialVersionUID = -3223113410248163686L;
+
+    /*
+     * This class implements extensions of the dual stack and dual
+     * queue algorithms described in "Nonblocking Concurrent Objects
+     * with Condition Synchronization", by W. N. Scherer III and
+     * M. L. Scott.  18th Annual Conf. on Distributed Computing,
+     * Oct. 2004 (see also
+     * http://www.cs.rochester.edu/u/scott/synchronization/pseudocode/duals.html).
+     * The (Lifo) stack is used for non-fair mode, and the (Fifo)
+     * queue for fair mode. The performance of the two is generally
+     * similar. Fifo usually supports higher throughput under
+     * contention but Lifo maintains higher thread locality in common
+     * applications.
+     *
+     * A dual queue (and similarly stack) is one that at any given
+     * time either holds "data" -- items provided by put operations,
+     * or "requests" -- slots representing take operations, or is
+     * empty. A call to "fulfill" (i.e., a call requesting an item
+     * from a queue holding data or vice versa) dequeues a
+     * complementary node.  The most interesting feature of these
+     * queues is that any operation can figure out which mode the
+     * queue is in, and act accordingly without needing locks.
+     *
+     * Both the queue and stack extend abstract class Transferer
+     * defining the single method transfer that does a put or a
+     * take. These are unified into a single method because in dual
+     * data structures, the put and take operations are symmetrical,
+     * so nearly all code can be combined. The resulting transfer
+     * methods are on the long side, but are easier to follow than
+     * they would be if broken up into nearly-duplicated parts.
+     *
+     * The queue and stack data structures share many conceptual
+     * similarities but very few concrete details. For simplicity,
+     * they are kept distinct so that they can later evolve
+     * separately.
+     *
+     * The algorithms here differ from the versions in the above paper
+     * in extending them for use in synchronous queues, as well as
+     * dealing with cancellation. The main differences include:
+     *
+     *  1. The original algorithms used bit-marked pointers, but
+     *     the ones here use mode bits in nodes, leading to a number
+     *     of further adaptations.
+     *  2. SynchronousQueues must block threads waiting to become
+     *     fulfilled.
+     *  3. Support for cancellation via timeout and interrupts,
+     *     including cleaning out cancelled nodes/threads
+     *     from lists to avoid garbage retention and memory depletion.
+     *
+     * Blocking is mainly accomplished using LockSupport park/unpark,
+     * except that nodes that appear to be the next ones to become
+     * fulfilled first spin a bit (on multiprocessors only). On very
+     * busy synchronous queues, spinning can dramatically improve
+     * throughput. And on less busy ones, the amount of spinning is
+     * small enough not to be noticeable.
+     *
+     * Cleaning is done in different ways in queues vs stacks.  For
+     * queues, we can almost always remove a node immediately in O(1)
+     * time (modulo retries for consistency checks) when it is
+     * cancelled. But if it may be pinned as the current tail, it must
+     * wait until some subsequent cancellation. For stacks, we need a
+     * potentially O(n) traversal to be sure that we can remove the
+     * node, but this can run concurrently with other threads
+     * accessing the stack.
+     *
+     * While garbage collection takes care of most node reclamation
+     * issues that otherwise complicate nonblocking algorithms, care
+     * is taken to "forget" references to data, other nodes, and
+     * threads that might be held on to long-term by blocked
+     * threads. In cases where setting to null would otherwise
+     * conflict with main algorithms, this is done by changing a
+     * node's link to now point to the node itself. This doesn't arise
+     * much for Stack nodes (because blocked threads do not hang on to
+     * old head pointers), but references in Queue nodes must be
+     * aggressively forgotten to avoid reachability of everything any
+     * node has ever referred to since arrival.
+     */
+
+    /**
+     * Shared internal API for dual stacks and queues.
+     */
+    static abstract class Transferer {
+        /**
+         * Performs a put or take.
+         *
+         * @param e if non-null, the item to be handed to a consumer;
+         *          if null, requests that transfer return an item
+         *          offered by producer.
+         * @param timed if this operation should timeout
+         * @param nanos the timeout, in nanoseconds
+         * @return if non-null, the item provided or received; if null,
+         *         the operation failed due to timeout or interrupt --
+         *         the caller can distinguish which of these occurred
+         *         by checking Thread.interrupted.
+         */
+        abstract Object transfer(Object e, boolean timed, long nanos);
+    }
+
+    /** The number of CPUs, for spin control */
+    static final int NCPUS = Runtime.getRuntime().availableProcessors();
+
+    /**
+     * The number of times to spin before blocking in timed waits.
+     * The value is empirically derived -- it works well across a
+     * variety of processors and OSes. Empirically, the best value
+     * seems not to vary with number of CPUs (beyond 2) so is just
+     * a constant.
+     */
+    static final int maxTimedSpins = (NCPUS < 2)? 0 : 32;
+
+    /**
+     * The number of times to spin before blocking in untimed waits.
+     * This is greater than timed value because untimed waits spin
+     * faster since they don't need to check times on each spin.
+     */
+    static final int maxUntimedSpins = maxTimedSpins * 16;
+
+    /**
+     * The number of nanoseconds for which it is faster to spin
+     * rather than to use timed park. A rough estimate suffices.
+     */
+    static final long spinForTimeoutThreshold = 1000L;
+
+    /** Dual stack */
+    static final class TransferStack extends Transferer {
+        /*
+         * This extends Scherer-Scott dual stack algorithm, differing,
+         * among other ways, by using "covering" nodes rather than
+         * bit-marked pointers: Fulfilling operations push on marker
+         * nodes (with FULFILLING bit set in mode) to reserve a spot
+         * to match a waiting node.
+         */
+
+        /* Modes for SNodes, ORed together in node fields */
+        /** Node represents an unfulfilled consumer */
+        static final int REQUEST    = 0;
+        /** Node represents an unfulfilled producer */
+        static final int DATA       = 1;
+        /** Node is fulfilling another unfulfilled DATA or REQUEST */
+        static final int FULFILLING = 2;
+
+        /** Return true if m has fulfilling bit set */
+        static boolean isFulfilling(int m) { return (m & FULFILLING) != 0; }
+
+        /** Node class for TransferStacks. */
+        static final class SNode {
+            volatile SNode next;        // next node in stack
+            volatile SNode match;       // the node matched to this
+            volatile Thread waiter;     // to control park/unpark
+            Object item;                // data; or null for REQUESTs
+            int mode;
+            // Note: item and mode fields don't need to be volatile
+            // since they are always written before, and read after,
+            // other volatile/atomic operations.
+
+            SNode(Object item) {
+                this.item = item;
+            }
+
+            static final AtomicReferenceFieldUpdater<SNode, SNode>
+                nextUpdater = AtomicReferenceFieldUpdater.newUpdater
+                (SNode.class, SNode.class, "next");
+
+            boolean casNext(SNode cmp, SNode val) {
+                return (cmp == next &&
+                        nextUpdater.compareAndSet(this, cmp, val));
+            }
+
+            static final AtomicReferenceFieldUpdater<SNode, SNode>
+                matchUpdater = AtomicReferenceFieldUpdater.newUpdater
+                (SNode.class, SNode.class, "match");
+
+            /**
+             * Tries to match node s to this node, if so, waking up thread.
+             * Fulfillers call tryMatch to identify their waiters.
+             * Waiters block until they have been matched.
+             *
+             * @param s the node to match
+             * @return true if successfully matched to s
+             */
+            boolean tryMatch(SNode s) {
+                if (match == null &&
+                    matchUpdater.compareAndSet(this, null, s)) {
+                    Thread w = waiter;
+                    if (w != null) {    // waiters need at most one unpark
+                        waiter = null;
+                        LockSupport.unpark(w);
+                    }
+                    return true;
+                }
+                return match == s;
+            }
+
+            /**
+             * Tries to cancel a wait by matching node to itself.
+             */
+            void tryCancel() {
+                matchUpdater.compareAndSet(this, null, this);
+            }
+
+            boolean isCancelled() {
+                return match == this;
+            }
+        }
+
+        /** The head (top) of the stack */
+        volatile SNode head;
+
+        static final AtomicReferenceFieldUpdater<TransferStack, SNode>
+            headUpdater = AtomicReferenceFieldUpdater.newUpdater
+            (TransferStack.class,  SNode.class, "head");
+
+        boolean casHead(SNode h, SNode nh) {
+            return h == head && headUpdater.compareAndSet(this, h, nh);
+        }
+
+        /**
+         * Creates or resets fields of a node. Called only from transfer
+         * where the node to push on stack is lazily created and
+         * reused when possible to help reduce intervals between reads
+         * and CASes of head and to avoid surges of garbage when CASes
+         * to push nodes fail due to contention.
+         */
+        static SNode snode(SNode s, Object e, SNode next, int mode) {
+            if (s == null) s = new SNode(e);
+            s.mode = mode;
+            s.next = next;
+            return s;
+        }
+
+        /**
+         * Puts or takes an item.
+         */
+        Object transfer(Object e, boolean timed, long nanos) {
+            /*
+             * Basic algorithm is to loop trying one of three actions:
+             *
+             * 1. If apparently empty or already containing nodes of same
+             *    mode, try to push node on stack and wait for a match,
+             *    returning it, or null if cancelled.
+             *
+             * 2. If apparently containing node of complementary mode,
+             *    try to push a fulfilling node on to stack, match
+             *    with corresponding waiting node, pop both from
+             *    stack, and return matched item. The matching or
+             *    unlinking might not actually be necessary because of
+             *    other threads performing action 3:
+             *
+             * 3. If top of stack already holds another fulfilling node,
+             *    help it out by doing its match and/or pop
+             *    operations, and then continue. The code for helping
+             *    is essentially the same as for fulfilling, except
+             *    that it doesn't return the item.
+             */
+
+            SNode s = null; // constructed/reused as needed
+            int mode = (e == null)? REQUEST : DATA;
+
+            for (;;) {
+                SNode h = head;
+                if (h == null || h.mode == mode) {  // empty or same-mode
+                    if (timed && nanos <= 0) {      // can't wait
+                        if (h != null && h.isCancelled())
+                            casHead(h, h.next);     // pop cancelled node
+                        else
+                            return null;
+                    } else if (casHead(h, s = snode(s, e, h, mode))) {
+                        SNode m = awaitFulfill(s, timed, nanos);
+                        if (m == s) {               // wait was cancelled
+                            clean(s);
+                            return null;
+                        }
+                        if ((h = head) != null && h.next == s)
+                            casHead(h, s.next);     // help s's fulfiller
+                        return mode == REQUEST? m.item : s.item;
+                    }
+                } else if (!isFulfilling(h.mode)) { // try to fulfill
+                    if (h.isCancelled())            // already cancelled
+                        casHead(h, h.next);         // pop and retry
+                    else if (casHead(h, s=snode(s, e, h, FULFILLING|mode))) {
+                        for (;;) { // loop until matched or waiters disappear
+                            SNode m = s.next;       // m is s's match
+                            if (m == null) {        // all waiters are gone
+                                casHead(s, null);   // pop fulfill node
+                                s = null;           // use new node next time
+                                break;              // restart main loop
+                            }
+                            SNode mn = m.next;
+                            if (m.tryMatch(s)) {
+                                casHead(s, mn);     // pop both s and m
+                                return (mode == REQUEST)? m.item : s.item;
+                            } else                  // lost match
+                                s.casNext(m, mn);   // help unlink
+                        }
+                    }
+                } else {                            // help a fulfiller
+                    SNode m = h.next;               // m is h's match
+                    if (m == null)                  // waiter is gone
+                        casHead(h, null);           // pop fulfilling node
+                    else {
+                        SNode mn = m.next;
+                        if (m.tryMatch(h))          // help match
+                            casHead(h, mn);         // pop both h and m
+                        else                        // lost match
+                            h.casNext(m, mn);       // help unlink
+                    }
+                }
+            }
+        }
+
+        /**
+         * Spins/blocks until node s is matched by a fulfill operation.
+         *
+         * @param s the waiting node
+         * @param timed true if timed wait
+         * @param nanos timeout value
+         * @return matched node, or s if cancelled
+         */
+        SNode awaitFulfill(SNode s, boolean timed, long nanos) {
+            /*
+             * When a node/thread is about to block, it sets its waiter
+             * field and then rechecks state at least one more time
+             * before actually parking, thus covering race vs
+             * fulfiller noticing that waiter is non-null so should be
+             * woken.
+             *
+             * When invoked by nodes that appear at the point of call
+             * to be at the head of the stack, calls to park are
+             * preceded by spins to avoid blocking when producers and
+             * consumers are arriving very close in time.  This can
+             * happen enough to bother only on multiprocessors.
+             *
+             * The order of checks for returning out of main loop
+             * reflects fact that interrupts have precedence over
+             * normal returns, which have precedence over
+             * timeouts. (So, on timeout, one last check for match is
+             * done before giving up.) Except that calls from untimed
+             * SynchronousQueue.{poll/offer} don't check interrupts
+             * and don't wait at all, so are trapped in transfer
+             * method rather than calling awaitFulfill.
+             */
+            long lastTime = (timed)? System.nanoTime() : 0;
+            Thread w = Thread.currentThread();
+            SNode h = head;
+            int spins = (shouldSpin(s)?
+                         (timed? maxTimedSpins : maxUntimedSpins) : 0);
+            for (;;) {
+                if (w.isInterrupted())
+                    s.tryCancel();
+                SNode m = s.match;
+                if (m != null)
+                    return m;
+                if (timed) {
+                    long now = System.nanoTime();
+                    nanos -= now - lastTime;
+                    lastTime = now;
+                    if (nanos <= 0) {
+                        s.tryCancel();
+                        continue;
+                    }
+                }
+                if (spins > 0)
+                    spins = shouldSpin(s)? (spins-1) : 0;
+                else if (s.waiter == null)
+                    s.waiter = w; // establish waiter so can park next iter
+                else if (!timed)
+                    LockSupport.park(this);
+                else if (nanos > spinForTimeoutThreshold)
+                    LockSupport.parkNanos(this, nanos);
+            }
+        }
+
+        /**
+         * Returns true if node s is at head or there is an active
+         * fulfiller.
+         */
+        boolean shouldSpin(SNode s) {
+            SNode h = head;
+            return (h == s || h == null || isFulfilling(h.mode));
+        }
+
+        /**
+         * Unlinks s from the stack.
+         */
+        void clean(SNode s) {
+            s.item = null;   // forget item
+            s.waiter = null; // forget thread
+
+            /*
+             * At worst we may need to traverse entire stack to unlink
+             * s. If there are multiple concurrent calls to clean, we
+             * might not see s if another thread has already removed
+             * it. But we can stop when we see any node known to
+             * follow s. We use s.next unless it too is cancelled, in
+             * which case we try the node one past. We don't check any
+             * further because we don't want to doubly traverse just to
+             * find sentinel.
+             */
+
+            SNode past = s.next;
+            if (past != null && past.isCancelled())
+                past = past.next;
+
+            // Absorb cancelled nodes at head
+            SNode p;
+            while ((p = head) != null && p != past && p.isCancelled())
+                casHead(p, p.next);
+
+            // Unsplice embedded nodes
+            while (p != null && p != past) {
+                SNode n = p.next;
+                if (n != null && n.isCancelled())
+                    p.casNext(n, n.next);
+                else
+                    p = n;
+            }
+        }
+    }
+
+    /** Dual Queue */
+    static final class TransferQueue extends Transferer {
+        /*
+         * This extends Scherer-Scott dual queue algorithm, differing,
+         * among other ways, by using modes within nodes rather than
+         * marked pointers. The algorithm is a little simpler than
+         * that for stacks because fulfillers do not need explicit
+         * nodes, and matching is done by CAS'ing QNode.item field
+         * from non-null to null (for put) or vice versa (for take).
+         */
+
+        /** Node class for TransferQueue. */
+        static final class QNode {
+            volatile QNode next;          // next node in queue
+            volatile Object item;         // CAS'ed to or from null
+            volatile Thread waiter;       // to control park/unpark
+            final boolean isData;
+
+            QNode(Object item, boolean isData) {
+                this.item = item;
+                this.isData = isData;
+            }
+
+            static final AtomicReferenceFieldUpdater<QNode, QNode>
+                nextUpdater = AtomicReferenceFieldUpdater.newUpdater
+                (QNode.class, QNode.class, "next");
+
+            boolean casNext(QNode cmp, QNode val) {
+                return (next == cmp &&
+                        nextUpdater.compareAndSet(this, cmp, val));
+            }
+
+            static final AtomicReferenceFieldUpdater<QNode, Object>
+                itemUpdater = AtomicReferenceFieldUpdater.newUpdater
+                (QNode.class, Object.class, "item");
+
+            boolean casItem(Object cmp, Object val) {
+                return (item == cmp &&
+                        itemUpdater.compareAndSet(this, cmp, val));
+            }
+
+            /**
+             * Tries to cancel by CAS'ing ref to this as item.
+             */
+            void tryCancel(Object cmp) {
+                itemUpdater.compareAndSet(this, cmp, this);
+            }
+
+            boolean isCancelled() {
+                return item == this;
+            }
+
+            /**
+             * Returns true if this node is known to be off the queue
+             * because its next pointer has been forgotten due to
+             * an advanceHead operation.
+             */
+            boolean isOffList() {
+                return next == this;
+            }
+        }
+
+        /** Head of queue */
+        transient volatile QNode head;
+        /** Tail of queue */
+        transient volatile QNode tail;
+        /**
+         * Reference to a cancelled node that might not yet have been
+         * unlinked from queue because it was the last inserted node
+         * when it cancelled.
+         */
+        transient volatile QNode cleanMe;
+
+        TransferQueue() {
+            QNode h = new QNode(null, false); // initialize to dummy node.
+            head = h;
+            tail = h;
+        }
+
+        static final AtomicReferenceFieldUpdater<TransferQueue, QNode>
+            headUpdater = AtomicReferenceFieldUpdater.newUpdater
+            (TransferQueue.class,  QNode.class, "head");
+
+        /**
+         * Tries to cas nh as new head; if successful, unlink
+         * old head's next node to avoid garbage retention.
+         */
+        void advanceHead(QNode h, QNode nh) {
+            if (h == head && headUpdater.compareAndSet(this, h, nh))
+                h.next = h; // forget old next
+        }
+
+        static final AtomicReferenceFieldUpdater<TransferQueue, QNode>
+            tailUpdater = AtomicReferenceFieldUpdater.newUpdater
+            (TransferQueue.class, QNode.class, "tail");
+
+        /**
+         * Tries to cas nt as new tail.
+         */
+        void advanceTail(QNode t, QNode nt) {
+            if (tail == t)
+                tailUpdater.compareAndSet(this, t, nt);
+        }
+
+        static final AtomicReferenceFieldUpdater<TransferQueue, QNode>
+            cleanMeUpdater = AtomicReferenceFieldUpdater.newUpdater
+            (TransferQueue.class, QNode.class, "cleanMe");
+
+        /**
+         * Tries to CAS cleanMe slot.
+         */
+        boolean casCleanMe(QNode cmp, QNode val) {
+            return (cleanMe == cmp &&
+                    cleanMeUpdater.compareAndSet(this, cmp, val));
+        }
+
+        /**
+         * Puts or takes an item.
+         */
+        Object transfer(Object e, boolean timed, long nanos) {
+            /* Basic algorithm is to loop trying to take either of
+             * two actions:
+             *
+             * 1. If queue apparently empty or holding same-mode nodes,
+             *    try to add node to queue of waiters, wait to be
+             *    fulfilled (or cancelled) and return matching item.
+             *
+             * 2. If queue apparently contains waiting items, and this
+             *    call is of complementary mode, try to fulfill by CAS'ing
+             *    item field of waiting node and dequeuing it, and then
+             *    returning matching item.
+             *
+             * In each case, along the way, check for and try to help
+             * advance head and tail on behalf of other stalled/slow
+             * threads.
+             *
+             * The loop starts off with a null check guarding against
+             * seeing uninitialized head or tail values. This never
+             * happens in current SynchronousQueue, but could if
+             * callers held non-volatile/final ref to the
+             * transferer. The check is here anyway because it places
+             * null checks at top of loop, which is usually faster
+             * than having them implicitly interspersed.
+             */
+
+            QNode s = null; // constructed/reused as needed
+            boolean isData = (e != null);
+
+            for (;;) {
+                QNode t = tail;
+                QNode h = head;
+                if (t == null || h == null)         // saw uninitialized value
+                    continue;                       // spin
+
+                if (h == t || t.isData == isData) { // empty or same-mode
+                    QNode tn = t.next;
+                    if (t != tail)                  // inconsistent read
+                        continue;
+                    if (tn != null) {               // lagging tail
+                        advanceTail(t, tn);
+                        continue;
+                    }
+                    if (timed && nanos <= 0)        // can't wait
+                        return null;
+                    if (s == null)
+                        s = new QNode(e, isData);
+                    if (!t.casNext(null, s))        // failed to link in
+                        continue;
+
+                    advanceTail(t, s);              // swing tail and wait
+                    Object x = awaitFulfill(s, e, timed, nanos);
+                    if (x == s) {                   // wait was cancelled
+                        clean(t, s);
+                        return null;
+                    }
+
+                    if (!s.isOffList()) {           // not already unlinked
+                        advanceHead(t, s);          // unlink if head
+                        if (x != null)              // and forget fields
+                            s.item = s;
+                        s.waiter = null;
+                    }
+                    return (x != null)? x : e;
+
+                } else {                            // complementary-mode
+                    QNode m = h.next;               // node to fulfill
+                    if (t != tail || m == null || h != head)
+                        continue;                   // inconsistent read
+
+                    Object x = m.item;
+                    if (isData == (x != null) ||    // m already fulfilled
+                        x == m ||                   // m cancelled
+                        !m.casItem(x, e)) {         // lost CAS
+                        advanceHead(h, m);          // dequeue and retry
+                        continue;
+                    }
+
+                    advanceHead(h, m);              // successfully fulfilled
+                    LockSupport.unpark(m.waiter);
+                    return (x != null)? x : e;
+                }
+            }
+        }
+
+        /**
+         * Spins/blocks until node s is fulfilled.
+         *
+         * @param s the waiting node
+         * @param e the comparison value for checking match
+         * @param timed true if timed wait
+         * @param nanos timeout value
+         * @return matched item, or s if cancelled
+         */
+        Object awaitFulfill(QNode s, Object e, boolean timed, long nanos) {
+            /* Same idea as TransferStack.awaitFulfill */
+            long lastTime = (timed)? System.nanoTime() : 0;
+            Thread w = Thread.currentThread();
+            int spins = ((head.next == s) ?
+                         (timed? maxTimedSpins : maxUntimedSpins) : 0);
+            for (;;) {
+                if (w.isInterrupted())
+                    s.tryCancel(e);
+                Object x = s.item;
+                if (x != e)
+                    return x;
+                if (timed) {
+                    long now = System.nanoTime();
+                    nanos -= now - lastTime;
+                    lastTime = now;
+                    if (nanos <= 0) {
+                        s.tryCancel(e);
+                        continue;
+                    }
+                }
+                if (spins > 0)
+                    --spins;
+                else if (s.waiter == null)
+                    s.waiter = w;
+                else if (!timed)
+                    LockSupport.park(this);
+                else if (nanos > spinForTimeoutThreshold)
+                    LockSupport.parkNanos(this, nanos);
+            }
+        }
+
+        /**
+         * Gets rid of cancelled node s with original predecessor pred.
+         */
+        void clean(QNode pred, QNode s) {
+            s.waiter = null; // forget thread
+            /*
+             * At any given time, exactly one node on list cannot be
+             * deleted -- the last inserted node. To accommodate this,
+             * if we cannot delete s, we save its predecessor as
+             * "cleanMe", deleting the previously saved version
+             * first. At least one of node s or the node previously
+             * saved can always be deleted, so this always terminates.
+             */
+            while (pred.next == s) { // Return early if already unlinked
+                QNode h = head;
+                QNode hn = h.next;   // Absorb cancelled first node as head
+                if (hn != null && hn.isCancelled()) {
+                    advanceHead(h, hn);
+                    continue;
+                }
+		QNode t = tail;      // Ensure consistent read for tail
+                if (t == h)
+                    return;
+		QNode tn = t.next;
+		if (t != tail)
+                    continue;
+                if (tn != null) {
+                    advanceTail(t, tn);
+                    continue;
+                }
+                if (s != t) {        // If not tail, try to unsplice
+                    QNode sn = s.next;
+                    if (sn == s || pred.casNext(s, sn))
+                        return;
+                }
+                QNode dp = cleanMe;
+                if (dp != null) {    // Try unlinking previous cancelled node
+                    QNode d = dp.next;
+                    QNode dn;
+                    if (d == null ||               // d is gone or
+                        d == dp ||                 // d is off list or
+                        !d.isCancelled() ||        // d not cancelled or
+                        (d != t &&                 // d not tail and
+                         (dn = d.next) != null &&  //   has successor
+                         dn != d &&                //   that is on list
+                         dp.casNext(d, dn)))       // d unspliced
+                        casCleanMe(dp, null);
+                    if (dp == pred)
+                        return;      // s is already saved node
+                } else if (casCleanMe(null, pred))
+                    return;          // Postpone cleaning s
+            }
+        }
+    }
+
+    /**
+     * The transferer. Set only in constructor, but cannot be declared
+     * as final without further complicating serialization.  Since
+     * this is accessed only at most once per public method, there
+     * isn't a noticeable performance penalty for using volatile
+     * instead of final here.
+     */
+    private transient volatile Transferer transferer;
+
+    /**
+     * Creates a <tt>SynchronousQueue</tt> with nonfair access policy.
+     */
+    public SynchronousQueue() {
+        this(false);
+    }
+
+    /**
+     * Creates a <tt>SynchronousQueue</tt> with the specified fairness policy.
+     *
+     * @param fair if true, waiting threads contend in FIFO order for
+     *        access; otherwise the order is unspecified.
+     */
+    public SynchronousQueue(boolean fair) {
+        transferer = (fair)? new TransferQueue() : new TransferStack();
+    }
+
+    /**
+     * Adds the specified element to this queue, waiting if necessary for
+     * another thread to receive it.
+     *
+     * @throws InterruptedException {@inheritDoc}
+     * @throws NullPointerException {@inheritDoc}
+     */
+    public void put(E o) throws InterruptedException {
+        if (o == null) throw new NullPointerException();
+        if (transferer.transfer(o, false, 0) == null) {
+	    Thread.interrupted();
+            throw new InterruptedException();
+	}
+    }
+
+    /**
+     * Inserts the specified element into this queue, waiting if necessary
+     * up to the specified wait time for another thread to receive it.
+     *
+     * @return <tt>true</tt> if successful, or <tt>false</tt> if the
+     *         specified waiting time elapses before a consumer appears.
+     * @throws InterruptedException {@inheritDoc}
+     * @throws NullPointerException {@inheritDoc}
+     */
+    public boolean offer(E o, long timeout, TimeUnit unit)
+        throws InterruptedException {
+        if (o == null) throw new NullPointerException();
+        if (transferer.transfer(o, true, unit.toNanos(timeout)) != null)
+            return true;
+        if (!Thread.interrupted())
+            return false;
+        throw new InterruptedException();
+    }
+
+    /**
+     * Inserts the specified element into this queue, if another thread is
+     * waiting to receive it.
+     *
+     * @param e the element to add
+     * @return <tt>true</tt> if the element was added to this queue, else
+     *         <tt>false</tt>
+     * @throws NullPointerException if the specified element is null
+     */
+    public boolean offer(E e) {
+        if (e == null) throw new NullPointerException();
+        return transferer.transfer(e, true, 0) != null;
+    }
+
+    /**
+     * Retrieves and removes the head of this queue, waiting if necessary
+     * for another thread to insert it.
+     *
+     * @return the head of this queue
+     * @throws InterruptedException {@inheritDoc}
+     */
+    public E take() throws InterruptedException {
+        Object e = transferer.transfer(null, false, 0);
+        if (e != null)
+            return (E)e;
+	Thread.interrupted();
+        throw new InterruptedException();
+    }
+
+    /**
+     * Retrieves and removes the head of this queue, waiting
+     * if necessary up to the specified wait time, for another thread
+     * to insert it.
+     *
+     * @return the head of this queue, or <tt>null</tt> if the
+     *         specified waiting time elapses before an element is present.
+     * @throws InterruptedException {@inheritDoc}
+     */
+    public E poll(long timeout, TimeUnit unit) throws InterruptedException {
+        Object e = transferer.transfer(null, true, unit.toNanos(timeout));
+        if (e != null || !Thread.interrupted())
+            return (E)e;
+        throw new InterruptedException();
+    }
+
+    /**
+     * Retrieves and removes the head of this queue, if another thread
+     * is currently making an element available.
+     *
+     * @return the head of this queue, or <tt>null</tt> if no
+     *         element is available.
+     */
+    public E poll() {
+        return (E)transferer.transfer(null, true, 0);
+    }
+
+    /**
+     * Always returns <tt>true</tt>.
+     * A <tt>SynchronousQueue</tt> has no internal capacity.
+     *
+     * @return <tt>true</tt>
+     */
+    public boolean isEmpty() {
+        return true;
+    }
+
+    /**
+     * Always returns zero.
+     * A <tt>SynchronousQueue</tt> has no internal capacity.
+     *
+     * @return zero.
+     */
+    public int size() {
+        return 0;
+    }
+
+    /**
+     * Always returns zero.
+     * A <tt>SynchronousQueue</tt> has no internal capacity.
+     *
+     * @return zero.
+     */
+    public int remainingCapacity() {
+        return 0;
+    }
+
+    /**
+     * Does nothing.
+     * A <tt>SynchronousQueue</tt> has no internal capacity.
+     */
+    public void clear() {
+    }
+
+    /**
+     * Always returns <tt>false</tt>.
+     * A <tt>SynchronousQueue</tt> has no internal capacity.
+     *
+     * @param o the element
+     * @return <tt>false</tt>
+     */
+    public boolean contains(Object o) {
+        return false;
+    }
+
+    /**
+     * Always returns <tt>false</tt>.
+     * A <tt>SynchronousQueue</tt> has no internal capacity.
+     *
+     * @param o the element to remove
+     * @return <tt>false</tt>
+     */
+    public boolean remove(Object o) {
+        return false;
+    }
+
+    /**
+     * Returns <tt>false</tt> unless the given collection is empty.
+     * A <tt>SynchronousQueue</tt> has no internal capacity.
+     *
+     * @param c the collection
+     * @return <tt>false</tt> unless given collection is empty
+     */
+    public boolean containsAll(Collection<?> c) {
+        return c.isEmpty();
+    }
+
+    /**
+     * Always returns <tt>false</tt>.
+     * A <tt>SynchronousQueue</tt> has no internal capacity.
+     *
+     * @param c the collection
+     * @return <tt>false</tt>
+     */
+    public boolean removeAll(Collection<?> c) {
+        return false;
+    }
+
+    /**
+     * Always returns <tt>false</tt>.
+     * A <tt>SynchronousQueue</tt> has no internal capacity.
+     *
+     * @param c the collection
+     * @return <tt>false</tt>
+     */
+    public boolean retainAll(Collection<?> c) {
+        return false;
+    }
+
+    /**
+     * Always returns <tt>null</tt>.
+     * A <tt>SynchronousQueue</tt> does not return elements
+     * unless actively waited on.
+     *
+     * @return <tt>null</tt>
+     */
+    public E peek() {
+        return null;
+    }
+
+    static class EmptyIterator<E> implements Iterator<E> {
+        public boolean hasNext() {
+            return false;
+        }
+        public E next() {
+            throw new NoSuchElementException();
+        }
+        public void remove() {
+            throw new IllegalStateException();
+        }
+    }
+
+    /**
+     * Returns an empty iterator in which <tt>hasNext</tt> always returns
+     * <tt>false</tt>.
+     *
+     * @return an empty iterator
+     */
+    public Iterator<E> iterator() {
+        return new EmptyIterator<E>();
+    }
+
+    /**
+     * Returns a zero-length array.
+     * @return a zero-length array
+     */
+    public Object[] toArray() {
+        return new Object[0];
+    }
+
+    /**
+     * Sets the zeroeth element of the specified array to <tt>null</tt>
+     * (if the array has non-zero length) and returns it.
+     *
+     * @param a the array
+     * @return the specified array
+     * @throws NullPointerException if the specified array is null
+     */
+    public <T> T[] toArray(T[] a) {
+        if (a.length > 0)
+            a[0] = null;
+        return a;
+    }
+
+    /**
+     * @throws UnsupportedOperationException {@inheritDoc}
+     * @throws ClassCastException            {@inheritDoc}
+     * @throws NullPointerException          {@inheritDoc}
+     * @throws IllegalArgumentException      {@inheritDoc}
+     */
+    public int drainTo(Collection<? super E> c) {
+        if (c == null)
+            throw new NullPointerException();
+        if (c == this)
+            throw new IllegalArgumentException();
+        int n = 0;
+        E e;
+        while ( (e = poll()) != null) {
+            c.add(e);
+            ++n;
+        }
+        return n;
+    }
+
+    /**
+     * @throws UnsupportedOperationException {@inheritDoc}
+     * @throws ClassCastException            {@inheritDoc}
+     * @throws NullPointerException          {@inheritDoc}
+     * @throws IllegalArgumentException      {@inheritDoc}
+     */
+    public int drainTo(Collection<? super E> c, int maxElements) {
+        if (c == null)
+            throw new NullPointerException();
+        if (c == this)
+            throw new IllegalArgumentException();
+        int n = 0;
+        E e;
+        while (n < maxElements && (e = poll()) != null) {
+            c.add(e);
+            ++n;
+        }
+        return n;
+    }
+
+    /*
+     * To cope with serialization strategy in the 1.5 version of
+     * SynchronousQueue, we declare some unused classes and fields
+     * that exist solely to enable serializability across versions.
+     * These fields are never used, so are initialized only if this
+     * object is ever serialized or deserialized.
+     */
+
+    static class WaitQueue implements java.io.Serializable { }
+    static class LifoWaitQueue extends WaitQueue {
+        private static final long serialVersionUID = -3633113410248163686L;
+    }
+    static class FifoWaitQueue extends WaitQueue {
+        private static final long serialVersionUID = -3623113410248163686L;
+    }
+    private ReentrantLock qlock;
+    private WaitQueue waitingProducers;
+    private WaitQueue waitingConsumers;
+
+    /**
+     * Save the state to a stream (that is, serialize it).
+     *
+     * @param s the stream
+     */
+    private void writeObject(java.io.ObjectOutputStream s)
+        throws java.io.IOException {
+        boolean fair = transferer instanceof TransferQueue;
+        if (fair) {
+            qlock = new ReentrantLock(true);
+            waitingProducers = new FifoWaitQueue();
+            waitingConsumers = new FifoWaitQueue();
+        }
+        else {
+            qlock = new ReentrantLock();
+            waitingProducers = new LifoWaitQueue();
+            waitingConsumers = new LifoWaitQueue();
+        }
+        s.defaultWriteObject();
+    }
+
+    private void readObject(final java.io.ObjectInputStream s)
+        throws java.io.IOException, ClassNotFoundException {
+        s.defaultReadObject();
+        if (waitingProducers instanceof FifoWaitQueue)
+            transferer = new TransferQueue();
+        else
+            transferer = new TransferStack();
+    }
+
+}
diff --git a/external/jsr166/java/util/concurrent/ThreadFactory.java b/external/jsr166/java/util/concurrent/ThreadFactory.java
new file mode 100644
index 000000000..eca8dceb0
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/ThreadFactory.java
@@ -0,0 +1,40 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+
+/**
+ * An object that creates new threads on demand.  Using thread factories
+ * removes hardwiring of calls to {@link Thread#Thread(Runnable) new Thread},
+ * enabling applications to use special thread subclasses, priorities, etc.
+ *
+ * <p>
+ * The simplest implementation of this interface is just:
+ * <pre>
+ * class SimpleThreadFactory implements ThreadFactory {
+ *   public Thread newThread(Runnable r) {
+ *     return new Thread(r);
+ *   }
+ * }
+ * </pre>
+ *
+ * The {@link Executors#defaultThreadFactory} method provides a more
+ * useful simple implementation, that sets the created thread context
+ * to known values before returning it.
+ * @since 1.5
+ * @author Doug Lea
+ */
+public interface ThreadFactory {
+
+    /**
+     * Constructs a new <tt>Thread</tt>.  Implementations may also initialize
+     * priority, name, daemon status, <tt>ThreadGroup</tt>, etc.
+     *
+     * @param r a runnable to be executed by new thread instance
+     * @return constructed thread
+     */
+    Thread newThread(Runnable r);
+}
diff --git a/external/jsr166/java/util/concurrent/ThreadPoolExecutor.java b/external/jsr166/java/util/concurrent/ThreadPoolExecutor.java
new file mode 100644
index 000000000..ea89a2c08
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/ThreadPoolExecutor.java
@@ -0,0 +1,1605 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+import java.util.concurrent.locks.*;
+import java.util.*;
+
+/**
+ * An {@link ExecutorService} that executes each submitted task using
+ * one of possibly several pooled threads, normally configured
+ * using {@link Executors} factory methods.
+ *
+ * <p>Thread pools address two different problems: they usually
+ * provide improved performance when executing large numbers of
+ * asynchronous tasks, due to reduced per-task invocation overhead,
+ * and they provide a means of bounding and managing the resources,
+ * including threads, consumed when executing a collection of tasks.
+ * Each <tt>ThreadPoolExecutor</tt> also maintains some basic
+ * statistics, such as the number of completed tasks.
+ *
+ * <p>To be useful across a wide range of contexts, this class
+ * provides many adjustable parameters and extensibility
+ * hooks. However, programmers are urged to use the more convenient
+ * {@link Executors} factory methods {@link
+ * Executors#newCachedThreadPool} (unbounded thread pool, with
+ * automatic thread reclamation), {@link Executors#newFixedThreadPool}
+ * (fixed size thread pool) and {@link
+ * Executors#newSingleThreadExecutor} (single background thread), that
+ * preconfigure settings for the most common usage
+ * scenarios. Otherwise, use the following guide when manually
+ * configuring and tuning this class:
+ *
+ * <dl>
+ *
+ * <dt>Core and maximum pool sizes</dt>
+ *
+ * <dd>A <tt>ThreadPoolExecutor</tt> will automatically adjust the
+ * pool size
+ * (see {@link ThreadPoolExecutor#getPoolSize})
+ * according to the bounds set by corePoolSize
+ * (see {@link ThreadPoolExecutor#getCorePoolSize})
+ * and
+ * maximumPoolSize
+ * (see {@link ThreadPoolExecutor#getMaximumPoolSize}).
+ * When a new task is submitted in method {@link
+ * ThreadPoolExecutor#execute}, and fewer than corePoolSize threads
+ * are running, a new thread is created to handle the request, even if
+ * other worker threads are idle.  If there are more than
+ * corePoolSize but less than maximumPoolSize threads running, a new
+ * thread will be created only if the queue is full.  By setting
+ * corePoolSize and maximumPoolSize the same, you create a fixed-size
+ * thread pool. By setting maximumPoolSize to an essentially unbounded
+ * value such as <tt>Integer.MAX_VALUE</tt>, you allow the pool to
+ * accommodate an arbitrary number of concurrent tasks. Most typically,
+ * core and maximum pool sizes are set only upon construction, but they
+ * may also be changed dynamically using {@link
+ * ThreadPoolExecutor#setCorePoolSize} and {@link
+ * ThreadPoolExecutor#setMaximumPoolSize}. <dd>
+ *
+ * <dt> On-demand construction
+ *
+ * <dd> By default, even core threads are initially created and
+ * started only when new tasks arrive, but this can be overridden
+ * dynamically using method {@link
+ * ThreadPoolExecutor#prestartCoreThread} or
+ * {@link ThreadPoolExecutor#prestartAllCoreThreads}.
+ * You probably want to prestart threads if you construct the
+ * pool with a non-empty queue. </dd>
+ *
+ * <dt>Creating new threads</dt>
+ *
+ * <dd>New threads are created using a {@link
+ * java.util.concurrent.ThreadFactory}.  If not otherwise specified, a
+ * {@link Executors#defaultThreadFactory} is used, that creates threads to all
+ * be in the same {@link ThreadGroup} and with the same
+ * <tt>NORM_PRIORITY</tt> priority and non-daemon status. By supplying
+ * a different ThreadFactory, you can alter the thread's name, thread
+ * group, priority, daemon status, etc. If a <tt>ThreadFactory</tt> fails to create
+ * a thread when asked by returning null from <tt>newThread</tt>,
+ * the executor will continue, but might
+ * not be able to execute any tasks. </dd>
+ *
+ * <dt>Keep-alive times</dt>
+ *
+ * <dd>If the pool currently has more than corePoolSize threads,
+ * excess threads will be terminated if they have been idle for more
+ * than the keepAliveTime (see {@link
+ * ThreadPoolExecutor#getKeepAliveTime}). This provides a means of
+ * reducing resource consumption when the pool is not being actively
+ * used. If the pool becomes more active later, new threads will be
+ * constructed. This parameter can also be changed dynamically using
+ * method {@link ThreadPoolExecutor#setKeepAliveTime}. Using a value
+ * of <tt>Long.MAX_VALUE</tt> {@link TimeUnit#NANOSECONDS} effectively
+ * disables idle threads from ever terminating prior to shut down. By
+ * default, the keep-alive policy applies only when there are more
+ * than corePoolSizeThreads. But method {@link
+ * ThreadPoolExecutor#allowCoreThreadTimeOut} can be used to apply
+ * this time-out policy to core threads as well, so long as
+ * the keepAliveTime value is non-zero. </dd>
+ *
+ * <dt>Queuing</dt>
+ *
+ * <dd>Any {@link BlockingQueue} may be used to transfer and hold
+ * submitted tasks.  The use of this queue interacts with pool sizing:
+ *
+ * <ul>
+ *
+ * <li> If fewer than corePoolSize threads are running, the Executor
+ * always prefers adding a new thread
+ * rather than queuing.</li>
+ *
+ * <li> If corePoolSize or more threads are running, the Executor
+ * always prefers queuing a request rather than adding a new
+ * thread.</li>
+ *
+ * <li> If a request cannot be queued, a new thread is created unless
+ * this would exceed maximumPoolSize, in which case, the task will be
+ * rejected.</li>
+ *
+ * </ul>
+ *
+ * There are three general strategies for queuing:
+ * <ol>
+ *
+ * <li> <em> Direct handoffs.</em> A good default choice for a work
+ * queue is a {@link SynchronousQueue} that hands off tasks to threads
+ * without otherwise holding them. Here, an attempt to queue a task
+ * will fail if no threads are immediately available to run it, so a
+ * new thread will be constructed. This policy avoids lockups when
+ * handling sets of requests that might have internal dependencies.
+ * Direct handoffs generally require unbounded maximumPoolSizes to
+ * avoid rejection of new submitted tasks. This in turn admits the
+ * possibility of unbounded thread growth when commands continue to
+ * arrive on average faster than they can be processed.  </li>
+ *
+ * <li><em> Unbounded queues.</em> Using an unbounded queue (for
+ * example a {@link LinkedBlockingQueue} without a predefined
+ * capacity) will cause new tasks to wait in the queue when all
+ * corePoolSize threads are busy. Thus, no more than corePoolSize
+ * threads will ever be created. (And the value of the maximumPoolSize
+ * therefore doesn't have any effect.)  This may be appropriate when
+ * each task is completely independent of others, so tasks cannot
+ * affect each others execution; for example, in a web page server.
+ * While this style of queuing can be useful in smoothing out
+ * transient bursts of requests, it admits the possibility of
+ * unbounded work queue growth when commands continue to arrive on
+ * average faster than they can be processed.  </li>
+ *
+ * <li><em>Bounded queues.</em> A bounded queue (for example, an
+ * {@link ArrayBlockingQueue}) helps prevent resource exhaustion when
+ * used with finite maximumPoolSizes, but can be more difficult to
+ * tune and control.  Queue sizes and maximum pool sizes may be traded
+ * off for each other: Using large queues and small pools minimizes
+ * CPU usage, OS resources, and context-switching overhead, but can
+ * lead to artificially low throughput.  If tasks frequently block (for
+ * example if they are I/O bound), a system may be able to schedule
+ * time for more threads than you otherwise allow. Use of small queues
+ * generally requires larger pool sizes, which keeps CPUs busier but
+ * may encounter unacceptable scheduling overhead, which also
+ * decreases throughput.  </li>
+ *
+ * </ol>
+ *
+ * </dd>
+ *
+ * <dt>Rejected tasks</dt>
+ *
+ * <dd> New tasks submitted in method {@link
+ * ThreadPoolExecutor#execute} will be <em>rejected</em> when the
+ * Executor has been shut down, and also when the Executor uses finite
+ * bounds for both maximum threads and work queue capacity, and is
+ * saturated.  In either case, the <tt>execute</tt> method invokes the
+ * {@link RejectedExecutionHandler#rejectedExecution} method of its
+ * {@link RejectedExecutionHandler}.  Four predefined handler policies
+ * are provided:
+ *
+ * <ol>
+ *
+ * <li> In the
+ * default {@link ThreadPoolExecutor.AbortPolicy}, the handler throws a
+ * runtime {@link RejectedExecutionException} upon rejection. </li>
+ *
+ * <li> In {@link
+ * ThreadPoolExecutor.CallerRunsPolicy}, the thread that invokes
+ * <tt>execute</tt> itself runs the task. This provides a simple
+ * feedback control mechanism that will slow down the rate that new
+ * tasks are submitted. </li>
+ *
+ * <li> In {@link ThreadPoolExecutor.DiscardPolicy},
+ * a task that cannot be executed is simply dropped.  </li>
+ *
+ * <li>In {@link
+ * ThreadPoolExecutor.DiscardOldestPolicy}, if the executor is not
+ * shut down, the task at the head of the work queue is dropped, and
+ * then execution is retried (which can fail again, causing this to be
+ * repeated.) </li>
+ *
+ * </ol>
+ *
+ * It is possible to define and use other kinds of {@link
+ * RejectedExecutionHandler} classes. Doing so requires some care
+ * especially when policies are designed to work only under particular
+ * capacity or queuing policies. </dd>
+ *
+ * <dt>Hook methods</dt>
+ *
+ * <dd>This class provides <tt>protected</tt> overridable {@link
+ * ThreadPoolExecutor#beforeExecute} and {@link
+ * ThreadPoolExecutor#afterExecute} methods that are called before and
+ * after execution of each task.  These can be used to manipulate the
+ * execution environment; for example, reinitializing ThreadLocals,
+ * gathering statistics, or adding log entries. Additionally, method
+ * {@link ThreadPoolExecutor#terminated} can be overridden to perform
+ * any special processing that needs to be done once the Executor has
+ * fully terminated.
+ *
+ * <p>If hook or callback methods throw
+ * exceptions, internal worker threads may in turn fail and
+ * abruptly terminate.</dd>
+ *
+ * <dt>Queue maintenance</dt>
+ *
+ * <dd> Method {@link ThreadPoolExecutor#getQueue} allows access to
+ * the work queue for purposes of monitoring and debugging.  Use of
+ * this method for any other purpose is strongly discouraged.  Two
+ * supplied methods, {@link ThreadPoolExecutor#remove} and {@link
+ * ThreadPoolExecutor#purge} are available to assist in storage
+ * reclamation when large numbers of queued tasks become
+ * cancelled.</dd>
+ *
+ * <dt>Finalization</dt>
+ *
+ * <dd> A pool that is no longer referenced in a program <em>AND</em>
+ * has no remaining threads will be <tt>shutdown</tt>
+ * automatically. If you would like to ensure that unreferenced pools
+ * are reclaimed even if users forget to call {@link
+ * ThreadPoolExecutor#shutdown}, then you must arrange that unused
+ * threads eventually die, by setting appropriate keep-alive times,
+ * using a lower bound of zero core threads and/or setting {@link
+ * ThreadPoolExecutor#allowCoreThreadTimeOut}.  </dd> </dl>
+ *
+ * <p> <b>Extension example</b>. Most extensions of this class
+ * override one or more of the protected hook methods. For example,
+ * here is a subclass that adds a simple pause/resume feature:
+ *
+ * <pre>
+ * class PausableThreadPoolExecutor extends ThreadPoolExecutor {
+ *   private boolean isPaused;
+ *   private ReentrantLock pauseLock = new ReentrantLock();
+ *   private Condition unpaused = pauseLock.newCondition();
+ *
+ *   public PausableThreadPoolExecutor(...) { super(...); }
+ *
+ *   protected void beforeExecute(Thread t, Runnable r) {
+ *     super.beforeExecute(t, r);
+ *     pauseLock.lock();
+ *     try {
+ *       while (isPaused) unpaused.await();
+ *     } catch (InterruptedException ie) {
+ *       t.interrupt();
+ *     } finally {
+ *       pauseLock.unlock();
+ *     }
+ *   }
+ *
+ *   public void pause() {
+ *     pauseLock.lock();
+ *     try {
+ *       isPaused = true;
+ *     } finally {
+ *       pauseLock.unlock();
+ *     }
+ *   }
+ *
+ *   public void resume() {
+ *     pauseLock.lock();
+ *     try {
+ *       isPaused = false;
+ *       unpaused.signalAll();
+ *     } finally {
+ *       pauseLock.unlock();
+ *     }
+ *   }
+ * }
+ * </pre>
+ * @since 1.5
+ * @author Doug Lea
+ */
+public class ThreadPoolExecutor extends AbstractExecutorService {
+    /**
+     * Only used to force toArray() to produce a Runnable[].
+     */
+    private static final Runnable[] EMPTY_RUNNABLE_ARRAY = new Runnable[0];
+
+    /**
+     * Permission for checking shutdown
+     */
+    private static final RuntimePermission shutdownPerm =
+        new RuntimePermission("modifyThread");
+
+    /**
+     * Queue used for holding tasks and handing off to worker threads.
+     */
+    private final BlockingQueue<Runnable> workQueue;
+
+    /**
+     * Lock held on updates to poolSize, corePoolSize, maximumPoolSize, and
+     * workers set.
+     */
+    private final ReentrantLock mainLock = new ReentrantLock();
+
+    /**
+     * Wait condition to support awaitTermination
+     */
+    private final Condition termination = mainLock.newCondition();
+
+    /**
+     * Set containing all worker threads in pool.
+     */
+    private final HashSet<Worker> workers = new HashSet<Worker>();
+
+    /**
+     * Timeout in nanoseconds for idle threads waiting for work.
+     * Threads use this timeout only when there are more than
+     * corePoolSize present. Otherwise they wait forever for new work.
+     */
+    private volatile long  keepAliveTime;
+
+    /**
+     * If false (default) core threads stay alive even when idle.
+     * If true, core threads use keepAliveTime to time out waiting for work.
+     */
+    private volatile boolean allowCoreThreadTimeOut;
+
+    /**
+     * Core pool size, updated only while holding mainLock,
+     * but volatile to allow concurrent readability even
+     * during updates.
+     */
+    private volatile int   corePoolSize;
+
+    /**
+     * Maximum pool size, updated only while holding mainLock
+     * but volatile to allow concurrent readability even
+     * during updates.
+     */
+    private volatile int   maximumPoolSize;
+
+    /**
+     * Current pool size, updated only while holding mainLock
+     * but volatile to allow concurrent readability even
+     * during updates.
+     */
+    private volatile int   poolSize;
+
+    /**
+     * Lifecycle state
+     */
+    volatile int runState;
+
+    // Special values for runState
+    /** Normal, not-shutdown mode */
+    static final int RUNNING    = 0;
+    /** Controlled shutdown mode */
+    static final int SHUTDOWN   = 1;
+    /** Immediate shutdown mode */
+    static final int STOP       = 2;
+    /** Final state */
+    static final int TERMINATED = 3;
+
+    /**
+     * Handler called when saturated or shutdown in execute.
+     */
+    private volatile RejectedExecutionHandler handler;
+
+    /**
+     * Factory for new threads.
+     */
+    private volatile ThreadFactory threadFactory;
+
+    /**
+     * Tracks largest attained pool size.
+     */
+    private int largestPoolSize;
+
+    /**
+     * Counter for completed tasks. Updated only on termination of
+     * worker threads.
+     */
+    private long completedTaskCount;
+
+    /**
+     * The default rejected execution handler
+     */
+    private static final RejectedExecutionHandler defaultHandler =
+        new AbortPolicy();
+
+    /**
+     * Invokes the rejected execution handler for the given command.
+     */
+    void reject(Runnable command) {
+        handler.rejectedExecution(command, this);
+    }
+
+    /**
+     * Creates and returns a new thread running firstTask as its first
+     * task. Call only while holding mainLock.
+     * @param firstTask the task the new thread should run first (or
+     * null if none)
+     * @return the new thread, or null if threadFactory fails to create thread
+     */
+    private Thread addThread(Runnable firstTask) {
+        if (runState == TERMINATED) // Don't create thread if terminated
+            return null;
+        Worker w = new Worker(firstTask);
+        Thread t = threadFactory.newThread(w);
+        if (t != null) {
+            w.thread = t;
+            workers.add(w);
+            int nt = ++poolSize;
+            if (nt > largestPoolSize)
+                largestPoolSize = nt;
+        }
+        return t;
+    }
+
+    /**
+     * Creates and starts a new thread running firstTask as its first
+     * task, only if fewer than corePoolSize threads are running.
+     * @param firstTask the task the new thread should run first (or
+     * null if none)
+     * @return true if successful.
+     */
+    private boolean addIfUnderCorePoolSize(Runnable firstTask) {
+        Thread t = null;
+        final ReentrantLock mainLock = this.mainLock;
+        mainLock.lock();
+        try {
+            if (poolSize < corePoolSize)
+                t = addThread(firstTask);
+        } finally {
+            mainLock.unlock();
+        }
+        if (t == null)
+            return false;
+        t.start();
+        return true;
+    }
+
+    /**
+     * Creates and starts a new thread only if fewer than maximumPoolSize
+     * threads are running.  The new thread runs as its first task the
+     * next task in queue, or if there is none, the given task.
+     * @param firstTask the task the new thread should run first (or
+     * null if none)
+     * @return 0 if a new thread cannot be created, a positive number
+     * if firstTask will be run in a new thread, or a negative number
+     * if a new thread was created but is running some other task, in
+     * which case the caller must try some other way to run firstTask
+     * (perhaps by calling this method again).
+     */
+    private int addIfUnderMaximumPoolSize(Runnable firstTask) {
+        Thread t = null;
+        int status = 0;
+        final ReentrantLock mainLock = this.mainLock;
+        mainLock.lock();
+        try {
+            if (poolSize < maximumPoolSize) {
+                Runnable next = workQueue.poll();
+                if (next == null) {
+                    next = firstTask;
+                    status = 1;
+                } else
+                    status = -1;
+                t = addThread(next);
+            }
+        } finally {
+            mainLock.unlock();
+        }
+        if (t == null)
+            return 0;
+        t.start();
+        return status;
+    }
+
+
+    /**
+     * Gets the next task for a worker thread to run.
+     * @return the task
+     */
+    Runnable getTask() {
+        for (;;) {
+            try {
+                switch (runState) {
+                case RUNNING: {
+                    // untimed wait if core and not allowing core timeout
+                    if (poolSize <= corePoolSize && !allowCoreThreadTimeOut)
+                        return workQueue.take();
+
+                    long timeout = keepAliveTime;
+                    if (timeout <= 0) // die immediately for 0 timeout
+                        return null;
+                    Runnable r = workQueue.poll(timeout, TimeUnit.NANOSECONDS);
+                    if (r != null)
+                        return r;
+                    if (poolSize > corePoolSize || allowCoreThreadTimeOut)
+                        return null; // timed out
+                    // Else, after timeout, the pool shrank. Retry
+                    break;
+                }
+
+                case SHUTDOWN: {
+                    // Help drain queue
+                    Runnable r = workQueue.poll();
+                    if (r != null)
+                        return r;
+
+                    // Check if can terminate
+                    if (workQueue.isEmpty()) {
+                        interruptIdleWorkers();
+                        return null;
+                    }
+
+                    // Else there could still be delayed tasks in queue.
+                    return workQueue.take();
+                }
+
+                case STOP:
+                    return null;
+                default:
+                    assert false;
+                }
+            } catch (InterruptedException ie) {
+                // On interruption, re-check runstate
+            }
+        }
+    }
+
+    /**
+     * Wakes up all threads that might be waiting for tasks.
+     */
+    void interruptIdleWorkers() {
+        final ReentrantLock mainLock = this.mainLock;
+        mainLock.lock();
+        try {
+            for (Worker w : workers)
+                w.interruptIfIdle();
+        } finally {
+            mainLock.unlock();
+        }
+    }
+
+    /**
+     * Performs bookkeeping for a terminated worker thread.
+     * @param w the worker
+     */
+    void workerDone(Worker w) {
+        final ReentrantLock mainLock = this.mainLock;
+        mainLock.lock();
+        try {
+            completedTaskCount += w.completedTasks;
+            workers.remove(w);
+            if (--poolSize > 0)
+                return;
+
+            // Else, this is the last thread. Deal with potential shutdown.
+
+            int state = runState;
+            assert state != TERMINATED;
+
+            if (state != STOP) {
+                // If there are queued tasks but no threads, create
+                // replacement thread. We must create it initially
+                // idle to avoid orphaned tasks in case addThread
+                // fails.  This also handles case of delayed tasks
+                // that will sometime later become runnable.
+                if (!workQueue.isEmpty()) {
+                    Thread t = addThread(null);
+                    if (t != null)
+                        t.start();
+                    return;
+                }
+
+                // Otherwise, we can exit without replacement
+                if (state == RUNNING)
+                    return;
+            }
+
+            // Either state is STOP, or state is SHUTDOWN and there is
+            // no work to do. So we can terminate.
+            termination.signalAll();
+            runState = TERMINATED;
+            // fall through to call terminate() outside of lock.
+        } finally {
+            mainLock.unlock();
+        }
+
+        assert runState == TERMINATED;
+        terminated();
+    }
+
+    /**
+     *  Worker threads
+     */
+    private class Worker implements Runnable {
+
+        /**
+         * The runLock is acquired and released surrounding each task
+         * execution. It mainly protects against interrupts that are
+         * intended to cancel the worker thread from instead
+         * interrupting the task being run.
+         */
+        private final ReentrantLock runLock = new ReentrantLock();
+
+        /**
+         * Initial task to run before entering run loop
+         */
+        private Runnable firstTask;
+
+        /**
+         * Per thread completed task counter; accumulated
+         * into completedTaskCount upon termination.
+         */
+        volatile long completedTasks;
+
+        /**
+         * Thread this worker is running in.  Acts as a final field,
+         * but cannot be set until thread is created.
+         */
+        Thread thread;
+
+        Worker(Runnable firstTask) {
+            this.firstTask = firstTask;
+        }
+
+        boolean isActive() {
+            return runLock.isLocked();
+        }
+
+        /**
+         * Interrupts thread if not running a task.
+         */
+        void interruptIfIdle() {
+            final ReentrantLock runLock = this.runLock;
+            if (runLock.tryLock()) {
+                try {
+                    thread.interrupt();
+                } finally {
+                    runLock.unlock();
+                }
+            }
+        }
+
+        /**
+         * Interrupts thread even if running a task.
+         */
+        void interruptNow() {
+            thread.interrupt();
+        }
+
+        /**
+         * Runs a single task between before/after methods.
+         */
+        private void runTask(Runnable task) {
+            final ReentrantLock runLock = this.runLock;
+            runLock.lock();
+            try {
+                // If not shutting down then clear an outstanding interrupt.
+                if (runState != STOP && 
+                    Thread.interrupted() && 
+                    runState == STOP) // Re-interrupt if stopped after clearing
+                    thread.interrupt();
+                boolean ran = false;
+                beforeExecute(thread, task);
+                try {
+                    task.run();
+                    ran = true;
+                    afterExecute(task, null);
+                    ++completedTasks;
+                } catch (RuntimeException ex) {
+                    if (!ran)
+                        afterExecute(task, ex);
+                    // Else the exception occurred within
+                    // afterExecute itself in which case we don't
+                    // want to call it again.
+                    throw ex;
+                }
+            } finally {
+                runLock.unlock();
+            }
+        }
+
+        /**
+         * Main run loop
+         */
+        public void run() {
+            try {
+                Runnable task = firstTask;
+                firstTask = null;
+                while (task != null || (task = getTask()) != null) {
+                    runTask(task);
+                    task = null; // unnecessary but can help GC
+                }
+            } finally {
+                workerDone(this);
+            }
+        }
+    }
+
+    // Public methods
+
+    /**
+     * Creates a new <tt>ThreadPoolExecutor</tt> with the given initial
+     * parameters and default thread factory and rejected execution handler.
+     * It may be more convenient to use one of the {@link Executors} factory
+     * methods instead of this general purpose constructor.
+     *
+     * @param corePoolSize the number of threads to keep in the
+     * pool, even if they are idle.
+     * @param maximumPoolSize the maximum number of threads to allow in the
+     * pool.
+     * @param keepAliveTime when the number of threads is greater than
+     * the core, this is the maximum time that excess idle threads
+     * will wait for new tasks before terminating.
+     * @param unit the time unit for the keepAliveTime
+     * argument.
+     * @param workQueue the queue to use for holding tasks before they
+     * are executed. This queue will hold only the <tt>Runnable</tt>
+     * tasks submitted by the <tt>execute</tt> method.
+     * @throws IllegalArgumentException if corePoolSize, or
+     * keepAliveTime less than zero, or if maximumPoolSize less than or
+     * equal to zero, or if corePoolSize greater than maximumPoolSize.
+     * @throws NullPointerException if <tt>workQueue</tt> is null
+     */
+    public ThreadPoolExecutor(int corePoolSize,
+                              int maximumPoolSize,
+                              long keepAliveTime,
+                              TimeUnit unit,
+                              BlockingQueue<Runnable> workQueue) {
+        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
+             Executors.defaultThreadFactory(), defaultHandler);
+    }
+
+    /**
+     * Creates a new <tt>ThreadPoolExecutor</tt> with the given initial
+     * parameters and default rejected execution handler.
+     *
+     * @param corePoolSize the number of threads to keep in the
+     * pool, even if they are idle.
+     * @param maximumPoolSize the maximum number of threads to allow in the
+     * pool.
+     * @param keepAliveTime when the number of threads is greater than
+     * the core, this is the maximum time that excess idle threads
+     * will wait for new tasks before terminating.
+     * @param unit the time unit for the keepAliveTime
+     * argument.
+     * @param workQueue the queue to use for holding tasks before they
+     * are executed. This queue will hold only the <tt>Runnable</tt>
+     * tasks submitted by the <tt>execute</tt> method.
+     * @param threadFactory the factory to use when the executor
+     * creates a new thread.
+     * @throws IllegalArgumentException if corePoolSize, or
+     * keepAliveTime less than zero, or if maximumPoolSize less than or
+     * equal to zero, or if corePoolSize greater than maximumPoolSize.
+     * @throws NullPointerException if <tt>workQueue</tt>
+     * or <tt>threadFactory</tt> are null.
+     */
+    public ThreadPoolExecutor(int corePoolSize,
+                              int maximumPoolSize,
+                              long keepAliveTime,
+                              TimeUnit unit,
+                              BlockingQueue<Runnable> workQueue,
+                              ThreadFactory threadFactory) {
+        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
+             threadFactory, defaultHandler);
+    }
+
+    /**
+     * Creates a new <tt>ThreadPoolExecutor</tt> with the given initial
+     * parameters and default thread factory.
+     *
+     * @param corePoolSize the number of threads to keep in the
+     * pool, even if they are idle.
+     * @param maximumPoolSize the maximum number of threads to allow in the
+     * pool.
+     * @param keepAliveTime when the number of threads is greater than
+     * the core, this is the maximum time that excess idle threads
+     * will wait for new tasks before terminating.
+     * @param unit the time unit for the keepAliveTime
+     * argument.
+     * @param workQueue the queue to use for holding tasks before they
+     * are executed. This queue will hold only the <tt>Runnable</tt>
+     * tasks submitted by the <tt>execute</tt> method.
+     * @param handler the handler to use when execution is blocked
+     * because the thread bounds and queue capacities are reached.
+     * @throws IllegalArgumentException if corePoolSize, or
+     * keepAliveTime less than zero, or if maximumPoolSize less than or
+     * equal to zero, or if corePoolSize greater than maximumPoolSize.
+     * @throws NullPointerException if <tt>workQueue</tt>
+     * or <tt>handler</tt> are null.
+     */
+    public ThreadPoolExecutor(int corePoolSize,
+                              int maximumPoolSize,
+                              long keepAliveTime,
+                              TimeUnit unit,
+                              BlockingQueue<Runnable> workQueue,
+                              RejectedExecutionHandler handler) {
+        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
+             Executors.defaultThreadFactory(), handler);
+    }
+
+    /**
+     * Creates a new <tt>ThreadPoolExecutor</tt> with the given initial
+     * parameters.
+     *
+     * @param corePoolSize the number of threads to keep in the
+     * pool, even if they are idle.
+     * @param maximumPoolSize the maximum number of threads to allow in the
+     * pool.
+     * @param keepAliveTime when the number of threads is greater than
+     * the core, this is the maximum time that excess idle threads
+     * will wait for new tasks before terminating.
+     * @param unit the time unit for the keepAliveTime
+     * argument.
+     * @param workQueue the queue to use for holding tasks before they
+     * are executed. This queue will hold only the <tt>Runnable</tt>
+     * tasks submitted by the <tt>execute</tt> method.
+     * @param threadFactory the factory to use when the executor
+     * creates a new thread.
+     * @param handler the handler to use when execution is blocked
+     * because the thread bounds and queue capacities are reached.
+     * @throws IllegalArgumentException if corePoolSize, or
+     * keepAliveTime less than zero, or if maximumPoolSize less than or
+     * equal to zero, or if corePoolSize greater than maximumPoolSize.
+     * @throws NullPointerException if <tt>workQueue</tt>
+     * or <tt>threadFactory</tt> or <tt>handler</tt> are null.
+     */
+    public ThreadPoolExecutor(int corePoolSize,
+                              int maximumPoolSize,
+                              long keepAliveTime,
+                              TimeUnit unit,
+                              BlockingQueue<Runnable> workQueue,
+                              ThreadFactory threadFactory,
+                              RejectedExecutionHandler handler) {
+        if (corePoolSize < 0 ||
+            maximumPoolSize <= 0 ||
+            maximumPoolSize < corePoolSize ||
+            keepAliveTime < 0)
+            throw new IllegalArgumentException();
+        if (workQueue == null || threadFactory == null || handler == null)
+            throw new NullPointerException();
+        this.corePoolSize = corePoolSize;
+        this.maximumPoolSize = maximumPoolSize;
+        this.workQueue = workQueue;
+        this.keepAliveTime = unit.toNanos(keepAliveTime);
+        this.threadFactory = threadFactory;
+        this.handler = handler;
+    }
+
+
+    /**
+     * Executes the given task sometime in the future.  The task
+     * may execute in a new thread or in an existing pooled thread.
+     *
+     * If the task cannot be submitted for execution, either because this
+     * executor has been shutdown or because its capacity has been reached,
+     * the task is handled by the current <tt>RejectedExecutionHandler</tt>.
+     *
+     * @param command the task to execute
+     * @throws RejectedExecutionException at discretion of
+     * <tt>RejectedExecutionHandler</tt>, if task cannot be accepted
+     * for execution
+     * @throws NullPointerException if command is null
+     */
+    public void execute(Runnable command) {
+        if (command == null)
+            throw new NullPointerException();
+        for (;;) {
+            if (runState != RUNNING) {
+                reject(command);
+                return;
+            }
+            if (poolSize < corePoolSize && addIfUnderCorePoolSize(command))
+                return;
+            if (workQueue.offer(command))
+                return;
+            int status = addIfUnderMaximumPoolSize(command);
+            if (status > 0)      // created new thread
+                return;
+            if (status == 0) {   // failed to create thread
+                reject(command);
+                return;
+            }
+            // Retry if created a new thread but it is busy with another task
+        }
+    }
+
+    /**
+     * Initiates an orderly shutdown in which previously submitted
+     * tasks are executed, but no new tasks will be
+     * accepted. Invocation has no additional effect if already shut
+     * down.
+     * @throws SecurityException if a security manager exists and
+     * shutting down this ExecutorService may manipulate threads that
+     * the caller is not permitted to modify because it does not hold
+     * {@link java.lang.RuntimePermission}<tt>("modifyThread")</tt>,
+     * or the security manager's <tt>checkAccess</tt> method denies access.
+     */
+    public void shutdown() {
+        // Fail if caller doesn't have modifyThread permission.
+	SecurityManager security = System.getSecurityManager();
+	if (security != null)
+            security.checkPermission(shutdownPerm);
+
+        boolean fullyTerminated = false;
+        final ReentrantLock mainLock = this.mainLock;
+        mainLock.lock();
+        try {
+            if (workers.size() > 0) {
+                // Check if caller can modify worker threads.  This
+                // might not be true even if passed above check, if
+                // the SecurityManager treats some threads specially.
+                if (security != null) {
+                    for (Worker w: workers)
+                        security.checkAccess(w.thread);
+                }
+
+                int state = runState;
+                if (state == RUNNING) // don't override shutdownNow
+                    runState = SHUTDOWN;
+
+                try {
+                    for (Worker w: workers)
+                        w.interruptIfIdle();
+                } catch (SecurityException se) {
+                    // If SecurityManager allows above checks, but
+                    // then unexpectedly throws exception when
+                    // interrupting threads (which it ought not do),
+                    // back out as cleanly as we can. Some threads may
+                    // have been killed but we remain in non-shutdown
+                    // state.
+                    runState = state;
+                    throw se;
+                }
+            }
+            else { // If no workers, trigger full termination now
+                fullyTerminated = true;
+                runState = TERMINATED;
+                termination.signalAll();
+            }
+        } finally {
+            mainLock.unlock();
+        }
+        if (fullyTerminated)
+            terminated();
+    }
+
+
+    /**
+     * Attempts to stop all actively executing tasks, halts the
+     * processing of waiting tasks, and returns a list of the tasks
+     * that were awaiting execution.
+     *
+     * <p>There are no guarantees beyond best-effort attempts to stop
+     * processing actively executing tasks.  This implementation
+     * cancels tasks via {@link Thread#interrupt}, so any task that
+     * fails to respond to interrupts may never terminate.
+     *
+     * @return list of tasks that never commenced execution
+     * @throws SecurityException if a security manager exists and
+     * shutting down this ExecutorService may manipulate threads that
+     * the caller is not permitted to modify because it does not hold
+     * {@link java.lang.RuntimePermission}<tt>("modifyThread")</tt>,
+     * or the security manager's <tt>checkAccess</tt> method denies access.
+     */
+    public List<Runnable> shutdownNow() {
+        // Almost the same code as shutdown()
+	SecurityManager security = System.getSecurityManager();
+	if (security != null)
+            security.checkPermission(shutdownPerm);
+
+        boolean fullyTerminated = false;
+        final ReentrantLock mainLock = this.mainLock;
+        mainLock.lock();
+        try {
+            if (workers.size() > 0) {
+                if (security != null) {
+                    for (Worker w: workers)
+                        security.checkAccess(w.thread);
+                }
+
+                int state = runState;
+                if (state != TERMINATED)
+                    runState = STOP;
+                try {
+                    for (Worker w : workers)
+                        w.interruptNow();
+                } catch (SecurityException se) {
+                    runState = state; // back out;
+                    throw se;
+                }
+            }
+            else { // If no workers, trigger full termination now
+                fullyTerminated = true;
+                runState = TERMINATED;
+                termination.signalAll();
+            }
+        } finally {
+            mainLock.unlock();
+        }
+        if (fullyTerminated)
+            terminated();
+        return Arrays.asList(workQueue.toArray(EMPTY_RUNNABLE_ARRAY));
+    }
+
+    public boolean isShutdown() {
+        return runState != RUNNING;
+    }
+
+    /**
+     * Returns true if this executor is in the process of terminating
+     * after <tt>shutdown</tt> or <tt>shutdownNow</tt> but has not
+     * completely terminated.  This method may be useful for
+     * debugging. A return of <tt>true</tt> reported a sufficient
+     * period after shutdown may indicate that submitted tasks have
+     * ignored or suppressed interruption, causing this executor not
+     * to properly terminate.
+     * @return true if terminating but not yet terminated.
+     */
+    public boolean isTerminating() {
+        return runState == STOP;
+    }
+
+    public boolean isTerminated() {
+        return runState == TERMINATED;
+    }
+
+    public boolean awaitTermination(long timeout, TimeUnit unit)
+        throws InterruptedException {
+        long nanos = unit.toNanos(timeout);
+        final ReentrantLock mainLock = this.mainLock;
+        mainLock.lock();
+        try {
+            for (;;) {
+                if (runState == TERMINATED)
+                    return true;
+                if (nanos <= 0)
+                    return false;
+                nanos = termination.awaitNanos(nanos);
+            }
+        } finally {
+            mainLock.unlock();
+        }
+    }
+
+    /**
+     * Invokes <tt>shutdown</tt> when this executor is no longer
+     * referenced.
+     */
+    protected void finalize()  {
+        shutdown();
+    }
+
+    /**
+     * Sets the thread factory used to create new threads.
+     *
+     * @param threadFactory the new thread factory
+     * @throws NullPointerException if threadFactory is null
+     * @see #getThreadFactory
+     */
+    public void setThreadFactory(ThreadFactory threadFactory) {
+        if (threadFactory == null)
+            throw new NullPointerException();
+        this.threadFactory = threadFactory;
+    }
+
+    /**
+     * Returns the thread factory used to create new threads.
+     *
+     * @return the current thread factory
+     * @see #setThreadFactory
+     */
+    public ThreadFactory getThreadFactory() {
+        return threadFactory;
+    }
+
+    /**
+     * Sets a new handler for unexecutable tasks.
+     *
+     * @param handler the new handler
+     * @throws NullPointerException if handler is null
+     * @see #getRejectedExecutionHandler
+     */
+    public void setRejectedExecutionHandler(RejectedExecutionHandler handler) {
+        if (handler == null)
+            throw new NullPointerException();
+        this.handler = handler;
+    }
+
+    /**
+     * Returns the current handler for unexecutable tasks.
+     *
+     * @return the current handler
+     * @see #setRejectedExecutionHandler
+     */
+    public RejectedExecutionHandler getRejectedExecutionHandler() {
+        return handler;
+    }
+
+    /**
+     * Returns the task queue used by this executor. Access to the
+     * task queue is intended primarily for debugging and monitoring.
+     * This queue may be in active use.  Retrieving the task queue
+     * does not prevent queued tasks from executing.
+     *
+     * @return the task queue
+     */
+    public BlockingQueue<Runnable> getQueue() {
+        return workQueue;
+    }
+
+    /**
+     * Removes this task from the executor's internal queue if it is
+     * present, thus causing it not to be run if it has not already
+     * started.
+     *
+     * <p> This method may be useful as one part of a cancellation
+     * scheme.  It may fail to remove tasks that have been converted
+     * into other forms before being placed on the internal queue. For
+     * example, a task entered using <tt>submit</tt> might be
+     * converted into a form that maintains <tt>Future</tt> status.
+     * However, in such cases, method {@link ThreadPoolExecutor#purge}
+     * may be used to remove those Futures that have been cancelled.
+     *
+     * @param task the task to remove
+     * @return true if the task was removed
+     */
+    public boolean remove(Runnable task) {
+        return getQueue().remove(task);
+    }
+
+
+    /**
+     * Tries to remove from the work queue all {@link Future}
+     * tasks that have been cancelled. This method can be useful as a
+     * storage reclamation operation, that has no other impact on
+     * functionality. Cancelled tasks are never executed, but may
+     * accumulate in work queues until worker threads can actively
+     * remove them. Invoking this method instead tries to remove them now.
+     * However, this method may fail to remove tasks in
+     * the presence of interference by other threads.
+     */
+    public void purge() {
+        // Fail if we encounter interference during traversal
+        try {
+            Iterator<Runnable> it = getQueue().iterator();
+            while (it.hasNext()) {
+                Runnable r = it.next();
+                if (r instanceof Future<?>) {
+                    Future<?> c = (Future<?>)r;
+                    if (c.isCancelled())
+                        it.remove();
+                }
+            }
+        }
+        catch (ConcurrentModificationException ex) {
+            return;
+        }
+    }
+
+    /**
+     * Sets the core number of threads.  This overrides any value set
+     * in the constructor.  If the new value is smaller than the
+     * current value, excess existing threads will be terminated when
+     * they next become idle. If larger, new threads will, if needed,
+     * be started to execute any queued tasks.
+     *
+     * @param corePoolSize the new core size
+     * @throws IllegalArgumentException if <tt>corePoolSize</tt>
+     * less than zero
+     * @see #getCorePoolSize
+     */
+    public void setCorePoolSize(int corePoolSize) {
+        if (corePoolSize < 0)
+            throw new IllegalArgumentException();
+        final ReentrantLock mainLock = this.mainLock;
+        mainLock.lock();
+        try {
+            int extra = this.corePoolSize - corePoolSize;
+            this.corePoolSize = corePoolSize;
+            if (extra < 0) {
+                int n = workQueue.size();
+                // We have to create initially-idle threads here
+                // because we otherwise have no recourse about
+                // what to do with a dequeued task if addThread fails.
+                while (extra++ < 0 && n-- > 0 && poolSize < corePoolSize ) {
+                    Thread t = addThread(null);
+                    if (t != null)
+                        t.start();
+                    else
+                        break;
+                }
+            }
+            else if (extra > 0 && poolSize > corePoolSize) {
+                Iterator<Worker> it = workers.iterator();
+                while (it.hasNext() &&
+                       extra-- > 0 &&
+                       poolSize > corePoolSize &&
+                       workQueue.remainingCapacity() == 0)
+                    it.next().interruptIfIdle();
+            }
+        } finally {
+            mainLock.unlock();
+        }
+    }
+
+    /**
+     * Returns the core number of threads.
+     *
+     * @return the core number of threads
+     * @see #setCorePoolSize
+     */
+    public int getCorePoolSize() {
+        return corePoolSize;
+    }
+
+    /**
+     * Starts a core thread, causing it to idly wait for work. This
+     * overrides the default policy of starting core threads only when
+     * new tasks are executed. This method will return <tt>false</tt>
+     * if all core threads have already been started.
+     * @return true if a thread was started
+     */
+    public boolean prestartCoreThread() {
+        return addIfUnderCorePoolSize(null);
+    }
+
+    /**
+     * Starts all core threads, causing them to idly wait for work. This
+     * overrides the default policy of starting core threads only when
+     * new tasks are executed.
+     * @return the number of threads started.
+     */
+    public int prestartAllCoreThreads() {
+        int n = 0;
+        while (addIfUnderCorePoolSize(null))
+            ++n;
+        return n;
+    }
+
+    /**
+     * Returns true if this pool allows core threads to time out and
+     * terminate if no tasks arrive within the keepAlive time, being
+     * replaced if needed when new tasks arrive. When true, the same
+     * keep-alive policy applying to non-core threads applies also to
+     * core threads. When false (the default), core threads are never
+     * terminated due to lack of incoming tasks.
+     * @return <tt>true</tt> if core threads are allowed to time out,
+     * else <tt>false</tt>
+     *
+     * @since 1.6
+     */
+    public boolean allowsCoreThreadTimeOut() {
+        return allowCoreThreadTimeOut;
+    }
+
+    /**
+     * Sets the policy governing whether core threads may time out and
+     * terminate if no tasks arrive within the keep-alive time, being
+     * replaced if needed when new tasks arrive. When false, core
+     * threads are never terminated due to lack of incoming
+     * tasks. When true, the same keep-alive policy applying to
+     * non-core threads applies also to core threads. To avoid
+     * continual thread replacement, the keep-alive time must be
+     * greater than zero when setting <tt>true</tt>. This method
+     * should in general be called before the pool is actively used.
+     * @param value <tt>true</tt> if should time out, else <tt>false</tt>
+     * @throws IllegalArgumentException if value is <tt>true</tt>
+     * and the current keep-alive time is not greater than zero.
+     *
+     * @since 1.6
+     */
+    public void allowCoreThreadTimeOut(boolean value) {
+        if (value && keepAliveTime <= 0)
+            throw new IllegalArgumentException("Core threads must have nonzero keep alive times");
+
+        allowCoreThreadTimeOut = value;
+    }
+
+    /**
+     * Sets the maximum allowed number of threads. This overrides any
+     * value set in the constructor. If the new value is smaller than
+     * the current value, excess existing threads will be
+     * terminated when they next become idle.
+     *
+     * @param maximumPoolSize the new maximum
+     * @throws IllegalArgumentException if the new maximum is
+     *         less than or equal to zero, or
+     *         less than the {@linkplain #getCorePoolSize core pool size}
+     * @see #getMaximumPoolSize
+     */
+    public void setMaximumPoolSize(int maximumPoolSize) {
+        if (maximumPoolSize <= 0 || maximumPoolSize < corePoolSize)
+            throw new IllegalArgumentException();
+        final ReentrantLock mainLock = this.mainLock;
+        mainLock.lock();
+        try {
+            int extra = this.maximumPoolSize - maximumPoolSize;
+            this.maximumPoolSize = maximumPoolSize;
+            if (extra > 0 && poolSize > maximumPoolSize) {
+                Iterator<Worker> it = workers.iterator();
+                while (it.hasNext() &&
+                       extra > 0 &&
+                       poolSize > maximumPoolSize) {
+                    it.next().interruptIfIdle();
+                    --extra;
+                }
+            }
+        } finally {
+            mainLock.unlock();
+        }
+    }
+
+    /**
+     * Returns the maximum allowed number of threads.
+     *
+     * @return the maximum allowed number of threads
+     * @see #setMaximumPoolSize
+     */
+    public int getMaximumPoolSize() {
+        return maximumPoolSize;
+    }
+
+    /**
+     * Sets the time limit for which threads may remain idle before
+     * being terminated.  If there are more than the core number of
+     * threads currently in the pool, after waiting this amount of
+     * time without processing a task, excess threads will be
+     * terminated.  This overrides any value set in the constructor.
+     * @param time the time to wait.  A time value of zero will cause
+     * excess threads to terminate immediately after executing tasks.
+     * @param unit  the time unit of the time argument
+     * @throws IllegalArgumentException if time less than zero or
+     * if time is zero and allowsCoreThreadTimeOut
+     * @see #getKeepAliveTime
+     */
+    public void setKeepAliveTime(long time, TimeUnit unit) {
+        if (time < 0)
+            throw new IllegalArgumentException();
+        if (time == 0 && allowsCoreThreadTimeOut())
+            throw new IllegalArgumentException("Core threads must have nonzero keep alive times");
+        this.keepAliveTime = unit.toNanos(time);
+    }
+
+    /**
+     * Returns the thread keep-alive time, which is the amount of time
+     * which threads in excess of the core pool size may remain
+     * idle before being terminated.
+     *
+     * @param unit the desired time unit of the result
+     * @return the time limit
+     * @see #setKeepAliveTime
+     */
+    public long getKeepAliveTime(TimeUnit unit) {
+        return unit.convert(keepAliveTime, TimeUnit.NANOSECONDS);
+    }
+
+    /* Statistics */
+
+    /**
+     * Returns the current number of threads in the pool.
+     *
+     * @return the number of threads
+     */
+    public int getPoolSize() {
+        return poolSize;
+    }
+
+    /**
+     * Returns the approximate number of threads that are actively
+     * executing tasks.
+     *
+     * @return the number of threads
+     */
+    public int getActiveCount() {
+        final ReentrantLock mainLock = this.mainLock;
+        mainLock.lock();
+        try {
+            int n = 0;
+            for (Worker w : workers) {
+                if (w.isActive())
+                    ++n;
+            }
+            return n;
+        } finally {
+            mainLock.unlock();
+        }
+    }
+
+    /**
+     * Returns the largest number of threads that have ever
+     * simultaneously been in the pool.
+     *
+     * @return the number of threads
+     */
+    public int getLargestPoolSize() {
+        final ReentrantLock mainLock = this.mainLock;
+        mainLock.lock();
+        try {
+            return largestPoolSize;
+        } finally {
+            mainLock.unlock();
+        }
+    }
+
+    /**
+     * Returns the approximate total number of tasks that have been
+     * scheduled for execution. Because the states of tasks and
+     * threads may change dynamically during computation, the returned
+     * value is only an approximation, but one that does not ever
+     * decrease across successive calls.
+     *
+     * @return the number of tasks
+     */
+    public long getTaskCount() {
+        final ReentrantLock mainLock = this.mainLock;
+        mainLock.lock();
+        try {
+            long n = completedTaskCount;
+            for (Worker w : workers) {
+                n += w.completedTasks;
+                if (w.isActive())
+                    ++n;
+            }
+            return n + workQueue.size();
+        } finally {
+            mainLock.unlock();
+        }
+    }
+
+    /**
+     * Returns the approximate total number of tasks that have
+     * completed execution. Because the states of tasks and threads
+     * may change dynamically during computation, the returned value
+     * is only an approximation, but one that does not ever decrease
+     * across successive calls.
+     *
+     * @return the number of tasks
+     */
+    public long getCompletedTaskCount() {
+        final ReentrantLock mainLock = this.mainLock;
+        mainLock.lock();
+        try {
+            long n = completedTaskCount;
+            for (Worker w : workers)
+                n += w.completedTasks;
+            return n;
+        } finally {
+            mainLock.unlock();
+        }
+    }
+
+    /**
+     * Method invoked prior to executing the given Runnable in the
+     * given thread.  This method is invoked by thread <tt>t</tt> that
+     * will execute task <tt>r</tt>, and may be used to re-initialize
+     * ThreadLocals, or to perform logging.
+     *
+     * <p>This implementation does nothing, but may be customized in
+     * subclasses. Note: To properly nest multiple overridings, subclasses
+     * should generally invoke <tt>super.beforeExecute</tt> at the end of
+     * this method.
+     *
+     * @param t the thread that will run task r.
+     * @param r the task that will be executed.
+     */
+    protected void beforeExecute(Thread t, Runnable r) { }
+
+    /**
+     * Method invoked upon completion of execution of the given Runnable.
+     * This method is invoked by the thread that executed the task. If
+     * non-null, the Throwable is the uncaught <tt>RuntimeException</tt>
+     * or <tt>Error</tt> that caused execution to terminate abruptly.
+     *
+     * <p><b>Note:</b> When actions are enclosed in tasks (such as
+     * {@link FutureTask}) either explicitly or via methods such as
+     * <tt>submit</tt>, these task objects catch and maintain
+     * computational exceptions, and so they do not cause abrupt
+     * termination, and the internal exceptions are <em>not</em>
+     * passed to this method.
+     *
+     * <p>This implementation does nothing, but may be customized in
+     * subclasses. Note: To properly nest multiple overridings, subclasses
+     * should generally invoke <tt>super.afterExecute</tt> at the
+     * beginning of this method.
+     *
+     * @param r the runnable that has completed.
+     * @param t the exception that caused termination, or null if
+     * execution completed normally.
+     */
+    protected void afterExecute(Runnable r, Throwable t) { }
+
+    /**
+     * Method invoked when the Executor has terminated.  Default
+     * implementation does nothing. Note: To properly nest multiple
+     * overridings, subclasses should generally invoke
+     * <tt>super.terminated</tt> within this method.
+     */
+    protected void terminated() { }
+
+    /**
+     * A handler for rejected tasks that runs the rejected task
+     * directly in the calling thread of the <tt>execute</tt> method,
+     * unless the executor has been shut down, in which case the task
+     * is discarded.
+     */
+    public static class CallerRunsPolicy implements RejectedExecutionHandler {
+        /**
+         * Creates a <tt>CallerRunsPolicy</tt>.
+         */
+        public CallerRunsPolicy() { }
+
+        /**
+         * Executes task r in the caller's thread, unless the executor
+         * has been shut down, in which case the task is discarded.
+         * @param r the runnable task requested to be executed
+         * @param e the executor attempting to execute this task
+         */
+        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
+            if (!e.isShutdown()) {
+                r.run();
+            }
+        }
+    }
+
+    /**
+     * A handler for rejected tasks that throws a
+     * <tt>RejectedExecutionException</tt>.
+     */
+    public static class AbortPolicy implements RejectedExecutionHandler {
+        /**
+         * Creates an <tt>AbortPolicy</tt>.
+         */
+        public AbortPolicy() { }
+
+        /**
+         * Always throws RejectedExecutionException.
+         * @param r the runnable task requested to be executed
+         * @param e the executor attempting to execute this task
+         * @throws RejectedExecutionException always.
+         */
+        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
+            throw new RejectedExecutionException();
+        }
+    }
+
+    /**
+     * A handler for rejected tasks that silently discards the
+     * rejected task.
+     */
+    public static class DiscardPolicy implements RejectedExecutionHandler {
+        /**
+         * Creates a <tt>DiscardPolicy</tt>.
+         */
+        public DiscardPolicy() { }
+
+        /**
+         * Does nothing, which has the effect of discarding task r.
+         * @param r the runnable task requested to be executed
+         * @param e the executor attempting to execute this task
+         */
+        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
+        }
+    }
+
+    /**
+     * A handler for rejected tasks that discards the oldest unhandled
+     * request and then retries <tt>execute</tt>, unless the executor
+     * is shut down, in which case the task is discarded.
+     */
+    public static class DiscardOldestPolicy implements RejectedExecutionHandler {
+        /**
+         * Creates a <tt>DiscardOldestPolicy</tt> for the given executor.
+         */
+        public DiscardOldestPolicy() { }
+
+        /**
+         * Obtains and ignores the next task that the executor
+         * would otherwise execute, if one is immediately available,
+         * and then retries execution of task r, unless the executor
+         * is shut down, in which case task r is instead discarded.
+         * @param r the runnable task requested to be executed
+         * @param e the executor attempting to execute this task
+         */
+        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
+            if (!e.isShutdown()) {
+                e.getQueue().poll();
+                e.execute(r);
+            }
+        }
+    }
+}
diff --git a/external/jsr166/java/util/concurrent/TimeUnit.java b/external/jsr166/java/util/concurrent/TimeUnit.java
new file mode 100644
index 000000000..2cd3d06ab
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/TimeUnit.java
@@ -0,0 +1,331 @@
+/*
+  * Written by Doug Lea with assistance from members of JCP JSR-166
+  * Expert Group and released to the public domain, as explained at
+  * http://creativecommons.org/licenses/publicdomain
+  */
+
+package java.util.concurrent;
+
+/**
+ * A <tt>TimeUnit</tt> represents time durations at a given unit of
+ * granularity and provides utility methods to convert across units,
+ * and to perform timing and delay operations in these units.  A
+ * <tt>TimeUnit</tt> does not maintain time information, but only
+ * helps organize and use time representations that may be maintained
+ * separately across various contexts.  A nanosecond is defined as one
+ * thousandth of a microsecond, a microsecond as one thousandth of a
+ * millisecond, a millisecond as one thousandth of a second, a minute
+ * as sixty seconds, an hour as sixty minutes, and a day as twenty four
+ * hours.
+ *
+ * <p>A <tt>TimeUnit</tt> is mainly used to inform time-based methods
+ * how a given timing parameter should be interpreted. For example,
+ * the following code will timeout in 50 milliseconds if the {@link
+ * java.util.concurrent.locks.Lock lock} is not available:
+ *
+ * <pre>  Lock lock = ...;
+ *  if ( lock.tryLock(50L, TimeUnit.MILLISECONDS) ) ...
+ * </pre>
+ * while this code will timeout in 50 seconds:
+ * <pre>
+ *  Lock lock = ...;
+ *  if ( lock.tryLock(50L, TimeUnit.SECONDS) ) ...
+ * </pre>
+ *
+ * Note however, that there is no guarantee that a particular timeout
+ * implementation will be able to notice the passage of time at the
+ * same granularity as the given <tt>TimeUnit</tt>.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ */
+public enum TimeUnit {
+    NANOSECONDS {
+        public long toNanos(long d)   { return d; }
+        public long toMicros(long d)  { return d/(C1/C0); }
+        public long toMillis(long d)  { return d/(C2/C0); }
+        public long toSeconds(long d) { return d/(C3/C0); }
+        public long toMinutes(long d) { return d/(C4/C0); }
+        public long toHours(long d)   { return d/(C5/C0); }
+        public long toDays(long d)    { return d/(C6/C0); }
+        public long convert(long d, TimeUnit u) { return u.toNanos(d); }
+        int excessNanos(long d, long m) { return (int)(d - (m*C2)); }
+    },
+    MICROSECONDS {
+        public long toNanos(long d)   { return x(d, C1/C0, MAX/(C1/C0)); }
+        public long toMicros(long d)  { return d; }
+        public long toMillis(long d)  { return d/(C2/C1); }
+        public long toSeconds(long d) { return d/(C3/C1); }
+        public long toMinutes(long d) { return d/(C4/C1); }
+        public long toHours(long d)   { return d/(C5/C1); }
+        public long toDays(long d)    { return d/(C6/C1); }
+        public long convert(long d, TimeUnit u) { return u.toMicros(d); }
+        int excessNanos(long d, long m) { return (int)((d*C1) - (m*C2)); }
+    },
+    MILLISECONDS {
+        public long toNanos(long d)   { return x(d, C2/C0, MAX/(C2/C0)); }
+        public long toMicros(long d)  { return x(d, C2/C1, MAX/(C2/C1)); }
+        public long toMillis(long d)  { return d; }
+        public long toSeconds(long d) { return d/(C3/C2); }
+        public long toMinutes(long d) { return d/(C4/C2); }
+        public long toHours(long d)   { return d/(C5/C2); }
+        public long toDays(long d)    { return d/(C6/C2); }
+        public long convert(long d, TimeUnit u) { return u.toMillis(d); }
+        int excessNanos(long d, long m) { return 0; }
+    },
+    SECONDS {
+        public long toNanos(long d)   { return x(d, C3/C0, MAX/(C3/C0)); }
+        public long toMicros(long d)  { return x(d, C3/C1, MAX/(C3/C1)); }
+        public long toMillis(long d)  { return x(d, C3/C2, MAX/(C3/C2)); }
+        public long toSeconds(long d) { return d; }
+        public long toMinutes(long d) { return d/(C4/C3); }
+        public long toHours(long d)   { return d/(C5/C3); }
+        public long toDays(long d)    { return d/(C6/C3); }
+        public long convert(long d, TimeUnit u) { return u.toSeconds(d); }
+        int excessNanos(long d, long m) { return 0; }
+    },
+    MINUTES {
+        public long toNanos(long d)   { return x(d, C4/C0, MAX/(C4/C0)); }
+        public long toMicros(long d)  { return x(d, C4/C1, MAX/(C4/C1)); }
+        public long toMillis(long d)  { return x(d, C4/C2, MAX/(C4/C2)); }
+        public long toSeconds(long d) { return x(d, C4/C3, MAX/(C4/C3)); }
+        public long toMinutes(long d) { return d; }
+        public long toHours(long d)   { return d/(C5/C4); }
+        public long toDays(long d)    { return d/(C6/C4); }
+        public long convert(long d, TimeUnit u) { return u.toMinutes(d); }
+        int excessNanos(long d, long m) { return 0; }
+    },
+    HOURS {
+        public long toNanos(long d)   { return x(d, C5/C0, MAX/(C5/C0)); }
+        public long toMicros(long d)  { return x(d, C5/C1, MAX/(C5/C1)); }
+        public long toMillis(long d)  { return x(d, C5/C2, MAX/(C5/C2)); }
+        public long toSeconds(long d) { return x(d, C5/C3, MAX/(C5/C3)); }
+        public long toMinutes(long d) { return x(d, C5/C4, MAX/(C5/C4)); }
+        public long toHours(long d)   { return d; }
+        public long toDays(long d)    { return d/(C6/C5); }
+        public long convert(long d, TimeUnit u) { return u.toHours(d); }
+        int excessNanos(long d, long m) { return 0; }
+    },
+    DAYS {
+        public long toNanos(long d)   { return x(d, C6/C0, MAX/(C6/C0)); }
+        public long toMicros(long d)  { return x(d, C6/C1, MAX/(C6/C1)); }
+        public long toMillis(long d)  { return x(d, C6/C2, MAX/(C6/C2)); }
+        public long toSeconds(long d) { return x(d, C6/C3, MAX/(C6/C3)); }
+        public long toMinutes(long d) { return x(d, C6/C4, MAX/(C6/C4)); }
+        public long toHours(long d)   { return x(d, C6/C5, MAX/(C6/C5)); }
+        public long toDays(long d)    { return d; }
+        public long convert(long d, TimeUnit u) { return u.toDays(d); }
+        int excessNanos(long d, long m) { return 0; }
+    };
+
+    // Handy constants for conversion methods
+    static final long C0 = 1L;
+    static final long C1 = C0 * 1000L;
+    static final long C2 = C1 * 1000L;
+    static final long C3 = C2 * 1000L;
+    static final long C4 = C3 * 60L;
+    static final long C5 = C4 * 60L;
+    static final long C6 = C5 * 24L;
+
+    static final long MAX = Long.MAX_VALUE;
+
+    /**
+     * Scale d by m, checking for overflow.
+     * This has a short name to make above code more readable.
+     */
+    static long x(long d, long m, long over) {
+        if (d >  over) return Long.MAX_VALUE;
+        if (d < -over) return Long.MIN_VALUE;
+        return d * m;
+    }
+
+    // To maintain full signature compatibility with 1.5, and to improve the
+    // clarity of the generated javadoc (see 6287639: Abstract methods in
+    // enum classes should not be listed as abstract), method convert
+    // etc. are not declared abstract but otherwise act as abstract methods.
+
+    /**
+     * Convert the given time duration in the given unit to this
+     * unit.  Conversions from finer to coarser granularities
+     * truncate, so lose precision. For example converting
+     * <tt>999</tt> milliseconds to seconds results in
+     * <tt>0</tt>. Conversions from coarser to finer granularities
+     * with arguments that would numerically overflow saturate to
+     * <tt>Long.MIN_VALUE</tt> if negative or <tt>Long.MAX_VALUE</tt>
+     * if positive.
+     *
+     * <p>For example, to convert 10 minutes to milliseconds, use:
+     * <tt>TimeUnit.MILLISECONDS.convert(10L, TimeUnit.MINUTES)</tt>
+     *
+     * @param sourceDuration the time duration in the given <tt>sourceUnit</tt>
+     * @param sourceUnit the unit of the <tt>sourceDuration</tt> argument
+     * @return the converted duration in this unit,
+     * or <tt>Long.MIN_VALUE</tt> if conversion would negatively
+     * overflow, or <tt>Long.MAX_VALUE</tt> if it would positively overflow.
+     */
+    public long convert(long sourceDuration, TimeUnit sourceUnit) {
+        throw new AbstractMethodError();
+    }
+
+    /**
+     * Equivalent to <tt>NANOSECONDS.convert(duration, this)</tt>.
+     * @param duration the duration
+     * @return the converted duration,
+     * or <tt>Long.MIN_VALUE</tt> if conversion would negatively
+     * overflow, or <tt>Long.MAX_VALUE</tt> if it would positively overflow.
+     * @see #convert
+     */
+    public long toNanos(long duration) {
+        throw new AbstractMethodError();
+    }
+
+    /**
+     * Equivalent to <tt>MICROSECONDS.convert(duration, this)</tt>.
+     * @param duration the duration
+     * @return the converted duration,
+     * or <tt>Long.MIN_VALUE</tt> if conversion would negatively
+     * overflow, or <tt>Long.MAX_VALUE</tt> if it would positively overflow.
+     * @see #convert
+     */
+    public long toMicros(long duration) {
+        throw new AbstractMethodError();
+    }
+
+    /**
+     * Equivalent to <tt>MILLISECONDS.convert(duration, this)</tt>.
+     * @param duration the duration
+     * @return the converted duration,
+     * or <tt>Long.MIN_VALUE</tt> if conversion would negatively
+     * overflow, or <tt>Long.MAX_VALUE</tt> if it would positively overflow.
+     * @see #convert
+     */
+    public long toMillis(long duration) {
+        throw new AbstractMethodError();
+    }
+
+    /**
+     * Equivalent to <tt>SECONDS.convert(duration, this)</tt>.
+     * @param duration the duration
+     * @return the converted duration,
+     * or <tt>Long.MIN_VALUE</tt> if conversion would negatively
+     * overflow, or <tt>Long.MAX_VALUE</tt> if it would positively overflow.
+     * @see #convert
+     */
+    public long toSeconds(long duration) {
+        throw new AbstractMethodError();
+    }
+
+    /**
+     * Equivalent to <tt>MINUTES.convert(duration, this)</tt>.
+     * @param duration the duration
+     * @return the converted duration,
+     * or <tt>Long.MIN_VALUE</tt> if conversion would negatively
+     * overflow, or <tt>Long.MAX_VALUE</tt> if it would positively overflow.
+     * @see #convert
+     * @since 1.6
+     */
+    public long toMinutes(long duration) {
+        throw new AbstractMethodError();
+    }
+
+    /**
+     * Equivalent to <tt>HOURS.convert(duration, this)</tt>.
+     * @param duration the duration
+     * @return the converted duration,
+     * or <tt>Long.MIN_VALUE</tt> if conversion would negatively
+     * overflow, or <tt>Long.MAX_VALUE</tt> if it would positively overflow.
+     * @see #convert
+     * @since 1.6
+     */
+    public long toHours(long duration) {
+        throw new AbstractMethodError();
+    }
+
+    /**
+     * Equivalent to <tt>DAYS.convert(duration, this)</tt>.
+     * @param duration the duration
+     * @return the converted duration
+     * @see #convert
+     * @since 1.6
+     */
+    public long toDays(long duration) {
+        throw new AbstractMethodError();
+    }
+
+    /**
+     * Utility to compute the excess-nanosecond argument to wait,
+     * sleep, join.
+     * @param d the duration
+     * @param m the number of milliseconds
+     * @return the number of nanoseconds
+     */
+    abstract int excessNanos(long d, long m);
+
+    /**
+     * Performs a timed <tt>Object.wait</tt> using this time unit.
+     * This is a convenience method that converts timeout arguments
+     * into the form required by the <tt>Object.wait</tt> method.
+     *
+     * <p>For example, you could implement a blocking <tt>poll</tt>
+     * method (see {@link BlockingQueue#poll BlockingQueue.poll})
+     * using:
+     *
+     * <pre>  public synchronized Object poll(long timeout, TimeUnit unit) throws InterruptedException {
+     *    while (empty) {
+     *      unit.timedWait(this, timeout);
+     *      ...
+     *    }
+     *  }</pre>
+     *
+     * @param obj the object to wait on
+     * @param timeout the maximum time to wait. If less than
+     * or equal to zero, do not wait at all.
+     * @throws InterruptedException if interrupted while waiting.
+     * @see Object#wait(long, int)
+     */
+    public void timedWait(Object obj, long timeout)
+    throws InterruptedException {
+        if (timeout > 0) {
+            long ms = toMillis(timeout);
+            int ns = excessNanos(timeout, ms);
+            obj.wait(ms, ns);
+        }
+    }
+
+    /**
+     * Performs a timed <tt>Thread.join</tt> using this time unit.
+     * This is a convenience method that converts time arguments into the
+     * form required by the <tt>Thread.join</tt> method.
+     * @param thread the thread to wait for
+     * @param timeout the maximum time to wait. If less than
+     * or equal to zero, do not wait at all.
+     * @throws InterruptedException if interrupted while waiting.
+     * @see Thread#join(long, int)
+     */
+    public void timedJoin(Thread thread, long timeout)
+    throws InterruptedException {
+        if (timeout > 0) {
+            long ms = toMillis(timeout);
+            int ns = excessNanos(timeout, ms);
+            thread.join(ms, ns);
+        }
+    }
+
+    /**
+     * Performs a <tt>Thread.sleep</tt> using this unit.
+     * This is a convenience method that converts time arguments into the
+     * form required by the <tt>Thread.sleep</tt> method.
+     * @param timeout the minimum time to sleep. If less than
+     * or equal to zero, do not sleep at all.
+     * @throws InterruptedException if interrupted while sleeping.
+     * @see Thread#sleep
+     */
+    public void sleep(long timeout) throws InterruptedException {
+        if (timeout > 0) {
+            long ms = toMillis(timeout);
+            int ns = excessNanos(timeout, ms);
+            Thread.sleep(ms, ns);
+        }
+    }
+
+}
diff --git a/external/jsr166/java/util/concurrent/TimeoutException.java b/external/jsr166/java/util/concurrent/TimeoutException.java
new file mode 100644
index 000000000..8b84f28e5
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/TimeoutException.java
@@ -0,0 +1,38 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent;
+
+/**
+ * Exception thrown when a blocking operation times out.  Blocking
+ * operations for which a timeout is specified need a means to
+ * indicate that the timeout has occurred. For many such operations it
+ * is possible to return a value that indicates timeout; when that is
+ * not possible or desirable then <tt>TimeoutException</tt> should be
+ * declared and thrown.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ */
+public class TimeoutException extends Exception {
+    private static final long serialVersionUID = 1900926677490660714L;
+
+    /**
+     * Constructs a <tt>TimeoutException</tt> with no specified detail
+     * message.
+     */
+    public TimeoutException() {}
+
+    /**
+     * Constructs a <tt>TimeoutException</tt> with the specified detail
+     * message.
+     *
+     * @param message the detail message
+     */
+    public TimeoutException(String message) {
+        super(message);
+    }
+}
diff --git a/external/jsr166/java/util/concurrent/atomic/AtomicBoolean.java b/external/jsr166/java/util/concurrent/atomic/AtomicBoolean.java
new file mode 100644
index 000000000..bd823bd2c
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/atomic/AtomicBoolean.java
@@ -0,0 +1,133 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent.atomic;
+import sun.misc.Unsafe;
+
+/**
+ * A <tt>boolean</tt> value that may be updated atomically. See the
+ * {@link java.util.concurrent.atomic} package specification for
+ * description of the properties of atomic variables. An
+ * <tt>AtomicBoolean</tt> is used in applications such as atomically
+ * updated flags, and cannot be used as a replacement for a
+ * {@link java.lang.Boolean}.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ */
+public class AtomicBoolean implements java.io.Serializable {
+    private static final long serialVersionUID = 4654671469794556979L;
+    // setup to use Unsafe.compareAndSwapInt for updates
+    private static final Unsafe unsafe = Unsafe.getUnsafe();
+    private static final long valueOffset;
+
+    static {
+      try {
+        valueOffset = unsafe.objectFieldOffset
+            (AtomicBoolean.class.getDeclaredField("value"));
+      } catch (Exception ex) { throw new Error(ex); }
+    }
+
+    private volatile int value;
+
+    /**
+     * Creates a new <tt>AtomicBoolean</tt> with the given initial value.
+     *
+     * @param initialValue the initial value
+     */
+    public AtomicBoolean(boolean initialValue) {
+        value = initialValue ? 1 : 0;
+    }
+
+    /**
+     * Creates a new <tt>AtomicBoolean</tt> with initial value <tt>false</tt>.
+     */
+    public AtomicBoolean() {
+    }
+
+    /**
+     * Returns the current value.
+     *
+     * @return the current value
+     */
+    public final boolean get() {
+        return value != 0;
+    }
+
+    /**
+     * Atomically sets the value to the given updated value
+     * if the current value <tt>==</tt> the expected value.
+     *
+     * @param expect the expected value
+     * @param update the new value
+     * @return true if successful. False return indicates that
+     * the actual value was not equal to the expected value.
+     */
+    public final boolean compareAndSet(boolean expect, boolean update) {
+        int e = expect ? 1 : 0;
+        int u = update ? 1 : 0;
+        return unsafe.compareAndSwapInt(this, valueOffset, e, u);
+    }
+
+    /**
+     * Atomically sets the value to the given updated value
+     * if the current value <tt>==</tt> the expected value.
+     * May fail spuriously and does not provide ordering guarantees,
+     * so is only rarely an appropriate alternative to <tt>compareAndSet</tt>.
+     *
+     * @param expect the expected value
+     * @param update the new value
+     * @return true if successful.
+     */
+    public boolean weakCompareAndSet(boolean expect, boolean update) {
+        int e = expect ? 1 : 0;
+        int u = update ? 1 : 0;
+        return unsafe.compareAndSwapInt(this, valueOffset, e, u);
+    }
+
+    /**
+     * Unconditionally sets to the given value.
+     *
+     * @param newValue the new value
+     */
+    public final void set(boolean newValue) {
+        value = newValue ? 1 : 0;
+    }
+
+    /**
+     * Eventually sets to the given value.
+     *
+     * @param newValue the new value
+     * @since 1.6
+     */
+    public final void lazySet(boolean newValue) {
+        int v = newValue ? 1 : 0;
+        unsafe.putOrderedInt(this, valueOffset, v);
+    }
+
+    /**
+     * Atomically sets to the given value and returns the previous value.
+     *
+     * @param newValue the new value
+     * @return the previous value
+     */
+    public final boolean getAndSet(boolean newValue) {
+        for (;;) {
+            boolean current = get();
+            if (compareAndSet(current, newValue))
+                return current;
+        }
+    }
+
+    /**
+     * Returns the String representation of the current value.
+     * @return the String representation of the current value.
+     */
+    public String toString() {
+        return Boolean.toString(get());
+    }
+
+}
diff --git a/external/jsr166/java/util/concurrent/atomic/AtomicInteger.java b/external/jsr166/java/util/concurrent/atomic/AtomicInteger.java
new file mode 100644
index 000000000..dc4d470c3
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/atomic/AtomicInteger.java
@@ -0,0 +1,234 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent.atomic;
+import sun.misc.Unsafe;
+
+/**
+ * An <tt>int</tt> value that may be updated atomically.  See the
+ * {@link java.util.concurrent.atomic} package specification for
+ * description of the properties of atomic variables. An
+ * <tt>AtomicInteger</tt> is used in applications such as atomically
+ * incremented counters, and cannot be used as a replacement for an
+ * {@link java.lang.Integer}. However, this class does extend
+ * <tt>Number</tt> to allow uniform access by tools and utilities that
+ * deal with numerically-based classes.
+ *
+ * @since 1.5
+ * @author Doug Lea
+*/
+public class AtomicInteger extends Number implements java.io.Serializable {
+    private static final long serialVersionUID = 6214790243416807050L;
+
+    // setup to use Unsafe.compareAndSwapInt for updates
+    private static final Unsafe unsafe = Unsafe.getUnsafe();
+    private static final long valueOffset;
+
+    static {
+      try {
+        valueOffset = unsafe.objectFieldOffset
+            (AtomicInteger.class.getDeclaredField("value"));
+      } catch (Exception ex) { throw new Error(ex); }
+    }
+
+    private volatile int value;
+
+    /**
+     * Creates a new AtomicInteger with the given initial value.
+     *
+     * @param initialValue the initial value
+     */
+    public AtomicInteger(int initialValue) {
+        value = initialValue;
+    }
+
+    /**
+     * Creates a new AtomicInteger with initial value <tt>0</tt>.
+     */
+    public AtomicInteger() {
+    }
+
+    /**
+     * Gets the current value.
+     *
+     * @return the current value
+     */
+    public final int get() {
+        return value;
+    }
+
+    /**
+     * Sets to the given value.
+     *
+     * @param newValue the new value
+     */
+    public final void set(int newValue) {
+        value = newValue;
+    }
+
+    /**
+     * Eventually sets to the given value.
+     *
+     * @param newValue the new value
+     * @since 1.6
+     */
+    public final void lazySet(int newValue) {
+        unsafe.putOrderedInt(this, valueOffset, newValue);
+    }
+
+    /**
+     * Atomically sets to the given value and returns the old value.
+     *
+     * @param newValue the new value
+     * @return the previous value
+     */
+    public final int getAndSet(int newValue) {
+        for (;;) {
+            int current = get();
+            if (compareAndSet(current, newValue))
+                return current;
+        }
+    }
+
+    /**
+     * Atomically sets the value to the given updated value
+     * if the current value <tt>==</tt> the expected value.
+     *
+     * @param expect the expected value
+     * @param update the new value
+     * @return true if successful. False return indicates that
+     * the actual value was not equal to the expected value.
+     */
+    public final boolean compareAndSet(int expect, int update) {
+	return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
+    }
+
+    /**
+     * Atomically sets the value to the given updated value
+     * if the current value <tt>==</tt> the expected value.
+     * May fail spuriously and does not provide ordering guarantees,
+     * so is only rarely an appropriate alternative to <tt>compareAndSet</tt>.
+     *
+     * @param expect the expected value
+     * @param update the new value
+     * @return true if successful.
+     */
+    public final boolean weakCompareAndSet(int expect, int update) {
+	return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
+    }
+
+    /**
+     * Atomically increments by one the current value.
+     *
+     * @return the previous value
+     */
+    public final int getAndIncrement() {
+        for (;;) {
+            int current = get();
+            int next = current + 1;
+            if (compareAndSet(current, next))
+                return current;
+        }
+    }
+
+    /**
+     * Atomically decrements by one the current value.
+     *
+     * @return the previous value
+     */
+    public final int getAndDecrement() {
+        for (;;) {
+            int current = get();
+            int next = current - 1;
+            if (compareAndSet(current, next))
+                return current;
+        }
+    }
+
+    /**
+     * Atomically adds the given value to the current value.
+     *
+     * @param delta the value to add
+     * @return the previous value
+     */
+    public final int getAndAdd(int delta) {
+        for (;;) {
+            int current = get();
+            int next = current + delta;
+            if (compareAndSet(current, next))
+                return current;
+        }
+    }
+
+    /**
+     * Atomically increments by one the current value.
+     *
+     * @return the updated value
+     */
+    public final int incrementAndGet() {
+        for (;;) {
+            int current = get();
+            int next = current + 1;
+            if (compareAndSet(current, next))
+                return next;
+        }
+    }
+
+    /**
+     * Atomically decrements by one the current value.
+     *
+     * @return the updated value
+     */
+    public final int decrementAndGet() {
+        for (;;) {
+            int current = get();
+            int next = current - 1;
+            if (compareAndSet(current, next))
+                return next;
+        }
+    }
+
+    /**
+     * Atomically adds the given value to the current value.
+     *
+     * @param delta the value to add
+     * @return the updated value
+     */
+    public final int addAndGet(int delta) {
+        for (;;) {
+            int current = get();
+            int next = current + delta;
+            if (compareAndSet(current, next))
+                return next;
+        }
+    }
+
+    /**
+     * Returns the String representation of the current value.
+     * @return the String representation of the current value.
+     */
+    public String toString() {
+        return Integer.toString(get());
+    }
+
+
+    public int intValue() {
+	return get();
+    }
+
+    public long longValue() {
+	return (long)get();
+    }
+
+    public float floatValue() {
+	return (float)get();
+    }
+
+    public double doubleValue() {
+	return (double)get();
+    }
+
+}
diff --git a/external/jsr166/java/util/concurrent/atomic/AtomicIntegerArray.java b/external/jsr166/java/util/concurrent/atomic/AtomicIntegerArray.java
new file mode 100644
index 000000000..2ad754fda
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/atomic/AtomicIntegerArray.java
@@ -0,0 +1,255 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent.atomic;
+import sun.misc.Unsafe;
+import java.util.*;
+
+/**
+ * An <tt>int</tt> array in which elements may be updated atomically.
+ * See the {@link java.util.concurrent.atomic} package
+ * specification for description of the properties of atomic
+ * variables.
+ * @since 1.5
+ * @author Doug Lea
+ */
+public class AtomicIntegerArray implements java.io.Serializable {
+    private static final long serialVersionUID = 2862133569453604235L;
+
+   // setup to use Unsafe.compareAndSwapInt for updates
+    private static final Unsafe unsafe = Unsafe.getUnsafe();
+    private static final int base = unsafe.arrayBaseOffset(int[].class);
+    private static final int scale = unsafe.arrayIndexScale(int[].class);
+    private final int[] array;
+
+    private long rawIndex(int i) {
+        if (i < 0 || i >= array.length)
+            throw new IndexOutOfBoundsException("index " + i);
+        return base + i * scale;
+    }
+
+    /**
+     * Creates a new AtomicIntegerArray of given length.
+     *
+     * @param length the length of the array
+     */
+    public AtomicIntegerArray(int length) {
+        array = new int[length];
+        // must perform at least one volatile write to conform to JMM
+        if (length > 0)
+            unsafe.putIntVolatile(array, rawIndex(0), 0);
+    }
+
+    /**
+     * Creates a new AtomicIntegerArray with the same length as, and
+     * all elements copied from, the given array.
+     *
+     * @param array the array to copy elements from
+     * @throws NullPointerException if array is null
+     */
+    public AtomicIntegerArray(int[] array) {
+        if (array == null)
+            throw new NullPointerException();
+        int length = array.length;
+        this.array = new int[length];
+        if (length > 0) {
+            int last = length-1;
+            for (int i = 0; i < last; ++i)
+                this.array[i] = array[i];
+            // Do the last write as volatile
+            unsafe.putIntVolatile(this.array, rawIndex(last), array[last]);
+        }
+    }
+
+    /**
+     * Returns the length of the array.
+     *
+     * @return the length of the array
+     */
+    public final int length() {
+        return array.length;
+    }
+
+    /**
+     * Gets the current value at position <tt>i</tt>.
+     *
+     * @param i the index
+     * @return the current value
+     */
+    public final int get(int i) {
+        return unsafe.getIntVolatile(array, rawIndex(i));
+    }
+
+    /**
+     * Sets the element at position <tt>i</tt> to the given value.
+     *
+     * @param i the index
+     * @param newValue the new value
+     */
+    public final void set(int i, int newValue) {
+        unsafe.putIntVolatile(array, rawIndex(i), newValue);
+    }
+
+    /**
+     * Eventually sets the element at position <tt>i</tt> to the given value.
+     *
+     * @param i the index
+     * @param newValue the new value
+     * @since 1.6
+     */
+    public final void lazySet(int i, int newValue) {
+        unsafe.putOrderedInt(array, rawIndex(i), newValue);
+    }
+
+    /**
+     * Atomically sets the element at position <tt>i</tt> to the given
+     * value and returns the old value.
+     *
+     * @param i the index
+     * @param newValue the new value
+     * @return the previous value
+     */
+    public final int getAndSet(int i, int newValue) {
+        while (true) {
+            int current = get(i);
+            if (compareAndSet(i, current, newValue))
+                return current;
+        }
+    }
+
+    /**
+     * Atomically sets the element at position <tt>i</tt> to the given
+     * updated value if the current value <tt>==</tt> the expected value.
+     *
+     * @param i the index
+     * @param expect the expected value
+     * @param update the new value
+     * @return true if successful. False return indicates that
+     * the actual value was not equal to the expected value.
+     */
+    public final boolean compareAndSet(int i, int expect, int update) {
+        return unsafe.compareAndSwapInt(array, rawIndex(i),
+                                        expect, update);
+    }
+
+    /**
+     * Atomically sets the element at position <tt>i</tt> to the given
+     * updated value if the current value <tt>==</tt> the expected value.
+     * May fail spuriously and does not provide ordering guarantees,
+     * so is only rarely an appropriate alternative to <tt>compareAndSet</tt>.
+     *
+     * @param i the index
+     * @param expect the expected value
+     * @param update the new value
+     * @return true if successful.
+     */
+    public final boolean weakCompareAndSet(int i, int expect, int update) {
+        return compareAndSet(i, expect, update);
+    }
+
+    /**
+     * Atomically increments by one the element at index <tt>i</tt>.
+     *
+     * @param i the index
+     * @return the previous value
+     */
+    public final int getAndIncrement(int i) {
+        while (true) {
+            int current = get(i);
+            int next = current + 1;
+            if (compareAndSet(i, current, next))
+                return current;
+        }
+    }
+
+    /**
+     * Atomically decrements by one the element at index <tt>i</tt>.
+     *
+     * @param i the index
+     * @return the previous value
+     */
+    public final int getAndDecrement(int i) {
+        while (true) {
+            int current = get(i);
+            int next = current - 1;
+            if (compareAndSet(i, current, next))
+                return current;
+        }
+    }
+
+    /**
+     * Atomically adds the given value to the element at index <tt>i</tt>.
+     *
+     * @param i the index
+     * @param delta the value to add
+     * @return the previous value
+     */
+    public final int getAndAdd(int i, int delta) {
+        while (true) {
+            int current = get(i);
+            int next = current + delta;
+            if (compareAndSet(i, current, next))
+                return current;
+        }
+    }
+
+    /**
+     * Atomically increments by one the element at index <tt>i</tt>.
+     *
+     * @param i the index
+     * @return the updated value
+     */
+    public final int incrementAndGet(int i) {
+        while (true) {
+            int current = get(i);
+            int next = current + 1;
+            if (compareAndSet(i, current, next))
+                return next;
+        }
+    }
+
+    /**
+     * Atomically decrements by one the element at index <tt>i</tt>.
+     *
+     * @param i the index
+     * @return the updated value
+     */
+    public final int decrementAndGet(int i) {
+        while (true) {
+            int current = get(i);
+            int next = current - 1;
+            if (compareAndSet(i, current, next))
+                return next;
+        }
+    }
+
+    /**
+     * Atomically adds the given value to the element at index <tt>i</tt>.
+     *
+     * @param i the index
+     * @param delta the value to add
+     * @return the updated value
+     */
+    public final int addAndGet(int i, int delta) {
+        while (true) {
+            int current = get(i);
+            int next = current + delta;
+            if (compareAndSet(i, current, next))
+                return next;
+        }
+    }
+
+    /**
+     * Returns the String representation of the current values of array.
+     * @return the String representation of the current values of array.
+     */
+    public String toString() {
+        if (array.length > 0) // force volatile read
+            get(0);
+        return Arrays.toString(array);
+    }
+
+}
diff --git a/external/jsr166/java/util/concurrent/atomic/AtomicIntegerFieldUpdater.java b/external/jsr166/java/util/concurrent/atomic/AtomicIntegerFieldUpdater.java
new file mode 100644
index 000000000..102c2a7c9
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/atomic/AtomicIntegerFieldUpdater.java
@@ -0,0 +1,316 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent.atomic;
+import sun.misc.Unsafe;
+import java.lang.reflect.*;
+
+/**
+ * A reflection-based utility that enables atomic updates to
+ * designated <tt>volatile int</tt> fields of designated classes.
+ * This class is designed for use in atomic data structures in which
+ * several fields of the same node are independently subject to atomic
+ * updates.
+ *
+ * <p>Note that the guarantees of the {@code compareAndSet}
+ * method in this class are weaker than in other atomic classes.
+ * Because this class cannot ensure that all uses of the field
+ * are appropriate for purposes of atomic access, it can
+ * guarantee atomicity only with respect to other invocations of
+ * {@code compareAndSet} and {@code set} on the same updater.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ * @param <T> The type of the object holding the updatable field
+ */
+public abstract class  AtomicIntegerFieldUpdater<T>  {
+    /**
+     * Creates and returns an updater for objects with the given field.
+     * The Class argument is needed to check that reflective types and
+     * generic types match.
+     *
+     * @param tclass the class of the objects holding the field
+     * @param fieldName the name of the field to be updated
+     * @return the updater
+     * @throws IllegalArgumentException if the field is not a
+     * volatile integer type
+     * @throws RuntimeException with a nested reflection-based
+     * exception if the class does not hold field or is the wrong type
+     */
+    public static <U> AtomicIntegerFieldUpdater<U> newUpdater(Class<U> tclass, String fieldName) {
+        return new AtomicIntegerFieldUpdaterImpl<U>(tclass, fieldName);
+    }
+
+    /**
+     * Protected do-nothing constructor for use by subclasses.
+     */
+    protected AtomicIntegerFieldUpdater() {
+    }
+
+    /**
+     * Atomically sets the field of the given object managed by this updater
+     * to the given updated value if the current value <tt>==</tt> the
+     * expected value. This method is guaranteed to be atomic with respect to
+     * other calls to <tt>compareAndSet</tt> and <tt>set</tt>, but not
+     * necessarily with respect to other changes in the field.
+     *
+     * @param obj An object whose field to conditionally set
+     * @param expect the expected value
+     * @param update the new value
+     * @return true if successful
+     * @throws ClassCastException if <tt>obj</tt> is not an instance
+     * of the class possessing the field established in the constructor
+     */
+    public abstract boolean compareAndSet(T obj, int expect, int update);
+
+    /**
+     * Atomically sets the field of the given object managed by this updater
+     * to the given updated value if the current value <tt>==</tt> the
+     * expected value. This method is guaranteed to be atomic with respect to
+     * other calls to <tt>compareAndSet</tt> and <tt>set</tt>, but not
+     * necessarily with respect to other changes in the field.
+     * May fail spuriously and does not provide ordering guarantees,
+     * so is only rarely an appropriate alternative to <tt>compareAndSet</tt>.
+     *
+     * @param obj An object whose field to conditionally set
+     * @param expect the expected value
+     * @param update the new value
+     * @return true if successful
+     * @throws ClassCastException if <tt>obj</tt> is not an instance
+     * of the class possessing the field established in the constructor
+     */
+    public abstract boolean weakCompareAndSet(T obj, int expect, int update);
+
+    /**
+     * Sets the field of the given object managed by this updater to the
+     * given updated value. This operation is guaranteed to act as a volatile
+     * store with respect to subsequent invocations of
+     * <tt>compareAndSet</tt>.
+     *
+     * @param obj An object whose field to set
+     * @param newValue the new value
+     */
+    public abstract void set(T obj, int newValue);
+
+    /**
+     * Eventually sets the field of the given object managed by this
+     * updater to the given updated value.
+     *
+     * @param obj An object whose field to set
+     * @param newValue the new value
+     * @since 1.6
+     */
+    public abstract void lazySet(T obj, int newValue);
+
+
+    /**
+     * Gets the current value held in the field of the given object managed
+     * by this updater.
+     *
+     * @param obj An object whose field to get
+     * @return the current value
+     */
+    public abstract int get(T obj);
+
+    /**
+     * Atomically sets the field of the given object managed by this updater
+     * to the given value and returns the old value.
+     *
+     * @param obj An object whose field to get and set
+     * @param newValue the new value
+     * @return the previous value
+     */
+    public int getAndSet(T obj, int newValue) {
+        for (;;) {
+            int current = get(obj);
+            if (compareAndSet(obj, current, newValue))
+                return current;
+        }
+    }
+
+    /**
+     * Atomically increments by one the current value of the field of the
+     * given object managed by this updater.
+     *
+     * @param obj An object whose field to get and set
+     * @return the previous value
+     */
+    public int getAndIncrement(T obj) {
+        for (;;) {
+            int current = get(obj);
+            int next = current + 1;
+            if (compareAndSet(obj, current, next))
+                return current;
+        }
+    }
+
+    /**
+     * Atomically decrements by one the current value of the field of the
+     * given object managed by this updater.
+     *
+     * @param obj An object whose field to get and set
+     * @return the previous value
+     */
+    public int getAndDecrement(T obj) {
+        for (;;) {
+            int current = get(obj);
+            int next = current - 1;
+            if (compareAndSet(obj, current, next))
+                return current;
+        }
+    }
+
+    /**
+     * Atomically adds the given value to the current value of the field of
+     * the given object managed by this updater.
+     *
+     * @param obj An object whose field to get and set
+     * @param delta the value to add
+     * @return the previous value
+     */
+    public int getAndAdd(T obj, int delta) {
+        for (;;) {
+            int current = get(obj);
+            int next = current + delta;
+            if (compareAndSet(obj, current, next))
+                return current;
+        }
+    }
+
+    /**
+     * Atomically increments by one the current value of the field of the
+     * given object managed by this updater.
+     *
+     * @param obj An object whose field to get and set
+     * @return the updated value
+     */
+    public int incrementAndGet(T obj) {
+        for (;;) {
+            int current = get(obj);
+            int next = current + 1;
+            if (compareAndSet(obj, current, next))
+                return next;
+        }
+    }
+
+    /**
+     * Atomically decrements by one the current value of the field of the
+     * given object managed by this updater.
+     *
+     * @param obj An object whose field to get and set
+     * @return the updated value
+     */
+    public int decrementAndGet(T obj) {
+        for (;;) {
+            int current = get(obj);
+            int next = current - 1;
+            if (compareAndSet(obj, current, next))
+                return next;
+        }
+    }
+
+    /**
+     * Atomically adds the given value to the current value of the field of
+     * the given object managed by this updater.
+     *
+     * @param obj An object whose field to get and set
+     * @param delta the value to add
+     * @return the updated value
+     */
+    public int addAndGet(T obj, int delta) {
+        for (;;) {
+            int current = get(obj);
+            int next = current + delta;
+            if (compareAndSet(obj, current, next))
+                return next;
+        }
+    }
+
+    /**
+     * Standard hotspot implementation using intrinsics
+     */
+    private static class AtomicIntegerFieldUpdaterImpl<T> extends AtomicIntegerFieldUpdater<T> {
+        private static final Unsafe unsafe = Unsafe.getUnsafe();
+        private final long offset;
+        private final Class<T> tclass;
+        private final Class cclass;
+
+        AtomicIntegerFieldUpdaterImpl(Class<T> tclass, String fieldName) {
+            Field field = null;
+	    Class caller = null;
+	    int modifiers = 0;
+            try {
+                field = tclass.getDeclaredField(fieldName);
+		caller = sun.reflect.Reflection.getCallerClass(3);
+		modifiers = field.getModifiers();
+                sun.reflect.misc.ReflectUtil.ensureMemberAccess(
+                    caller, tclass, null, modifiers); 
+		sun.reflect.misc.ReflectUtil.checkPackageAccess(tclass);
+            } catch(Exception ex) {
+                throw new RuntimeException(ex);
+            }
+
+            Class fieldt = field.getType();
+            if (fieldt != int.class)
+                throw new IllegalArgumentException("Must be integer type");
+            
+            if (!Modifier.isVolatile(modifiers))
+                throw new IllegalArgumentException("Must be volatile type");
+         
+	    this.cclass = (Modifier.isProtected(modifiers) &&
+			   caller != tclass) ? caller : null;
+            this.tclass = tclass;
+            offset = unsafe.objectFieldOffset(field);
+        }
+
+        private void fullCheck(T obj) {
+            if (!tclass.isInstance(obj))
+                throw new ClassCastException();
+	    if (cclass != null)
+		ensureProtectedAccess(obj);
+        }
+
+        public boolean compareAndSet(T obj, int expect, int update) {
+            if (obj == null || obj.getClass() != tclass || cclass != null) fullCheck(obj);
+            return unsafe.compareAndSwapInt(obj, offset, expect, update);
+        }
+
+        public boolean weakCompareAndSet(T obj, int expect, int update) {
+            if (obj == null || obj.getClass() != tclass || cclass != null) fullCheck(obj);
+            return unsafe.compareAndSwapInt(obj, offset, expect, update);
+        }
+
+        public void set(T obj, int newValue) {
+            if (obj == null || obj.getClass() != tclass || cclass != null) fullCheck(obj);
+            unsafe.putIntVolatile(obj, offset, newValue);
+        }
+
+        public void lazySet(T obj, int newValue) {
+            if (obj == null || obj.getClass() != tclass || cclass != null) fullCheck(obj);
+            unsafe.putOrderedInt(obj, offset, newValue);
+        }
+
+        public final int get(T obj) {
+            if (obj == null || obj.getClass() != tclass || cclass != null) fullCheck(obj);
+            return unsafe.getIntVolatile(obj, offset);
+        }
+
+	private void ensureProtectedAccess(T obj) {
+	    if (cclass.isInstance(obj)) {
+		return;
+	    }
+	    throw new RuntimeException(
+                new IllegalAccessException("Class " +
+		    cclass.getName() +
+                    " can not access a protected member of class " +
+                    tclass.getName() +
+		    " using an instance of " +
+                    obj.getClass().getName()
+		)
+	    );
+	}
+    }
+}
diff --git a/external/jsr166/java/util/concurrent/atomic/AtomicLong.java b/external/jsr166/java/util/concurrent/atomic/AtomicLong.java
new file mode 100644
index 000000000..136dc60fb
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/atomic/AtomicLong.java
@@ -0,0 +1,248 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent.atomic;
+import sun.misc.Unsafe;
+
+/**
+ * A <tt>long</tt> value that may be updated atomically.  See the
+ * {@link java.util.concurrent.atomic} package specification for
+ * description of the properties of atomic variables. An
+ * <tt>AtomicLong</tt> is used in applications such as atomically
+ * incremented sequence numbers, and cannot be used as a replacement
+ * for a {@link java.lang.Long}. However, this class does extend
+ * <tt>Number</tt> to allow uniform access by tools and utilities that
+ * deal with numerically-based classes.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ */
+public class AtomicLong extends Number implements java.io.Serializable {
+    private static final long serialVersionUID = 1927816293512124184L;
+
+    // setup to use Unsafe.compareAndSwapLong for updates
+    private static final Unsafe unsafe = Unsafe.getUnsafe();
+    private static final long valueOffset;
+
+    /**
+     * Records whether the underlying JVM supports lockless
+     * CompareAndSet for longs. While the unsafe.CompareAndSetLong
+     * method works in either case, some constructions should be
+     * handled at Java level to avoid locking user-visible locks.
+     */
+    static final boolean VM_SUPPORTS_LONG_CAS = VMSupportsCS8();
+
+    /**
+     * Returns whether underlying JVM supports lockless CompareAndSet
+     * for longs. Called only once and cached in VM_SUPPORTS_LONG_CAS.
+     */
+    private static native boolean VMSupportsCS8();
+
+    static {
+      try {
+        valueOffset = unsafe.objectFieldOffset
+            (AtomicLong.class.getDeclaredField("value"));
+      } catch (Exception ex) { throw new Error(ex); }
+    }
+
+    private volatile long value;
+
+    /**
+     * Creates a new AtomicLong with the given initial value.
+     *
+     * @param initialValue the initial value
+     */
+    public AtomicLong(long initialValue) {
+        value = initialValue;
+    }
+
+    /**
+     * Creates a new AtomicLong with initial value <tt>0</tt>.
+     */
+    public AtomicLong() {
+    }
+
+    /**
+     * Gets the current value.
+     *
+     * @return the current value
+     */
+    public final long get() {
+        return value;
+    }
+
+    /**
+     * Sets to the given value.
+     *
+     * @param newValue the new value
+     */
+    public final void set(long newValue) {
+        value = newValue;
+    }
+
+    /**
+     * Eventually sets to the given value.
+     *
+     * @param newValue the new value
+     * @since 1.6
+     */
+    public final void lazySet(long newValue) {
+        unsafe.putOrderedLong(this, valueOffset, newValue);
+    }
+
+    /**
+     * Atomically sets to the given value and returns the old value.
+     *
+     * @param newValue the new value
+     * @return the previous value
+     */
+    public final long getAndSet(long newValue) {
+        while (true) {
+            long current = get();
+            if (compareAndSet(current, newValue))
+                return current;
+        }
+    }
+
+    /**
+     * Atomically sets the value to the given updated value
+     * if the current value <tt>==</tt> the expected value.
+     *
+     * @param expect the expected value
+     * @param update the new value
+     * @return true if successful. False return indicates that
+     * the actual value was not equal to the expected value.
+     */
+    public final boolean compareAndSet(long expect, long update) {
+	return unsafe.compareAndSwapLong(this, valueOffset, expect, update);
+    }
+
+    /**
+     * Atomically sets the value to the given updated value
+     * if the current value <tt>==</tt> the expected value.
+     * May fail spuriously and does not provide ordering guarantees,
+     * so is only rarely an appropriate alternative to <tt>compareAndSet</tt>.
+     *
+     * @param expect the expected value
+     * @param update the new value
+     * @return true if successful.
+     */
+    public final boolean weakCompareAndSet(long expect, long update) {
+	return unsafe.compareAndSwapLong(this, valueOffset, expect, update);
+    }
+
+    /**
+     * Atomically increments by one the current value.
+     *
+     * @return the previous value
+     */
+    public final long getAndIncrement() {
+        while (true) {
+            long current = get();
+            long next = current + 1;
+            if (compareAndSet(current, next))
+                return current;
+        }
+    }
+
+    /**
+     * Atomically decrements by one the current value.
+     *
+     * @return the previous value
+     */
+    public final long getAndDecrement() {
+        while (true) {
+            long current = get();
+            long next = current - 1;
+            if (compareAndSet(current, next))
+                return current;
+        }
+    }
+
+    /**
+     * Atomically adds the given value to the current value.
+     *
+     * @param delta the value to add
+     * @return the previous value
+     */
+    public final long getAndAdd(long delta) {
+        while (true) {
+            long current = get();
+            long next = current + delta;
+            if (compareAndSet(current, next))
+                return current;
+        }
+    }
+
+    /**
+     * Atomically increments by one the current value.
+     *
+     * @return the updated value
+     */
+    public final long incrementAndGet() {
+        for (;;) {
+            long current = get();
+            long next = current + 1;
+            if (compareAndSet(current, next))
+                return next;
+        }
+    }
+
+    /**
+     * Atomically decrements by one the current value.
+     *
+     * @return the updated value
+     */
+    public final long decrementAndGet() {
+        for (;;) {
+            long current = get();
+            long next = current - 1;
+            if (compareAndSet(current, next))
+                return next;
+        }
+    }
+
+    /**
+     * Atomically adds the given value to the current value.
+     *
+     * @param delta the value to add
+     * @return the updated value
+     */
+    public final long addAndGet(long delta) {
+        for (;;) {
+            long current = get();
+            long next = current + delta;
+            if (compareAndSet(current, next))
+                return next;
+        }
+    }
+
+    /**
+     * Returns the String representation of the current value.
+     * @return the String representation of the current value.
+     */
+    public String toString() {
+        return Long.toString(get());
+    }
+
+
+    public int intValue() {
+	return (int)get();
+    }
+
+    public long longValue() {
+	return (long)get();
+    }
+
+    public float floatValue() {
+	return (float)get();
+    }
+
+    public double doubleValue() {
+	return (double)get();
+    }
+
+}
diff --git a/external/jsr166/java/util/concurrent/atomic/AtomicLongArray.java b/external/jsr166/java/util/concurrent/atomic/AtomicLongArray.java
new file mode 100644
index 000000000..c582cba54
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/atomic/AtomicLongArray.java
@@ -0,0 +1,255 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent.atomic;
+import sun.misc.Unsafe;
+import java.util.*;
+
+/**
+ * A <tt>long</tt> array in which elements may be updated atomically.
+ * See the {@link java.util.concurrent.atomic} package specification
+ * for description of the properties of atomic variables.
+ * @since 1.5
+ * @author Doug Lea
+ */
+public class AtomicLongArray implements java.io.Serializable {
+    private static final long serialVersionUID = -2308431214976778248L;
+
+    // setup to use Unsafe.compareAndSwapInt for updates
+    private static final Unsafe unsafe = Unsafe.getUnsafe();
+    private static final int base = unsafe.arrayBaseOffset(long[].class);
+    private static final int scale = unsafe.arrayIndexScale(long[].class);
+    private final long[] array;
+
+    private long rawIndex(int i) {
+        if (i < 0 || i >= array.length)
+            throw new IndexOutOfBoundsException("index " + i);
+        return base + i * scale;
+    }
+
+    /**
+     * Creates a new AtomicLongArray of given length.
+     *
+     * @param length the length of the array
+     */
+    public AtomicLongArray(int length) {
+        array = new long[length];
+        // must perform at least one volatile write to conform to JMM
+        if (length > 0)
+            unsafe.putLongVolatile(array, rawIndex(0), 0);
+    }
+
+    /**
+     * Creates a new AtomicLongArray with the same length as, and
+     * all elements copied from, the given array.
+     *
+     * @param array the array to copy elements from
+     * @throws NullPointerException if array is null
+     */
+    public AtomicLongArray(long[] array) {
+        if (array == null)
+            throw new NullPointerException();
+        int length = array.length;
+        this.array = new long[length];
+        if (length > 0) {
+            int last = length-1;
+            for (int i = 0; i < last; ++i)
+                this.array[i] = array[i];
+            // Do the last write as volatile
+            unsafe.putLongVolatile(this.array, rawIndex(last), array[last]);
+        }
+    }
+
+    /**
+     * Returns the length of the array.
+     *
+     * @return the length of the array
+     */
+    public final int length() {
+        return array.length;
+    }
+
+    /**
+     * Gets the current value at position <tt>i</tt>.
+     *
+     * @param i the index
+     * @return the current value
+     */
+    public final long get(int i) {
+        return unsafe.getLongVolatile(array, rawIndex(i));
+    }
+
+    /**
+     * Sets the element at position <tt>i</tt> to the given value.
+     *
+     * @param i the index
+     * @param newValue the new value
+     */
+    public final void set(int i, long newValue) {
+        unsafe.putLongVolatile(array, rawIndex(i), newValue);
+    }
+
+    /**
+     * Eventually sets the element at position <tt>i</tt> to the given value.
+     *
+     * @param i the index
+     * @param newValue the new value
+     * @since 1.6
+     */
+    public final void lazySet(int i, long newValue) {
+        unsafe.putOrderedLong(array, rawIndex(i), newValue);
+    }
+
+
+    /**
+     * Atomically sets the element at position <tt>i</tt> to the given value
+     * and returns the old value.
+     *
+     * @param i the index
+     * @param newValue the new value
+     * @return the previous value
+     */
+    public final long getAndSet(int i, long newValue) {
+        while (true) {
+            long current = get(i);
+            if (compareAndSet(i, current, newValue))
+                return current;
+        }
+    }
+
+    /**
+     * Atomically sets the value to the given updated value
+     * if the current value <tt>==</tt> the expected value.
+     *
+     * @param i the index
+     * @param expect the expected value
+     * @param update the new value
+     * @return true if successful. False return indicates that
+     * the actual value was not equal to the expected value.
+     */
+    public final boolean compareAndSet(int i, long expect, long update) {
+        return unsafe.compareAndSwapLong(array, rawIndex(i),
+                                         expect, update);
+    }
+
+    /**
+     * Atomically sets the value to the given updated value
+     * if the current value <tt>==</tt> the expected value.
+     * May fail spuriously and does not provide ordering guarantees,
+     * so is only rarely an appropriate alternative to <tt>compareAndSet</tt>.
+     *
+     * @param i the index
+     * @param expect the expected value
+     * @param update the new value
+     * @return true if successful.
+     */
+    public final boolean weakCompareAndSet(int i, long expect, long update) {
+        return compareAndSet(i, expect, update);
+    }
+
+    /**
+     * Atomically increments by one the element at index <tt>i</tt>.
+     *
+     * @param i the index
+     * @return the previous value
+     */
+    public final long getAndIncrement(int i) {
+        while (true) {
+            long current = get(i);
+            long next = current + 1;
+            if (compareAndSet(i, current, next))
+                return current;
+        }
+    }
+
+    /**
+     * Atomically decrements by one the element at index <tt>i</tt>.
+     *
+     * @param i the index
+     * @return the previous value
+     */
+    public final long getAndDecrement(int i) {
+        while (true) {
+            long current = get(i);
+            long next = current - 1;
+            if (compareAndSet(i, current, next))
+                return current;
+        }
+    }
+
+    /**
+     * Atomically adds the given value to the element at index <tt>i</tt>.
+     *
+     * @param i the index
+     * @param delta the value to add
+     * @return the previous value
+     */
+    public final long getAndAdd(int i, long delta) {
+        while (true) {
+            long current = get(i);
+            long next = current + delta;
+            if (compareAndSet(i, current, next))
+                return current;
+        }
+    }
+
+    /**
+     * Atomically increments by one the element at index <tt>i</tt>.
+     *
+     * @param i the index
+     * @return the updated value
+     */
+    public final long incrementAndGet(int i) {
+        while (true) {
+            long current = get(i);
+            long next = current + 1;
+            if (compareAndSet(i, current, next))
+                return next;
+        }
+    }
+
+    /**
+     * Atomically decrements by one the element at index <tt>i</tt>.
+     *
+     * @param i the index
+     * @return the updated value
+     */
+    public final long decrementAndGet(int i) {
+        while (true) {
+            long current = get(i);
+            long next = current - 1;
+            if (compareAndSet(i, current, next))
+                return next;
+        }
+    }
+
+    /**
+     * Atomically adds the given value to the element at index <tt>i</tt>.
+     *
+     * @param i the index
+     * @param delta the value to add
+     * @return the updated value
+     */
+    public long addAndGet(int i, long delta) {
+        while (true) {
+            long current = get(i);
+            long next = current + delta;
+            if (compareAndSet(i, current, next))
+                return next;
+        }
+    }
+
+    /**
+     * Returns the String representation of the current values of array.
+     * @return the String representation of the current values of array.
+     */
+    public String toString() {
+        if (array.length > 0) // force volatile read
+            get(0);
+        return Arrays.toString(array);
+    }
+
+}
diff --git a/external/jsr166/java/util/concurrent/atomic/AtomicLongFieldUpdater.java b/external/jsr166/java/util/concurrent/atomic/AtomicLongFieldUpdater.java
new file mode 100644
index 000000000..dafd08923
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/atomic/AtomicLongFieldUpdater.java
@@ -0,0 +1,406 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent.atomic;
+import sun.misc.Unsafe;
+import java.lang.reflect.*;
+
+/**
+ * A reflection-based utility that enables atomic updates to
+ * designated <tt>volatile long</tt> fields of designated classes.
+ * This class is designed for use in atomic data structures in which
+ * several fields of the same node are independently subject to atomic
+ * updates.
+ *
+ * <p>Note that the guarantees of the {@code compareAndSet}
+ * method in this class are weaker than in other atomic classes.
+ * Because this class cannot ensure that all uses of the field
+ * are appropriate for purposes of atomic access, it can
+ * guarantee atomicity only with respect to other invocations of
+ * {@code compareAndSet} and {@code set} on the same updater.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ * @param <T> The type of the object holding the updatable field
+ */
+public abstract class  AtomicLongFieldUpdater<T>  {
+    /**
+     * Creates and returns an updater for objects with the given field.
+     * The Class argument is needed to check that reflective types and
+     * generic types match.
+     *
+     * @param tclass the class of the objects holding the field
+     * @param fieldName the name of the field to be updated.
+     * @return the updater
+     * @throws IllegalArgumentException if the field is not a
+     * volatile long type.
+     * @throws RuntimeException with a nested reflection-based
+     * exception if the class does not hold field or is the wrong type.
+     */
+    public static <U> AtomicLongFieldUpdater<U> newUpdater(Class<U> tclass, String fieldName) {
+        if (AtomicLong.VM_SUPPORTS_LONG_CAS)
+            return new CASUpdater<U>(tclass, fieldName);
+        else
+            return new LockedUpdater<U>(tclass, fieldName);
+    }
+
+    /**
+     * Protected do-nothing constructor for use by subclasses.
+     */
+    protected AtomicLongFieldUpdater() {
+    }
+
+    /**
+     * Atomically sets the field of the given object managed by this updater
+     * to the given updated value if the current value <tt>==</tt> the
+     * expected value. This method is guaranteed to be atomic with respect to
+     * other calls to <tt>compareAndSet</tt> and <tt>set</tt>, but not
+     * necessarily with respect to other changes in the field.
+     *
+     * @param obj An object whose field to conditionally set
+     * @param expect the expected value
+     * @param update the new value
+     * @return true if successful.
+     * @throws ClassCastException if <tt>obj</tt> is not an instance
+     * of the class possessing the field established in the constructor.
+     */
+    public abstract boolean compareAndSet(T obj, long expect, long update);
+
+    /**
+     * Atomically sets the field of the given object managed by this updater
+     * to the given updated value if the current value <tt>==</tt> the
+     * expected value. This method is guaranteed to be atomic with respect to
+     * other calls to <tt>compareAndSet</tt> and <tt>set</tt>, but not
+     * necessarily with respect to other changes in the field.
+     * May fail spuriously and does not provide ordering guarantees,
+     * so is only rarely an appropriate alternative to <tt>compareAndSet</tt>.
+     *
+     * @param obj An object whose field to conditionally set
+     * @param expect the expected value
+     * @param update the new value
+     * @return true if successful.
+     * @throws ClassCastException if <tt>obj</tt> is not an instance
+     * of the class possessing the field established in the constructor.
+     */
+    public abstract boolean weakCompareAndSet(T obj, long expect, long update);
+
+    /**
+     * Sets the field of the given object managed by this updater to the
+     * given updated value. This operation is guaranteed to act as a volatile
+     * store with respect to subsequent invocations of
+     * <tt>compareAndSet</tt>.
+     *
+     * @param obj An object whose field to set
+     * @param newValue the new value
+     */
+    public abstract void set(T obj, long newValue);
+
+    /**
+     * Eventually sets the field of the given object managed by this
+     * updater to the given updated value.
+     *
+     * @param obj An object whose field to set
+     * @param newValue the new value
+     * @since 1.6
+     */
+    public abstract void lazySet(T obj, long newValue);
+
+    /**
+     * Gets the current value held in the field of the given object managed
+     * by this updater.
+     *
+     * @param obj An object whose field to get
+     * @return the current value
+     */
+    public abstract long get(T obj);
+
+    /**
+     * Atomically sets the field of the given object managed by this updater
+     * to the given value and returns the old value.
+     *
+     * @param obj An object whose field to get and set
+     * @param newValue the new value
+     * @return the previous value
+     */
+    public long getAndSet(T obj, long newValue) {
+        for (;;) {
+            long current = get(obj);
+            if (compareAndSet(obj, current, newValue))
+                return current;
+        }
+    }
+
+    /**
+     * Atomically increments by one the current value of the field of the
+     * given object managed by this updater.
+     *
+     * @param obj An object whose field to get and set
+     * @return the previous value
+     */
+    public long getAndIncrement(T obj) {
+        for (;;) {
+            long current = get(obj);
+            long next = current + 1;
+            if (compareAndSet(obj, current, next))
+                return current;
+        }
+    }
+
+    /**
+     * Atomically decrements by one the current value of the field of the
+     * given object managed by this updater.
+     *
+     * @param obj An object whose field to get and set
+     * @return the previous value
+     */
+    public long getAndDecrement(T obj) {
+        for (;;) {
+            long current = get(obj);
+            long next = current - 1;
+            if (compareAndSet(obj, current, next))
+                return current;
+        }
+    }
+
+    /**
+     * Atomically adds the given value to the current value of the field of
+     * the given object managed by this updater.
+     *
+     * @param obj An object whose field to get and set
+     * @param delta the value to add
+     * @return the previous value
+     */
+    public long getAndAdd(T obj, long delta) {
+        for (;;) {
+            long current = get(obj);
+            long next = current + delta;
+            if (compareAndSet(obj, current, next))
+                return current;
+        }
+    }
+
+    /**
+     * Atomically increments by one the current value of the field of the
+     * given object managed by this updater.
+     *
+     * @param obj An object whose field to get and set
+     * @return the updated value
+     */
+    public long incrementAndGet(T obj) {
+        for (;;) {
+            long current = get(obj);
+            long next = current + 1;
+            if (compareAndSet(obj, current, next))
+                return next;
+        }
+    }
+
+    /**
+     * Atomically decrements by one the current value of the field of the
+     * given object managed by this updater.
+     *
+     * @param obj An object whose field to get and set
+     * @return the updated value
+     */
+    public long decrementAndGet(T obj) {
+        for (;;) {
+            long current = get(obj);
+            long next = current - 1;
+            if (compareAndSet(obj, current, next))
+                return next;
+        }
+    }
+
+    /**
+     * Atomically adds the given value to the current value of the field of
+     * the given object managed by this updater.
+     *
+     * @param obj An object whose field to get and set
+     * @param delta the value to add
+     * @return the updated value
+     */
+    public long addAndGet(T obj, long delta) {
+        for (;;) {
+            long current = get(obj);
+            long next = current + delta;
+            if (compareAndSet(obj, current, next))
+                return next;
+        }
+    }
+
+    private static class CASUpdater<T> extends AtomicLongFieldUpdater<T> {
+        private static final Unsafe unsafe = Unsafe.getUnsafe();
+        private final long offset;
+        private final Class<T> tclass;
+        private final Class cclass;
+
+        CASUpdater(Class<T> tclass, String fieldName) {
+            Field field = null;
+	    Class caller = null;
+	    int modifiers = 0;
+            try {
+                field = tclass.getDeclaredField(fieldName);
+		caller = sun.reflect.Reflection.getCallerClass(3);
+		modifiers = field.getModifiers();
+                sun.reflect.misc.ReflectUtil.ensureMemberAccess(
+                    caller, tclass, null, modifiers); 
+		sun.reflect.misc.ReflectUtil.checkPackageAccess(tclass);
+            } catch(Exception ex) {
+                throw new RuntimeException(ex);
+            }
+
+            Class fieldt = field.getType();
+            if (fieldt != long.class)
+                throw new IllegalArgumentException("Must be long type");
+
+            if (!Modifier.isVolatile(modifiers))
+                throw new IllegalArgumentException("Must be volatile type");
+
+	    this.cclass = (Modifier.isProtected(modifiers) &&
+			   caller != tclass) ? caller : null;
+            this.tclass = tclass;
+            offset = unsafe.objectFieldOffset(field);
+        }
+
+        private void fullCheck(T obj) {
+            if (!tclass.isInstance(obj))
+                throw new ClassCastException();
+	    if (cclass != null)
+	        ensureProtectedAccess(obj);
+	}
+
+        public boolean compareAndSet(T obj, long expect, long update) {
+            if (obj == null || obj.getClass() != tclass || cclass != null) fullCheck(obj);
+            return unsafe.compareAndSwapLong(obj, offset, expect, update);
+        }
+
+        public boolean weakCompareAndSet(T obj, long expect, long update) {
+            if (obj == null || obj.getClass() != tclass || cclass != null) fullCheck(obj);
+            return unsafe.compareAndSwapLong(obj, offset, expect, update);
+        }
+
+        public void set(T obj, long newValue) {
+            if (obj == null || obj.getClass() != tclass || cclass != null) fullCheck(obj);
+            unsafe.putLongVolatile(obj, offset, newValue);
+        }
+
+        public void lazySet(T obj, long newValue) {
+            if (obj == null || obj.getClass() != tclass || cclass != null) fullCheck(obj);
+            unsafe.putOrderedLong(obj, offset, newValue);
+        }
+
+        public long get(T obj) {
+            if (obj == null || obj.getClass() != tclass || cclass != null) fullCheck(obj);
+            return unsafe.getLongVolatile(obj, offset);
+        }
+
+	private void ensureProtectedAccess(T obj) {
+	    if (cclass.isInstance(obj)) {
+		return;
+	    }
+	    throw new RuntimeException (
+                new IllegalAccessException("Class " +
+		    cclass.getName() +
+                    " can not access a protected member of class " +
+                    tclass.getName() +
+		    " using an instance of " +
+                    obj.getClass().getName()
+		)
+	    );
+	}
+    }
+
+
+    private static class LockedUpdater<T> extends AtomicLongFieldUpdater<T> {
+        private static final Unsafe unsafe = Unsafe.getUnsafe();
+        private final long offset;
+        private final Class<T> tclass;
+        private final Class cclass;
+
+        LockedUpdater(Class<T> tclass, String fieldName) {
+            Field field = null;
+	    Class caller = null;
+	    int modifiers = 0;
+            try {
+                field = tclass.getDeclaredField(fieldName);
+		caller = sun.reflect.Reflection.getCallerClass(3);
+		modifiers = field.getModifiers();
+                sun.reflect.misc.ReflectUtil.ensureMemberAccess(
+                    caller, tclass, null, modifiers); 
+		sun.reflect.misc.ReflectUtil.checkPackageAccess(tclass);
+            } catch(Exception ex) {
+                throw new RuntimeException(ex);
+            }
+
+            Class fieldt = field.getType();
+            if (fieldt != long.class)
+                throw new IllegalArgumentException("Must be long type");
+
+            if (!Modifier.isVolatile(modifiers))
+                throw new IllegalArgumentException("Must be volatile type");
+
+	    this.cclass = (Modifier.isProtected(modifiers) &&
+			   caller != tclass) ? caller : null;
+            this.tclass = tclass;
+            offset = unsafe.objectFieldOffset(field);
+        }
+
+        private void fullCheck(T obj) {
+            if (!tclass.isInstance(obj))
+                throw new ClassCastException();
+	    if (cclass != null)
+	        ensureProtectedAccess(obj);
+	}
+
+        public boolean compareAndSet(T obj, long expect, long update) {
+            if (obj == null || obj.getClass() != tclass || cclass != null) fullCheck(obj);
+            synchronized(this) {
+                long v = unsafe.getLong(obj, offset);
+                if (v != expect)
+                    return false;
+                unsafe.putLong(obj, offset, update);
+                return true;
+            }
+        }
+
+        public boolean weakCompareAndSet(T obj, long expect, long update) {
+            return compareAndSet(obj, expect, update);
+        }
+
+        public void set(T obj, long newValue) {
+            if (obj == null || obj.getClass() != tclass || cclass != null) fullCheck(obj);
+            synchronized(this) {
+                unsafe.putLong(obj, offset, newValue);
+            }
+        }
+
+        public void lazySet(T obj, long newValue) {
+	    set(obj, newValue);
+        }
+
+        public long get(T obj) {
+            if (obj == null || obj.getClass() != tclass || cclass != null) fullCheck(obj);
+            synchronized(this) {
+                return unsafe.getLong(obj, offset);
+            }
+        }
+
+	private void ensureProtectedAccess(T obj) {
+	    if (cclass.isInstance(obj)) {
+		return;
+	    }
+	    throw new RuntimeException (
+                new IllegalAccessException("Class " +
+		    cclass.getName() +
+                    " can not access a protected member of class " +
+                    tclass.getName() +
+		    " using an instance of " +
+                    obj.getClass().getName()
+		)
+	    );
+	}
+    }
+}
diff --git a/external/jsr166/java/util/concurrent/atomic/AtomicMarkableReference.java b/external/jsr166/java/util/concurrent/atomic/AtomicMarkableReference.java
new file mode 100644
index 000000000..85335b737
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/atomic/AtomicMarkableReference.java
@@ -0,0 +1,161 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent.atomic;
+
+/**
+ * An <tt>AtomicMarkableReference</tt> maintains an object reference
+ * along with a mark bit, that can be updated atomically.
+ * <p>
+ * <p> Implementation note. This implementation maintains markable
+ * references by creating internal objects representing "boxed"
+ * [reference, boolean] pairs.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ * @param <V> The type of object referred to by this reference
+ */
+public class AtomicMarkableReference<V>  {
+
+    private static class ReferenceBooleanPair<T> {
+        private final T reference;
+        private final boolean bit;
+        ReferenceBooleanPair(T r, boolean i) {
+            reference = r; bit = i;
+        }
+    }
+
+    private final AtomicReference<ReferenceBooleanPair<V>>  atomicRef;
+
+    /**
+     * Creates a new <tt>AtomicMarkableReference</tt> with the given
+     * initial values.
+     *
+     * @param initialRef the initial reference
+     * @param initialMark the initial mark
+     */
+    public AtomicMarkableReference(V initialRef, boolean initialMark) {
+        atomicRef = new AtomicReference<ReferenceBooleanPair<V>> (new ReferenceBooleanPair<V>(initialRef, initialMark));
+    }
+
+    /**
+     * Returns the current value of the reference.
+     *
+     * @return the current value of the reference
+     */
+    public V getReference() {
+        return atomicRef.get().reference;
+    }
+
+    /**
+     * Returns the current value of the mark.
+     *
+     * @return the current value of the mark
+     */
+    public boolean isMarked() {
+        return atomicRef.get().bit;
+    }
+
+    /**
+     * Returns the current values of both the reference and the mark.
+     * Typical usage is <tt>boolean[1] holder; ref = v.get(holder); </tt>.
+     *
+     * @param markHolder an array of size of at least one. On return,
+     * <tt>markholder[0]</tt> will hold the value of the mark.
+     * @return the current value of the reference
+     */
+    public V get(boolean[] markHolder) {
+        ReferenceBooleanPair<V> p = atomicRef.get();
+        markHolder[0] = p.bit;
+        return p.reference;
+    }
+
+    /**
+     * Atomically sets the value of both the reference and mark
+     * to the given update values if the
+     * current reference is <tt>==</tt> to the expected reference
+     * and the current mark is equal to the expected mark.
+     * May fail spuriously and does not provide ordering guarantees,
+     * so is only rarely an appropriate alternative to <tt>compareAndSet</tt>.
+     *
+     * @param expectedReference the expected value of the reference
+     * @param newReference the new value for the reference
+     * @param expectedMark the expected value of the mark
+     * @param newMark the new value for the mark
+     * @return true if successful
+     */
+    public boolean weakCompareAndSet(V       expectedReference,
+                                     V       newReference,
+                                     boolean expectedMark,
+                                     boolean newMark) {
+        ReferenceBooleanPair<V> current = atomicRef.get();
+        return  expectedReference == current.reference &&
+            expectedMark == current.bit &&
+            ((newReference == current.reference && newMark == current.bit) ||
+             atomicRef.weakCompareAndSet(current,
+                                     new ReferenceBooleanPair<V>(newReference,
+                                                              newMark)));
+    }
+
+    /**
+     * Atomically sets the value of both the reference and mark
+     * to the given update values if the
+     * current reference is <tt>==</tt> to the expected reference
+     * and the current mark is equal to the expected mark.
+     *
+     * @param expectedReference the expected value of the reference
+     * @param newReference the new value for the reference
+     * @param expectedMark the expected value of the mark
+     * @param newMark the new value for the mark
+     * @return true if successful
+     */
+    public boolean compareAndSet(V       expectedReference,
+                                 V       newReference,
+                                 boolean expectedMark,
+                                 boolean newMark) {
+        ReferenceBooleanPair<V> current = atomicRef.get();
+        return  expectedReference == current.reference &&
+            expectedMark == current.bit &&
+            ((newReference == current.reference && newMark == current.bit) ||
+             atomicRef.compareAndSet(current,
+                                     new ReferenceBooleanPair<V>(newReference,
+                                                              newMark)));
+    }
+
+    /**
+     * Unconditionally sets the value of both the reference and mark.
+     *
+     * @param newReference the new value for the reference
+     * @param newMark the new value for the mark
+     */
+    public void set(V newReference, boolean newMark) {
+        ReferenceBooleanPair<V> current = atomicRef.get();
+        if (newReference != current.reference || newMark != current.bit)
+            atomicRef.set(new ReferenceBooleanPair<V>(newReference, newMark));
+    }
+
+    /**
+     * Atomically sets the value of the mark to the given update value
+     * if the current reference is <tt>==</tt> to the expected
+     * reference.  Any given invocation of this operation may fail
+     * (return <tt>false</tt>) spuriously, but repeated invocation
+     * when the current value holds the expected value and no other
+     * thread is also attempting to set the value will eventually
+     * succeed.
+     *
+     * @param expectedReference the expected value of the reference
+     * @param newMark the new value for the mark
+     * @return true if successful
+     */
+    public boolean attemptMark(V expectedReference, boolean newMark) {
+        ReferenceBooleanPair<V> current = atomicRef.get();
+        return  expectedReference == current.reference &&
+            (newMark == current.bit ||
+             atomicRef.compareAndSet
+             (current, new ReferenceBooleanPair<V>(expectedReference,
+                                                newMark)));
+    }
+}
diff --git a/external/jsr166/java/util/concurrent/atomic/AtomicReference.java b/external/jsr166/java/util/concurrent/atomic/AtomicReference.java
new file mode 100644
index 000000000..e7c989c2b
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/atomic/AtomicReference.java
@@ -0,0 +1,124 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent.atomic;
+import sun.misc.Unsafe;
+
+/**
+ * An object reference that may be updated atomically. See the {@link
+ * java.util.concurrent.atomic} package specification for description
+ * of the properties of atomic variables.
+ * @since 1.5
+ * @author Doug Lea
+ * @param <V> The type of object referred to by this reference
+ */
+public class AtomicReference<V>  implements java.io.Serializable {
+    private static final long serialVersionUID = -1848883965231344442L;
+
+    private static final Unsafe unsafe = Unsafe.getUnsafe();
+    private static final long valueOffset;
+
+    static {
+      try {
+        valueOffset = unsafe.objectFieldOffset
+            (AtomicReference.class.getDeclaredField("value"));
+      } catch (Exception ex) { throw new Error(ex); }
+    }
+
+    private volatile V value;
+
+    /**
+     * Creates a new AtomicReference with the given initial value.
+     *
+     * @param initialValue the initial value
+     */
+    public AtomicReference(V initialValue) {
+        value = initialValue;
+    }
+
+    /**
+     * Creates a new AtomicReference with null initial value.
+     */
+    public AtomicReference() {
+    }
+
+    /**
+     * Gets the current value.
+     *
+     * @return the current value
+     */
+    public final V get() {
+        return value;
+    }
+
+    /**
+     * Sets to the given value.
+     *
+     * @param newValue the new value
+     */
+    public final void set(V newValue) {
+        value = newValue;
+    }
+
+    /**
+     * Eventually sets to the given value.
+     *
+     * @param newValue the new value
+     * @since 1.6
+     */
+    public final void lazySet(V newValue) {
+        unsafe.putOrderedObject(this, valueOffset, newValue);
+    }
+
+    /**
+     * Atomically sets the value to the given updated value
+     * if the current value <tt>==</tt> the expected value.
+     * @param expect the expected value
+     * @param update the new value
+     * @return true if successful. False return indicates that
+     * the actual value was not equal to the expected value.
+     */
+    public final boolean compareAndSet(V expect, V update) {
+        return unsafe.compareAndSwapObject(this, valueOffset, expect, update);
+    }
+
+    /**
+     * Atomically sets the value to the given updated value
+     * if the current value <tt>==</tt> the expected value.
+     * May fail spuriously and does not provide ordering guarantees,
+     * so is only rarely an appropriate alternative to <tt>compareAndSet</tt>.
+     *
+     * @param expect the expected value
+     * @param update the new value
+     * @return true if successful.
+     */
+    public final boolean weakCompareAndSet(V expect, V update) {
+        return unsafe.compareAndSwapObject(this, valueOffset, expect, update);
+    }
+
+    /**
+     * Atomically sets to the given value and returns the old value.
+     *
+     * @param newValue the new value
+     * @return the previous value
+     */
+    public final V getAndSet(V newValue) {
+        while (true) {
+            V x = get();
+            if (compareAndSet(x, newValue))
+                return x;
+        }
+    }
+
+    /**
+     * Returns the String representation of the current value.
+     * @return the String representation of the current value.
+     */
+    public String toString() {
+        return String.valueOf(get());
+    }
+
+}
diff --git a/external/jsr166/java/util/concurrent/atomic/AtomicReferenceArray.java b/external/jsr166/java/util/concurrent/atomic/AtomicReferenceArray.java
new file mode 100644
index 000000000..91b601ed9
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/atomic/AtomicReferenceArray.java
@@ -0,0 +1,163 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent.atomic;
+import sun.misc.Unsafe;
+import java.util.*;
+
+/**
+ * An array of object references in which elements may be updated
+ * atomically.  See the {@link java.util.concurrent.atomic} package
+ * specification for description of the properties of atomic
+ * variables.
+ * @since 1.5
+ * @author Doug Lea
+ * @param <E> The base class of elements held in this array
+ */
+public class AtomicReferenceArray<E> implements java.io.Serializable {
+    private static final long serialVersionUID = -6209656149925076980L;
+
+    private static final Unsafe unsafe = Unsafe.getUnsafe();
+    private static final int base = unsafe.arrayBaseOffset(Object[].class);
+    private static final int scale = unsafe.arrayIndexScale(Object[].class);
+    private final Object[] array;
+
+    private long rawIndex(int i) {
+        if (i < 0 || i >= array.length)
+            throw new IndexOutOfBoundsException("index " + i);
+        return base + i * scale;
+    }
+
+    /**
+     * Creates a new AtomicReferenceArray of given length.
+     * @param length the length of the array
+     */
+    public AtomicReferenceArray(int length) {
+        array = new Object[length];
+        // must perform at least one volatile write to conform to JMM
+        if (length > 0)
+            unsafe.putObjectVolatile(array, rawIndex(0), null);
+    }
+
+    /**
+     * Creates a new AtomicReferenceArray with the same length as, and
+     * all elements copied from, the given array.
+     *
+     * @param array the array to copy elements from
+     * @throws NullPointerException if array is null
+     */
+    public AtomicReferenceArray(E[] array) {
+        if (array == null)
+            throw new NullPointerException();
+        int length = array.length;
+        this.array = new Object[length];
+        if (length > 0) {
+            int last = length-1;
+            for (int i = 0; i < last; ++i)
+                this.array[i] = array[i];
+            // Do the last write as volatile
+            E e = array[last];
+            unsafe.putObjectVolatile(this.array, rawIndex(last), e);
+        }
+    }
+
+    /**
+     * Returns the length of the array.
+     *
+     * @return the length of the array
+     */
+    public final int length() {
+        return array.length;
+    }
+
+    /**
+     * Gets the current value at position <tt>i</tt>.
+     *
+     * @param i the index
+     * @return the current value
+     */
+    public final E get(int i) {
+        return (E) unsafe.getObjectVolatile(array, rawIndex(i));
+    }
+
+    /**
+     * Sets the element at position <tt>i</tt> to the given value.
+     *
+     * @param i the index
+     * @param newValue the new value
+     */
+    public final void set(int i, E newValue) {
+        unsafe.putObjectVolatile(array, rawIndex(i), newValue);
+    }
+
+    /**
+     * Eventually sets the element at position <tt>i</tt> to the given value.
+     *
+     * @param i the index
+     * @param newValue the new value
+     * @since 1.6
+     */
+    public final void lazySet(int i, E newValue) {
+        unsafe.putOrderedObject(array, rawIndex(i), newValue);
+    }
+
+
+    /**
+     * Atomically sets the element at position <tt>i</tt> to the given
+     * value and returns the old value.
+     *
+     * @param i the index
+     * @param newValue the new value
+     * @return the previous value
+     */
+    public final E getAndSet(int i, E newValue) {
+        while (true) {
+            E current = get(i);
+            if (compareAndSet(i, current, newValue))
+                return current;
+        }
+    }
+
+    /**
+     * Atomically sets the element at position <tt>i</tt> to the given
+     * updated value if the current value <tt>==</tt> the expected value.
+     * @param i the index
+     * @param expect the expected value
+     * @param update the new value
+     * @return true if successful. False return indicates that
+     * the actual value was not equal to the expected value.
+     */
+    public final boolean compareAndSet(int i, E expect, E update) {
+        return unsafe.compareAndSwapObject(array, rawIndex(i),
+                                         expect, update);
+    }
+
+    /**
+     * Atomically sets the element at position <tt>i</tt> to the given
+     * updated value if the current value <tt>==</tt> the expected value.
+     * May fail spuriously and does not provide ordering guarantees,
+     * so is only rarely an appropriate alternative to <tt>compareAndSet</tt>.
+     *
+     * @param i the index
+     * @param expect the expected value
+     * @param update the new value
+     * @return true if successful.
+     */
+    public final boolean weakCompareAndSet(int i, E expect, E update) {
+        return compareAndSet(i, expect, update);
+    }
+
+    /**
+     * Returns the String representation of the current values of array.
+     * @return the String representation of the current values of array.
+     */
+    public String toString() {
+        if (array.length > 0) // force volatile read
+            get(0);
+        return Arrays.toString(array);
+    }
+
+}
diff --git a/external/jsr166/java/util/concurrent/atomic/AtomicReferenceFieldUpdater.java b/external/jsr166/java/util/concurrent/atomic/AtomicReferenceFieldUpdater.java
new file mode 100644
index 000000000..5c18eca33
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/atomic/AtomicReferenceFieldUpdater.java
@@ -0,0 +1,275 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent.atomic;
+import sun.misc.Unsafe;
+import java.lang.reflect.*;
+
+/**
+ * A reflection-based utility that enables atomic updates to
+ * designated <tt>volatile</tt> reference fields of designated
+ * classes.  This class is designed for use in atomic data structures
+ * in which several reference fields of the same node are
+ * independently subject to atomic updates. For example, a tree node
+ * might be declared as
+ *
+ * <pre>
+ * class Node {
+ *   private volatile Node left, right;
+ *
+ *   private static final AtomicReferenceFieldUpdater&lt;Node, Node&gt; leftUpdater =
+ *     AtomicReferenceFieldUpdater.newUpdater(Node.class, Node.class, "left");
+ *   private static AtomicReferenceFieldUpdater&lt;Node, Node&gt; rightUpdater =
+ *     AtomicReferenceFieldUpdater.newUpdater(Node.class, Node.class, "right");
+ *
+ *   Node getLeft() { return left;  }
+ *   boolean compareAndSetLeft(Node expect, Node update) {
+ *     return leftUpdater.compareAndSet(this, expect, update);
+ *   }
+ *   // ... and so on
+ * }
+ * </pre>
+ *
+ * <p>Note that the guarantees of the {@code compareAndSet}
+ * method in this class are weaker than in other atomic classes.
+ * Because this class cannot ensure that all uses of the field
+ * are appropriate for purposes of atomic access, it can
+ * guarantee atomicity only with respect to other invocations of
+ * {@code compareAndSet} and {@code set} on the same updater.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ * @param <T> The type of the object holding the updatable field
+ * @param <V> The type of the field
+ */
+public abstract class AtomicReferenceFieldUpdater<T, V>  {
+
+    /**
+     * Creates and returns an updater for objects with the given field.
+     * The Class arguments are needed to check that reflective types and
+     * generic types match.
+     *
+     * @param tclass the class of the objects holding the field.
+     * @param vclass the class of the field
+     * @param fieldName the name of the field to be updated.
+     * @return the updater
+     * @throws IllegalArgumentException if the field is not a volatile reference type.
+     * @throws RuntimeException with a nested reflection-based
+     * exception if the class does not hold field or is the wrong type.
+     */
+    public static <U, W> AtomicReferenceFieldUpdater<U,W> newUpdater(Class<U> tclass, Class<W> vclass, String fieldName) {
+        return new AtomicReferenceFieldUpdaterImpl<U,W>(tclass,
+                                                        vclass,
+                                                        fieldName);
+    }
+
+    /**
+     * Protected do-nothing constructor for use by subclasses.
+     */
+    protected AtomicReferenceFieldUpdater() {
+    }
+
+    /**
+     * Atomically sets the field of the given object managed by this updater
+     * to the given updated value if the current value <tt>==</tt> the
+     * expected value. This method is guaranteed to be atomic with respect to
+     * other calls to <tt>compareAndSet</tt> and <tt>set</tt>, but not
+     * necessarily with respect to other changes in the field.
+     *
+     * @param obj An object whose field to conditionally set
+     * @param expect the expected value
+     * @param update the new value
+     * @return true if successful.
+     */
+    public abstract boolean compareAndSet(T obj, V expect, V update);
+
+    /**
+     * Atomically sets the field of the given object managed by this updater
+     * to the given updated value if the current value <tt>==</tt> the
+     * expected value. This method is guaranteed to be atomic with respect to
+     * other calls to <tt>compareAndSet</tt> and <tt>set</tt>, but not
+     * necessarily with respect to other changes in the field.
+     * May fail spuriously and does not provide ordering guarantees,
+     * so is only rarely an appropriate alternative to <tt>compareAndSet</tt>.
+     *
+     * @param obj An object whose field to conditionally set
+     * @param expect the expected value
+     * @param update the new value
+     * @return true if successful.
+     */
+    public abstract boolean weakCompareAndSet(T obj, V expect, V update);
+
+    /**
+     * Sets the field of the given object managed by this updater to the
+     * given updated value. This operation is guaranteed to act as a volatile
+     * store with respect to subsequent invocations of
+     * <tt>compareAndSet</tt>.
+     *
+     * @param obj An object whose field to set
+     * @param newValue the new value
+     */
+    public abstract void set(T obj, V newValue);
+
+    /**
+     * Eventually sets the field of the given object managed by this
+     * updater to the given updated value.
+     *
+     * @param obj An object whose field to set
+     * @param newValue the new value
+     * @since 1.6
+     */
+    public abstract void lazySet(T obj, V newValue);
+
+    /**
+     * Gets the current value held in the field of the given object managed
+     * by this updater.
+     *
+     * @param obj An object whose field to get
+     * @return the current value
+     */
+    public abstract V get(T obj);
+
+    /**
+     * Atomically sets the field of the given object managed by this updater
+     * to the given value and returns the old value.
+     *
+     * @param obj An object whose field to get and set
+     * @param newValue the new value
+     * @return the previous value
+     */
+    public V getAndSet(T obj, V newValue) {
+        for (;;) {
+            V current = get(obj);
+            if (compareAndSet(obj, current, newValue))
+                return current;
+        }
+    }
+
+    private static final class AtomicReferenceFieldUpdaterImpl<T,V>
+	extends AtomicReferenceFieldUpdater<T,V> {
+        private static final Unsafe unsafe = Unsafe.getUnsafe();
+        private final long offset;
+        private final Class<T> tclass;
+        private final Class<V> vclass;
+        private final Class cclass;
+
+        /*
+         * Internal type checks within all update methods contain
+         * internal inlined optimizations checking for the common
+         * cases where the class is final (in which case a simple
+         * getClass comparison suffices) or is of type Object (in
+         * which case no check is needed because all objects are
+         * instances of Object). The Object case is handled simply by
+         * setting vclass to null in constructor.  The targetCheck and
+         * updateCheck methods are invoked when these faster
+         * screenings fail.
+         */
+
+        AtomicReferenceFieldUpdaterImpl(Class<T> tclass,
+					Class<V> vclass,
+					String fieldName) {
+            Field field = null;
+            Class fieldClass = null;
+	    Class caller = null;
+	    int modifiers = 0;
+            try {
+                field = tclass.getDeclaredField(fieldName);
+		caller = sun.reflect.Reflection.getCallerClass(3);
+		modifiers = field.getModifiers();
+                sun.reflect.misc.ReflectUtil.ensureMemberAccess(
+                    caller, tclass, null, modifiers); 
+		sun.reflect.misc.ReflectUtil.checkPackageAccess(tclass);
+                fieldClass = field.getType();
+            } catch (Exception ex) {
+                throw new RuntimeException(ex);
+            }
+
+            if (vclass != fieldClass)
+                throw new ClassCastException();
+            
+            if (!Modifier.isVolatile(modifiers))
+                throw new IllegalArgumentException("Must be volatile type");
+
+	    this.cclass = (Modifier.isProtected(modifiers) &&
+			   caller != tclass) ? caller : null;
+            this.tclass = tclass;
+            if (vclass == Object.class)
+                this.vclass = null;
+            else
+                this.vclass = vclass;
+            offset = unsafe.objectFieldOffset(field);
+        }
+
+        void targetCheck(T obj) {
+            if (!tclass.isInstance(obj))
+                throw new ClassCastException();
+	    if (cclass != null)
+		ensureProtectedAccess(obj);
+        }
+
+        void updateCheck(T obj, V update) {
+            if (!tclass.isInstance(obj) ||
+                (update != null && vclass != null && !vclass.isInstance(update)))
+                throw new ClassCastException();
+	    if (cclass != null)
+		ensureProtectedAccess(obj);
+        }
+
+        public boolean compareAndSet(T obj, V expect, V update) {
+            if (obj == null || obj.getClass() != tclass || cclass != null ||
+                (update != null && vclass != null &&
+                 vclass != update.getClass()))
+                updateCheck(obj, update);
+            return unsafe.compareAndSwapObject(obj, offset, expect, update);
+        }
+
+        public boolean weakCompareAndSet(T obj, V expect, V update) {
+            // same implementation as strong form for now
+            if (obj == null || obj.getClass() != tclass || cclass != null ||
+                (update != null && vclass != null &&
+                 vclass != update.getClass()))
+                updateCheck(obj, update);
+            return unsafe.compareAndSwapObject(obj, offset, expect, update);
+        }
+
+        public void set(T obj, V newValue) {
+            if (obj == null || obj.getClass() != tclass || cclass != null ||
+                (newValue != null && vclass != null &&
+                 vclass != newValue.getClass()))
+                updateCheck(obj, newValue);
+            unsafe.putObjectVolatile(obj, offset, newValue);
+        }
+
+        public void lazySet(T obj, V newValue) {
+            if (obj == null || obj.getClass() != tclass || cclass != null ||
+                (newValue != null && vclass != null &&
+                 vclass != newValue.getClass()))
+                updateCheck(obj, newValue);
+            unsafe.putOrderedObject(obj, offset, newValue);
+        }
+
+        public V get(T obj) {
+            if (obj == null || obj.getClass() != tclass || cclass != null)
+                targetCheck(obj);
+            return (V)unsafe.getObjectVolatile(obj, offset);
+        }
+
+	private void ensureProtectedAccess(T obj) {
+	    if (cclass.isInstance(obj)) {
+		return;
+	    }
+	    throw new RuntimeException (
+                new IllegalAccessException("Class " +
+		    cclass.getName() +
+                    " can not access a protected member of class " +
+                    tclass.getName() +
+		    " using an instance of " +
+                    obj.getClass().getName()
+		)
+	    );
+	}
+    }
+}
diff --git a/external/jsr166/java/util/concurrent/atomic/AtomicStampedReference.java b/external/jsr166/java/util/concurrent/atomic/AtomicStampedReference.java
new file mode 100644
index 000000000..558808216
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/atomic/AtomicStampedReference.java
@@ -0,0 +1,165 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent.atomic;
+
+/**
+ * An <tt>AtomicStampedReference</tt> maintains an object reference
+ * along with an integer "stamp", that can be updated atomically.
+ *
+ * <p> Implementation note. This implementation maintains stamped
+ * references by creating internal objects representing "boxed"
+ * [reference, integer] pairs.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ * @param <V> The type of object referred to by this reference
+ */
+public class AtomicStampedReference<V>  {
+
+    private static class ReferenceIntegerPair<T> {
+        private final T reference;
+        private final int integer;
+        ReferenceIntegerPair(T r, int i) {
+            reference = r; integer = i;
+        }
+    }
+
+    private final AtomicReference<ReferenceIntegerPair<V>>  atomicRef;
+
+    /**
+     * Creates a new <tt>AtomicStampedReference</tt> with the given
+     * initial values.
+     *
+     * @param initialRef the initial reference
+     * @param initialStamp the initial stamp
+     */
+    public AtomicStampedReference(V initialRef, int initialStamp) {
+        atomicRef = new AtomicReference<ReferenceIntegerPair<V>>
+            (new ReferenceIntegerPair<V>(initialRef, initialStamp));
+    }
+
+    /**
+     * Returns the current value of the reference.
+     *
+     * @return the current value of the reference
+     */
+    public V getReference() {
+        return atomicRef.get().reference;
+    }
+
+    /**
+     * Returns the current value of the stamp.
+     *
+     * @return the current value of the stamp
+     */
+    public int getStamp() {
+        return atomicRef.get().integer;
+    }
+
+    /**
+     * Returns the current values of both the reference and the stamp.
+     * Typical usage is <tt>int[1] holder; ref = v.get(holder); </tt>.
+     *
+     * @param stampHolder an array of size of at least one.  On return,
+     * <tt>stampholder[0]</tt> will hold the value of the stamp.
+     * @return the current value of the reference
+     */
+    public V get(int[] stampHolder) {
+        ReferenceIntegerPair<V> p = atomicRef.get();
+        stampHolder[0] = p.integer;
+        return p.reference;
+    }
+
+    /**
+     * Atomically sets the value of both the reference and stamp
+     * to the given update values if the
+     * current reference is <tt>==</tt> to the expected reference
+     * and the current stamp is equal to the expected stamp.
+     * May fail spuriously and does not provide ordering guarantees,
+     * so is only rarely an appropriate alternative to <tt>compareAndSet</tt>.
+     *
+     * @param expectedReference the expected value of the reference
+     * @param newReference the new value for the reference
+     * @param expectedStamp the expected value of the stamp
+     * @param newStamp the new value for the stamp
+     * @return true if successful
+     */
+    public boolean weakCompareAndSet(V      expectedReference,
+                                     V      newReference,
+                                     int    expectedStamp,
+                                     int    newStamp) {
+        ReferenceIntegerPair<V> current = atomicRef.get();
+        return  expectedReference == current.reference &&
+            expectedStamp == current.integer &&
+            ((newReference == current.reference &&
+              newStamp == current.integer) ||
+             atomicRef.weakCompareAndSet(current,
+                                     new ReferenceIntegerPair<V>(newReference,
+                                                              newStamp)));
+    }
+
+    /**
+     * Atomically sets the value of both the reference and stamp
+     * to the given update values if the
+     * current reference is <tt>==</tt> to the expected reference
+     * and the current stamp is equal to the expected stamp.
+     *
+     * @param expectedReference the expected value of the reference
+     * @param newReference the new value for the reference
+     * @param expectedStamp the expected value of the stamp
+     * @param newStamp the new value for the stamp
+     * @return true if successful
+     */
+    public boolean compareAndSet(V      expectedReference,
+                                 V      newReference,
+                                 int    expectedStamp,
+                                 int    newStamp) {
+        ReferenceIntegerPair<V> current = atomicRef.get();
+        return  expectedReference == current.reference &&
+            expectedStamp == current.integer &&
+            ((newReference == current.reference &&
+              newStamp == current.integer) ||
+             atomicRef.compareAndSet(current,
+                                     new ReferenceIntegerPair<V>(newReference,
+                                                              newStamp)));
+    }
+
+
+    /**
+     * Unconditionally sets the value of both the reference and stamp.
+     *
+     * @param newReference the new value for the reference
+     * @param newStamp the new value for the stamp
+     */
+    public void set(V newReference, int newStamp) {
+        ReferenceIntegerPair<V> current = atomicRef.get();
+        if (newReference != current.reference || newStamp != current.integer)
+            atomicRef.set(new ReferenceIntegerPair<V>(newReference, newStamp));
+    }
+
+    /**
+     * Atomically sets the value of the stamp to the given update value
+     * if the current reference is <tt>==</tt> to the expected
+     * reference.  Any given invocation of this operation may fail
+     * (return <tt>false</tt>) spuriously, but repeated invocation
+     * when the current value holds the expected value and no other
+     * thread is also attempting to set the value will eventually
+     * succeed.
+     *
+     * @param expectedReference the expected value of the reference
+     * @param newStamp the new value for the stamp
+     * @return true if successful
+     */
+    public boolean attemptStamp(V expectedReference, int newStamp) {
+        ReferenceIntegerPair<V> current = atomicRef.get();
+        return  expectedReference == current.reference &&
+            (newStamp == current.integer ||
+             atomicRef.compareAndSet(current,
+                                     new ReferenceIntegerPair<V>(expectedReference,
+                                                              newStamp)));
+    }
+}
diff --git a/external/jsr166/java/util/concurrent/atomic/package.html b/external/jsr166/java/util/concurrent/atomic/package.html
new file mode 100644
index 000000000..6bc191ddf
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/atomic/package.html
@@ -0,0 +1,158 @@
+<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
+<html> <head>
+<title>Atomics</title>
+</head>
+
+<body>
+
+A small toolkit of classes that support lock-free thread-safe
+programming on single variables. In essence, the classes in this
+package extend the notion of <tt>volatile</tt> values, fields, and
+array elements to those that also provide an atomic conditional update
+operation of the form:
+
+<pre>
+  boolean compareAndSet(expectedValue, updateValue);
+</pre>
+
+<p> This method (which varies in argument types across different
+classes) atomically sets a variable to the <tt>updateValue</tt> if it
+currently holds the <tt>expectedValue</tt>, reporting <tt>true</tt> on
+success.  The classes in this package also contain methods to get and
+unconditionally set values, as well as a weaker conditional atomic
+update operation <tt>weakCompareAndSet</tt> desribed below.
+
+<p> The specifications of these methods enable implementations to
+employ efficient machine-level atomic instructions that are available
+on contemporary processors. However on some platforms, support may
+entail some form of internal locking. Thus the methods are not
+strictly guaranteed to be non-blocking --
+a thread may block transiently before performing the operation.
+
+<p> Instances of classes {@link
+java.util.concurrent.atomic.AtomicBoolean}, {@link
+java.util.concurrent.atomic.AtomicInteger}, {@link
+java.util.concurrent.atomic.AtomicLong}, and {@link
+java.util.concurrent.atomic.AtomicReference} each provide access and
+updates to a single variable of the corresponding type.  Each class
+also provides appropriate utility methods for that type.  For example,
+classes <tt>AtomicLong</tt> and <tt>AtomicInteger</tt> provide atomic
+increment methods.  One application is to generate sequence numbers,
+as in:
+
+<pre>
+class Sequencer {
+  private AtomicLong sequenceNumber = new AtomicLong(0);
+  public long next() { return sequenceNumber.getAndIncrement(); }
+}
+</pre>
+
+<p>The memory effects for accesses and updates of atomics generally
+follow the rules for volatiles, as stated in <a
+href="http://java.sun.com/docs/books/jls/"> The Java Language
+Specification, Third Edition (17.4 Memory Model)</a>:
+
+<ul>
+
+  <li> <tt>get</tt> has the memory effects of reading a
+<tt>volatile</tt> variable.
+
+  <li> <tt>set</tt> has the memory effects of writing (assigning) a
+<tt>volatile</tt> variable.
+
+  <li> <tt>lazySet</tt> has the memory effects of writing (assigning)
+  a <tt>volatile</tt> variable except that it permits reorderings with
+  subsequent (but not previous) memory actions that do not themselves
+  impose reordering constraints with ordinary non-<tt>volatile</tt>
+  writes.  Among other usage contexts, <tt>lazySet</tt> may apply when
+  nulling out, for the sake of garbage collection, a reference that is
+  never accessed again.
+
+  <li><tt>weakCompareAndSet</tt> atomically reads and conditionally
+  writes a variable but does <em>not</em>
+  create any happens-before orderings, so provides no guarantees
+  with respect to previous or subsequent reads and writes of any
+  variables other than the target of the <tt>weakCompareAndSet</tt>.
+
+  <li> <tt>compareAndSet</tt>
+  and all other read-and-update operations such as <tt>getAndIncrement</tt>
+  have the memory effects of both reading and
+  writing <tt>volatile</tt> variables.
+</ul>
+
+<p>In addition to classes representing single values, this package
+contains <em>Updater</em> classes that can be used to obtain
+<tt>compareAndSet</tt> operations on any selected <tt>volatile</tt>
+field of any selected class.  {@link
+java.util.concurrent.atomic.AtomicReferenceFieldUpdater}, {@link
+java.util.concurrent.atomic.AtomicIntegerFieldUpdater}, and {@link
+java.util.concurrent.atomic.AtomicLongFieldUpdater} are
+reflection-based utilities that provide access to the associated field
+types. These are mainly of use in atomic data structures in which
+several <tt>volatile</tt> fields of the same node (for example, the
+links of a tree node) are independently subject to atomic
+updates. These classes enable greater flexibility in how and when to
+use atomic updates, at the expense of more awkward reflection-based
+setup, less convenient usage, and weaker guarantees.
+
+<p>The {@link java.util.concurrent.atomic.AtomicIntegerArray}, {@link
+java.util.concurrent.atomic.AtomicLongArray}, and {@link
+java.util.concurrent.atomic.AtomicReferenceArray} classes further
+extend atomic operation support to arrays of these types. These
+classes are also notable in providing <tt>volatile</tt> access
+semantics for their array elements, which is not supported for
+ordinary arrays.
+
+<p>The atomic classes also support method <tt>weakCompareAndSet</tt>,
+which has limited applicability. On some platforms, the weak version
+may be more efficient than <tt>compareAndSet</tt> in the normal case,
+but differs in that any given invocation of <tt>weakCompareAndSet</tt>
+method may return <tt>false</tt> spuriously (that is, for no apparent
+reason). A <tt>false</tt> return means only that the operation may be
+retried if desired, relying on the guarantee that repeated invocation
+when the variable holds <tt>expectedValue</tt> and no other thread is
+also attempting to set the variable will eventually succeed.  (Such
+spurious failures may for example be due to memory contention effects
+that are unrelated to whether the expected and current values are
+equal.)  Additionally <tt>weakCompareAndSet</tt> does not provide
+ordering guarantees that are usually needed for synchronization
+control.  However, the method may be useful for updating counters and
+statistics when such updates are unrelated to the other happens-before
+orderings of a program. When a thread sees an update to an atomic
+variable caused by a <tt>weakCompareAndSet</tt>, it does not
+necessarily see updates to any <em>other</em> variables that occurred
+before the <tt>weakCompareAndSet</tt>.  This may be acceptable when
+for example updating performance statistics, but rarely otherwise.
+
+<p> The {@link java.util.concurrent.atomic.AtomicMarkableReference}
+class associates a single boolean with a reference. For example, this
+bit might be used inside a data structure to mean that the object
+being referenced has logically been deleted. The {@link
+java.util.concurrent.atomic.AtomicStampedReference} class associates
+an integer value with a reference. This may be used for example, to
+represent version numbers corresponding to series of updates.
+
+<p> Atomic classes are designed primarily as building blocks for
+implementing non-blocking data structures and related infrastructure
+classes.  The <tt>compareAndSet</tt> method is not a general
+replacement for locking. It applies only when critical updates for an
+object are confined to a <em>single</em> variable.
+
+<p> Atomic classes are not general purpose replacements for
+<tt>java.lang.Integer</tt> and related classes. They do <em>not</em>
+define methods such as <tt>hashCode</tt> and
+<tt>compareTo</tt>. (Because atomic variables are expected to be
+mutated, they are poor choices for hash table keys.)  Additionally,
+classes are provided only for those types that are commonly useful in
+intended applications. For example, there is no atomic class for
+representing <tt>byte</tt>. In those infrequent cases where you would
+like to do so, you can use an <tt>AtomicInteger</tt> to hold
+<tt>byte</tt> values, and cast appropriately. You can also hold floats
+using <tt>Float.floatToIntBits</tt> and <tt>Float.intBitstoFloat</tt>
+conversions, and doubles using <tt>Double.doubleToLongBits</tt> and
+<tt>Double.longBitsToDouble</tt> conversions.
+
+
+@since 1.5
+
+</body> </html>
diff --git a/external/jsr166/java/util/concurrent/locks/AbstractOwnableSynchronizer.java b/external/jsr166/java/util/concurrent/locks/AbstractOwnableSynchronizer.java
new file mode 100644
index 000000000..f3780e5a6
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/locks/AbstractOwnableSynchronizer.java
@@ -0,0 +1,57 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent.locks;
+
+/**
+ * A synchronizer that may be exclusively owned by a thread.  This
+ * class provides a basis for creating locks and related synchronizers
+ * that may entail a notion of ownership.  The
+ * <tt>AbstractOwnableSynchronizer</tt> class itself does not manage or
+ * use this information. However, subclasses and tools may use
+ * appropriately maintained values to help control and monitor access
+ * and provide diagnostics.
+ *
+ * @since 1.6
+ * @author Doug Lea
+ */
+public abstract class AbstractOwnableSynchronizer
+    implements java.io.Serializable {
+
+    /** Use serial ID even though all fields transient. */
+    private static final long serialVersionUID = 3737899427754241961L;
+
+    /**
+     * Empty constructor for use by subclasses.
+     */
+    protected AbstractOwnableSynchronizer() { }
+
+    /**
+     * The current owner of exclusive mode synchronization.
+     */
+    private transient Thread exclusiveOwnerThread;
+
+    /**
+     * Sets the thread that currently owns exclusive access. A
+     * <tt>null</tt> argument indicates that no thread owns access.
+     * This method does not otherwise impose any synchronization or
+     * <tt>volatile</tt> field accesses.
+     */
+    protected final void setExclusiveOwnerThread(Thread t) {
+        exclusiveOwnerThread = t;
+    }
+
+    /**
+     * Returns the thread last set by
+     * <tt>setExclusiveOwnerThread</tt>, or <tt>null</tt> if never
+     * set.  This method does not otherwise impose any synchronization
+     * or <tt>volatile</tt> field accesses.
+     * @return the owner thread
+     */
+    protected final Thread getExclusiveOwnerThread() {
+        return exclusiveOwnerThread;
+    }
+}
diff --git a/external/jsr166/java/util/concurrent/locks/AbstractQueuedLongSynchronizer.java b/external/jsr166/java/util/concurrent/locks/AbstractQueuedLongSynchronizer.java
new file mode 100644
index 000000000..88a4354bc
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/locks/AbstractQueuedLongSynchronizer.java
@@ -0,0 +1,1934 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent.locks;
+import java.util.*;
+import java.util.concurrent.*;
+import java.util.concurrent.atomic.*;
+import sun.misc.Unsafe;
+
+/**
+ * A version of {@link AbstractQueuedSynchronizer} in
+ * which synchronization state is maintained as a <tt>long</tt>.
+ * This class has exactly the same structure, properties, and methods
+ * as <tt>AbstractQueuedSynchronizer</tt> with the exception
+ * that all state-related parameters and results are defined
+ * as <tt>long</tt> rather than <tt>int</tt>. This class
+ * may be useful when creating synchronizers such as
+ * multilevel locks and barriers that require
+ * 64 bits of state.
+ *
+ * <p>See {@link AbstractQueuedSynchronizer} for usage
+ * notes and examples.
+ *
+ * @since 1.6
+ * @author Doug Lea
+ */
+public abstract class AbstractQueuedLongSynchronizer
+    extends AbstractOwnableSynchronizer
+    implements java.io.Serializable {
+
+    private static final long serialVersionUID = 7373984972572414692L;
+
+    /*
+      To keep sources in sync, the remainder of this source file is
+      exactly cloned from AbstractQueuedSynchronizer, replacing class
+      name and changing ints related with sync state to longs. Please
+      keep it that way.
+    */
+
+    /**
+     * Creates a new <tt>AbstractQueuedLongSynchronizer</tt> instance
+     * with initial synchronization state of zero.
+     */
+    protected AbstractQueuedLongSynchronizer() { }
+
+    /**
+     * Wait queue node class.
+     *
+     * <p>The wait queue is a variant of a "CLH" (Craig, Landin, and
+     * Hagersten) lock queue. CLH locks are normally used for
+     * spinlocks.  We instead use them for blocking synchronizers, but
+     * use the same basic tactic of holding some of the control
+     * information about a thread in the predecessor of its node.  A
+     * "status" field in each node keeps track of whether a thread
+     * should block.  A node is signalled when its predecessor
+     * releases.  Each node of the queue otherwise serves as a
+     * specific-notification-style monitor holding a single waiting
+     * thread. The status field does NOT control whether threads are
+     * granted locks etc though.  A thread may try to acquire if it is
+     * first in the queue. But being first does not guarantee success;
+     * it only gives the right to contend.  So the currently released
+     * contender thread may need to rewait.
+     *
+     * <p>To enqueue into a CLH lock, you atomically splice it in as new
+     * tail. To dequeue, you just set the head field.
+     * <pre>
+     *      +------+  prev +-----+       +-----+
+     * head |      | <---- |     | <---- |     |  tail
+     *      +------+       +-----+       +-----+
+     * </pre>
+     *
+     * <p>Insertion into a CLH queue requires only a single atomic
+     * operation on "tail", so there is a simple atomic point of
+     * demarcation from unqueued to queued. Similarly, dequeing
+     * involves only updating the "head". However, it takes a bit
+     * more work for nodes to determine who their successors are,
+     * in part to deal with possible cancellation due to timeouts
+     * and interrupts.
+     *
+     * <p>The "prev" links (not used in original CLH locks), are mainly
+     * needed to handle cancellation. If a node is cancelled, its
+     * successor is (normally) relinked to a non-cancelled
+     * predecessor. For explanation of similar mechanics in the case
+     * of spin locks, see the papers by Scott and Scherer at
+     * http://www.cs.rochester.edu/u/scott/synchronization/
+     *
+     * <p>We also use "next" links to implement blocking mechanics.
+     * The thread id for each node is kept in its own node, so a
+     * predecessor signals the next node to wake up by traversing
+     * next link to determine which thread it is.  Determination of
+     * successor must avoid races with newly queued nodes to set
+     * the "next" fields of their predecessors.  This is solved
+     * when necessary by checking backwards from the atomically
+     * updated "tail" when a node's successor appears to be null.
+     * (Or, said differently, the next-links are an optimization
+     * so that we don't usually need a backward scan.)
+     *
+     * <p>Cancellation introduces some conservatism to the basic
+     * algorithms.  Since we must poll for cancellation of other
+     * nodes, we can miss noticing whether a cancelled node is
+     * ahead or behind us. This is dealt with by always unparking
+     * successors upon cancellation, allowing them to stabilize on
+     * a new predecessor.
+     *
+     * <p>CLH queues need a dummy header node to get started. But
+     * we don't create them on construction, because it would be wasted
+     * effort if there is never contention. Instead, the node
+     * is constructed and head and tail pointers are set upon first
+     * contention.
+     *
+     * <p>Threads waiting on Conditions use the same nodes, but
+     * use an additional link. Conditions only need to link nodes
+     * in simple (non-concurrent) linked queues because they are
+     * only accessed when exclusively held.  Upon await, a node is
+     * inserted into a condition queue.  Upon signal, the node is
+     * transferred to the main queue.  A special value of status
+     * field is used to mark which queue a node is on.
+     *
+     * <p>Thanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill
+     * Scherer and Michael Scott, along with members of JSR-166
+     * expert group, for helpful ideas, discussions, and critiques
+     * on the design of this class.
+     */
+    static final class Node {
+        /** waitStatus value to indicate thread has cancelled */
+        static final int CANCELLED =  1;
+        /** waitStatus value to indicate successor's thread needs unparking */
+        static final int SIGNAL    = -1;
+        /** waitStatus value to indicate thread is waiting on condition */
+        static final int CONDITION = -2;
+        /** Marker to indicate a node is waiting in shared mode */
+        static final Node SHARED = new Node();
+        /** Marker to indicate a node is waiting in exclusive mode */
+        static final Node EXCLUSIVE = null;
+
+        /**
+         * Status field, taking on only the values:
+         *   SIGNAL:     The successor of this node is (or will soon be)
+         *               blocked (via park), so the current node must
+         *               unpark its successor when it releases or
+         *               cancels. To avoid races, acquire methods must
+         *               first indicate they need a signal,
+         *               then retry the atomic acquire, and then,
+         *               on failure, block.
+         *   CANCELLED:  This node is cancelled due to timeout or interrupt.
+         *               Nodes never leave this state. In particular,
+         *               a thread with cancelled node never again blocks.
+         *   CONDITION:  This node is currently on a condition queue.
+         *               It will not be used as a sync queue node until
+         *               transferred. (Use of this value here
+         *               has nothing to do with the other uses
+         *               of the field, but simplifies mechanics.)
+         *   0:          None of the above
+         *
+         * The values are arranged numerically to simplify use.
+         * Non-negative values mean that a node doesn't need to
+         * signal. So, most code doesn't need to check for particular
+         * values, just for sign.
+         *
+         * The field is initialized to 0 for normal sync nodes, and
+         * CONDITION for condition nodes.  It is modified only using
+         * CAS.
+         */
+        volatile int waitStatus;
+
+        /**
+         * Link to predecessor node that current node/thread relies on
+         * for checking waitStatus. Assigned during enqueing, and nulled
+         * out (for sake of GC) only upon dequeuing.  Also, upon
+         * cancellation of a predecessor, we short-circuit while
+         * finding a non-cancelled one, which will always exist
+         * because the head node is never cancelled: A node becomes
+         * head only as a result of successful acquire. A
+         * cancelled thread never succeeds in acquiring, and a thread only
+         * cancels itself, not any other node.
+         */
+        volatile Node prev;
+
+        /**
+         * Link to the successor node that the current node/thread
+         * unparks upon release. Assigned once during enqueuing, and
+         * nulled out (for sake of GC) when no longer needed.  Upon
+         * cancellation, we cannot adjust this field, but can notice
+         * status and bypass the node if cancelled.  The enq operation
+         * does not assign next field of a predecessor until after
+         * attachment, so seeing a null next field does not
+         * necessarily mean that node is at end of queue. However, if
+         * a next field appears to be null, we can scan prev's from
+         * the tail to double-check.
+         */
+        volatile Node next;
+
+        /**
+         * The thread that enqueued this node.  Initialized on
+         * construction and nulled out after use.
+         */
+        volatile Thread thread;
+
+        /**
+         * Link to next node waiting on condition, or the special
+         * value SHARED.  Because condition queues are accessed only
+         * when holding in exclusive mode, we just need a simple
+         * linked queue to hold nodes while they are waiting on
+         * conditions. They are then transferred to the queue to
+         * re-acquire. And because conditions can only be exclusive,
+         * we save a field by using special value to indicate shared
+         * mode.
+         */
+        Node nextWaiter;
+
+        /**
+         * Returns true if node is waiting in shared mode
+         */
+        final boolean isShared() {
+            return nextWaiter == SHARED;
+        }
+
+        /**
+         * Returns previous node, or throws NullPointerException if
+         * null.  Use when predecessor cannot be null.
+         * @return the predecessor of this node
+         */
+        final Node predecessor() throws NullPointerException {
+            Node p = prev;
+            if (p == null)
+                throw new NullPointerException();
+            else
+                return p;
+        }
+
+        Node() {    // Used to establish initial head or SHARED marker
+        }
+
+        Node(Thread thread, Node mode) {     // Used by addWaiter
+            this.nextWaiter = mode;
+            this.thread = thread;
+        }
+
+        Node(Thread thread, int waitStatus) { // Used by Condition
+            this.waitStatus = waitStatus;
+            this.thread = thread;
+        }
+    }
+
+    /**
+     * Head of the wait queue, lazily initialized.  Except for
+     * initialization, it is modified only via method setHead.  Note:
+     * If head exists, its waitStatus is guaranteed not to be
+     * CANCELLED.
+     */
+    private transient volatile Node head;
+
+    /**
+     * Tail of the wait queue, lazily initialized.  Modified only via
+     * method enq to add new wait node.
+     */
+    private transient volatile Node tail;
+
+    /**
+     * The synchronization state.
+     */
+    private volatile long state;
+
+    /**
+     * Returns the current value of synchronization state.
+     * This operation has memory semantics of a <tt>volatile</tt> read.
+     * @return current state value
+     */
+    protected final long getState() {
+        return state;
+    }
+
+    /**
+     * Sets the value of synchronization state.
+     * This operation has memory semantics of a <tt>volatile</tt> write.
+     * @param newState the new state value
+     */
+    protected final void setState(long newState) {
+        state = newState;
+    }
+
+    /**
+     * Atomically sets synchronization state to the given updated
+     * value if the current state value equals the expected value.
+     * This operation has memory semantics of a <tt>volatile</tt> read
+     * and write.
+     *
+     * @param expect the expected value
+     * @param update the new value
+     * @return true if successful. False return indicates that the actual
+     *         value was not equal to the expected value.
+     */
+    protected final boolean compareAndSetState(long expect, long update) {
+        // See below for intrinsics setup to support this
+        return unsafe.compareAndSwapLong(this, stateOffset, expect, update);
+    }
+
+    // Queuing utilities
+
+    /**
+     * The number of nanoseconds for which it is faster to spin
+     * rather than to use timed park. A rough estimate suffices
+     * to improve responsiveness with very short timeouts.
+     */
+    static final long spinForTimeoutThreshold = 1000L;
+
+    /**
+     * Inserts node into queue, initializing if necessary. See picture above.
+     * @param node the node to insert
+     * @return node's predecessor
+     */
+    private Node enq(final Node node) {
+        for (;;) {
+            Node t = tail;
+            if (t == null) { // Must initialize
+                Node h = new Node(); // Dummy header
+                h.next = node;
+                node.prev = h;
+                if (compareAndSetHead(h)) {
+                    tail = node;
+                    return h;
+                }
+            }
+            else {
+                node.prev = t;
+                if (compareAndSetTail(t, node)) {
+                    t.next = node;
+                    return t;
+                }
+            }
+        }
+    }
+
+    /**
+     * Creates and enqueues node for given thread and mode.
+     *
+     * @param current the thread
+     * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
+     * @return the new node
+     */
+    private Node addWaiter(Node mode) {
+        Node node = new Node(Thread.currentThread(), mode);
+        // Try the fast path of enq; backup to full enq on failure
+        Node pred = tail;
+        if (pred != null) {
+            node.prev = pred;
+            if (compareAndSetTail(pred, node)) {
+                pred.next = node;
+                return node;
+            }
+        }
+        enq(node);
+        return node;
+    }
+
+    /**
+     * Sets head of queue to be node, thus dequeuing. Called only by
+     * acquire methods.  Also nulls out unused fields for sake of GC
+     * and to suppress unnecessary signals and traversals.
+     *
+     * @param node the node
+     */
+    private void setHead(Node node) {
+        head = node;
+        node.thread = null;
+        node.prev = null;
+    }
+
+    /**
+     * Wakes up node's successor, if one exists.
+     *
+     * @param node the node
+     */
+    private void unparkSuccessor(Node node) {
+        /*
+         * Try to clear status in anticipation of signalling.  It is
+         * OK if this fails or if status is changed by waiting thread.
+         */
+        compareAndSetWaitStatus(node, Node.SIGNAL, 0);
+
+        /*
+         * Thread to unpark is held in successor, which is normally
+         * just the next node.  But if cancelled or apparently null,
+         * traverse backwards from tail to find the actual
+         * non-cancelled successor.
+         */
+        Node s = node.next;
+        if (s == null || s.waitStatus > 0) {
+            s = null;
+            for (Node t = tail; t != null && t != node; t = t.prev)
+                if (t.waitStatus <= 0)
+                    s = t;
+        }
+        if (s != null)
+            LockSupport.unpark(s.thread);
+    }
+
+    /**
+     * Sets head of queue, and checks if successor may be waiting
+     * in shared mode, if so propagating if propagate > 0.
+     *
+     * @param pred the node holding waitStatus for node
+     * @param node the node
+     * @param propagate the return value from a tryAcquireShared
+     */
+    private void setHeadAndPropagate(Node node, long propagate) {
+        setHead(node);
+        if (propagate > 0 && node.waitStatus != 0) {
+            /*
+             * Don't bother fully figuring out successor.  If it
+             * looks null, call unparkSuccessor anyway to be safe.
+             */
+            Node s = node.next;
+            if (s == null || s.isShared())
+                unparkSuccessor(node);
+        }
+    }
+
+    // Utilities for various versions of acquire
+
+    /**
+     * Cancels an ongoing attempt to acquire.
+     *
+     * @param node the node
+     */
+    private void cancelAcquire(Node node) {
+        if (node != null) { // Ignore if node doesn't exist
+            node.thread = null;
+            // Can use unconditional write instead of CAS here
+            node.waitStatus = Node.CANCELLED;
+            unparkSuccessor(node);
+        }
+    }
+
+    /**
+     * Checks and updates status for a node that failed to acquire.
+     * Returns true if thread should block. This is the main signal
+     * control in all acquire loops.  Requires that pred == node.prev
+     *
+     * @param pred node's predecessor holding status
+     * @param node the node
+     * @return {@code true} if thread should block
+     */
+    private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
+        int s = pred.waitStatus;
+        if (s < 0)
+            /*
+             * This node has already set status asking a release
+             * to signal it, so it can safely park
+             */
+            return true;
+        if (s > 0)
+            /*
+             * Predecessor was cancelled. Move up to its predecessor
+             * and indicate retry.
+             */
+            node.prev = pred.prev;
+        else
+            /*
+             * Indicate that we need a signal, but don't park yet. Caller
+             * will need to retry to make sure it cannot acquire before
+             * parking.
+             */
+            compareAndSetWaitStatus(pred, 0, Node.SIGNAL);
+        return false;
+    }
+
+    /**
+     * Convenience method to interrupt current thread.
+     */
+    private static void selfInterrupt() {
+        Thread.currentThread().interrupt();
+    }
+
+    /**
+     * Convenience method to park and then check if interrupted
+     *
+     * @return {@code true} if interrupted
+     */
+    private final boolean parkAndCheckInterrupt() {
+        LockSupport.park(this);
+        return Thread.interrupted();
+    }
+
+    /*
+     * Various flavors of acquire, varying in exclusive/shared and
+     * control modes.  Each is mostly the same, but annoyingly
+     * different.  Only a little bit of factoring is possible due to
+     * interactions of exception mechanics (including ensuring that we
+     * cancel if tryAcquire throws exception) and other control, at
+     * least not without hurting performance too much.
+     */
+
+    /**
+     * Acquires in exclusive uninterruptible mode for thread already in
+     * queue. Used by condition wait methods as well as acquire.
+     *
+     * @param node the node
+     * @param arg the acquire argument
+     * @return {@code true} if interrupted while waiting
+     */
+    final boolean acquireQueued(final Node node, long arg) {
+        try {
+            boolean interrupted = false;
+            for (;;) {
+                final Node p = node.predecessor();
+                if (p == head && tryAcquire(arg)) {
+                    setHead(node);
+                    p.next = null; // help GC
+                    return interrupted;
+                }
+                if (shouldParkAfterFailedAcquire(p, node) &&
+                    parkAndCheckInterrupt())
+                    interrupted = true;
+            }
+        } catch (RuntimeException ex) {
+            cancelAcquire(node);
+            throw ex;
+        }
+    }
+
+    /**
+     * Acquires in exclusive interruptible mode.
+     * @param arg the acquire argument
+     */
+    private void doAcquireInterruptibly(long arg)
+        throws InterruptedException {
+        final Node node = addWaiter(Node.EXCLUSIVE);
+        try {
+            for (;;) {
+                final Node p = node.predecessor();
+                if (p == head && tryAcquire(arg)) {
+                    setHead(node);
+                    p.next = null; // help GC
+                    return;
+                }
+                if (shouldParkAfterFailedAcquire(p, node) &&
+                    parkAndCheckInterrupt())
+                    break;
+            }
+        } catch (RuntimeException ex) {
+            cancelAcquire(node);
+            throw ex;
+        }
+        // Arrive here only if interrupted
+        cancelAcquire(node);
+        throw new InterruptedException();
+    }
+
+    /**
+     * Acquires in exclusive timed mode.
+     *
+     * @param arg the acquire argument
+     * @param nanosTimeout max wait time
+     * @return {@code true} if acquired
+     */
+    private boolean doAcquireNanos(long arg, long nanosTimeout)
+        throws InterruptedException {
+        long lastTime = System.nanoTime();
+        final Node node = addWaiter(Node.EXCLUSIVE);
+        try {
+            for (;;) {
+                final Node p = node.predecessor();
+                if (p == head && tryAcquire(arg)) {
+                    setHead(node);
+                    p.next = null; // help GC
+                    return true;
+                }
+                if (nanosTimeout <= 0) {
+                    cancelAcquire(node);
+                    return false;
+                }
+                if (nanosTimeout > spinForTimeoutThreshold &&
+                    shouldParkAfterFailedAcquire(p, node))
+                    LockSupport.parkNanos(this, nanosTimeout);
+                long now = System.nanoTime();
+                nanosTimeout -= now - lastTime;
+                lastTime = now;
+                if (Thread.interrupted())
+                    break;
+            }
+        } catch (RuntimeException ex) {
+            cancelAcquire(node);
+            throw ex;
+        }
+        // Arrive here only if interrupted
+        cancelAcquire(node);
+        throw new InterruptedException();
+    }
+
+    /**
+     * Acquires in shared uninterruptible mode.
+     * @param arg the acquire argument
+     */
+    private void doAcquireShared(long arg) {
+        final Node node = addWaiter(Node.SHARED);
+        try {
+            boolean interrupted = false;
+            for (;;) {
+                final Node p = node.predecessor();
+                if (p == head) {
+                    long r = tryAcquireShared(arg);
+                    if (r >= 0) {
+                        setHeadAndPropagate(node, r);
+                        p.next = null; // help GC
+                        if (interrupted)
+                            selfInterrupt();
+                        return;
+                    }
+                }
+                if (shouldParkAfterFailedAcquire(p, node) &&
+                    parkAndCheckInterrupt())
+                    interrupted = true;
+            }
+        } catch (RuntimeException ex) {
+            cancelAcquire(node);
+            throw ex;
+        }
+    }
+
+    /**
+     * Acquires in shared interruptible mode.
+     * @param arg the acquire argument
+     */
+    private void doAcquireSharedInterruptibly(long arg)
+        throws InterruptedException {
+        final Node node = addWaiter(Node.SHARED);
+        try {
+            for (;;) {
+                final Node p = node.predecessor();
+                if (p == head) {
+                    long r = tryAcquireShared(arg);
+                    if (r >= 0) {
+                        setHeadAndPropagate(node, r);
+                        p.next = null; // help GC
+                        return;
+                    }
+                }
+                if (shouldParkAfterFailedAcquire(p, node) &&
+                    parkAndCheckInterrupt())
+                    break;
+            }
+        } catch (RuntimeException ex) {
+            cancelAcquire(node);
+            throw ex;
+        }
+        // Arrive here only if interrupted
+        cancelAcquire(node);
+        throw new InterruptedException();
+    }
+
+    /**
+     * Acquires in shared timed mode.
+     *
+     * @param arg the acquire argument
+     * @param nanosTimeout max wait time
+     * @return {@code true} if acquired
+     */
+    private boolean doAcquireSharedNanos(long arg, long nanosTimeout)
+        throws InterruptedException {
+
+        long lastTime = System.nanoTime();
+        final Node node = addWaiter(Node.SHARED);
+        try {
+            for (;;) {
+                final Node p = node.predecessor();
+                if (p == head) {
+                    long r = tryAcquireShared(arg);
+                    if (r >= 0) {
+                        setHeadAndPropagate(node, r);
+                        p.next = null; // help GC
+                        return true;
+                    }
+                }
+                if (nanosTimeout <= 0) {
+                    cancelAcquire(node);
+                    return false;
+                }
+                if (nanosTimeout > spinForTimeoutThreshold &&
+                    shouldParkAfterFailedAcquire(p, node))
+                    LockSupport.parkNanos(this, nanosTimeout);
+                long now = System.nanoTime();
+                nanosTimeout -= now - lastTime;
+                lastTime = now;
+                if (Thread.interrupted())
+                    break;
+            }
+        } catch (RuntimeException ex) {
+            cancelAcquire(node);
+            throw ex;
+        }
+        // Arrive here only if interrupted
+        cancelAcquire(node);
+        throw new InterruptedException();
+    }
+
+    // Main exported methods
+
+    /**
+     * Attempts to acquire in exclusive mode. This method should query
+     * if the state of the object permits it to be acquired in the
+     * exclusive mode, and if so to acquire it.
+     *
+     * <p>This method is always invoked by the thread performing
+     * acquire.  If this method reports failure, the acquire method
+     * may queue the thread, if it is not already queued, until it is
+     * signalled by a release from some other thread. This can be used
+     * to implement method {@link Lock#tryLock()}.
+     *
+     * <p>The default
+     * implementation throws {@link UnsupportedOperationException}.
+     *
+     * @param arg the acquire argument. This value is always the one
+     *        passed to an acquire method, or is the value saved on entry
+     *        to a condition wait.  The value is otherwise uninterpreted
+     *        and can represent anything you like.
+     * @return {@code true} if successful. Upon success, this object has
+     *         been acquired.
+     * @throws IllegalMonitorStateException if acquiring would place this
+     *         synchronizer in an illegal state. This exception must be
+     *         thrown in a consistent fashion for synchronization to work
+     *         correctly.
+     * @throws UnsupportedOperationException if exclusive mode is not supported
+     */
+    protected boolean tryAcquire(long arg) {
+        throw new UnsupportedOperationException();
+    }
+
+    /**
+     * Attempts to set the state to reflect a release in exclusive
+     * mode.
+     *
+     * <p>This method is always invoked by the thread performing release.
+     *
+     * <p>The default implementation throws
+     * {@link UnsupportedOperationException}.
+     *
+     * @param arg the release argument. This value is always the one
+     *        passed to a release method, or the current state value upon
+     *        entry to a condition wait.  The value is otherwise
+     *        uninterpreted and can represent anything you like.
+     * @return {@code true} if this object is now in a fully released
+     *         state, so that any waiting threads may attempt to acquire;
+     *         and {@code false} otherwise.
+     * @throws IllegalMonitorStateException if releasing would place this
+     *         synchronizer in an illegal state. This exception must be
+     *         thrown in a consistent fashion for synchronization to work
+     *         correctly.
+     * @throws UnsupportedOperationException if exclusive mode is not supported
+     */
+    protected boolean tryRelease(long arg) {
+        throw new UnsupportedOperationException();
+    }
+
+    /**
+     * Attempts to acquire in shared mode. This method should query if
+     * the state of the object permits it to be acquired in the shared
+     * mode, and if so to acquire it.
+     *
+     * <p>This method is always invoked by the thread performing
+     * acquire.  If this method reports failure, the acquire method
+     * may queue the thread, if it is not already queued, until it is
+     * signalled by a release from some other thread.
+     *
+     * <p>The default implementation throws {@link
+     * UnsupportedOperationException}.
+     *
+     * @param arg the acquire argument. This value is always the one
+     *        passed to an acquire method, or is the value saved on entry
+     *        to a condition wait.  The value is otherwise uninterpreted
+     *        and can represent anything you like.
+     * @return a negative value on failure; zero if acquisition in shared
+     *         mode succeeded but no subsequent shared-mode acquire can
+     *         succeed; and a positive value if acquisition in shared
+     *         mode succeeded and subsequent shared-mode acquires might
+     *         also succeed, in which case a subsequent waiting thread
+     *         must check availability. (Support for three different
+     *         return values enables this method to be used in contexts
+     *         where acquires only sometimes act exclusively.)  Upon
+     *         success, this object has been acquired.
+     * @throws IllegalMonitorStateException if acquiring would place this
+     *         synchronizer in an illegal state. This exception must be
+     *         thrown in a consistent fashion for synchronization to work
+     *         correctly.
+     * @throws UnsupportedOperationException if shared mode is not supported
+     */
+    protected long tryAcquireShared(long arg) {
+        throw new UnsupportedOperationException();
+    }
+
+    /**
+     * Attempts to set the state to reflect a release in shared mode.
+     *
+     * <p>This method is always invoked by the thread performing release.
+     *
+     * <p>The default implementation throws
+     * {@link UnsupportedOperationException}.
+     *
+     * @param arg the release argument. This value is always the one
+     *        passed to a release method, or the current state value upon
+     *        entry to a condition wait.  The value is otherwise
+     *        uninterpreted and can represent anything you like.
+     * @return {@code true} if this release of shared mode may permit a
+     *         waiting acquire (shared or exclusive) to succeed; and
+     *         {@code false} otherwise
+     * @throws IllegalMonitorStateException if releasing would place this
+     *         synchronizer in an illegal state. This exception must be
+     *         thrown in a consistent fashion for synchronization to work
+     *         correctly.
+     * @throws UnsupportedOperationException if shared mode is not supported
+     */
+    protected boolean tryReleaseShared(long arg) {
+        throw new UnsupportedOperationException();
+    }
+
+    /**
+     * Returns {@code true} if synchronization is held exclusively with
+     * respect to the current (calling) thread.  This method is invoked
+     * upon each call to a non-waiting {@link ConditionObject} method.
+     * (Waiting methods instead invoke {@link #release}.)
+     *
+     * <p>The default implementation throws {@link
+     * UnsupportedOperationException}. This method is invoked
+     * internally only within {@link ConditionObject} methods, so need
+     * not be defined if conditions are not used.
+     *
+     * @return {@code true} if synchronization is held exclusively;
+     *         {@code false} otherwise
+     * @throws UnsupportedOperationException if conditions are not supported
+     */
+    protected boolean isHeldExclusively() {
+        throw new UnsupportedOperationException();
+    }
+
+    /**
+     * Acquires in exclusive mode, ignoring interrupts.  Implemented
+     * by invoking at least once {@link #tryAcquire},
+     * returning on success.  Otherwise the thread is queued, possibly
+     * repeatedly blocking and unblocking, invoking {@link
+     * #tryAcquire} until success.  This method can be used
+     * to implement method {@link Lock#lock}.
+     *
+     * @param arg the acquire argument.  This value is conveyed to
+     *        {@link #tryAcquire} but is otherwise uninterpreted and
+     *        can represent anything you like.
+     */
+    public final void acquire(long arg) {
+        if (!tryAcquire(arg) &&
+            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
+            selfInterrupt();
+    }
+
+    /**
+     * Acquires in exclusive mode, aborting if interrupted.
+     * Implemented by first checking interrupt status, then invoking
+     * at least once {@link #tryAcquire}, returning on
+     * success.  Otherwise the thread is queued, possibly repeatedly
+     * blocking and unblocking, invoking {@link #tryAcquire}
+     * until success or the thread is interrupted.  This method can be
+     * used to implement method {@link Lock#lockInterruptibly}.
+     *
+     * @param arg the acquire argument.  This value is conveyed to
+     *        {@link #tryAcquire} but is otherwise uninterpreted and
+     *        can represent anything you like.
+     * @throws InterruptedException if the current thread is interrupted
+     */
+    public final void acquireInterruptibly(long arg) throws InterruptedException {
+        if (Thread.interrupted())
+            throw new InterruptedException();
+        if (!tryAcquire(arg))
+            doAcquireInterruptibly(arg);
+    }
+
+    /**
+     * Attempts to acquire in exclusive mode, aborting if interrupted,
+     * and failing if the given timeout elapses.  Implemented by first
+     * checking interrupt status, then invoking at least once {@link
+     * #tryAcquire}, returning on success.  Otherwise, the thread is
+     * queued, possibly repeatedly blocking and unblocking, invoking
+     * {@link #tryAcquire} until success or the thread is interrupted
+     * or the timeout elapses.  This method can be used to implement
+     * method {@link Lock#tryLock(long, TimeUnit)}.
+     *
+     * @param arg the acquire argument.  This value is conveyed to
+     *        {@link #tryAcquire} but is otherwise uninterpreted and
+     *        can represent anything you like.
+     * @param nanosTimeout the maximum number of nanoseconds to wait
+     * @return {@code true} if acquired; {@code false} if timed out
+     * @throws InterruptedException if the current thread is interrupted
+     */
+    public final boolean tryAcquireNanos(long arg, long nanosTimeout) throws InterruptedException {
+	if (Thread.interrupted())
+	    throw new InterruptedException();
+	return tryAcquire(arg) ||
+	    doAcquireNanos(arg, nanosTimeout);
+    }
+
+    /**
+     * Releases in exclusive mode.  Implemented by unblocking one or
+     * more threads if {@link #tryRelease} returns true.
+     * This method can be used to implement method {@link Lock#unlock}.
+     *
+     * @param arg the release argument.  This value is conveyed to
+     *        {@link #tryRelease} but is otherwise uninterpreted and
+     *        can represent anything you like.
+     * @return the value returned from {@link #tryRelease}
+     */
+    public final boolean release(long arg) {
+        if (tryRelease(arg)) {
+            Node h = head;
+            if (h != null && h.waitStatus != 0)
+                unparkSuccessor(h);
+            return true;
+        }
+        return false;
+    }
+
+    /**
+     * Acquires in shared mode, ignoring interrupts.  Implemented by
+     * first invoking at least once {@link #tryAcquireShared},
+     * returning on success.  Otherwise the thread is queued, possibly
+     * repeatedly blocking and unblocking, invoking {@link
+     * #tryAcquireShared} until success.
+     *
+     * @param arg the acquire argument.  This value is conveyed to
+     *        {@link #tryAcquireShared} but is otherwise uninterpreted
+     *        and can represent anything you like.
+     */
+    public final void acquireShared(long arg) {
+        if (tryAcquireShared(arg) < 0)
+            doAcquireShared(arg);
+    }
+
+    /**
+     * Acquires in shared mode, aborting if interrupted.  Implemented
+     * by first checking interrupt status, then invoking at least once
+     * {@link #tryAcquireShared}, returning on success.  Otherwise the
+     * thread is queued, possibly repeatedly blocking and unblocking,
+     * invoking {@link #tryAcquireShared} until success or the thread
+     * is interrupted.
+     * @param arg the acquire argument.
+     * This value is conveyed to {@link #tryAcquireShared} but is
+     * otherwise uninterpreted and can represent anything
+     * you like.
+     * @throws InterruptedException if the current thread is interrupted
+     */
+    public final void acquireSharedInterruptibly(long arg) throws InterruptedException {
+        if (Thread.interrupted())
+            throw new InterruptedException();
+        if (tryAcquireShared(arg) < 0)
+            doAcquireSharedInterruptibly(arg);
+    }
+
+    /**
+     * Attempts to acquire in shared mode, aborting if interrupted, and
+     * failing if the given timeout elapses.  Implemented by first
+     * checking interrupt status, then invoking at least once {@link
+     * #tryAcquireShared}, returning on success.  Otherwise, the
+     * thread is queued, possibly repeatedly blocking and unblocking,
+     * invoking {@link #tryAcquireShared} until success or the thread
+     * is interrupted or the timeout elapses.
+     *
+     * @param arg the acquire argument.  This value is conveyed to
+     *        {@link #tryAcquireShared} but is otherwise uninterpreted
+     *        and can represent anything you like.
+     * @param nanosTimeout the maximum number of nanoseconds to wait
+     * @return {@code true} if acquired; {@code false} if timed out
+     * @throws InterruptedException if the current thread is interrupted
+     */
+    public final boolean tryAcquireSharedNanos(long arg, long nanosTimeout) throws InterruptedException {
+	if (Thread.interrupted())
+	    throw new InterruptedException();
+	return tryAcquireShared(arg) >= 0 ||
+	    doAcquireSharedNanos(arg, nanosTimeout);
+    }
+
+    /**
+     * Releases in shared mode.  Implemented by unblocking one or more
+     * threads if {@link #tryReleaseShared} returns true.
+     *
+     * @param arg the release argument.  This value is conveyed to
+     *        {@link #tryReleaseShared} but is otherwise uninterpreted
+     *        and can represent anything you like.
+     * @return the value returned from {@link #tryReleaseShared}
+     */
+    public final boolean releaseShared(long arg) {
+        if (tryReleaseShared(arg)) {
+            Node h = head;
+            if (h != null && h.waitStatus != 0)
+                unparkSuccessor(h);
+            return true;
+        }
+        return false;
+    }
+
+    // Queue inspection methods
+
+    /**
+     * Queries whether any threads are waiting to acquire. Note that
+     * because cancellations due to interrupts and timeouts may occur
+     * at any time, a {@code true} return does not guarantee that any
+     * other thread will ever acquire.
+     *
+     * <p>In this implementation, this operation returns in
+     * constant time.
+     *
+     * @return {@code true} if there may be other threads waiting to acquire
+     */
+    public final boolean hasQueuedThreads() {
+        return head != tail;
+    }
+
+    /**
+     * Queries whether any threads have ever contended to acquire this
+     * synchronizer; that is if an acquire method has ever blocked.
+     *
+     * <p>In this implementation, this operation returns in
+     * constant time.
+     *
+     * @return {@code true} if there has ever been contention
+     */
+    public final boolean hasContended() {
+        return head != null;
+    }
+
+    /**
+     * Returns the first (longest-waiting) thread in the queue, or
+     * {@code null} if no threads are currently queued.
+     *
+     * <p>In this implementation, this operation normally returns in
+     * constant time, but may iterate upon contention if other threads are
+     * concurrently modifying the queue.
+     *
+     * @return the first (longest-waiting) thread in the queue, or
+     *         {@code null} if no threads are currently queued
+     */
+    public final Thread getFirstQueuedThread() {
+        // handle only fast path, else relay
+        return (head == tail)? null : fullGetFirstQueuedThread();
+    }
+
+    /**
+     * Version of getFirstQueuedThread called when fastpath fails
+     */
+    private Thread fullGetFirstQueuedThread() {
+        /*
+         * The first node is normally h.next. Try to get its
+         * thread field, ensuring consistent reads: If thread
+         * field is nulled out or s.prev is no longer head, then
+         * some other thread(s) concurrently performed setHead in
+         * between some of our reads. We try this twice before
+         * resorting to traversal.
+         */
+        Node h, s;
+        Thread st;
+        if (((h = head) != null && (s = h.next) != null &&
+             s.prev == head && (st = s.thread) != null) ||
+            ((h = head) != null && (s = h.next) != null &&
+             s.prev == head && (st = s.thread) != null))
+            return st;
+
+        /*
+         * Head's next field might not have been set yet, or may have
+         * been unset after setHead. So we must check to see if tail
+         * is actually first node. If not, we continue on, safely
+         * traversing from tail back to head to find first,
+         * guaranteeing termination.
+         */
+
+        Node t = tail;
+        Thread firstThread = null;
+        while (t != null && t != head) {
+            Thread tt = t.thread;
+            if (tt != null)
+                firstThread = tt;
+            t = t.prev;
+        }
+        return firstThread;
+    }
+
+    /**
+     * Returns true if the given thread is currently queued.
+     *
+     * <p>This implementation traverses the queue to determine
+     * presence of the given thread.
+     *
+     * @param thread the thread
+     * @return {@code true} if the given thread is on the queue
+     * @throws NullPointerException if the thread is null
+     */
+    public final boolean isQueued(Thread thread) {
+        if (thread == null)
+            throw new NullPointerException();
+        for (Node p = tail; p != null; p = p.prev)
+            if (p.thread == thread)
+                return true;
+        return false;
+    }
+
+    /**
+     * Return {@code true} if the apparent first queued thread, if one
+     * exists, is not waiting in exclusive mode. Used only as a heuristic
+     * in ReentrantReadWriteLock.
+     */
+    final boolean apparentlyFirstQueuedIsExclusive() {
+        Node h, s;
+        return ((h = head) != null && (s = h.next) != null &&
+                s.nextWaiter != Node.SHARED);
+    }
+
+    /**
+     * Return {@code true} if the queue is empty or if the given thread
+     * is at the head of the queue. This is reliable only if
+     * <tt>current</tt> is actually Thread.currentThread() of caller.
+     */
+    final boolean isFirst(Thread current) {
+        Node h, s;
+        return ((h = head) == null ||
+                ((s = h.next) != null && s.thread == current) ||
+                fullIsFirst(current));
+    }
+
+    final boolean fullIsFirst(Thread current) {
+        // same idea as fullGetFirstQueuedThread
+        Node h, s;
+        Thread firstThread = null;
+        if (((h = head) != null && (s = h.next) != null &&
+             s.prev == head && (firstThread = s.thread) != null))
+            return firstThread == current;
+        Node t = tail;
+        while (t != null && t != head) {
+            Thread tt = t.thread;
+            if (tt != null)
+                firstThread = tt;
+            t = t.prev;
+        }
+        return firstThread == current || firstThread == null;
+    }
+
+
+    // Instrumentation and monitoring methods
+
+    /**
+     * Returns an estimate of the number of threads waiting to
+     * acquire.  The value is only an estimate because the number of
+     * threads may change dynamically while this method traverses
+     * internal data structures.  This method is designed for use in
+     * monitoring system state, not for synchronization
+     * control.
+     *
+     * @return the estimated number of threads waiting to acquire
+     */
+    public final int getQueueLength() {
+        int n = 0;
+        for (Node p = tail; p != null; p = p.prev) {
+            if (p.thread != null)
+                ++n;
+        }
+        return n;
+    }
+
+    /**
+     * Returns a collection containing threads that may be waiting to
+     * acquire.  Because the actual set of threads may change
+     * dynamically while constructing this result, the returned
+     * collection is only a best-effort estimate.  The elements of the
+     * returned collection are in no particular order.  This method is
+     * designed to facilitate construction of subclasses that provide
+     * more extensive monitoring facilities.
+     *
+     * @return the collection of threads
+     */
+    public final Collection<Thread> getQueuedThreads() {
+        ArrayList<Thread> list = new ArrayList<Thread>();
+        for (Node p = tail; p != null; p = p.prev) {
+            Thread t = p.thread;
+            if (t != null)
+                list.add(t);
+        }
+        return list;
+    }
+
+    /**
+     * Returns a collection containing threads that may be waiting to
+     * acquire in exclusive mode. This has the same properties
+     * as {@link #getQueuedThreads} except that it only returns
+     * those threads waiting due to an exclusive acquire.
+     *
+     * @return the collection of threads
+     */
+    public final Collection<Thread> getExclusiveQueuedThreads() {
+        ArrayList<Thread> list = new ArrayList<Thread>();
+        for (Node p = tail; p != null; p = p.prev) {
+            if (!p.isShared()) {
+                Thread t = p.thread;
+                if (t != null)
+                    list.add(t);
+            }
+        }
+        return list;
+    }
+
+    /**
+     * Returns a collection containing threads that may be waiting to
+     * acquire in shared mode. This has the same properties
+     * as {@link #getQueuedThreads} except that it only returns
+     * those threads waiting due to a shared acquire.
+     *
+     * @return the collection of threads
+     */
+    public final Collection<Thread> getSharedQueuedThreads() {
+        ArrayList<Thread> list = new ArrayList<Thread>();
+        for (Node p = tail; p != null; p = p.prev) {
+            if (p.isShared()) {
+                Thread t = p.thread;
+                if (t != null)
+                    list.add(t);
+            }
+        }
+        return list;
+    }
+
+    /**
+     * Returns a string identifying this synchronizer, as well as its state.
+     * The state, in brackets, includes the String {@code "State ="}
+     * followed by the current value of {@link #getState}, and either
+     * {@code "nonempty"} or {@code "empty"} depending on whether the
+     * queue is empty.
+     *
+     * @return a string identifying this synchronizer, as well as its state
+     */
+    public String toString() {
+        long s = getState();
+        String q  = hasQueuedThreads()? "non" : "";
+        return super.toString() +
+            "[State = " + s + ", " + q + "empty queue]";
+    }
+
+
+    // Internal support methods for Conditions
+
+    /**
+     * Returns true if a node, always one that was initially placed on
+     * a condition queue, is now waiting to reacquire on sync queue.
+     * @param node the node
+     * @return true if is reacquiring
+     */
+    final boolean isOnSyncQueue(Node node) {
+        if (node.waitStatus == Node.CONDITION || node.prev == null)
+            return false;
+        if (node.next != null) // If has successor, it must be on queue
+            return true;
+        /*
+         * node.prev can be non-null, but not yet on queue because
+         * the CAS to place it on queue can fail. So we have to
+         * traverse from tail to make sure it actually made it.  It
+         * will always be near the tail in calls to this method, and
+         * unless the CAS failed (which is unlikely), it will be
+         * there, so we hardly ever traverse much.
+         */
+        return findNodeFromTail(node);
+    }
+
+    /**
+     * Returns true if node is on sync queue by searching backwards from tail.
+     * Called only when needed by isOnSyncQueue.
+     * @return true if present
+     */
+    private boolean findNodeFromTail(Node node) {
+        Node t = tail;
+        for (;;) {
+            if (t == node)
+                return true;
+            if (t == null)
+                return false;
+            t = t.prev;
+        }
+    }
+
+    /**
+     * Transfers a node from a condition queue onto sync queue.
+     * Returns true if successful.
+     * @param node the node
+     * @return true if successfully transferred (else the node was
+     * cancelled before signal).
+     */
+    final boolean transferForSignal(Node node) {
+        /*
+         * If cannot change waitStatus, the node has been cancelled.
+         */
+        if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
+            return false;
+
+        /*
+         * Splice onto queue and try to set waitStatus of predecessor to
+         * indicate that thread is (probably) waiting. If cancelled or
+         * attempt to set waitStatus fails, wake up to resync (in which
+         * case the waitStatus can be transiently and harmlessly wrong).
+         */
+        Node p = enq(node);
+        int c = p.waitStatus;
+        if (c > 0 || !compareAndSetWaitStatus(p, c, Node.SIGNAL))
+            LockSupport.unpark(node.thread);
+        return true;
+    }
+
+    /**
+     * Transfers node, if necessary, to sync queue after a cancelled
+     * wait. Returns true if thread was cancelled before being
+     * signalled.
+     * @param current the waiting thread
+     * @param node its node
+     * @return true if cancelled before the node was signalled.
+     */
+    final boolean transferAfterCancelledWait(Node node) {
+        if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
+            enq(node);
+            return true;
+        }
+        /*
+         * If we lost out to a signal(), then we can't proceed
+         * until it finishes its enq().  Cancelling during an
+         * incomplete transfer is both rare and transient, so just
+         * spin.
+         */
+        while (!isOnSyncQueue(node))
+            Thread.yield();
+        return false;
+    }
+
+    /**
+     * Invokes release with current state value; returns saved state.
+     * Cancels node and throws exception on failure.
+     * @param node the condition node for this wait
+     * @return previous sync state
+     */
+    final long fullyRelease(Node node) {
+        try {
+            long savedState = getState();
+            if (release(savedState))
+                return savedState;
+        } catch (RuntimeException ex) {
+            node.waitStatus = Node.CANCELLED;
+            throw ex;
+        }
+        // reach here if release fails
+        node.waitStatus = Node.CANCELLED;
+        throw new IllegalMonitorStateException();
+    }
+
+    // Instrumentation methods for conditions
+
+    /**
+     * Queries whether the given ConditionObject
+     * uses this synchronizer as its lock.
+     *
+     * @param condition the condition
+     * @return <tt>true</tt> if owned
+     * @throws NullPointerException if the condition is null
+     */
+    public final boolean owns(ConditionObject condition) {
+        if (condition == null)
+            throw new NullPointerException();
+        return condition.isOwnedBy(this);
+    }
+
+    /**
+     * Queries whether any threads are waiting on the given condition
+     * associated with this synchronizer. Note that because timeouts
+     * and interrupts may occur at any time, a <tt>true</tt> return
+     * does not guarantee that a future <tt>signal</tt> will awaken
+     * any threads.  This method is designed primarily for use in
+     * monitoring of the system state.
+     *
+     * @param condition the condition
+     * @return <tt>true</tt> if there are any waiting threads
+     * @throws IllegalMonitorStateException if exclusive synchronization
+     *         is not held
+     * @throws IllegalArgumentException if the given condition is
+     *         not associated with this synchronizer
+     * @throws NullPointerException if the condition is null
+     */
+    public final boolean hasWaiters(ConditionObject condition) {
+        if (!owns(condition))
+            throw new IllegalArgumentException("Not owner");
+        return condition.hasWaiters();
+    }
+
+    /**
+     * Returns an estimate of the number of threads waiting on the
+     * given condition associated with this synchronizer. Note that
+     * because timeouts and interrupts may occur at any time, the
+     * estimate serves only as an upper bound on the actual number of
+     * waiters.  This method is designed for use in monitoring of the
+     * system state, not for synchronization control.
+     *
+     * @param condition the condition
+     * @return the estimated number of waiting threads
+     * @throws IllegalMonitorStateException if exclusive synchronization
+     *         is not held
+     * @throws IllegalArgumentException if the given condition is
+     *         not associated with this synchronizer
+     * @throws NullPointerException if the condition is null
+     */
+    public final int getWaitQueueLength(ConditionObject condition) {
+        if (!owns(condition))
+            throw new IllegalArgumentException("Not owner");
+        return condition.getWaitQueueLength();
+    }
+
+    /**
+     * Returns a collection containing those threads that may be
+     * waiting on the given condition associated with this
+     * synchronizer.  Because the actual set of threads may change
+     * dynamically while constructing this result, the returned
+     * collection is only a best-effort estimate. The elements of the
+     * returned collection are in no particular order.
+     *
+     * @param condition the condition
+     * @return the collection of threads
+     * @throws IllegalMonitorStateException if exclusive synchronization
+     *         is not held
+     * @throws IllegalArgumentException if the given condition is
+     *         not associated with this synchronizer
+     * @throws NullPointerException if the condition is null
+     */
+    public final Collection<Thread> getWaitingThreads(ConditionObject condition) {
+        if (!owns(condition))
+            throw new IllegalArgumentException("Not owner");
+        return condition.getWaitingThreads();
+    }
+
+    /**
+     * Condition implementation for a {@link
+     * AbstractQueuedLongSynchronizer} serving as the basis of a {@link
+     * Lock} implementation.
+     *
+     * <p>Method documentation for this class describes mechanics,
+     * not behavioral specifications from the point of view of Lock
+     * and Condition users. Exported versions of this class will in
+     * general need to be accompanied by documentation describing
+     * condition semantics that rely on those of the associated
+     * <tt>AbstractQueuedLongSynchronizer</tt>.
+     *
+     * <p>This class is Serializable, but all fields are transient,
+     * so deserialized conditions have no waiters.
+     *
+     * @since 1.6
+     */
+    public class ConditionObject implements Condition, java.io.Serializable {
+        private static final long serialVersionUID = 1173984872572414699L;
+        /** First node of condition queue. */
+        private transient Node firstWaiter;
+        /** Last node of condition queue. */
+        private transient Node lastWaiter;
+
+        /**
+         * Creates a new <tt>ConditionObject</tt> instance.
+         */
+        public ConditionObject() { }
+
+        // Internal methods
+
+        /**
+         * Adds a new waiter to wait queue.
+         * @return its new wait node
+         */
+        private Node addConditionWaiter() {
+            Node node = new Node(Thread.currentThread(), Node.CONDITION);
+            Node t = lastWaiter;
+            if (t == null)
+                firstWaiter = node;
+            else
+                t.nextWaiter = node;
+            lastWaiter = node;
+            return node;
+        }
+
+        /**
+         * Removes and transfers nodes until hit non-cancelled one or
+         * null. Split out from signal in part to encourage compilers
+         * to inline the case of no waiters.
+         * @param first (non-null) the first node on condition queue
+         */
+        private void doSignal(Node first) {
+            do {
+                if ( (firstWaiter = first.nextWaiter) == null)
+                    lastWaiter = null;
+                first.nextWaiter = null;
+            } while (!transferForSignal(first) &&
+                     (first = firstWaiter) != null);
+        }
+
+        /**
+         * Removes and transfers all nodes.
+         * @param first (non-null) the first node on condition queue
+         */
+        private void doSignalAll(Node first) {
+            lastWaiter = firstWaiter  = null;
+            do {
+                Node next = first.nextWaiter;
+                first.nextWaiter = null;
+                transferForSignal(first);
+                first = next;
+            } while (first != null);
+        }
+
+        /**
+         * Returns true if given node is on this condition queue.
+         * Call only when holding lock.
+         */
+        private boolean isOnConditionQueue(Node node) {
+            return node.next != null || node == lastWaiter;
+        }
+
+        /**
+         * Unlinks a cancelled waiter node from condition queue.  This
+         * is called when cancellation occurred during condition wait,
+         * not lock wait, and is called only after lock has been
+         * re-acquired by a cancelled waiter and the node is not known
+         * to already have been dequeued.  It is needed to avoid
+         * garbage retention in the absence of signals. So even though
+         * it may require a full traversal, it comes into play only
+         * when timeouts or cancellations occur in the absence of
+         * signals.
+         */
+        private void unlinkCancelledWaiter(Node node) {
+            Node t = firstWaiter;
+            Node trail = null;
+            while (t != null) {
+                if (t == node) {
+                    Node next = t.nextWaiter;
+                    if (trail == null)
+                        firstWaiter = next;
+                    else
+                        trail.nextWaiter = next;
+                    if (lastWaiter == node)
+                        lastWaiter = trail;
+                    break;
+                }
+                trail = t;
+                t = t.nextWaiter;
+            }
+        }
+
+        // public methods
+
+        /**
+         * Moves the longest-waiting thread, if one exists, from the
+         * wait queue for this condition to the wait queue for the
+         * owning lock.
+         *
+         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
+         *         returns {@code false}
+         */
+        public final void signal() {
+            if (!isHeldExclusively())
+                throw new IllegalMonitorStateException();
+            Node first = firstWaiter;
+            if (first != null)
+                doSignal(first);
+        }
+
+        /**
+         * Moves all threads from the wait queue for this condition to
+         * the wait queue for the owning lock.
+         *
+         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
+         *         returns {@code false}
+         */
+        public final void signalAll() {
+            if (!isHeldExclusively())
+                throw new IllegalMonitorStateException();
+            Node first = firstWaiter;
+            if (first != null)
+                doSignalAll(first);
+        }
+
+        /**
+         * Implements uninterruptible condition wait.
+         * <ol>
+         * <li> Save lock state returned by {@link #getState}
+         * <li> Invoke {@link #release} with
+         *      saved state as argument, throwing
+         *      IllegalMonitorStateException if it fails.
+         * <li> Block until signalled
+         * <li> Reacquire by invoking specialized version of
+         *      {@link #acquire} with saved state as argument.
+         * </ol>
+         */
+        public final void awaitUninterruptibly() {
+            Node node = addConditionWaiter();
+            long savedState = fullyRelease(node);
+            boolean interrupted = false;
+            while (!isOnSyncQueue(node)) {
+                LockSupport.park(this);
+                if (Thread.interrupted())
+                    interrupted = true;
+            }
+            if (acquireQueued(node, savedState) || interrupted)
+                selfInterrupt();
+        }
+
+        /*
+         * For interruptible waits, we need to track whether to throw
+         * InterruptedException, if interrupted while blocked on
+         * condition, versus reinterrupt current thread, if
+         * interrupted while blocked waiting to re-acquire.
+         */
+
+        /** Mode meaning to reinterrupt on exit from wait */
+        private static final int REINTERRUPT =  1;
+        /** Mode meaning to throw InterruptedException on exit from wait */
+        private static final int THROW_IE    = -1;
+
+        /**
+         * Checks for interrupt, returning THROW_IE if interrupted
+         * before signalled, REINTERRUPT if after signalled, or
+         * 0 if not interrupted.
+         */
+        private int checkInterruptWhileWaiting(Node node) {
+            return (Thread.interrupted()) ?
+                ((transferAfterCancelledWait(node))? THROW_IE : REINTERRUPT) :
+                0;
+        }
+
+        /**
+         * Throws InterruptedException, reinterrupts current thread, or
+         * does nothing, depending on mode.
+         */
+        private void reportInterruptAfterWait(int interruptMode)
+            throws InterruptedException {
+            if (interruptMode == THROW_IE)
+                throw new InterruptedException();
+            else if (interruptMode == REINTERRUPT)
+                selfInterrupt();
+        }
+
+        /**
+         * Implements interruptible condition wait.
+         * <ol>
+         * <li> If current thread is interrupted, throw InterruptedException
+         * <li> Save lock state returned by {@link #getState}
+         * <li> Invoke {@link #release} with
+         *      saved state as argument, throwing
+         *      IllegalMonitorStateException  if it fails.
+         * <li> Block until signalled or interrupted
+         * <li> Reacquire by invoking specialized version of
+         *      {@link #acquire} with saved state as argument.
+         * <li> If interrupted while blocked in step 4, throw exception
+         * </ol>
+         */
+        public final void await() throws InterruptedException {
+            if (Thread.interrupted())
+                throw new InterruptedException();
+            Node node = addConditionWaiter();
+            long savedState = fullyRelease(node);
+            int interruptMode = 0;
+            while (!isOnSyncQueue(node)) {
+                LockSupport.park(this);
+                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
+                    break;
+            }
+            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
+                interruptMode = REINTERRUPT;
+            if (isOnConditionQueue(node))
+                unlinkCancelledWaiter(node);
+            if (interruptMode != 0)
+                reportInterruptAfterWait(interruptMode);
+        }
+
+        /**
+         * Implements timed condition wait.
+         * <ol>
+         * <li> If current thread is interrupted, throw InterruptedException
+         * <li> Save lock state returned by {@link #getState}
+         * <li> Invoke {@link #release} with
+         *      saved state as argument, throwing
+         *      IllegalMonitorStateException  if it fails.
+         * <li> Block until signalled, interrupted, or timed out
+         * <li> Reacquire by invoking specialized version of
+         *      {@link #acquire} with saved state as argument.
+         * <li> If interrupted while blocked in step 4, throw InterruptedException
+         * </ol>
+         */
+        public final long awaitNanos(long nanosTimeout) throws InterruptedException {
+            if (Thread.interrupted())
+                throw new InterruptedException();
+            Node node = addConditionWaiter();
+            long savedState = fullyRelease(node);
+            long lastTime = System.nanoTime();
+            int interruptMode = 0;
+            while (!isOnSyncQueue(node)) {
+                if (nanosTimeout <= 0L) {
+                    transferAfterCancelledWait(node);
+                    break;
+                }
+                LockSupport.parkNanos(this, nanosTimeout);
+                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
+                    break;
+
+                long now = System.nanoTime();
+                nanosTimeout -= now - lastTime;
+                lastTime = now;
+            }
+            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
+                interruptMode = REINTERRUPT;
+            if (isOnConditionQueue(node))
+                unlinkCancelledWaiter(node);
+            if (interruptMode != 0)
+                reportInterruptAfterWait(interruptMode);
+            return nanosTimeout - (System.nanoTime() - lastTime);
+        }
+
+        /**
+         * Implements absolute timed condition wait.
+         * <ol>
+         * <li> If current thread is interrupted, throw InterruptedException
+         * <li> Save lock state returned by {@link #getState}
+         * <li> Invoke {@link #release} with
+         *      saved state as argument, throwing
+         *      IllegalMonitorStateException  if it fails.
+         * <li> Block until signalled, interrupted, or timed out
+         * <li> Reacquire by invoking specialized version of
+         *      {@link #acquire} with saved state as argument.
+         * <li> If interrupted while blocked in step 4, throw InterruptedException
+         * <li> If timed out while blocked in step 4, return false, else true
+         * </ol>
+         */
+        public final boolean awaitUntil(Date deadline) throws InterruptedException {
+            if (deadline == null)
+                throw new NullPointerException();
+            long abstime = deadline.getTime();
+            if (Thread.interrupted())
+                throw new InterruptedException();
+            Node node = addConditionWaiter();
+            long savedState = fullyRelease(node);
+            boolean timedout = false;
+            int interruptMode = 0;
+            while (!isOnSyncQueue(node)) {
+                if (System.currentTimeMillis() > abstime) {
+                    timedout = transferAfterCancelledWait(node);
+                    break;
+                }
+                LockSupport.parkUntil(this, abstime);
+                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
+                    break;
+            }
+            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
+                interruptMode = REINTERRUPT;
+            if (isOnConditionQueue(node))
+                unlinkCancelledWaiter(node);
+            if (interruptMode != 0)
+                reportInterruptAfterWait(interruptMode);
+            return !timedout;
+        }
+
+        /**
+         * Implements timed condition wait.
+         * <ol>
+         * <li> If current thread is interrupted, throw InterruptedException
+         * <li> Save lock state returned by {@link #getState}
+         * <li> Invoke {@link #release} with
+         *      saved state as argument, throwing
+         *      IllegalMonitorStateException  if it fails.
+         * <li> Block until signalled, interrupted, or timed out
+         * <li> Reacquire by invoking specialized version of
+         *      {@link #acquire} with saved state as argument.
+         * <li> If interrupted while blocked in step 4, throw InterruptedException
+         * <li> If timed out while blocked in step 4, return false, else true
+         * </ol>
+         */
+        public final boolean await(long time, TimeUnit unit) throws InterruptedException {
+            if (unit == null)
+                throw new NullPointerException();
+            long nanosTimeout = unit.toNanos(time);
+            if (Thread.interrupted())
+                throw new InterruptedException();
+            Node node = addConditionWaiter();
+            long savedState = fullyRelease(node);
+            long lastTime = System.nanoTime();
+            boolean timedout = false;
+            int interruptMode = 0;
+            while (!isOnSyncQueue(node)) {
+                if (nanosTimeout <= 0L) {
+                    timedout = transferAfterCancelledWait(node);
+                    break;
+                }
+                LockSupport.parkNanos(this, nanosTimeout);
+                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
+                    break;
+                long now = System.nanoTime();
+                nanosTimeout -= now - lastTime;
+                lastTime = now;
+            }
+            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
+                interruptMode = REINTERRUPT;
+            if (isOnConditionQueue(node))
+                unlinkCancelledWaiter(node);
+            if (interruptMode != 0)
+                reportInterruptAfterWait(interruptMode);
+            return !timedout;
+        }
+
+        //  support for instrumentation
+
+        /**
+         * Returns true if this condition was created by the given
+         * synchronization object.
+         *
+         * @return {@code true} if owned
+         */
+        final boolean isOwnedBy(AbstractQueuedLongSynchronizer sync) {
+            return sync == AbstractQueuedLongSynchronizer.this;
+        }
+
+        /**
+         * Queries whether any threads are waiting on this condition.
+         * Implements {@link AbstractQueuedLongSynchronizer#hasWaiters}.
+         *
+         * @return {@code true} if there are any waiting threads
+         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
+         *         returns {@code false}
+         */
+        protected final boolean hasWaiters() {
+            if (!isHeldExclusively())
+                throw new IllegalMonitorStateException();
+            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
+                if (w.waitStatus == Node.CONDITION)
+                    return true;
+            }
+            return false;
+        }
+
+        /**
+         * Returns an estimate of the number of threads waiting on
+         * this condition.
+         * Implements {@link AbstractQueuedLongSynchronizer#getWaitQueueLength}.
+         *
+         * @return the estimated number of waiting threads
+         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
+         *         returns {@code false}
+         */
+        protected final int getWaitQueueLength() {
+            if (!isHeldExclusively())
+                throw new IllegalMonitorStateException();
+            int n = 0;
+            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
+                if (w.waitStatus == Node.CONDITION)
+                    ++n;
+            }
+            return n;
+        }
+
+        /**
+         * Returns a collection containing those threads that may be
+         * waiting on this Condition.
+         * Implements {@link AbstractQueuedLongSynchronizer#getWaitingThreads}.
+         *
+         * @return the collection of threads
+         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
+         *         returns {@code false}
+         */
+        protected final Collection<Thread> getWaitingThreads() {
+            if (!isHeldExclusively())
+                throw new IllegalMonitorStateException();
+            ArrayList<Thread> list = new ArrayList<Thread>();
+            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
+                if (w.waitStatus == Node.CONDITION) {
+                    Thread t = w.thread;
+                    if (t != null)
+                        list.add(t);
+                }
+            }
+            return list;
+        }
+    }
+
+    /**
+     * Setup to support compareAndSet. We need to natively implement
+     * this here: For the sake of permitting future enhancements, we
+     * cannot explicitly subclass AtomicLong, which would be
+     * efficient and useful otherwise. So, as the lesser of evils, we
+     * natively implement using hotspot intrinsics API. And while we
+     * are at it, we do the same for other CASable fields (which could
+     * otherwise be done with atomic field updaters).
+     */
+    private static final Unsafe unsafe = Unsafe.getUnsafe();
+    private static final long stateOffset;
+    private static final long headOffset;
+    private static final long tailOffset;
+    private static final long waitStatusOffset;
+
+    static {
+        try {
+            stateOffset = unsafe.objectFieldOffset
+                (AbstractQueuedLongSynchronizer.class.getDeclaredField("state"));
+            headOffset = unsafe.objectFieldOffset
+                (AbstractQueuedLongSynchronizer.class.getDeclaredField("head"));
+            tailOffset = unsafe.objectFieldOffset
+                (AbstractQueuedLongSynchronizer.class.getDeclaredField("tail"));
+            waitStatusOffset = unsafe.objectFieldOffset
+                (Node.class.getDeclaredField("waitStatus"));
+
+        } catch (Exception ex) { throw new Error(ex); }
+    }
+
+    /**
+     * CAS head field. Used only by enq
+     */
+    private final boolean compareAndSetHead(Node update) {
+        return unsafe.compareAndSwapObject(this, headOffset, null, update);
+    }
+
+    /**
+     * CAS tail field. Used only by enq
+     */
+    private final boolean compareAndSetTail(Node expect, Node update) {
+        return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
+    }
+
+    /**
+     * CAS waitStatus field of a node.
+     */
+    private final static boolean compareAndSetWaitStatus(Node node,
+                                                         int expect,
+                                                         int update) {
+        return unsafe.compareAndSwapInt(node, waitStatusOffset,
+                                        expect, update);
+    }
+}
diff --git a/external/jsr166/java/util/concurrent/locks/AbstractQueuedSynchronizer.java b/external/jsr166/java/util/concurrent/locks/AbstractQueuedSynchronizer.java
new file mode 100644
index 000000000..a3abb2cf5
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/locks/AbstractQueuedSynchronizer.java
@@ -0,0 +1,2159 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent.locks;
+import java.util.*;
+import java.util.concurrent.*;
+import java.util.concurrent.atomic.*;
+import sun.misc.Unsafe;
+
+/**
+ * Provides a framework for implementing blocking locks and related
+ * synchronizers (semaphores, events, etc) that rely on
+ * first-in-first-out (FIFO) wait queues.  This class is designed to
+ * be a useful basis for most kinds of synchronizers that rely on a
+ * single atomic <tt>int</tt> value to represent state. Subclasses
+ * must define the protected methods that change this state, and which
+ * define what that state means in terms of this object being acquired
+ * or released.  Given these, the other methods in this class carry
+ * out all queuing and blocking mechanics. Subclasses can maintain
+ * other state fields, but only the atomically updated <tt>int</tt>
+ * value manipulated using methods {@link #getState}, {@link
+ * #setState} and {@link #compareAndSetState} is tracked with respect
+ * to synchronization.
+ *
+ * <p>Subclasses should be defined as non-public internal helper
+ * classes that are used to implement the synchronization properties
+ * of their enclosing class.  Class
+ * <tt>AbstractQueuedSynchronizer</tt> does not implement any
+ * synchronization interface.  Instead it defines methods such as
+ * {@link #acquireInterruptibly} that can be invoked as
+ * appropriate by concrete locks and related synchronizers to
+ * implement their public methods.
+ *
+ * <p>This class supports either or both a default <em>exclusive</em>
+ * mode and a <em>shared</em> mode. When acquired in exclusive mode,
+ * attempted acquires by other threads cannot succeed. Shared mode
+ * acquires by multiple threads may (but need not) succeed. This class
+ * does not &quot;understand&quot; these differences except in the
+ * mechanical sense that when a shared mode acquire succeeds, the next
+ * waiting thread (if one exists) must also determine whether it can
+ * acquire as well. Threads waiting in the different modes share the
+ * same FIFO queue. Usually, implementation subclasses support only
+ * one of these modes, but both can come into play for example in a
+ * {@link ReadWriteLock}. Subclasses that support only exclusive or
+ * only shared modes need not define the methods supporting the unused mode.
+ *
+ * <p>This class defines a nested {@link ConditionObject} class that
+ * can be used as a {@link Condition} implementation by subclasses
+ * supporting exclusive mode for which method {@link
+ * #isHeldExclusively} reports whether synchronization is exclusively
+ * held with respect to the current thread, method {@link #release}
+ * invoked with the current {@link #getState} value fully releases
+ * this object, and {@link #acquire}, given this saved state value,
+ * eventually restores this object to its previous acquired state.  No
+ * <tt>AbstractQueuedSynchronizer</tt> method otherwise creates such a
+ * condition, so if this constraint cannot be met, do not use it.  The
+ * behavior of {@link ConditionObject} depends of course on the
+ * semantics of its synchronizer implementation.
+ *
+ * <p>This class provides inspection, instrumentation, and monitoring
+ * methods for the internal queue, as well as similar methods for
+ * condition objects. These can be exported as desired into classes
+ * using an <tt>AbstractQueuedSynchronizer</tt> for their
+ * synchronization mechanics.
+ *
+ * <p>Serialization of this class stores only the underlying atomic
+ * integer maintaining state, so deserialized objects have empty
+ * thread queues. Typical subclasses requiring serializability will
+ * define a <tt>readObject</tt> method that restores this to a known
+ * initial state upon deserialization.
+ *
+ * <h3>Usage</h3>
+ *
+ * <p>To use this class as the basis of a synchronizer, redefine the
+ * following methods, as applicable, by inspecting and/or modifying
+ * the synchronization state using {@link #getState}, {@link
+ * #setState} and/or {@link #compareAndSetState}:
+ *
+ * <ul>
+ * <li> {@link #tryAcquire}
+ * <li> {@link #tryRelease}
+ * <li> {@link #tryAcquireShared}
+ * <li> {@link #tryReleaseShared}
+ * <li> {@link #isHeldExclusively}
+ *</ul>
+ *
+ * Each of these methods by default throws {@link
+ * UnsupportedOperationException}.  Implementations of these methods
+ * must be internally thread-safe, and should in general be short and
+ * not block. Defining these methods is the <em>only</em> supported
+ * means of using this class. All other methods are declared
+ * <tt>final</tt> because they cannot be independently varied.
+ *
+ * <p>You may also find the inherited methods from {@link
+ * AbstractOwnableSynchronizer} useful to keep track of the thread
+ * owning an exclusive synchronizer.  You are encouraged to use them
+ * -- this enables monitoring and diagnostic tools to assist users in
+ * determining which threads hold locks.
+ *
+ * <p>Even though this class is based on an internal FIFO queue, it
+ * does not automatically enforce FIFO acquisition policies.  The core
+ * of exclusive synchronization takes the form:
+ *
+ * <pre>
+ * Acquire:
+ *     while (!tryAcquire(arg)) {
+ *        <em>enqueue thread if it is not already queued</em>;
+ *        <em>possibly block current thread</em>;
+ *     }
+ *
+ * Release:
+ *     if (tryRelease(arg))
+ *        <em>unblock the first queued thread</em>;
+ * </pre>
+ *
+ * (Shared mode is similar but may involve cascading signals.)
+ *
+ * <p>Because checks in acquire are invoked before enqueuing, a newly
+ * acquiring thread may <em>barge</em> ahead of others that are
+ * blocked and queued. However, you can, if desired, define
+ * <tt>tryAcquire</tt> and/or <tt>tryAcquireShared</tt> to disable
+ * barging by internally invoking one or more of the inspection
+ * methods. In particular, a strict FIFO lock can define
+ * <tt>tryAcquire</tt> to immediately return <tt>false</tt> if {@link
+ * #getFirstQueuedThread} does not return the current thread.  A
+ * normally preferable non-strict fair version can immediately return
+ * <tt>false</tt> only if {@link #hasQueuedThreads} returns
+ * <tt>true</tt> and <tt>getFirstQueuedThread</tt> is not the current
+ * thread; or equivalently, that <tt>getFirstQueuedThread</tt> is both
+ * non-null and not the current thread.  Further variations are
+ * possible.
+ *
+ * <p>Throughput and scalability are generally highest for the
+ * default barging (also known as <em>greedy</em>,
+ * <em>renouncement</em>, and <em>convoy-avoidance</em>) strategy.
+ * While this is not guaranteed to be fair or starvation-free, earlier
+ * queued threads are allowed to recontend before later queued
+ * threads, and each recontention has an unbiased chance to succeed
+ * against incoming threads.  Also, while acquires do not
+ * &quot;spin&quot; in the usual sense, they may perform multiple
+ * invocations of <tt>tryAcquire</tt> interspersed with other
+ * computations before blocking.  This gives most of the benefits of
+ * spins when exclusive synchronization is only briefly held, without
+ * most of the liabilities when it isn't. If so desired, you can
+ * augment this by preceding calls to acquire methods with
+ * "fast-path" checks, possibly prechecking {@link #hasContended}
+ * and/or {@link #hasQueuedThreads} to only do so if the synchronizer
+ * is likely not to be contended.
+ *
+ * <p>This class provides an efficient and scalable basis for
+ * synchronization in part by specializing its range of use to
+ * synchronizers that can rely on <tt>int</tt> state, acquire, and
+ * release parameters, and an internal FIFO wait queue. When this does
+ * not suffice, you can build synchronizers from a lower level using
+ * {@link java.util.concurrent.atomic atomic} classes, your own custom
+ * {@link java.util.Queue} classes, and {@link LockSupport} blocking
+ * support.
+ *
+ * <h3>Usage Examples</h3>
+ *
+ * <p>Here is a non-reentrant mutual exclusion lock class that uses
+ * the value zero to represent the unlocked state, and one to
+ * represent the locked state. While a non-reentrant lock
+ * does not strictly require recording of the current owner
+ * thread, this class does so anyway to make usage easier to monitor.
+ * It also supports conditions and exposes
+ * one of the instrumentation methods:
+ *
+ * <pre>
+ * class Mutex implements Lock, java.io.Serializable {
+ *
+ *   // Our internal helper class
+ *   private static class Sync extends AbstractQueuedSynchronizer {
+ *     // Report whether in locked state
+ *     protected boolean isHeldExclusively() {
+ *       return getState() == 1;
+ *     }
+ *
+ *     // Acquire the lock if state is zero
+ *     public boolean tryAcquire(int acquires) {
+ *       assert acquires == 1; // Otherwise unused
+ *       if (compareAndSetState(0, 1)) {
+ *         setExclusiveOwnerThread(Thread.currentThread());
+ *         return true;
+ *       }
+ *       return false;
+ *     }
+ *
+ *     // Release the lock by setting state to zero
+ *     protected boolean tryRelease(int releases) {
+ *       assert releases == 1; // Otherwise unused
+ *       if (getState() == 0) throw new IllegalMonitorStateException();
+ *       setExclusiveOwnerThread(null);
+ *       setState(0);
+ *       return true;
+ *     }
+ *
+ *     // Provide a Condition
+ *     Condition newCondition() { return new ConditionObject(); }
+ *
+ *     // Deserialize properly
+ *     private void readObject(ObjectInputStream s)
+ *         throws IOException, ClassNotFoundException {
+ *       s.defaultReadObject();
+ *       setState(0); // reset to unlocked state
+ *     }
+ *   }
+ *
+ *   // The sync object does all the hard work. We just forward to it.
+ *   private final Sync sync = new Sync();
+ *
+ *   public void lock()                { sync.acquire(1); }
+ *   public boolean tryLock()          { return sync.tryAcquire(1); }
+ *   public void unlock()              { sync.release(1); }
+ *   public Condition newCondition()   { return sync.newCondition(); }
+ *   public boolean isLocked()         { return sync.isHeldExclusively(); }
+ *   public boolean hasQueuedThreads() { return sync.hasQueuedThreads(); }
+ *   public void lockInterruptibly() throws InterruptedException {
+ *     sync.acquireInterruptibly(1);
+ *   }
+ *   public boolean tryLock(long timeout, TimeUnit unit)
+ *       throws InterruptedException {
+ *     return sync.tryAcquireNanos(1, unit.toNanos(timeout));
+ *   }
+ * }
+ * </pre>
+ *
+ * <p>Here is a latch class that is like a {@link CountDownLatch}
+ * except that it only requires a single <tt>signal</tt> to
+ * fire. Because a latch is non-exclusive, it uses the <tt>shared</tt>
+ * acquire and release methods.
+ *
+ * <pre>
+ * class BooleanLatch {
+ *
+ *   private static class Sync extends AbstractQueuedSynchronizer {
+ *     boolean isSignalled() { return getState() != 0; }
+ *
+ *     protected int tryAcquireShared(int ignore) {
+ *       return isSignalled()? 1 : -1;
+ *     }
+ *
+ *     protected boolean tryReleaseShared(int ignore) {
+ *       setState(1);
+ *       return true;
+ *     }
+ *   }
+ *
+ *   private final Sync sync = new Sync();
+ *   public boolean isSignalled() { return sync.isSignalled(); }
+ *   public void signal()         { sync.releaseShared(1); }
+ *   public void await() throws InterruptedException {
+ *     sync.acquireSharedInterruptibly(1);
+ *   }
+ * }
+ * </pre>
+ *
+ * @since 1.5
+ * @author Doug Lea
+ */
+public abstract class AbstractQueuedSynchronizer
+    extends AbstractOwnableSynchronizer
+    implements java.io.Serializable {
+
+    private static final long serialVersionUID = 7373984972572414691L;
+
+    /**
+     * Creates a new <tt>AbstractQueuedSynchronizer</tt> instance
+     * with initial synchronization state of zero.
+     */
+    protected AbstractQueuedSynchronizer() { }
+
+    /**
+     * Wait queue node class.
+     *
+     * <p>The wait queue is a variant of a "CLH" (Craig, Landin, and
+     * Hagersten) lock queue. CLH locks are normally used for
+     * spinlocks.  We instead use them for blocking synchronizers, but
+     * use the same basic tactic of holding some of the control
+     * information about a thread in the predecessor of its node.  A
+     * "status" field in each node keeps track of whether a thread
+     * should block.  A node is signalled when its predecessor
+     * releases.  Each node of the queue otherwise serves as a
+     * specific-notification-style monitor holding a single waiting
+     * thread. The status field does NOT control whether threads are
+     * granted locks etc though.  A thread may try to acquire if it is
+     * first in the queue. But being first does not guarantee success;
+     * it only gives the right to contend.  So the currently released
+     * contender thread may need to rewait.
+     *
+     * <p>To enqueue into a CLH lock, you atomically splice it in as new
+     * tail. To dequeue, you just set the head field.
+     * <pre>
+     *      +------+  prev +-----+       +-----+
+     * head |      | <---- |     | <---- |     |  tail
+     *      +------+       +-----+       +-----+
+     * </pre>
+     *
+     * <p>Insertion into a CLH queue requires only a single atomic
+     * operation on "tail", so there is a simple atomic point of
+     * demarcation from unqueued to queued. Similarly, dequeing
+     * involves only updating the "head". However, it takes a bit
+     * more work for nodes to determine who their successors are,
+     * in part to deal with possible cancellation due to timeouts
+     * and interrupts.
+     *
+     * <p>The "prev" links (not used in original CLH locks), are mainly
+     * needed to handle cancellation. If a node is cancelled, its
+     * successor is (normally) relinked to a non-cancelled
+     * predecessor. For explanation of similar mechanics in the case
+     * of spin locks, see the papers by Scott and Scherer at
+     * http://www.cs.rochester.edu/u/scott/synchronization/
+     *
+     * <p>We also use "next" links to implement blocking mechanics.
+     * The thread id for each node is kept in its own node, so a
+     * predecessor signals the next node to wake up by traversing
+     * next link to determine which thread it is.  Determination of
+     * successor must avoid races with newly queued nodes to set
+     * the "next" fields of their predecessors.  This is solved
+     * when necessary by checking backwards from the atomically
+     * updated "tail" when a node's successor appears to be null.
+     * (Or, said differently, the next-links are an optimization
+     * so that we don't usually need a backward scan.)
+     *
+     * <p>Cancellation introduces some conservatism to the basic
+     * algorithms.  Since we must poll for cancellation of other
+     * nodes, we can miss noticing whether a cancelled node is
+     * ahead or behind us. This is dealt with by always unparking
+     * successors upon cancellation, allowing them to stabilize on
+     * a new predecessor.
+     *
+     * <p>CLH queues need a dummy header node to get started. But
+     * we don't create them on construction, because it would be wasted
+     * effort if there is never contention. Instead, the node
+     * is constructed and head and tail pointers are set upon first
+     * contention.
+     *
+     * <p>Threads waiting on Conditions use the same nodes, but
+     * use an additional link. Conditions only need to link nodes
+     * in simple (non-concurrent) linked queues because they are
+     * only accessed when exclusively held.  Upon await, a node is
+     * inserted into a condition queue.  Upon signal, the node is
+     * transferred to the main queue.  A special value of status
+     * field is used to mark which queue a node is on.
+     *
+     * <p>Thanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill
+     * Scherer and Michael Scott, along with members of JSR-166
+     * expert group, for helpful ideas, discussions, and critiques
+     * on the design of this class.
+     */
+    static final class Node {
+        /** waitStatus value to indicate thread has cancelled */
+        static final int CANCELLED =  1;
+        /** waitStatus value to indicate successor's thread needs unparking */
+        static final int SIGNAL    = -1;
+        /** waitStatus value to indicate thread is waiting on condition */
+        static final int CONDITION = -2;
+        /** Marker to indicate a node is waiting in shared mode */
+        static final Node SHARED = new Node();
+        /** Marker to indicate a node is waiting in exclusive mode */
+        static final Node EXCLUSIVE = null;
+
+        /**
+         * Status field, taking on only the values:
+         *   SIGNAL:     The successor of this node is (or will soon be)
+         *               blocked (via park), so the current node must
+         *               unpark its successor when it releases or
+         *               cancels. To avoid races, acquire methods must
+         *               first indicate they need a signal,
+         *               then retry the atomic acquire, and then,
+         *               on failure, block.
+         *   CANCELLED:  This node is cancelled due to timeout or interrupt.
+         *               Nodes never leave this state. In particular,
+         *               a thread with cancelled node never again blocks.
+         *   CONDITION:  This node is currently on a condition queue.
+         *               It will not be used as a sync queue node until
+         *               transferred. (Use of this value here
+         *               has nothing to do with the other uses
+         *               of the field, but simplifies mechanics.)
+         *   0:          None of the above
+         *
+         * The values are arranged numerically to simplify use.
+         * Non-negative values mean that a node doesn't need to
+         * signal. So, most code doesn't need to check for particular
+         * values, just for sign.
+         *
+         * The field is initialized to 0 for normal sync nodes, and
+         * CONDITION for condition nodes.  It is modified only using
+         * CAS.
+         */
+        volatile int waitStatus;
+
+        /**
+         * Link to predecessor node that current node/thread relies on
+         * for checking waitStatus. Assigned during enqueing, and nulled
+         * out (for sake of GC) only upon dequeuing.  Also, upon
+         * cancellation of a predecessor, we short-circuit while
+         * finding a non-cancelled one, which will always exist
+         * because the head node is never cancelled: A node becomes
+         * head only as a result of successful acquire. A
+         * cancelled thread never succeeds in acquiring, and a thread only
+         * cancels itself, not any other node.
+         */
+        volatile Node prev;
+
+        /**
+         * Link to the successor node that the current node/thread
+         * unparks upon release. Assigned once during enqueuing, and
+         * nulled out (for sake of GC) when no longer needed.  Upon
+         * cancellation, we cannot adjust this field, but can notice
+         * status and bypass the node if cancelled.  The enq operation
+         * does not assign next field of a predecessor until after
+         * attachment, so seeing a null next field does not
+         * necessarily mean that node is at end of queue. However, if
+         * a next field appears to be null, we can scan prev's from
+         * the tail to double-check.
+         */
+        volatile Node next;
+
+        /**
+         * The thread that enqueued this node.  Initialized on
+         * construction and nulled out after use.
+         */
+        volatile Thread thread;
+
+        /**
+         * Link to next node waiting on condition, or the special
+         * value SHARED.  Because condition queues are accessed only
+         * when holding in exclusive mode, we just need a simple
+         * linked queue to hold nodes while they are waiting on
+         * conditions. They are then transferred to the queue to
+         * re-acquire. And because conditions can only be exclusive,
+         * we save a field by using special value to indicate shared
+         * mode.
+         */
+        Node nextWaiter;
+
+        /**
+         * Returns true if node is waiting in shared mode
+         */
+        final boolean isShared() {
+            return nextWaiter == SHARED;
+        }
+
+        /**
+         * Returns previous node, or throws NullPointerException if
+         * null.  Use when predecessor cannot be null.
+         * @return the predecessor of this node
+         */
+        final Node predecessor() throws NullPointerException {
+            Node p = prev;
+            if (p == null)
+                throw new NullPointerException();
+            else
+                return p;
+        }
+
+        Node() {    // Used to establish initial head or SHARED marker
+        }
+
+        Node(Thread thread, Node mode) {     // Used by addWaiter
+            this.nextWaiter = mode;
+            this.thread = thread;
+        }
+
+        Node(Thread thread, int waitStatus) { // Used by Condition
+            this.waitStatus = waitStatus;
+            this.thread = thread;
+        }
+    }
+
+    /**
+     * Head of the wait queue, lazily initialized.  Except for
+     * initialization, it is modified only via method setHead.  Note:
+     * If head exists, its waitStatus is guaranteed not to be
+     * CANCELLED.
+     */
+    private transient volatile Node head;
+
+    /**
+     * Tail of the wait queue, lazily initialized.  Modified only via
+     * method enq to add new wait node.
+     */
+    private transient volatile Node tail;
+
+    /**
+     * The synchronization state.
+     */
+    private volatile int state;
+
+    /**
+     * Returns the current value of synchronization state.
+     * This operation has memory semantics of a <tt>volatile</tt> read.
+     * @return current state value
+     */
+    protected final int getState() {
+        return state;
+    }
+
+    /**
+     * Sets the value of synchronization state.
+     * This operation has memory semantics of a <tt>volatile</tt> write.
+     * @param newState the new state value
+     */
+    protected final void setState(int newState) {
+        state = newState;
+    }
+
+    /**
+     * Atomically sets synchronization state to the given updated
+     * value if the current state value equals the expected value.
+     * This operation has memory semantics of a <tt>volatile</tt> read
+     * and write.
+     *
+     * @param expect the expected value
+     * @param update the new value
+     * @return true if successful. False return indicates that the actual
+     *         value was not equal to the expected value.
+     */
+    protected final boolean compareAndSetState(int expect, int update) {
+        // See below for intrinsics setup to support this
+        return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
+    }
+
+    // Queuing utilities
+
+    /**
+     * The number of nanoseconds for which it is faster to spin
+     * rather than to use timed park. A rough estimate suffices
+     * to improve responsiveness with very short timeouts.
+     */
+    static final long spinForTimeoutThreshold = 1000L;
+
+    /**
+     * Inserts node into queue, initializing if necessary. See picture above.
+     * @param node the node to insert
+     * @return node's predecessor
+     */
+    private Node enq(final Node node) {
+        for (;;) {
+            Node t = tail;
+            if (t == null) { // Must initialize
+                Node h = new Node(); // Dummy header
+                h.next = node;
+                node.prev = h;
+                if (compareAndSetHead(h)) {
+                    tail = node;
+                    return h;
+                }
+            }
+            else {
+                node.prev = t;
+                if (compareAndSetTail(t, node)) {
+                    t.next = node;
+                    return t;
+                }
+            }
+        }
+    }
+
+    /**
+     * Creates and enqueues node for given thread and mode.
+     *
+     * @param current the thread
+     * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
+     * @return the new node
+     */
+    private Node addWaiter(Node mode) {
+        Node node = new Node(Thread.currentThread(), mode);
+        // Try the fast path of enq; backup to full enq on failure
+        Node pred = tail;
+        if (pred != null) {
+            node.prev = pred;
+            if (compareAndSetTail(pred, node)) {
+                pred.next = node;
+                return node;
+            }
+        }
+        enq(node);
+        return node;
+    }
+
+    /**
+     * Sets head of queue to be node, thus dequeuing. Called only by
+     * acquire methods.  Also nulls out unused fields for sake of GC
+     * and to suppress unnecessary signals and traversals.
+     *
+     * @param node the node
+     */
+    private void setHead(Node node) {
+        head = node;
+        node.thread = null;
+        node.prev = null;
+    }
+
+    /**
+     * Wakes up node's successor, if one exists.
+     *
+     * @param node the node
+     */
+    private void unparkSuccessor(Node node) {
+        /*
+         * Try to clear status in anticipation of signalling.  It is
+         * OK if this fails or if status is changed by waiting thread.
+         */
+        compareAndSetWaitStatus(node, Node.SIGNAL, 0);
+
+        /*
+         * Thread to unpark is held in successor, which is normally
+         * just the next node.  But if cancelled or apparently null,
+         * traverse backwards from tail to find the actual
+         * non-cancelled successor.
+         */
+        Node s = node.next;
+        if (s == null || s.waitStatus > 0) {
+            s = null;
+            for (Node t = tail; t != null && t != node; t = t.prev)
+                if (t.waitStatus <= 0)
+                    s = t;
+        }
+        if (s != null)
+            LockSupport.unpark(s.thread);
+    }
+
+    /**
+     * Sets head of queue, and checks if successor may be waiting
+     * in shared mode, if so propagating if propagate > 0.
+     *
+     * @param pred the node holding waitStatus for node
+     * @param node the node
+     * @param propagate the return value from a tryAcquireShared
+     */
+    private void setHeadAndPropagate(Node node, int propagate) {
+        setHead(node);
+        if (propagate > 0 && node.waitStatus != 0) {
+            /*
+             * Don't bother fully figuring out successor.  If it
+             * looks null, call unparkSuccessor anyway to be safe.
+             */
+            Node s = node.next;
+            if (s == null || s.isShared())
+                unparkSuccessor(node);
+        }
+    }
+
+    // Utilities for various versions of acquire
+
+    /**
+     * Cancels an ongoing attempt to acquire.
+     *
+     * @param node the node
+     */
+    private void cancelAcquire(Node node) {
+        if (node != null) { // Ignore if node doesn't exist
+            node.thread = null;
+            // Can use unconditional write instead of CAS here
+            node.waitStatus = Node.CANCELLED;
+            unparkSuccessor(node);
+        }
+    }
+
+    /**
+     * Checks and updates status for a node that failed to acquire.
+     * Returns true if thread should block. This is the main signal
+     * control in all acquire loops.  Requires that pred == node.prev
+     *
+     * @param pred node's predecessor holding status
+     * @param node the node
+     * @return {@code true} if thread should block
+     */
+    private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
+        int s = pred.waitStatus;
+        if (s < 0)
+            /*
+             * This node has already set status asking a release
+             * to signal it, so it can safely park
+             */
+            return true;
+        if (s > 0)
+            /*
+             * Predecessor was cancelled. Move up to its predecessor
+             * and indicate retry.
+             */
+            node.prev = pred.prev;
+        else
+            /*
+             * Indicate that we need a signal, but don't park yet. Caller
+             * will need to retry to make sure it cannot acquire before
+             * parking.
+             */
+            compareAndSetWaitStatus(pred, 0, Node.SIGNAL);
+        return false;
+    }
+
+    /**
+     * Convenience method to interrupt current thread.
+     */
+    private static void selfInterrupt() {
+        Thread.currentThread().interrupt();
+    }
+
+    /**
+     * Convenience method to park and then check if interrupted
+     *
+     * @return {@code true} if interrupted
+     */
+    private final boolean parkAndCheckInterrupt() {
+        LockSupport.park(this);
+        return Thread.interrupted();
+    }
+
+    /*
+     * Various flavors of acquire, varying in exclusive/shared and
+     * control modes.  Each is mostly the same, but annoyingly
+     * different.  Only a little bit of factoring is possible due to
+     * interactions of exception mechanics (including ensuring that we
+     * cancel if tryAcquire throws exception) and other control, at
+     * least not without hurting performance too much.
+     */
+
+    /**
+     * Acquires in exclusive uninterruptible mode for thread already in
+     * queue. Used by condition wait methods as well as acquire.
+     *
+     * @param node the node
+     * @param arg the acquire argument
+     * @return {@code true} if interrupted while waiting
+     */
+    final boolean acquireQueued(final Node node, int arg) {
+        try {
+            boolean interrupted = false;
+            for (;;) {
+                final Node p = node.predecessor();
+                if (p == head && tryAcquire(arg)) {
+                    setHead(node);
+                    p.next = null; // help GC
+                    return interrupted;
+                }
+                if (shouldParkAfterFailedAcquire(p, node) &&
+                    parkAndCheckInterrupt())
+                    interrupted = true;
+            }
+        } catch (RuntimeException ex) {
+            cancelAcquire(node);
+            throw ex;
+        }
+    }
+
+    /**
+     * Acquires in exclusive interruptible mode.
+     * @param arg the acquire argument
+     */
+    private void doAcquireInterruptibly(int arg)
+        throws InterruptedException {
+        final Node node = addWaiter(Node.EXCLUSIVE);
+        try {
+            for (;;) {
+                final Node p = node.predecessor();
+                if (p == head && tryAcquire(arg)) {
+                    setHead(node);
+                    p.next = null; // help GC
+                    return;
+                }
+                if (shouldParkAfterFailedAcquire(p, node) &&
+                    parkAndCheckInterrupt())
+                    break;
+            }
+        } catch (RuntimeException ex) {
+            cancelAcquire(node);
+            throw ex;
+        }
+        // Arrive here only if interrupted
+        cancelAcquire(node);
+        throw new InterruptedException();
+    }
+
+    /**
+     * Acquires in exclusive timed mode.
+     *
+     * @param arg the acquire argument
+     * @param nanosTimeout max wait time
+     * @return {@code true} if acquired
+     */
+    private boolean doAcquireNanos(int arg, long nanosTimeout)
+        throws InterruptedException {
+        long lastTime = System.nanoTime();
+        final Node node = addWaiter(Node.EXCLUSIVE);
+        try {
+            for (;;) {
+                final Node p = node.predecessor();
+                if (p == head && tryAcquire(arg)) {
+                    setHead(node);
+                    p.next = null; // help GC
+                    return true;
+                }
+                if (nanosTimeout <= 0) {
+                    cancelAcquire(node);
+                    return false;
+                }
+                if (nanosTimeout > spinForTimeoutThreshold &&
+                    shouldParkAfterFailedAcquire(p, node))
+                    LockSupport.parkNanos(this, nanosTimeout);
+                long now = System.nanoTime();
+                nanosTimeout -= now - lastTime;
+                lastTime = now;
+                if (Thread.interrupted())
+                    break;
+            }
+        } catch (RuntimeException ex) {
+            cancelAcquire(node);
+            throw ex;
+        }
+        // Arrive here only if interrupted
+        cancelAcquire(node);
+        throw new InterruptedException();
+    }
+
+    /**
+     * Acquires in shared uninterruptible mode.
+     * @param arg the acquire argument
+     */
+    private void doAcquireShared(int arg) {
+        final Node node = addWaiter(Node.SHARED);
+        try {
+            boolean interrupted = false;
+            for (;;) {
+                final Node p = node.predecessor();
+                if (p == head) {
+                    int r = tryAcquireShared(arg);
+                    if (r >= 0) {
+                        setHeadAndPropagate(node, r);
+                        p.next = null; // help GC
+                        if (interrupted)
+                            selfInterrupt();
+                        return;
+                    }
+                }
+                if (shouldParkAfterFailedAcquire(p, node) &&
+                    parkAndCheckInterrupt())
+                    interrupted = true;
+            }
+        } catch (RuntimeException ex) {
+            cancelAcquire(node);
+            throw ex;
+        }
+    }
+
+    /**
+     * Acquires in shared interruptible mode.
+     * @param arg the acquire argument
+     */
+    private void doAcquireSharedInterruptibly(int arg)
+        throws InterruptedException {
+        final Node node = addWaiter(Node.SHARED);
+        try {
+            for (;;) {
+                final Node p = node.predecessor();
+                if (p == head) {
+                    int r = tryAcquireShared(arg);
+                    if (r >= 0) {
+                        setHeadAndPropagate(node, r);
+                        p.next = null; // help GC
+                        return;
+                    }
+                }
+                if (shouldParkAfterFailedAcquire(p, node) &&
+                    parkAndCheckInterrupt())
+                    break;
+            }
+        } catch (RuntimeException ex) {
+            cancelAcquire(node);
+            throw ex;
+        }
+        // Arrive here only if interrupted
+        cancelAcquire(node);
+        throw new InterruptedException();
+    }
+
+    /**
+     * Acquires in shared timed mode.
+     *
+     * @param arg the acquire argument
+     * @param nanosTimeout max wait time
+     * @return {@code true} if acquired
+     */
+    private boolean doAcquireSharedNanos(int arg, long nanosTimeout)
+        throws InterruptedException {
+
+        long lastTime = System.nanoTime();
+        final Node node = addWaiter(Node.SHARED);
+        try {
+            for (;;) {
+                final Node p = node.predecessor();
+                if (p == head) {
+                    int r = tryAcquireShared(arg);
+                    if (r >= 0) {
+                        setHeadAndPropagate(node, r);
+                        p.next = null; // help GC
+                        return true;
+                    }
+                }
+                if (nanosTimeout <= 0) {
+                    cancelAcquire(node);
+                    return false;
+                }
+                if (nanosTimeout > spinForTimeoutThreshold &&
+                    shouldParkAfterFailedAcquire(p, node))
+                    LockSupport.parkNanos(this, nanosTimeout);
+                long now = System.nanoTime();
+                nanosTimeout -= now - lastTime;
+                lastTime = now;
+                if (Thread.interrupted())
+                    break;
+            }
+        } catch (RuntimeException ex) {
+            cancelAcquire(node);
+            throw ex;
+        }
+        // Arrive here only if interrupted
+        cancelAcquire(node);
+        throw new InterruptedException();
+    }
+
+    // Main exported methods
+
+    /**
+     * Attempts to acquire in exclusive mode. This method should query
+     * if the state of the object permits it to be acquired in the
+     * exclusive mode, and if so to acquire it.
+     *
+     * <p>This method is always invoked by the thread performing
+     * acquire.  If this method reports failure, the acquire method
+     * may queue the thread, if it is not already queued, until it is
+     * signalled by a release from some other thread. This can be used
+     * to implement method {@link Lock#tryLock()}.
+     *
+     * <p>The default
+     * implementation throws {@link UnsupportedOperationException}.
+     *
+     * @param arg the acquire argument. This value is always the one
+     *        passed to an acquire method, or is the value saved on entry
+     *        to a condition wait.  The value is otherwise uninterpreted
+     *        and can represent anything you like.
+     * @return {@code true} if successful. Upon success, this object has
+     *         been acquired.
+     * @throws IllegalMonitorStateException if acquiring would place this
+     *         synchronizer in an illegal state. This exception must be
+     *         thrown in a consistent fashion for synchronization to work
+     *         correctly.
+     * @throws UnsupportedOperationException if exclusive mode is not supported
+     */
+    protected boolean tryAcquire(int arg) {
+        throw new UnsupportedOperationException();
+    }
+
+    /**
+     * Attempts to set the state to reflect a release in exclusive
+     * mode.
+     *
+     * <p>This method is always invoked by the thread performing release.
+     *
+     * <p>The default implementation throws
+     * {@link UnsupportedOperationException}.
+     *
+     * @param arg the release argument. This value is always the one
+     *        passed to a release method, or the current state value upon
+     *        entry to a condition wait.  The value is otherwise
+     *        uninterpreted and can represent anything you like.
+     * @return {@code true} if this object is now in a fully released
+     *         state, so that any waiting threads may attempt to acquire;
+     *         and {@code false} otherwise.
+     * @throws IllegalMonitorStateException if releasing would place this
+     *         synchronizer in an illegal state. This exception must be
+     *         thrown in a consistent fashion for synchronization to work
+     *         correctly.
+     * @throws UnsupportedOperationException if exclusive mode is not supported
+     */
+    protected boolean tryRelease(int arg) {
+        throw new UnsupportedOperationException();
+    }
+
+    /**
+     * Attempts to acquire in shared mode. This method should query if
+     * the state of the object permits it to be acquired in the shared
+     * mode, and if so to acquire it.
+     *
+     * <p>This method is always invoked by the thread performing
+     * acquire.  If this method reports failure, the acquire method
+     * may queue the thread, if it is not already queued, until it is
+     * signalled by a release from some other thread.
+     *
+     * <p>The default implementation throws {@link
+     * UnsupportedOperationException}.
+     *
+     * @param arg the acquire argument. This value is always the one
+     *        passed to an acquire method, or is the value saved on entry
+     *        to a condition wait.  The value is otherwise uninterpreted
+     *        and can represent anything you like.
+     * @return a negative value on failure; zero if acquisition in shared
+     *         mode succeeded but no subsequent shared-mode acquire can
+     *         succeed; and a positive value if acquisition in shared
+     *         mode succeeded and subsequent shared-mode acquires might
+     *         also succeed, in which case a subsequent waiting thread
+     *         must check availability. (Support for three different
+     *         return values enables this method to be used in contexts
+     *         where acquires only sometimes act exclusively.)  Upon
+     *         success, this object has been acquired.
+     * @throws IllegalMonitorStateException if acquiring would place this
+     *         synchronizer in an illegal state. This exception must be
+     *         thrown in a consistent fashion for synchronization to work
+     *         correctly.
+     * @throws UnsupportedOperationException if shared mode is not supported
+     */
+    protected int tryAcquireShared(int arg) {
+        throw new UnsupportedOperationException();
+    }
+
+    /**
+     * Attempts to set the state to reflect a release in shared mode.
+     *
+     * <p>This method is always invoked by the thread performing release.
+     *
+     * <p>The default implementation throws
+     * {@link UnsupportedOperationException}.
+     *
+     * @param arg the release argument. This value is always the one
+     *        passed to a release method, or the current state value upon
+     *        entry to a condition wait.  The value is otherwise
+     *        uninterpreted and can represent anything you like.
+     * @return {@code true} if this release of shared mode may permit a
+     *         waiting acquire (shared or exclusive) to succeed; and
+     *         {@code false} otherwise
+     * @throws IllegalMonitorStateException if releasing would place this
+     *         synchronizer in an illegal state. This exception must be
+     *         thrown in a consistent fashion for synchronization to work
+     *         correctly.
+     * @throws UnsupportedOperationException if shared mode is not supported
+     */
+    protected boolean tryReleaseShared(int arg) {
+        throw new UnsupportedOperationException();
+    }
+
+    /**
+     * Returns {@code true} if synchronization is held exclusively with
+     * respect to the current (calling) thread.  This method is invoked
+     * upon each call to a non-waiting {@link ConditionObject} method.
+     * (Waiting methods instead invoke {@link #release}.)
+     *
+     * <p>The default implementation throws {@link
+     * UnsupportedOperationException}. This method is invoked
+     * internally only within {@link ConditionObject} methods, so need
+     * not be defined if conditions are not used.
+     *
+     * @return {@code true} if synchronization is held exclusively;
+     *         {@code false} otherwise
+     * @throws UnsupportedOperationException if conditions are not supported
+     */
+    protected boolean isHeldExclusively() {
+        throw new UnsupportedOperationException();
+    }
+
+    /**
+     * Acquires in exclusive mode, ignoring interrupts.  Implemented
+     * by invoking at least once {@link #tryAcquire},
+     * returning on success.  Otherwise the thread is queued, possibly
+     * repeatedly blocking and unblocking, invoking {@link
+     * #tryAcquire} until success.  This method can be used
+     * to implement method {@link Lock#lock}.
+     *
+     * @param arg the acquire argument.  This value is conveyed to
+     *        {@link #tryAcquire} but is otherwise uninterpreted and
+     *        can represent anything you like.
+     */
+    public final void acquire(int arg) {
+        if (!tryAcquire(arg) &&
+            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
+            selfInterrupt();
+    }
+
+    /**
+     * Acquires in exclusive mode, aborting if interrupted.
+     * Implemented by first checking interrupt status, then invoking
+     * at least once {@link #tryAcquire}, returning on
+     * success.  Otherwise the thread is queued, possibly repeatedly
+     * blocking and unblocking, invoking {@link #tryAcquire}
+     * until success or the thread is interrupted.  This method can be
+     * used to implement method {@link Lock#lockInterruptibly}.
+     *
+     * @param arg the acquire argument.  This value is conveyed to
+     *        {@link #tryAcquire} but is otherwise uninterpreted and
+     *        can represent anything you like.
+     * @throws InterruptedException if the current thread is interrupted
+     */
+    public final void acquireInterruptibly(int arg) throws InterruptedException {
+        if (Thread.interrupted())
+            throw new InterruptedException();
+        if (!tryAcquire(arg))
+            doAcquireInterruptibly(arg);
+    }
+
+    /**
+     * Attempts to acquire in exclusive mode, aborting if interrupted,
+     * and failing if the given timeout elapses.  Implemented by first
+     * checking interrupt status, then invoking at least once {@link
+     * #tryAcquire}, returning on success.  Otherwise, the thread is
+     * queued, possibly repeatedly blocking and unblocking, invoking
+     * {@link #tryAcquire} until success or the thread is interrupted
+     * or the timeout elapses.  This method can be used to implement
+     * method {@link Lock#tryLock(long, TimeUnit)}.
+     *
+     * @param arg the acquire argument.  This value is conveyed to
+     *        {@link #tryAcquire} but is otherwise uninterpreted and
+     *        can represent anything you like.
+     * @param nanosTimeout the maximum number of nanoseconds to wait
+     * @return {@code true} if acquired; {@code false} if timed out
+     * @throws InterruptedException if the current thread is interrupted
+     */
+    public final boolean tryAcquireNanos(int arg, long nanosTimeout) throws InterruptedException {
+	if (Thread.interrupted())
+	    throw new InterruptedException();
+	return tryAcquire(arg) ||
+	    doAcquireNanos(arg, nanosTimeout);
+    }
+
+    /**
+     * Releases in exclusive mode.  Implemented by unblocking one or
+     * more threads if {@link #tryRelease} returns true.
+     * This method can be used to implement method {@link Lock#unlock}.
+     *
+     * @param arg the release argument.  This value is conveyed to
+     *        {@link #tryRelease} but is otherwise uninterpreted and
+     *        can represent anything you like.
+     * @return the value returned from {@link #tryRelease}
+     */
+    public final boolean release(int arg) {
+        if (tryRelease(arg)) {
+            Node h = head;
+            if (h != null && h.waitStatus != 0)
+                unparkSuccessor(h);
+            return true;
+        }
+        return false;
+    }
+
+    /**
+     * Acquires in shared mode, ignoring interrupts.  Implemented by
+     * first invoking at least once {@link #tryAcquireShared},
+     * returning on success.  Otherwise the thread is queued, possibly
+     * repeatedly blocking and unblocking, invoking {@link
+     * #tryAcquireShared} until success.
+     *
+     * @param arg the acquire argument.  This value is conveyed to
+     *        {@link #tryAcquireShared} but is otherwise uninterpreted
+     *        and can represent anything you like.
+     */
+    public final void acquireShared(int arg) {
+        if (tryAcquireShared(arg) < 0)
+            doAcquireShared(arg);
+    }
+
+    /**
+     * Acquires in shared mode, aborting if interrupted.  Implemented
+     * by first checking interrupt status, then invoking at least once
+     * {@link #tryAcquireShared}, returning on success.  Otherwise the
+     * thread is queued, possibly repeatedly blocking and unblocking,
+     * invoking {@link #tryAcquireShared} until success or the thread
+     * is interrupted.
+     * @param arg the acquire argument.
+     * This value is conveyed to {@link #tryAcquireShared} but is
+     * otherwise uninterpreted and can represent anything
+     * you like.
+     * @throws InterruptedException if the current thread is interrupted
+     */
+    public final void acquireSharedInterruptibly(int arg) throws InterruptedException {
+        if (Thread.interrupted())
+            throw new InterruptedException();
+        if (tryAcquireShared(arg) < 0)
+            doAcquireSharedInterruptibly(arg);
+    }
+
+    /**
+     * Attempts to acquire in shared mode, aborting if interrupted, and
+     * failing if the given timeout elapses.  Implemented by first
+     * checking interrupt status, then invoking at least once {@link
+     * #tryAcquireShared}, returning on success.  Otherwise, the
+     * thread is queued, possibly repeatedly blocking and unblocking,
+     * invoking {@link #tryAcquireShared} until success or the thread
+     * is interrupted or the timeout elapses.
+     *
+     * @param arg the acquire argument.  This value is conveyed to
+     *        {@link #tryAcquireShared} but is otherwise uninterpreted
+     *        and can represent anything you like.
+     * @param nanosTimeout the maximum number of nanoseconds to wait
+     * @return {@code true} if acquired; {@code false} if timed out
+     * @throws InterruptedException if the current thread is interrupted
+     */
+    public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout) throws InterruptedException {
+	if (Thread.interrupted())
+	    throw new InterruptedException();
+	return tryAcquireShared(arg) >= 0 ||
+	    doAcquireSharedNanos(arg, nanosTimeout);
+    }
+
+    /**
+     * Releases in shared mode.  Implemented by unblocking one or more
+     * threads if {@link #tryReleaseShared} returns true.
+     *
+     * @param arg the release argument.  This value is conveyed to
+     *        {@link #tryReleaseShared} but is otherwise uninterpreted
+     *        and can represent anything you like.
+     * @return the value returned from {@link #tryReleaseShared}
+     */
+    public final boolean releaseShared(int arg) {
+        if (tryReleaseShared(arg)) {
+            Node h = head;
+            if (h != null && h.waitStatus != 0)
+                unparkSuccessor(h);
+            return true;
+        }
+        return false;
+    }
+
+    // Queue inspection methods
+
+    /**
+     * Queries whether any threads are waiting to acquire. Note that
+     * because cancellations due to interrupts and timeouts may occur
+     * at any time, a {@code true} return does not guarantee that any
+     * other thread will ever acquire.
+     *
+     * <p>In this implementation, this operation returns in
+     * constant time.
+     *
+     * @return {@code true} if there may be other threads waiting to acquire
+     */
+    public final boolean hasQueuedThreads() {
+        return head != tail;
+    }
+
+    /**
+     * Queries whether any threads have ever contended to acquire this
+     * synchronizer; that is if an acquire method has ever blocked.
+     *
+     * <p>In this implementation, this operation returns in
+     * constant time.
+     *
+     * @return {@code true} if there has ever been contention
+     */
+    public final boolean hasContended() {
+        return head != null;
+    }
+
+    /**
+     * Returns the first (longest-waiting) thread in the queue, or
+     * {@code null} if no threads are currently queued.
+     *
+     * <p>In this implementation, this operation normally returns in
+     * constant time, but may iterate upon contention if other threads are
+     * concurrently modifying the queue.
+     *
+     * @return the first (longest-waiting) thread in the queue, or
+     *         {@code null} if no threads are currently queued
+     */
+    public final Thread getFirstQueuedThread() {
+        // handle only fast path, else relay
+        return (head == tail)? null : fullGetFirstQueuedThread();
+    }
+
+    /**
+     * Version of getFirstQueuedThread called when fastpath fails
+     */
+    private Thread fullGetFirstQueuedThread() {
+        /*
+         * The first node is normally h.next. Try to get its
+         * thread field, ensuring consistent reads: If thread
+         * field is nulled out or s.prev is no longer head, then
+         * some other thread(s) concurrently performed setHead in
+         * between some of our reads. We try this twice before
+         * resorting to traversal.
+         */
+        Node h, s;
+        Thread st;
+        if (((h = head) != null && (s = h.next) != null &&
+             s.prev == head && (st = s.thread) != null) ||
+            ((h = head) != null && (s = h.next) != null &&
+             s.prev == head && (st = s.thread) != null))
+            return st;
+
+        /*
+         * Head's next field might not have been set yet, or may have
+         * been unset after setHead. So we must check to see if tail
+         * is actually first node. If not, we continue on, safely
+         * traversing from tail back to head to find first,
+         * guaranteeing termination.
+         */
+
+        Node t = tail;
+        Thread firstThread = null;
+        while (t != null && t != head) {
+            Thread tt = t.thread;
+            if (tt != null)
+                firstThread = tt;
+            t = t.prev;
+        }
+        return firstThread;
+    }
+
+    /**
+     * Returns true if the given thread is currently queued.
+     *
+     * <p>This implementation traverses the queue to determine
+     * presence of the given thread.
+     *
+     * @param thread the thread
+     * @return {@code true} if the given thread is on the queue
+     * @throws NullPointerException if the thread is null
+     */
+    public final boolean isQueued(Thread thread) {
+        if (thread == null)
+            throw new NullPointerException();
+        for (Node p = tail; p != null; p = p.prev)
+            if (p.thread == thread)
+                return true;
+        return false;
+    }
+
+    /**
+     * Return {@code true} if the apparent first queued thread, if one
+     * exists, is not waiting in exclusive mode. Used only as a heuristic
+     * in ReentrantReadWriteLock.
+     */
+    final boolean apparentlyFirstQueuedIsExclusive() {
+        Node h, s;
+        return ((h = head) != null && (s = h.next) != null &&
+                s.nextWaiter != Node.SHARED);
+    }
+
+    /**
+     * Return {@code true} if the queue is empty or if the given thread
+     * is at the head of the queue. This is reliable only if
+     * <tt>current</tt> is actually Thread.currentThread() of caller.
+     */
+    final boolean isFirst(Thread current) {
+        Node h, s;
+        return ((h = head) == null ||
+                ((s = h.next) != null && s.thread == current) ||
+                fullIsFirst(current));
+    }
+
+    final boolean fullIsFirst(Thread current) {
+        // same idea as fullGetFirstQueuedThread
+        Node h, s;
+        Thread firstThread = null;
+        if (((h = head) != null && (s = h.next) != null &&
+             s.prev == head && (firstThread = s.thread) != null))
+            return firstThread == current;
+        Node t = tail;
+        while (t != null && t != head) {
+            Thread tt = t.thread;
+            if (tt != null)
+                firstThread = tt;
+            t = t.prev;
+        }
+        return firstThread == current || firstThread == null;
+    }
+
+
+    // Instrumentation and monitoring methods
+
+    /**
+     * Returns an estimate of the number of threads waiting to
+     * acquire.  The value is only an estimate because the number of
+     * threads may change dynamically while this method traverses
+     * internal data structures.  This method is designed for use in
+     * monitoring system state, not for synchronization
+     * control.
+     *
+     * @return the estimated number of threads waiting to acquire
+     */
+    public final int getQueueLength() {
+        int n = 0;
+        for (Node p = tail; p != null; p = p.prev) {
+            if (p.thread != null)
+                ++n;
+        }
+        return n;
+    }
+
+    /**
+     * Returns a collection containing threads that may be waiting to
+     * acquire.  Because the actual set of threads may change
+     * dynamically while constructing this result, the returned
+     * collection is only a best-effort estimate.  The elements of the
+     * returned collection are in no particular order.  This method is
+     * designed to facilitate construction of subclasses that provide
+     * more extensive monitoring facilities.
+     *
+     * @return the collection of threads
+     */
+    public final Collection<Thread> getQueuedThreads() {
+        ArrayList<Thread> list = new ArrayList<Thread>();
+        for (Node p = tail; p != null; p = p.prev) {
+            Thread t = p.thread;
+            if (t != null)
+                list.add(t);
+        }
+        return list;
+    }
+
+    /**
+     * Returns a collection containing threads that may be waiting to
+     * acquire in exclusive mode. This has the same properties
+     * as {@link #getQueuedThreads} except that it only returns
+     * those threads waiting due to an exclusive acquire.
+     *
+     * @return the collection of threads
+     */
+    public final Collection<Thread> getExclusiveQueuedThreads() {
+        ArrayList<Thread> list = new ArrayList<Thread>();
+        for (Node p = tail; p != null; p = p.prev) {
+            if (!p.isShared()) {
+                Thread t = p.thread;
+                if (t != null)
+                    list.add(t);
+            }
+        }
+        return list;
+    }
+
+    /**
+     * Returns a collection containing threads that may be waiting to
+     * acquire in shared mode. This has the same properties
+     * as {@link #getQueuedThreads} except that it only returns
+     * those threads waiting due to a shared acquire.
+     *
+     * @return the collection of threads
+     */
+    public final Collection<Thread> getSharedQueuedThreads() {
+        ArrayList<Thread> list = new ArrayList<Thread>();
+        for (Node p = tail; p != null; p = p.prev) {
+            if (p.isShared()) {
+                Thread t = p.thread;
+                if (t != null)
+                    list.add(t);
+            }
+        }
+        return list;
+    }
+
+    /**
+     * Returns a string identifying this synchronizer, as well as its state.
+     * The state, in brackets, includes the String {@code "State ="}
+     * followed by the current value of {@link #getState}, and either
+     * {@code "nonempty"} or {@code "empty"} depending on whether the
+     * queue is empty.
+     *
+     * @return a string identifying this synchronizer, as well as its state
+     */
+    public String toString() {
+        int s = getState();
+        String q  = hasQueuedThreads()? "non" : "";
+        return super.toString() +
+            "[State = " + s + ", " + q + "empty queue]";
+    }
+
+
+    // Internal support methods for Conditions
+
+    /**
+     * Returns true if a node, always one that was initially placed on
+     * a condition queue, is now waiting to reacquire on sync queue.
+     * @param node the node
+     * @return true if is reacquiring
+     */
+    final boolean isOnSyncQueue(Node node) {
+        if (node.waitStatus == Node.CONDITION || node.prev == null)
+            return false;
+        if (node.next != null) // If has successor, it must be on queue
+            return true;
+        /*
+         * node.prev can be non-null, but not yet on queue because
+         * the CAS to place it on queue can fail. So we have to
+         * traverse from tail to make sure it actually made it.  It
+         * will always be near the tail in calls to this method, and
+         * unless the CAS failed (which is unlikely), it will be
+         * there, so we hardly ever traverse much.
+         */
+        return findNodeFromTail(node);
+    }
+
+    /**
+     * Returns true if node is on sync queue by searching backwards from tail.
+     * Called only when needed by isOnSyncQueue.
+     * @return true if present
+     */
+    private boolean findNodeFromTail(Node node) {
+        Node t = tail;
+        for (;;) {
+            if (t == node)
+                return true;
+            if (t == null)
+                return false;
+            t = t.prev;
+        }
+    }
+
+    /**
+     * Transfers a node from a condition queue onto sync queue.
+     * Returns true if successful.
+     * @param node the node
+     * @return true if successfully transferred (else the node was
+     * cancelled before signal).
+     */
+    final boolean transferForSignal(Node node) {
+        /*
+         * If cannot change waitStatus, the node has been cancelled.
+         */
+        if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
+            return false;
+
+        /*
+         * Splice onto queue and try to set waitStatus of predecessor to
+         * indicate that thread is (probably) waiting. If cancelled or
+         * attempt to set waitStatus fails, wake up to resync (in which
+         * case the waitStatus can be transiently and harmlessly wrong).
+         */
+        Node p = enq(node);
+        int c = p.waitStatus;
+        if (c > 0 || !compareAndSetWaitStatus(p, c, Node.SIGNAL))
+            LockSupport.unpark(node.thread);
+        return true;
+    }
+
+    /**
+     * Transfers node, if necessary, to sync queue after a cancelled
+     * wait. Returns true if thread was cancelled before being
+     * signalled.
+     * @param current the waiting thread
+     * @param node its node
+     * @return true if cancelled before the node was signalled.
+     */
+    final boolean transferAfterCancelledWait(Node node) {
+        if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
+            enq(node);
+            return true;
+        }
+        /*
+         * If we lost out to a signal(), then we can't proceed
+         * until it finishes its enq().  Cancelling during an
+         * incomplete transfer is both rare and transient, so just
+         * spin.
+         */
+        while (!isOnSyncQueue(node))
+            Thread.yield();
+        return false;
+    }
+
+    /**
+     * Invokes release with current state value; returns saved state.
+     * Cancels node and throws exception on failure.
+     * @param node the condition node for this wait
+     * @return previous sync state
+     */
+    final int fullyRelease(Node node) {
+        try {
+            int savedState = getState();
+            if (release(savedState))
+                return savedState;
+        } catch (RuntimeException ex) {
+            node.waitStatus = Node.CANCELLED;
+            throw ex;
+        }
+        // reach here if release fails
+        node.waitStatus = Node.CANCELLED;
+        throw new IllegalMonitorStateException();
+    }
+
+    // Instrumentation methods for conditions
+
+    /**
+     * Queries whether the given ConditionObject
+     * uses this synchronizer as its lock.
+     *
+     * @param condition the condition
+     * @return <tt>true</tt> if owned
+     * @throws NullPointerException if the condition is null
+     */
+    public final boolean owns(ConditionObject condition) {
+        if (condition == null)
+            throw new NullPointerException();
+        return condition.isOwnedBy(this);
+    }
+
+    /**
+     * Queries whether any threads are waiting on the given condition
+     * associated with this synchronizer. Note that because timeouts
+     * and interrupts may occur at any time, a <tt>true</tt> return
+     * does not guarantee that a future <tt>signal</tt> will awaken
+     * any threads.  This method is designed primarily for use in
+     * monitoring of the system state.
+     *
+     * @param condition the condition
+     * @return <tt>true</tt> if there are any waiting threads
+     * @throws IllegalMonitorStateException if exclusive synchronization
+     *         is not held
+     * @throws IllegalArgumentException if the given condition is
+     *         not associated with this synchronizer
+     * @throws NullPointerException if the condition is null
+     */
+    public final boolean hasWaiters(ConditionObject condition) {
+        if (!owns(condition))
+            throw new IllegalArgumentException("Not owner");
+        return condition.hasWaiters();
+    }
+
+    /**
+     * Returns an estimate of the number of threads waiting on the
+     * given condition associated with this synchronizer. Note that
+     * because timeouts and interrupts may occur at any time, the
+     * estimate serves only as an upper bound on the actual number of
+     * waiters.  This method is designed for use in monitoring of the
+     * system state, not for synchronization control.
+     *
+     * @param condition the condition
+     * @return the estimated number of waiting threads
+     * @throws IllegalMonitorStateException if exclusive synchronization
+     *         is not held
+     * @throws IllegalArgumentException if the given condition is
+     *         not associated with this synchronizer
+     * @throws NullPointerException if the condition is null
+     */
+    public final int getWaitQueueLength(ConditionObject condition) {
+        if (!owns(condition))
+            throw new IllegalArgumentException("Not owner");
+        return condition.getWaitQueueLength();
+    }
+
+    /**
+     * Returns a collection containing those threads that may be
+     * waiting on the given condition associated with this
+     * synchronizer.  Because the actual set of threads may change
+     * dynamically while constructing this result, the returned
+     * collection is only a best-effort estimate. The elements of the
+     * returned collection are in no particular order.
+     *
+     * @param condition the condition
+     * @return the collection of threads
+     * @throws IllegalMonitorStateException if exclusive synchronization
+     *         is not held
+     * @throws IllegalArgumentException if the given condition is
+     *         not associated with this synchronizer
+     * @throws NullPointerException if the condition is null
+     */
+    public final Collection<Thread> getWaitingThreads(ConditionObject condition) {
+        if (!owns(condition))
+            throw new IllegalArgumentException("Not owner");
+        return condition.getWaitingThreads();
+    }
+
+    /**
+     * Condition implementation for a {@link
+     * AbstractQueuedSynchronizer} serving as the basis of a {@link
+     * Lock} implementation.
+     *
+     * <p>Method documentation for this class describes mechanics,
+     * not behavioral specifications from the point of view of Lock
+     * and Condition users. Exported versions of this class will in
+     * general need to be accompanied by documentation describing
+     * condition semantics that rely on those of the associated
+     * <tt>AbstractQueuedSynchronizer</tt>.
+     *
+     * <p>This class is Serializable, but all fields are transient,
+     * so deserialized conditions have no waiters.
+     */
+    public class ConditionObject implements Condition, java.io.Serializable {
+        private static final long serialVersionUID = 1173984872572414699L;
+        /** First node of condition queue. */
+        private transient Node firstWaiter;
+        /** Last node of condition queue. */
+        private transient Node lastWaiter;
+
+        /**
+         * Creates a new <tt>ConditionObject</tt> instance.
+         */
+        public ConditionObject() { }
+
+        // Internal methods
+
+        /**
+         * Adds a new waiter to wait queue.
+         * @return its new wait node
+         */
+        private Node addConditionWaiter() {
+            Node node = new Node(Thread.currentThread(), Node.CONDITION);
+            Node t = lastWaiter;
+            if (t == null)
+                firstWaiter = node;
+            else
+                t.nextWaiter = node;
+            lastWaiter = node;
+            return node;
+        }
+
+        /**
+         * Removes and transfers nodes until hit non-cancelled one or
+         * null. Split out from signal in part to encourage compilers
+         * to inline the case of no waiters.
+         * @param first (non-null) the first node on condition queue
+         */
+        private void doSignal(Node first) {
+            do {
+                if ( (firstWaiter = first.nextWaiter) == null)
+                    lastWaiter = null;
+                first.nextWaiter = null;
+            } while (!transferForSignal(first) &&
+                     (first = firstWaiter) != null);
+        }
+
+        /**
+         * Removes and transfers all nodes.
+         * @param first (non-null) the first node on condition queue
+         */
+        private void doSignalAll(Node first) {
+            lastWaiter = firstWaiter  = null;
+            do {
+                Node next = first.nextWaiter;
+                first.nextWaiter = null;
+                transferForSignal(first);
+                first = next;
+            } while (first != null);
+        }
+
+        /**
+         * Returns true if given node is on this condition queue.
+         * Call only when holding lock.
+         */
+        private boolean isOnConditionQueue(Node node) {
+            return node.next != null || node == lastWaiter;
+        }
+
+        /**
+         * Unlinks a cancelled waiter node from condition queue.  This
+         * is called when cancellation occurred during condition wait,
+         * not lock wait, and is called only after lock has been
+         * re-acquired by a cancelled waiter and the node is not known
+         * to already have been dequeued.  It is needed to avoid
+         * garbage retention in the absence of signals. So even though
+         * it may require a full traversal, it comes into play only
+         * when timeouts or cancellations occur in the absence of
+         * signals.
+         */
+        private void unlinkCancelledWaiter(Node node) {
+            Node t = firstWaiter;
+            Node trail = null;
+            while (t != null) {
+                if (t == node) {
+                    Node next = t.nextWaiter;
+                    if (trail == null)
+                        firstWaiter = next;
+                    else
+                        trail.nextWaiter = next;
+                    if (lastWaiter == node)
+                        lastWaiter = trail;
+                    break;
+                }
+                trail = t;
+                t = t.nextWaiter;
+            }
+        }
+
+        // public methods
+
+        /**
+         * Moves the longest-waiting thread, if one exists, from the
+         * wait queue for this condition to the wait queue for the
+         * owning lock.
+         *
+         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
+         *         returns {@code false}
+         */
+        public final void signal() {
+            if (!isHeldExclusively())
+                throw new IllegalMonitorStateException();
+            Node first = firstWaiter;
+            if (first != null)
+                doSignal(first);
+        }
+
+        /**
+         * Moves all threads from the wait queue for this condition to
+         * the wait queue for the owning lock.
+         *
+         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
+         *         returns {@code false}
+         */
+        public final void signalAll() {
+            if (!isHeldExclusively())
+                throw new IllegalMonitorStateException();
+            Node first = firstWaiter;
+            if (first != null)
+                doSignalAll(first);
+        }
+
+        /**
+         * Implements uninterruptible condition wait.
+         * <ol>
+         * <li> Save lock state returned by {@link #getState}
+         * <li> Invoke {@link #release} with
+         *      saved state as argument, throwing
+         *      IllegalMonitorStateException if it fails.
+         * <li> Block until signalled
+         * <li> Reacquire by invoking specialized version of
+         *      {@link #acquire} with saved state as argument.
+         * </ol>
+         */
+        public final void awaitUninterruptibly() {
+            Node node = addConditionWaiter();
+            int savedState = fullyRelease(node);
+            boolean interrupted = false;
+            while (!isOnSyncQueue(node)) {
+                LockSupport.park(this);
+                if (Thread.interrupted())
+                    interrupted = true;
+            }
+            if (acquireQueued(node, savedState) || interrupted)
+                selfInterrupt();
+        }
+
+        /*
+         * For interruptible waits, we need to track whether to throw
+         * InterruptedException, if interrupted while blocked on
+         * condition, versus reinterrupt current thread, if
+         * interrupted while blocked waiting to re-acquire.
+         */
+
+        /** Mode meaning to reinterrupt on exit from wait */
+        private static final int REINTERRUPT =  1;
+        /** Mode meaning to throw InterruptedException on exit from wait */
+        private static final int THROW_IE    = -1;
+
+        /**
+         * Checks for interrupt, returning THROW_IE if interrupted
+         * before signalled, REINTERRUPT if after signalled, or
+         * 0 if not interrupted.
+         */
+        private int checkInterruptWhileWaiting(Node node) {
+            return (Thread.interrupted()) ?
+                ((transferAfterCancelledWait(node))? THROW_IE : REINTERRUPT) :
+                0;
+        }
+
+        /**
+         * Throws InterruptedException, reinterrupts current thread, or
+         * does nothing, depending on mode.
+         */
+        private void reportInterruptAfterWait(int interruptMode)
+            throws InterruptedException {
+            if (interruptMode == THROW_IE)
+                throw new InterruptedException();
+            else if (interruptMode == REINTERRUPT)
+                selfInterrupt();
+        }
+
+        /**
+         * Implements interruptible condition wait.
+         * <ol>
+         * <li> If current thread is interrupted, throw InterruptedException
+         * <li> Save lock state returned by {@link #getState}
+         * <li> Invoke {@link #release} with
+         *      saved state as argument, throwing
+         *      IllegalMonitorStateException  if it fails.
+         * <li> Block until signalled or interrupted
+         * <li> Reacquire by invoking specialized version of
+         *      {@link #acquire} with saved state as argument.
+         * <li> If interrupted while blocked in step 4, throw exception
+         * </ol>
+         */
+        public final void await() throws InterruptedException {
+            if (Thread.interrupted())
+                throw new InterruptedException();
+            Node node = addConditionWaiter();
+            int savedState = fullyRelease(node);
+            int interruptMode = 0;
+            while (!isOnSyncQueue(node)) {
+                LockSupport.park(this);
+                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
+                    break;
+            }
+            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
+                interruptMode = REINTERRUPT;
+            if (isOnConditionQueue(node))
+                unlinkCancelledWaiter(node);
+            if (interruptMode != 0)
+                reportInterruptAfterWait(interruptMode);
+        }
+
+        /**
+         * Implements timed condition wait.
+         * <ol>
+         * <li> If current thread is interrupted, throw InterruptedException
+         * <li> Save lock state returned by {@link #getState}
+         * <li> Invoke {@link #release} with
+         *      saved state as argument, throwing
+         *      IllegalMonitorStateException  if it fails.
+         * <li> Block until signalled, interrupted, or timed out
+         * <li> Reacquire by invoking specialized version of
+         *      {@link #acquire} with saved state as argument.
+         * <li> If interrupted while blocked in step 4, throw InterruptedException
+         * </ol>
+         */
+        public final long awaitNanos(long nanosTimeout) throws InterruptedException {
+            if (Thread.interrupted())
+                throw new InterruptedException();
+            Node node = addConditionWaiter();
+            int savedState = fullyRelease(node);
+            long lastTime = System.nanoTime();
+            int interruptMode = 0;
+            while (!isOnSyncQueue(node)) {
+                if (nanosTimeout <= 0L) {
+                    transferAfterCancelledWait(node);
+                    break;
+                }
+                LockSupport.parkNanos(this, nanosTimeout);
+                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
+                    break;
+
+                long now = System.nanoTime();
+                nanosTimeout -= now - lastTime;
+                lastTime = now;
+            }
+            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
+                interruptMode = REINTERRUPT;
+            if (isOnConditionQueue(node))
+                unlinkCancelledWaiter(node);
+            if (interruptMode != 0)
+                reportInterruptAfterWait(interruptMode);
+            return nanosTimeout - (System.nanoTime() - lastTime);
+        }
+
+        /**
+         * Implements absolute timed condition wait.
+         * <ol>
+         * <li> If current thread is interrupted, throw InterruptedException
+         * <li> Save lock state returned by {@link #getState}
+         * <li> Invoke {@link #release} with
+         *      saved state as argument, throwing
+         *      IllegalMonitorStateException  if it fails.
+         * <li> Block until signalled, interrupted, or timed out
+         * <li> Reacquire by invoking specialized version of
+         *      {@link #acquire} with saved state as argument.
+         * <li> If interrupted while blocked in step 4, throw InterruptedException
+         * <li> If timed out while blocked in step 4, return false, else true
+         * </ol>
+         */
+        public final boolean awaitUntil(Date deadline) throws InterruptedException {
+            if (deadline == null)
+                throw new NullPointerException();
+            long abstime = deadline.getTime();
+            if (Thread.interrupted())
+                throw new InterruptedException();
+            Node node = addConditionWaiter();
+            int savedState = fullyRelease(node);
+            boolean timedout = false;
+            int interruptMode = 0;
+            while (!isOnSyncQueue(node)) {
+                if (System.currentTimeMillis() > abstime) {
+                    timedout = transferAfterCancelledWait(node);
+                    break;
+                }
+                LockSupport.parkUntil(this, abstime);
+                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
+                    break;
+            }
+            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
+                interruptMode = REINTERRUPT;
+            if (isOnConditionQueue(node))
+                unlinkCancelledWaiter(node);
+            if (interruptMode != 0)
+                reportInterruptAfterWait(interruptMode);
+            return !timedout;
+        }
+
+        /**
+         * Implements timed condition wait.
+         * <ol>
+         * <li> If current thread is interrupted, throw InterruptedException
+         * <li> Save lock state returned by {@link #getState}
+         * <li> Invoke {@link #release} with
+         *      saved state as argument, throwing
+         *      IllegalMonitorStateException  if it fails.
+         * <li> Block until signalled, interrupted, or timed out
+         * <li> Reacquire by invoking specialized version of
+         *      {@link #acquire} with saved state as argument.
+         * <li> If interrupted while blocked in step 4, throw InterruptedException
+         * <li> If timed out while blocked in step 4, return false, else true
+         * </ol>
+         */
+        public final boolean await(long time, TimeUnit unit) throws InterruptedException {
+            if (unit == null)
+                throw new NullPointerException();
+            long nanosTimeout = unit.toNanos(time);
+            if (Thread.interrupted())
+                throw new InterruptedException();
+            Node node = addConditionWaiter();
+            int savedState = fullyRelease(node);
+            long lastTime = System.nanoTime();
+            boolean timedout = false;
+            int interruptMode = 0;
+            while (!isOnSyncQueue(node)) {
+                if (nanosTimeout <= 0L) {
+                    timedout = transferAfterCancelledWait(node);
+                    break;
+                }
+                LockSupport.parkNanos(this, nanosTimeout);
+                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
+                    break;
+                long now = System.nanoTime();
+                nanosTimeout -= now - lastTime;
+                lastTime = now;
+            }
+            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
+                interruptMode = REINTERRUPT;
+            if (isOnConditionQueue(node))
+                unlinkCancelledWaiter(node);
+            if (interruptMode != 0)
+                reportInterruptAfterWait(interruptMode);
+            return !timedout;
+        }
+
+        //  support for instrumentation
+
+        /**
+         * Returns true if this condition was created by the given
+         * synchronization object.
+         *
+         * @return {@code true} if owned
+         */
+        final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
+            return sync == AbstractQueuedSynchronizer.this;
+        }
+
+        /**
+         * Queries whether any threads are waiting on this condition.
+         * Implements {@link AbstractQueuedSynchronizer#hasWaiters}.
+         *
+         * @return {@code true} if there are any waiting threads
+         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
+         *         returns {@code false}
+         */
+        protected final boolean hasWaiters() {
+            if (!isHeldExclusively())
+                throw new IllegalMonitorStateException();
+            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
+                if (w.waitStatus == Node.CONDITION)
+                    return true;
+            }
+            return false;
+        }
+
+        /**
+         * Returns an estimate of the number of threads waiting on
+         * this condition.
+         * Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength}.
+         *
+         * @return the estimated number of waiting threads
+         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
+         *         returns {@code false}
+         */
+        protected final int getWaitQueueLength() {
+            if (!isHeldExclusively())
+                throw new IllegalMonitorStateException();
+            int n = 0;
+            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
+                if (w.waitStatus == Node.CONDITION)
+                    ++n;
+            }
+            return n;
+        }
+
+        /**
+         * Returns a collection containing those threads that may be
+         * waiting on this Condition.
+         * Implements {@link AbstractQueuedSynchronizer#getWaitingThreads}.
+         *
+         * @return the collection of threads
+         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
+         *         returns {@code false}
+         */
+        protected final Collection<Thread> getWaitingThreads() {
+            if (!isHeldExclusively())
+                throw new IllegalMonitorStateException();
+            ArrayList<Thread> list = new ArrayList<Thread>();
+            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
+                if (w.waitStatus == Node.CONDITION) {
+                    Thread t = w.thread;
+                    if (t != null)
+                        list.add(t);
+                }
+            }
+            return list;
+        }
+    }
+
+    /**
+     * Setup to support compareAndSet. We need to natively implement
+     * this here: For the sake of permitting future enhancements, we
+     * cannot explicitly subclass AtomicInteger, which would be
+     * efficient and useful otherwise. So, as the lesser of evils, we
+     * natively implement using hotspot intrinsics API. And while we
+     * are at it, we do the same for other CASable fields (which could
+     * otherwise be done with atomic field updaters).
+     */
+    private static final Unsafe unsafe = Unsafe.getUnsafe();
+    private static final long stateOffset;
+    private static final long headOffset;
+    private static final long tailOffset;
+    private static final long waitStatusOffset;
+
+    static {
+        try {
+            stateOffset = unsafe.objectFieldOffset
+                (AbstractQueuedSynchronizer.class.getDeclaredField("state"));
+            headOffset = unsafe.objectFieldOffset
+                (AbstractQueuedSynchronizer.class.getDeclaredField("head"));
+            tailOffset = unsafe.objectFieldOffset
+                (AbstractQueuedSynchronizer.class.getDeclaredField("tail"));
+            waitStatusOffset = unsafe.objectFieldOffset
+                (Node.class.getDeclaredField("waitStatus"));
+
+        } catch (Exception ex) { throw new Error(ex); }
+    }
+
+    /**
+     * CAS head field. Used only by enq
+     */
+    private final boolean compareAndSetHead(Node update) {
+        return unsafe.compareAndSwapObject(this, headOffset, null, update);
+    }
+
+    /**
+     * CAS tail field. Used only by enq
+     */
+    private final boolean compareAndSetTail(Node expect, Node update) {
+        return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
+    }
+
+    /**
+     * CAS waitStatus field of a node.
+     */
+    private final static boolean compareAndSetWaitStatus(Node node,
+                                                         int expect,
+                                                         int update) {
+        return unsafe.compareAndSwapInt(node, waitStatusOffset,
+                                        expect, update);
+    }
+}
diff --git a/external/jsr166/java/util/concurrent/locks/Condition.java b/external/jsr166/java/util/concurrent/locks/Condition.java
new file mode 100644
index 000000000..5d24128e1
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/locks/Condition.java
@@ -0,0 +1,435 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent.locks;
+import java.util.concurrent.*;
+import java.util.Date;
+
+/**
+ * {@code Condition} factors out the {@code Object} monitor
+ * methods ({@link Object#wait() wait}, {@link Object#notify notify}
+ * and {@link Object#notifyAll notifyAll}) into distinct objects to
+ * give the effect of having multiple wait-sets per object, by
+ * combining them with the use of arbitrary {@link Lock} implementations.
+ * Where a {@code Lock} replaces the use of {@code synchronized} methods
+ * and statements, a {@code Condition} replaces the use of the Object
+ * monitor methods.
+ *
+ * <p>Conditions (also known as <em>condition queues</em> or
+ * <em>condition variables</em>) provide a means for one thread to
+ * suspend execution (to &quot;wait&quot;) until notified by another
+ * thread that some state condition may now be true.  Because access
+ * to this shared state information occurs in different threads, it
+ * must be protected, so a lock of some form is associated with the
+ * condition. The key property that waiting for a condition provides
+ * is that it <em>atomically</em> releases the associated lock and
+ * suspends the current thread, just like {@code Object.wait}.
+ *
+ * <p>A {@code Condition} instance is intrinsically bound to a lock.
+ * To obtain a {@code Condition} instance for a particular {@link Lock}
+ * instance use its {@link Lock#newCondition newCondition()} method.
+ *
+ * <p>As an example, suppose we have a bounded buffer which supports
+ * {@code put} and {@code take} methods.  If a
+ * {@code take} is attempted on an empty buffer, then the thread will block
+ * until an item becomes available; if a {@code put} is attempted on a
+ * full buffer, then the thread will block until a space becomes available.
+ * We would like to keep waiting {@code put} threads and {@code take}
+ * threads in separate wait-sets so that we can use the optimization of
+ * only notifying a single thread at a time when items or spaces become
+ * available in the buffer. This can be achieved using two
+ * {@link Condition} instances.
+ * <pre>
+ * class BoundedBuffer {
+ *   <b>final Lock lock = new ReentrantLock();</b>
+ *   final Condition notFull  = <b>lock.newCondition(); </b>
+ *   final Condition notEmpty = <b>lock.newCondition(); </b>
+ *
+ *   final Object[] items = new Object[100];
+ *   int putptr, takeptr, count;
+ *
+ *   public void put(Object x) throws InterruptedException {
+ *     <b>lock.lock();
+ *     try {</b>
+ *       while (count == items.length)
+ *         <b>notFull.await();</b>
+ *       items[putptr] = x;
+ *       if (++putptr == items.length) putptr = 0;
+ *       ++count;
+ *       <b>notEmpty.signal();</b>
+ *     <b>} finally {
+ *       lock.unlock();
+ *     }</b>
+ *   }
+ *
+ *   public Object take() throws InterruptedException {
+ *     <b>lock.lock();
+ *     try {</b>
+ *       while (count == 0)
+ *         <b>notEmpty.await();</b>
+ *       Object x = items[takeptr];
+ *       if (++takeptr == items.length) takeptr = 0;
+ *       --count;
+ *       <b>notFull.signal();</b>
+ *       return x;
+ *     <b>} finally {
+ *       lock.unlock();
+ *     }</b>
+ *   }
+ * }
+ * </pre>
+ *
+ * (The {@link java.util.concurrent.ArrayBlockingQueue} class provides
+ * this functionality, so there is no reason to implement this
+ * sample usage class.)
+ *
+ * <p>A {@code Condition} implementation can provide behavior and semantics
+ * that is
+ * different from that of the {@code Object} monitor methods, such as
+ * guaranteed ordering for notifications, or not requiring a lock to be held
+ * when performing notifications.
+ * If an implementation provides such specialized semantics then the
+ * implementation must document those semantics.
+ *
+ * <p>Note that {@code Condition} instances are just normal objects and can
+ * themselves be used as the target in a {@code synchronized} statement,
+ * and can have their own monitor {@link Object#wait wait} and
+ * {@link Object#notify notification} methods invoked.
+ * Acquiring the monitor lock of a {@code Condition} instance, or using its
+ * monitor methods, has no specified relationship with acquiring the
+ * {@link Lock} associated with that {@code Condition} or the use of its
+ * {@linkplain #await waiting} and {@linkplain #signal signalling} methods.
+ * It is recommended that to avoid confusion you never use {@code Condition}
+ * instances in this way, except perhaps within their own implementation.
+ *
+ * <p>Except where noted, passing a {@code null} value for any parameter
+ * will result in a {@link NullPointerException} being thrown.
+ *
+ * <h3>Implementation Considerations</h3>
+ *
+ * <p>When waiting upon a {@code Condition}, a &quot;<em>spurious
+ * wakeup</em>&quot; is permitted to occur, in
+ * general, as a concession to the underlying platform semantics.
+ * This has little practical impact on most application programs as a
+ * {@code Condition} should always be waited upon in a loop, testing
+ * the state predicate that is being waited for.  An implementation is
+ * free to remove the possibility of spurious wakeups but it is
+ * recommended that applications programmers always assume that they can
+ * occur and so always wait in a loop.
+ *
+ * <p>The three forms of condition waiting
+ * (interruptible, non-interruptible, and timed) may differ in their ease of
+ * implementation on some platforms and in their performance characteristics.
+ * In particular, it may be difficult to provide these features and maintain
+ * specific semantics such as ordering guarantees.
+ * Further, the ability to interrupt the actual suspension of the thread may
+ * not always be feasible to implement on all platforms.
+ *
+ * <p>Consequently, an implementation is not required to define exactly the
+ * same guarantees or semantics for all three forms of waiting, nor is it
+ * required to support interruption of the actual suspension of the thread.
+ *
+ * <p>An implementation is required to
+ * clearly document the semantics and guarantees provided by each of the
+ * waiting methods, and when an implementation does support interruption of
+ * thread suspension then it must obey the interruption semantics as defined
+ * in this interface.
+ *
+ * <p>As interruption generally implies cancellation, and checks for
+ * interruption are often infrequent, an implementation can favor responding
+ * to an interrupt over normal method return. This is true even if it can be
+ * shown that the interrupt occurred after another action may have unblocked
+ * the thread. An implementation should document this behavior.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ */
+public interface Condition {
+
+    /**
+     * Causes the current thread to wait until it is signalled or
+     * {@linkplain Thread#interrupt interrupted}.
+     *
+     * <p>The lock associated with this {@code Condition} is atomically
+     * released and the current thread becomes disabled for thread scheduling
+     * purposes and lies dormant until <em>one</em> of four things happens:
+     * <ul>
+     * <li>Some other thread invokes the {@link #signal} method for this
+     * {@code Condition} and the current thread happens to be chosen as the
+     * thread to be awakened; or
+     * <li>Some other thread invokes the {@link #signalAll} method for this
+     * {@code Condition}; or
+     * <li>Some other thread {@linkplain Thread#interrupt interrupts} the
+     * current thread, and interruption of thread suspension is supported; or
+     * <li>A &quot;<em>spurious wakeup</em>&quot; occurs.
+     * </ul>
+     *
+     * <p>In all cases, before this method can return the current thread must
+     * re-acquire the lock associated with this condition. When the
+     * thread returns it is <em>guaranteed</em> to hold this lock.
+     *
+     * <p>If the current thread:
+     * <ul>
+     * <li>has its interrupted status set on entry to this method; or
+     * <li>is {@linkplain Thread#interrupt interrupted} while waiting
+     * and interruption of thread suspension is supported,
+     * </ul>
+     * then {@link InterruptedException} is thrown and the current thread's
+     * interrupted status is cleared. It is not specified, in the first
+     * case, whether or not the test for interruption occurs before the lock
+     * is released.
+     *
+     * <p><b>Implementation Considerations</b>
+     *
+     * <p>The current thread is assumed to hold the lock associated with this
+     * {@code Condition} when this method is called.
+     * It is up to the implementation to determine if this is
+     * the case and if not, how to respond. Typically, an exception will be
+     * thrown (such as {@link IllegalMonitorStateException}) and the
+     * implementation must document that fact.
+     *
+     * <p>An implementation can favor responding to an interrupt over normal
+     * method return in response to a signal. In that case the implementation
+     * must ensure that the signal is redirected to another waiting thread, if
+     * there is one.
+     *
+     * @throws InterruptedException if the current thread is interrupted
+     *         (and interruption of thread suspension is supported)
+     */
+    void await() throws InterruptedException;
+
+    /**
+     * Causes the current thread to wait until it is signalled.
+     *
+     * <p>The lock associated with this condition is atomically
+     * released and the current thread becomes disabled for thread scheduling
+     * purposes and lies dormant until <em>one</em> of three things happens:
+     * <ul>
+     * <li>Some other thread invokes the {@link #signal} method for this
+     * {@code Condition} and the current thread happens to be chosen as the
+     * thread to be awakened; or
+     * <li>Some other thread invokes the {@link #signalAll} method for this
+     * {@code Condition}; or
+     * <li>A &quot;<em>spurious wakeup</em>&quot; occurs.
+     * </ul>
+     *
+     * <p>In all cases, before this method can return the current thread must
+     * re-acquire the lock associated with this condition. When the
+     * thread returns it is <em>guaranteed</em> to hold this lock.
+     *
+     * <p>If the current thread's interrupted status is set when it enters
+     * this method, or it is {@linkplain Thread#interrupt interrupted}
+     * while waiting, it will continue to wait until signalled. When it finally
+     * returns from this method its interrupted status will still
+     * be set.
+     *
+     * <p><b>Implementation Considerations</b>
+     *
+     * <p>The current thread is assumed to hold the lock associated with this
+     * {@code Condition} when this method is called.
+     * It is up to the implementation to determine if this is
+     * the case and if not, how to respond. Typically, an exception will be
+     * thrown (such as {@link IllegalMonitorStateException}) and the
+     * implementation must document that fact.
+     */
+    void awaitUninterruptibly();
+
+    /**
+     * Causes the current thread to wait until it is signalled or interrupted,
+     * or the specified waiting time elapses.
+     *
+     * <p>The lock associated with this condition is atomically
+     * released and the current thread becomes disabled for thread scheduling
+     * purposes and lies dormant until <em>one</em> of five things happens:
+     * <ul>
+     * <li>Some other thread invokes the {@link #signal} method for this
+     * {@code Condition} and the current thread happens to be chosen as the
+     * thread to be awakened; or
+     * <li>Some other thread invokes the {@link #signalAll} method for this
+     * {@code Condition}; or
+     * <li>Some other thread {@linkplain Thread#interrupt interrupts} the
+     * current thread, and interruption of thread suspension is supported; or
+     * <li>The specified waiting time elapses; or
+     * <li>A &quot;<em>spurious wakeup</em>&quot; occurs.
+     * </ul>
+     *
+     * <p>In all cases, before this method can return the current thread must
+     * re-acquire the lock associated with this condition. When the
+     * thread returns it is <em>guaranteed</em> to hold this lock.
+     *
+     * <p>If the current thread:
+     * <ul>
+     * <li>has its interrupted status set on entry to this method; or
+     * <li>is {@linkplain Thread#interrupt interrupted} while waiting
+     * and interruption of thread suspension is supported,
+     * </ul>
+     * then {@link InterruptedException} is thrown and the current thread's
+     * interrupted status is cleared. It is not specified, in the first
+     * case, whether or not the test for interruption occurs before the lock
+     * is released.
+     *
+     * <p>The method returns an estimate of the number of nanoseconds
+     * remaining to wait given the supplied {@code nanosTimeout}
+     * value upon return, or a value less than or equal to zero if it
+     * timed out. This value can be used to determine whether and how
+     * long to re-wait in cases where the wait returns but an awaited
+     * condition still does not hold. Typical uses of this method take
+     * the following form:
+     *
+     * <pre>
+     * synchronized boolean aMethod(long timeout, TimeUnit unit) {
+     *   long nanosTimeout = unit.toNanos(timeout);
+     *   while (!conditionBeingWaitedFor) {
+     *     if (nanosTimeout &gt; 0)
+     *         nanosTimeout = theCondition.awaitNanos(nanosTimeout);
+     *      else
+     *        return false;
+     *   }
+     *   // ...
+     * }
+     * </pre>
+     *
+     * <p> Design note: This method requires a nanosecond argument so
+     * as to avoid truncation errors in reporting remaining times.
+     * Such precision loss would make it difficult for programmers to
+     * ensure that total waiting times are not systematically shorter
+     * than specified when re-waits occur.
+     *
+     * <p><b>Implementation Considerations</b>
+     *
+     * <p>The current thread is assumed to hold the lock associated with this
+     * {@code Condition} when this method is called.
+     * It is up to the implementation to determine if this is
+     * the case and if not, how to respond. Typically, an exception will be
+     * thrown (such as {@link IllegalMonitorStateException}) and the
+     * implementation must document that fact.
+     *
+     * <p>An implementation can favor responding to an interrupt over normal
+     * method return in response to a signal, or over indicating the elapse
+     * of the specified waiting time. In either case the implementation
+     * must ensure that the signal is redirected to another waiting thread, if
+     * there is one.
+     *
+     * @param nanosTimeout the maximum time to wait, in nanoseconds
+     * @return an estimate of the {@code nanosTimeout} value minus
+     *         the time spent waiting upon return from this method.
+     *         A positive value may be used as the argument to a
+     *         subsequent call to this method to finish waiting out
+     *         the desired time.  A value less than or equal to zero
+     *         indicates that no time remains.
+     * @throws InterruptedException if the current thread is interrupted
+     *         (and interruption of thread suspension is supported)
+     */
+    long awaitNanos(long nanosTimeout) throws InterruptedException;
+
+    /**
+     * Causes the current thread to wait until it is signalled or interrupted,
+     * or the specified waiting time elapses. This method is behaviorally
+     * equivalent to:<br>
+     * <pre>
+     *   awaitNanos(unit.toNanos(time)) &gt; 0
+     * </pre>
+     * @param time the maximum time to wait
+     * @param unit the time unit of the {@code time} argument
+     * @return {@code false} if the waiting time detectably elapsed
+     *         before return from the method, else {@code true}
+     * @throws InterruptedException if the current thread is interrupted
+     *         (and interruption of thread suspension is supported)
+     */
+    boolean await(long time, TimeUnit unit) throws InterruptedException;
+
+    /**
+     * Causes the current thread to wait until it is signalled or interrupted,
+     * or the specified deadline elapses.
+     *
+     * <p>The lock associated with this condition is atomically
+     * released and the current thread becomes disabled for thread scheduling
+     * purposes and lies dormant until <em>one</em> of five things happens:
+     * <ul>
+     * <li>Some other thread invokes the {@link #signal} method for this
+     * {@code Condition} and the current thread happens to be chosen as the
+     * thread to be awakened; or
+     * <li>Some other thread invokes the {@link #signalAll} method for this
+     * {@code Condition}; or
+     * <li>Some other thread {@linkplain Thread#interrupt interrupts} the
+     * current thread, and interruption of thread suspension is supported; or
+     * <li>The specified deadline elapses; or
+     * <li>A &quot;<em>spurious wakeup</em>&quot; occurs.
+     * </ul>
+     *
+     * <p>In all cases, before this method can return the current thread must
+     * re-acquire the lock associated with this condition. When the
+     * thread returns it is <em>guaranteed</em> to hold this lock.
+     *
+     *
+     * <p>If the current thread:
+     * <ul>
+     * <li>has its interrupted status set on entry to this method; or
+     * <li>is {@linkplain Thread#interrupt interrupted} while waiting
+     * and interruption of thread suspension is supported,
+     * </ul>
+     * then {@link InterruptedException} is thrown and the current thread's
+     * interrupted status is cleared. It is not specified, in the first
+     * case, whether or not the test for interruption occurs before the lock
+     * is released.
+     *
+     *
+     * <p>The return value indicates whether the deadline has elapsed,
+     * which can be used as follows:
+     * <pre>
+     * synchronized boolean aMethod(Date deadline) {
+     *   boolean stillWaiting = true;
+     *   while (!conditionBeingWaitedFor) {
+     *     if (stillWaiting)
+     *         stillWaiting = theCondition.awaitUntil(deadline);
+     *      else
+     *        return false;
+     *   }
+     *   // ...
+     * }
+     * </pre>
+     *
+     * <p><b>Implementation Considerations</b>
+     *
+     * <p>The current thread is assumed to hold the lock associated with this
+     * {@code Condition} when this method is called.
+     * It is up to the implementation to determine if this is
+     * the case and if not, how to respond. Typically, an exception will be
+     * thrown (such as {@link IllegalMonitorStateException}) and the
+     * implementation must document that fact.
+     *
+     * <p>An implementation can favor responding to an interrupt over normal
+     * method return in response to a signal, or over indicating the passing
+     * of the specified deadline. In either case the implementation
+     * must ensure that the signal is redirected to another waiting thread, if
+     * there is one.
+     *
+     * @param deadline the absolute time to wait until
+     * @return {@code false} if the deadline has elapsed upon return, else
+     *         {@code true}
+     * @throws InterruptedException if the current thread is interrupted
+     *         (and interruption of thread suspension is supported)
+     */
+    boolean awaitUntil(Date deadline) throws InterruptedException;
+
+    /**
+     * Wakes up one waiting thread.
+     *
+     * <p>If any threads are waiting on this condition then one
+     * is selected for waking up. That thread must then re-acquire the
+     * lock before returning from {@code await}.
+     */
+    void signal();
+
+    /**
+     * Wakes up all waiting threads.
+     *
+     * <p>If any threads are waiting on this condition then they are
+     * all woken up. Each thread must re-acquire the lock before it can
+     * return from {@code await}.
+     */
+    void signalAll();
+}
diff --git a/external/jsr166/java/util/concurrent/locks/Lock.java b/external/jsr166/java/util/concurrent/locks/Lock.java
new file mode 100644
index 000000000..4b9abd665
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/locks/Lock.java
@@ -0,0 +1,327 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent.locks;
+import java.util.concurrent.TimeUnit;
+
+/**
+ * {@code Lock} implementations provide more extensive locking
+ * operations than can be obtained using {@code synchronized} methods
+ * and statements.  They allow more flexible structuring, may have
+ * quite different properties, and may support multiple associated
+ * {@link Condition} objects.
+ *
+ * <p>A lock is a tool for controlling access to a shared resource by
+ * multiple threads. Commonly, a lock provides exclusive access to a
+ * shared resource: only one thread at a time can acquire the lock and
+ * all access to the shared resource requires that the lock be
+ * acquired first. However, some locks may allow concurrent access to
+ * a shared resource, such as the read lock of a {@link ReadWriteLock}.
+ *
+ * <p>The use of {@code synchronized} methods or statements provides
+ * access to the implicit monitor lock associated with every object, but
+ * forces all lock acquisition and release to occur in a block-structured way:
+ * when multiple locks are acquired they must be released in the opposite
+ * order, and all locks must be released in the same lexical scope in which
+ * they were acquired.
+ *
+ * <p>While the scoping mechanism for {@code synchronized} methods
+ * and statements makes it much easier to program with monitor locks,
+ * and helps avoid many common programming errors involving locks,
+ * there are occasions where you need to work with locks in a more
+ * flexible way. For example, some algorithms for traversing
+ * concurrently accessed data structures require the use of
+ * &quot;hand-over-hand&quot; or &quot;chain locking&quot;: you
+ * acquire the lock of node A, then node B, then release A and acquire
+ * C, then release B and acquire D and so on.  Implementations of the
+ * {@code Lock} interface enable the use of such techniques by
+ * allowing a lock to be acquired and released in different scopes,
+ * and allowing multiple locks to be acquired and released in any
+ * order.
+ *
+ * <p>With this increased flexibility comes additional
+ * responsibility. The absence of block-structured locking removes the
+ * automatic release of locks that occurs with {@code synchronized}
+ * methods and statements. In most cases, the following idiom
+ * should be used:
+ *
+ * <pre><tt>     Lock l = ...;
+ *     l.lock();
+ *     try {
+ *         // access the resource protected by this lock
+ *     } finally {
+ *         l.unlock();
+ *     }
+ * </tt></pre>
+ *
+ * When locking and unlocking occur in different scopes, care must be
+ * taken to ensure that all code that is executed while the lock is
+ * held is protected by try-finally or try-catch to ensure that the
+ * lock is released when necessary.
+ *
+ * <p>{@code Lock} implementations provide additional functionality
+ * over the use of {@code synchronized} methods and statements by
+ * providing a non-blocking attempt to acquire a lock ({@link
+ * #tryLock()}), an attempt to acquire the lock that can be
+ * interrupted ({@link #lockInterruptibly}, and an attempt to acquire
+ * the lock that can timeout ({@link #tryLock(long, TimeUnit)}).
+ *
+ * <p>A {@code Lock} class can also provide behavior and semantics
+ * that is quite different from that of the implicit monitor lock,
+ * such as guaranteed ordering, non-reentrant usage, or deadlock
+ * detection. If an implementation provides such specialized semantics
+ * then the implementation must document those semantics.
+ *
+ * <p>Note that {@code Lock} instances are just normal objects and can
+ * themselves be used as the target in a {@code synchronized} statement.
+ * Acquiring the
+ * monitor lock of a {@code Lock} instance has no specified relationship
+ * with invoking any of the {@link #lock} methods of that instance.
+ * It is recommended that to avoid confusion you never use {@code Lock}
+ * instances in this way, except within their own implementation.
+ *
+ * <p>Except where noted, passing a {@code null} value for any
+ * parameter will result in a {@link NullPointerException} being
+ * thrown.
+ *
+ * <h3>Memory Synchronization</h3>
+ *
+ * <p>All {@code Lock} implementations <em>must</em> enforce the same
+ * memory synchronization semantics as provided by the built-in monitor
+ * lock, as described in <a href="http://java.sun.com/docs/books/jls/">
+ * The Java Language Specification, Third Edition (17.4 Memory Model)</a>:
+ * <ul>
+ * <li>A successful {@code lock} operation has the same memory
+ * synchronization effects as a successful <em>Lock</em> action.
+ * <li>A successful {@code unlock} operation has the same
+ * memory synchronization effects as a successful <em>Unlock</em> action.
+ * </ul>
+ *
+ * Unsuccessful locking and unlocking operations, and reentrant
+ * locking/unlocking operations, do not require any memory
+ * synchronization effects.
+ *
+ * <h3>Implementation Considerations</h3>
+ *
+ * <p> The three forms of lock acquisition (interruptible,
+ * non-interruptible, and timed) may differ in their performance
+ * characteristics, ordering guarantees, or other implementation
+ * qualities.  Further, the ability to interrupt the <em>ongoing</em>
+ * acquisition of a lock may not be available in a given {@code Lock}
+ * class.  Consequently, an implementation is not required to define
+ * exactly the same guarantees or semantics for all three forms of
+ * lock acquisition, nor is it required to support interruption of an
+ * ongoing lock acquisition.  An implementation is required to clearly
+ * document the semantics and guarantees provided by each of the
+ * locking methods. It must also obey the interruption semantics as
+ * defined in this interface, to the extent that interruption of lock
+ * acquisition is supported: which is either totally, or only on
+ * method entry.
+ *
+ * <p>As interruption generally implies cancellation, and checks for
+ * interruption are often infrequent, an implementation can favor responding
+ * to an interrupt over normal method return. This is true even if it can be
+ * shown that the interrupt occurred after another action may have unblocked
+ * the thread. An implementation should document this behavior.
+ *
+ * @see ReentrantLock
+ * @see Condition
+ * @see ReadWriteLock
+ *
+ * @since 1.5
+ * @author Doug Lea
+ */
+public interface Lock {
+
+    /**
+     * Acquires the lock.
+     *
+     * <p>If the lock is not available then the current thread becomes
+     * disabled for thread scheduling purposes and lies dormant until the
+     * lock has been acquired.
+     *
+     * <p><b>Implementation Considerations</b>
+     *
+     * <p>A {@code Lock} implementation may be able to detect erroneous use
+     * of the lock, such as an invocation that would cause deadlock, and
+     * may throw an (unchecked) exception in such circumstances.  The
+     * circumstances and the exception type must be documented by that
+     * {@code Lock} implementation.
+     */
+    void lock();
+
+    /**
+     * Acquires the lock unless the current thread is
+     * {@linkplain Thread#interrupt interrupted}.
+     *
+     * <p>Acquires the lock if it is available and returns immediately.
+     *
+     * <p>If the lock is not available then the current thread becomes
+     * disabled for thread scheduling purposes and lies dormant until
+     * one of two things happens:
+     *
+     * <ul>
+     * <li>The lock is acquired by the current thread; or
+     * <li>Some other thread {@linkplain Thread#interrupt interrupts} the
+     * current thread, and interruption of lock acquisition is supported.
+     * </ul>
+     *
+     * <p>If the current thread:
+     * <ul>
+     * <li>has its interrupted status set on entry to this method; or
+     * <li>is {@linkplain Thread#interrupt interrupted} while acquiring the
+     * lock, and interruption of lock acquisition is supported,
+     * </ul>
+     * then {@link InterruptedException} is thrown and the current thread's
+     * interrupted status is cleared.
+     *
+     * <p><b>Implementation Considerations</b>
+     *
+     * <p>The ability to interrupt a lock acquisition in some
+     * implementations may not be possible, and if possible may be an
+     * expensive operation.  The programmer should be aware that this
+     * may be the case. An implementation should document when this is
+     * the case.
+     *
+     * <p>An implementation can favor responding to an interrupt over
+     * normal method return.
+     *
+     * <p>A {@code Lock} implementation may be able to detect
+     * erroneous use of the lock, such as an invocation that would
+     * cause deadlock, and may throw an (unchecked) exception in such
+     * circumstances.  The circumstances and the exception type must
+     * be documented by that {@code Lock} implementation.
+     *
+     * @throws InterruptedException if the current thread is
+     *         interrupted while acquiring the lock (and interruption
+     *         of lock acquisition is supported).
+     */
+    void lockInterruptibly() throws InterruptedException;
+
+    /**
+     * Acquires the lock only if it is free at the time of invocation.
+     *
+     * <p>Acquires the lock if it is available and returns immediately
+     * with the value {@code true}.
+     * If the lock is not available then this method will return
+     * immediately with the value {@code false}.
+     *
+     * <p>A typical usage idiom for this method would be:
+     * <pre>
+     *      Lock lock = ...;
+     *      if (lock.tryLock()) {
+     *          try {
+     *              // manipulate protected state
+     *          } finally {
+     *              lock.unlock();
+     *          }
+     *      } else {
+     *          // perform alternative actions
+     *      }
+     * </pre>
+     * This usage ensures that the lock is unlocked if it was acquired, and
+     * doesn't try to unlock if the lock was not acquired.
+     *
+     * @return {@code true} if the lock was acquired and
+     *         {@code false} otherwise
+     */
+    boolean tryLock();
+
+    /**
+     * Acquires the lock if it is free within the given waiting time and the
+     * current thread has not been {@linkplain Thread#interrupt interrupted}.
+     *
+     * <p>If the lock is available this method returns immediately
+     * with the value {@code true}.
+     * If the lock is not available then
+     * the current thread becomes disabled for thread scheduling
+     * purposes and lies dormant until one of three things happens:
+     * <ul>
+     * <li>The lock is acquired by the current thread; or
+     * <li>Some other thread {@linkplain Thread#interrupt interrupts} the
+     * current thread, and interruption of lock acquisition is supported; or
+     * <li>The specified waiting time elapses
+     * </ul>
+     *
+     * <p>If the lock is acquired then the value {@code true} is returned.
+     *
+     * <p>If the current thread:
+     * <ul>
+     * <li>has its interrupted status set on entry to this method; or
+     * <li>is {@linkplain Thread#interrupt interrupted} while acquiring
+     * the lock, and interruption of lock acquisition is supported,
+     * </ul>
+     * then {@link InterruptedException} is thrown and the current thread's
+     * interrupted status is cleared.
+     *
+     * <p>If the specified waiting time elapses then the value {@code false}
+     * is returned.
+     * If the time is
+     * less than or equal to zero, the method will not wait at all.
+     *
+     * <p><b>Implementation Considerations</b>
+     *
+     * <p>The ability to interrupt a lock acquisition in some implementations
+     * may not be possible, and if possible may
+     * be an expensive operation.
+     * The programmer should be aware that this may be the case. An
+     * implementation should document when this is the case.
+     *
+     * <p>An implementation can favor responding to an interrupt over normal
+     * method return, or reporting a timeout.
+     *
+     * <p>A {@code Lock} implementation may be able to detect
+     * erroneous use of the lock, such as an invocation that would cause
+     * deadlock, and may throw an (unchecked) exception in such circumstances.
+     * The circumstances and the exception type must be documented by that
+     * {@code Lock} implementation.
+     *
+     * @param time the maximum time to wait for the lock
+     * @param unit the time unit of the {@code time} argument
+     * @return {@code true} if the lock was acquired and {@code false}
+     *         if the waiting time elapsed before the lock was acquired
+     *
+     * @throws InterruptedException if the current thread is interrupted
+     *         while acquiring the lock (and interruption of lock
+     *         acquisition is supported)
+     */
+    boolean tryLock(long time, TimeUnit unit) throws InterruptedException;
+
+    /**
+     * Releases the lock.
+     *
+     * <p><b>Implementation Considerations</b>
+     *
+     * <p>A {@code Lock} implementation will usually impose
+     * restrictions on which thread can release a lock (typically only the
+     * holder of the lock can release it) and may throw
+     * an (unchecked) exception if the restriction is violated.
+     * Any restrictions and the exception
+     * type must be documented by that {@code Lock} implementation.
+     */
+    void unlock();
+
+    /**
+     * Returns a new {@link Condition} instance that is bound to this
+     * {@code Lock} instance.
+     *
+     * <p>Before waiting on the condition the lock must be held by the
+     * current thread.
+     * A call to {@link Condition#await()} will atomically release the lock
+     * before waiting and re-acquire the lock before the wait returns.
+     *
+     * <p><b>Implementation Considerations</b>
+     *
+     * <p>The exact operation of the {@link Condition} instance depends on
+     * the {@code Lock} implementation and must be documented by that
+     * implementation.
+     *
+     * @return A new {@link Condition} instance for this {@code Lock} instance
+     * @throws UnsupportedOperationException if this {@code Lock}
+     *         implementation does not support conditions
+     */
+    Condition newCondition();
+}
diff --git a/external/jsr166/java/util/concurrent/locks/LockSupport.java b/external/jsr166/java/util/concurrent/locks/LockSupport.java
new file mode 100644
index 000000000..28728ae2b
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/locks/LockSupport.java
@@ -0,0 +1,352 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent.locks;
+import java.util.concurrent.*;
+import sun.misc.Unsafe;
+
+
+/**
+ * Basic thread blocking primitives for creating locks and other
+ * synchronization classes.
+ *
+ * <p>This class associates, with each thread that uses it, a permit
+ * (in the sense of the {@link java.util.concurrent.Semaphore
+ * Semaphore} class). A call to {@code park} will return immediately
+ * if the permit is available, consuming it in the process; otherwise
+ * it <em>may</em> block.  A call to {@code unpark} makes the permit
+ * available, if it was not already available. (Unlike with Semaphores
+ * though, permits do not accumulate. There is at most one.)
+ *
+ * <p>Methods {@code park} and {@code unpark} provide efficient
+ * means of blocking and unblocking threads that do not encounter the
+ * problems that cause the deprecated methods {@code Thread.suspend}
+ * and {@code Thread.resume} to be unusable for such purposes: Races
+ * between one thread invoking {@code park} and another thread trying
+ * to {@code unpark} it will preserve liveness, due to the
+ * permit. Additionally, {@code park} will return if the caller's
+ * thread was interrupted, and timeout versions are supported. The
+ * {@code park} method may also return at any other time, for "no
+ * reason", so in general must be invoked within a loop that rechecks
+ * conditions upon return. In this sense {@code park} serves as an
+ * optimization of a "busy wait" that does not waste as much time
+ * spinning, but must be paired with an {@code unpark} to be
+ * effective.
+ *
+ * <p>The three forms of {@code park} each also support a
+ * {@code blocker} object parameter. This object is recorded while
+ * the thread is blocked to permit monitoring and diagnostic tools to
+ * identify the reasons that threads are blocked. (Such tools may
+ * access blockers using method {@link #getBlocker}.) The use of these
+ * forms rather than the original forms without this parameter is
+ * strongly encouraged. The normal argument to supply as a
+ * {@code blocker} within a lock implementation is {@code this}.
+ *
+ * <p>These methods are designed to be used as tools for creating
+ * higher-level synchronization utilities, and are not in themselves
+ * useful for most concurrency control applications.  The {@code park}
+ * method is designed for use only in constructions of the form:
+ * <pre>while (!canProceed()) { ... LockSupport.park(this); }</pre>
+ * where neither {@code canProceed} nor any other actions prior to the
+ * call to {@code park} entail locking or blocking.  Because only one
+ * permit is associated with each thread, any intermediary uses of
+ * {@code park} could interfere with its intended effects.
+ *
+ * <p><b>Sample Usage.</b> Here is a sketch of a first-in-first-out
+ * non-reentrant lock class:
+ * <pre>{@code
+ * class FIFOMutex {
+ *   private final AtomicBoolean locked = new AtomicBoolean(false);
+ *   private final Queue<Thread> waiters
+ *     = new ConcurrentLinkedQueue<Thread>();
+ *
+ *   public void lock() {
+ *     boolean wasInterrupted = false;
+ *     Thread current = Thread.currentThread();
+ *     waiters.add(current);
+ *
+ *     // Block while not first in queue or cannot acquire lock
+ *     while (waiters.peek() != current ||
+ *            !locked.compareAndSet(false, true)) {
+ *        LockSupport.park(this);
+ *        if (Thread.interrupted()) // ignore interrupts while waiting
+ *          wasInterrupted = true;
+ *     }
+ *
+ *     waiters.remove();
+ *     if (wasInterrupted)          // reassert interrupt status on exit
+ *        current.interrupt();
+ *   }
+ *
+ *   public void unlock() {
+ *     locked.set(false);
+ *     LockSupport.unpark(waiters.peek());
+ *   }
+ * }}</pre>
+ */
+
+public class LockSupport {
+    private LockSupport() {} // Cannot be instantiated.
+
+    // Hotspot implementation via intrinsics API
+    private static final Unsafe unsafe = Unsafe.getUnsafe();
+    private static final long parkBlockerOffset;
+
+    static {
+        try {
+            parkBlockerOffset = unsafe.objectFieldOffset
+                (java.lang.Thread.class.getDeclaredField("parkBlocker"));
+        } catch (Exception ex) { throw new Error(ex); }
+    }
+
+    private static void setBlocker(Thread t, Object arg) {
+        // Even though volatile, hotspot doesn't need a write barrier here.
+        unsafe.putObject(t, parkBlockerOffset, arg);
+    }
+
+    /**
+     * Makes available the permit for the given thread, if it
+     * was not already available.  If the thread was blocked on
+     * {@code park} then it will unblock.  Otherwise, its next call
+     * to {@code park} is guaranteed not to block. This operation
+     * is not guaranteed to have any effect at all if the given
+     * thread has not been started.
+     *
+     * @param thread the thread to unpark, or {@code null}, in which case
+     *        this operation has no effect
+     */
+    public static void unpark(Thread thread) {
+        if (thread != null)
+            unsafe.unpark(thread);
+    }
+
+    /**
+     * Disables the current thread for thread scheduling purposes unless the
+     * permit is available.
+     *
+     * <p>If the permit is available then it is consumed and the call returns
+     * immediately; otherwise
+     * the current thread becomes disabled for thread scheduling
+     * purposes and lies dormant until one of three things happens:
+     *
+     * <ul>
+     * <li>Some other thread invokes {@link #unpark unpark} with the
+     * current thread as the target; or
+     *
+     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
+     * the current thread; or
+     *
+     * <li>The call spuriously (that is, for no reason) returns.
+     * </ul>
+     *
+     * <p>This method does <em>not</em> report which of these caused the
+     * method to return. Callers should re-check the conditions which caused
+     * the thread to park in the first place. Callers may also determine,
+     * for example, the interrupt status of the thread upon return.
+     *
+     * @param blocker the synchronization object responsible for this
+     *        thread parking
+     * @since 1.6
+     */
+    public static void park(Object blocker) {
+        Thread t = Thread.currentThread();
+        setBlocker(t, blocker);
+        unsafe.park(false, 0L);
+        setBlocker(t, null);
+    }
+
+    /**
+     * Disables the current thread for thread scheduling purposes, for up to
+     * the specified waiting time, unless the permit is available.
+     *
+     * <p>If the permit is available then it is consumed and the call
+     * returns immediately; otherwise the current thread becomes disabled
+     * for thread scheduling purposes and lies dormant until one of four
+     * things happens:
+     *
+     * <ul>
+     * <li>Some other thread invokes {@link #unpark unpark} with the
+     * current thread as the target; or
+     *
+     * <li>Some other thread {@linkplain Thread#interrupt interrupts} the current
+     * thread; or
+     *
+     * <li>The specified waiting time elapses; or
+     *
+     * <li>The call spuriously (that is, for no reason) returns.
+     * </ul>
+     *
+     * <p>This method does <em>not</em> report which of these caused the
+     * method to return. Callers should re-check the conditions which caused
+     * the thread to park in the first place. Callers may also determine,
+     * for example, the interrupt status of the thread, or the elapsed time
+     * upon return.
+     *
+     * @param blocker the synchronization object responsible for this
+     *        thread parking
+     * @param nanos the maximum number of nanoseconds to wait
+     * @since 1.6
+     */
+    public static void parkNanos(Object blocker, long nanos) {
+        if (nanos > 0) {
+            Thread t = Thread.currentThread();
+            setBlocker(t, blocker);
+            unsafe.park(false, nanos);
+            setBlocker(t, null);
+        }
+    }
+
+    /**
+     * Disables the current thread for thread scheduling purposes, until
+     * the specified deadline, unless the permit is available.
+     *
+     * <p>If the permit is available then it is consumed and the call
+     * returns immediately; otherwise the current thread becomes disabled
+     * for thread scheduling purposes and lies dormant until one of four
+     * things happens:
+     *
+     * <ul>
+     * <li>Some other thread invokes {@link #unpark unpark} with the
+     * current thread as the target; or
+     *
+     * <li>Some other thread {@linkplain Thread#interrupt interrupts} the
+     * current thread; or
+     *
+     * <li>The specified deadline passes; or
+     *
+     * <li>The call spuriously (that is, for no reason) returns.
+     * </ul>
+     *
+     * <p>This method does <em>not</em> report which of these caused the
+     * method to return. Callers should re-check the conditions which caused
+     * the thread to park in the first place. Callers may also determine,
+     * for example, the interrupt status of the thread, or the current time
+     * upon return.
+     *
+     * @param blocker the synchronization object responsible for this
+     *        thread parking
+     * @param deadline the absolute time, in milliseconds from the Epoch,
+     *        to wait until
+     * @since 1.6
+     */
+    public static void parkUntil(Object blocker, long deadline) {
+        Thread t = Thread.currentThread();
+        setBlocker(t, blocker);
+        unsafe.park(true, deadline);
+        setBlocker(t, null);
+    }
+
+    /**
+     * Returns the blocker object supplied to the most recent
+     * invocation of a park method that has not yet unblocked, or null
+     * if not blocked.  The value returned is just a momentary
+     * snapshot -- the thread may have since unblocked or blocked on a
+     * different blocker object.
+     *
+     * @return the blocker
+     * @since 1.6
+     */
+    public static Object getBlocker(Thread t) {
+        return unsafe.getObjectVolatile(t, parkBlockerOffset);
+    }
+
+    /**
+     * Disables the current thread for thread scheduling purposes unless the
+     * permit is available.
+     *
+     * <p>If the permit is available then it is consumed and the call
+     * returns immediately; otherwise the current thread becomes disabled
+     * for thread scheduling purposes and lies dormant until one of three
+     * things happens:
+     *
+     * <ul>
+     *
+     * <li>Some other thread invokes {@link #unpark unpark} with the
+     * current thread as the target; or
+     *
+     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
+     * the current thread; or
+     *
+     * <li>The call spuriously (that is, for no reason) returns.
+     * </ul>
+     *
+     * <p>This method does <em>not</em> report which of these caused the
+     * method to return. Callers should re-check the conditions which caused
+     * the thread to park in the first place. Callers may also determine,
+     * for example, the interrupt status of the thread upon return.
+     */
+    public static void park() {
+        unsafe.park(false, 0L);
+    }
+
+    /**
+     * Disables the current thread for thread scheduling purposes, for up to
+     * the specified waiting time, unless the permit is available.
+     *
+     * <p>If the permit is available then it is consumed and the call
+     * returns immediately; otherwise the current thread becomes disabled
+     * for thread scheduling purposes and lies dormant until one of four
+     * things happens:
+     *
+     * <ul>
+     * <li>Some other thread invokes {@link #unpark unpark} with the
+     * current thread as the target; or
+     *
+     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
+     * the current thread; or
+     *
+     * <li>The specified waiting time elapses; or
+     *
+     * <li>The call spuriously (that is, for no reason) returns.
+     * </ul>
+     *
+     * <p>This method does <em>not</em> report which of these caused the
+     * method to return. Callers should re-check the conditions which caused
+     * the thread to park in the first place. Callers may also determine,
+     * for example, the interrupt status of the thread, or the elapsed time
+     * upon return.
+     *
+     * @param nanos the maximum number of nanoseconds to wait
+     */
+    public static void parkNanos(long nanos) {
+        if (nanos > 0)
+            unsafe.park(false, nanos);
+    }
+
+    /**
+     * Disables the current thread for thread scheduling purposes, until
+     * the specified deadline, unless the permit is available.
+     *
+     * <p>If the permit is available then it is consumed and the call
+     * returns immediately; otherwise the current thread becomes disabled
+     * for thread scheduling purposes and lies dormant until one of four
+     * things happens:
+     *
+     * <ul>
+     * <li>Some other thread invokes {@link #unpark unpark} with the
+     * current thread as the target; or
+     *
+     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
+     * the current thread; or
+     *
+     * <li>The specified deadline passes; or
+     *
+     * <li>The call spuriously (that is, for no reason) returns.
+     * </ul>
+     *
+     * <p>This method does <em>not</em> report which of these caused the
+     * method to return. Callers should re-check the conditions which caused
+     * the thread to park in the first place. Callers may also determine,
+     * for example, the interrupt status of the thread, or the current time
+     * upon return.
+     *
+     * @param deadline the absolute time, in milliseconds from the Epoch,
+     *        to wait until
+     */
+    public static void parkUntil(long deadline) {
+        unsafe.park(true, deadline);
+    }
+}
diff --git a/external/jsr166/java/util/concurrent/locks/ReadWriteLock.java b/external/jsr166/java/util/concurrent/locks/ReadWriteLock.java
new file mode 100644
index 000000000..484f68d15
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/locks/ReadWriteLock.java
@@ -0,0 +1,104 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent.locks;
+
+/**
+ * A <tt>ReadWriteLock</tt> maintains a pair of associated {@link
+ * Lock locks}, one for read-only operations and one for writing.
+ * The {@link #readLock read lock} may be held simultaneously by
+ * multiple reader threads, so long as there are no writers.  The
+ * {@link #writeLock write lock} is exclusive.
+ *
+ * <p>All <tt>ReadWriteLock</tt> implementations must guarantee that
+ * the memory synchronization effects of <tt>writeLock</tt> operations
+ * (as specified in the {@link Lock} interface) also hold with respect
+ * to the associated <tt>readLock</tt>. That is, a thread successfully
+ * acquiring the read lock will see all updates made upon previous
+ * release of the write lock.
+ *
+ * <p>A read-write lock allows for a greater level of concurrency in
+ * accessing shared data than that permitted by a mutual exclusion lock.
+ * It exploits the fact that while only a single thread at a time (a
+ * <em>writer</em> thread) can modify the shared data, in many cases any
+ * number of threads can concurrently read the data (hence <em>reader</em>
+ * threads).
+ * In theory, the increase in concurrency permitted by the use of a read-write
+ * lock will lead to performance improvements over the use of a mutual
+ * exclusion lock. In practice this increase in concurrency will only be fully
+ * realized on a multi-processor, and then only if the access patterns for
+ * the shared data are suitable.
+ *
+ * <p>Whether or not a read-write lock will improve performance over the use
+ * of a mutual exclusion lock depends on the frequency that the data is
+ * read compared to being modified, the duration of the read and write
+ * operations, and the contention for the data - that is, the number of
+ * threads that will try to read or write the data at the same time.
+ * For example, a collection that is initially populated with data and
+ * thereafter infrequently modified, while being frequently searched
+ * (such as a directory of some kind) is an ideal candidate for the use of
+ * a read-write lock. However, if updates become frequent then the data
+ * spends most of its time being exclusively locked and there is little, if any
+ * increase in concurrency. Further, if the read operations are too short
+ * the overhead of the read-write lock implementation (which is inherently
+ * more complex than a mutual exclusion lock) can dominate the execution
+ * cost, particularly as many read-write lock implementations still serialize
+ * all threads through a small section of code. Ultimately, only profiling
+ * and measurement will establish whether the use of a read-write lock is
+ * suitable for your application.
+ *
+ *
+ * <p>Although the basic operation of a read-write lock is straight-forward,
+ * there are many policy decisions that an implementation must make, which
+ * may affect the effectiveness of the read-write lock in a given application.
+ * Examples of these policies include:
+ * <ul>
+ * <li>Determining whether to grant the read lock or the write lock, when
+ * both readers and writers are waiting, at the time that a writer releases
+ * the write lock. Writer preference is common, as writes are expected to be
+ * short and infrequent. Reader preference is less common as it can lead to
+ * lengthy delays for a write if the readers are frequent and long-lived as
+ * expected. Fair, or &quot;in-order&quot; implementations are also possible.
+ *
+ * <li>Determining whether readers that request the read lock while a
+ * reader is active and a writer is waiting, are granted the read lock.
+ * Preference to the reader can delay the writer indefinitely, while
+ * preference to the writer can reduce the potential for concurrency.
+ *
+ * <li>Determining whether the locks are reentrant: can a thread with the
+ * write lock reacquire it? Can it acquire a read lock while holding the
+ * write lock? Is the read lock itself reentrant?
+ *
+ * <li>Can the write lock be downgraded to a read lock without allowing
+ * an intervening writer? Can a read lock be upgraded to a write lock,
+ * in preference to other waiting readers or writers?
+ *
+ * </ul>
+ * You should consider all of these things when evaluating the suitability
+ * of a given implementation for your application.
+ *
+ * @see ReentrantReadWriteLock
+ * @see Lock
+ * @see ReentrantLock
+ *
+ * @since 1.5
+ * @author Doug Lea
+ */
+public interface ReadWriteLock {
+    /**
+     * Returns the lock used for reading.
+     *
+     * @return the lock used for reading.
+     */
+    Lock readLock();
+
+    /**
+     * Returns the lock used for writing.
+     *
+     * @return the lock used for writing.
+     */
+    Lock writeLock();
+}
diff --git a/external/jsr166/java/util/concurrent/locks/ReentrantLock.java b/external/jsr166/java/util/concurrent/locks/ReentrantLock.java
new file mode 100644
index 000000000..4a2fc175c
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/locks/ReentrantLock.java
@@ -0,0 +1,740 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent.locks;
+import java.util.*;
+import java.util.concurrent.*;
+import java.util.concurrent.atomic.*;
+
+/**
+ * A reentrant mutual exclusion {@link Lock} with the same basic
+ * behavior and semantics as the implicit monitor lock accessed using
+ * {@code synchronized} methods and statements, but with extended
+ * capabilities.
+ *
+ * <p>A {@code ReentrantLock} is <em>owned</em> by the thread last
+ * successfully locking, but not yet unlocking it. A thread invoking
+ * {@code lock} will return, successfully acquiring the lock, when
+ * the lock is not owned by another thread. The method will return
+ * immediately if the current thread already owns the lock. This can
+ * be checked using methods {@link #isHeldByCurrentThread}, and {@link
+ * #getHoldCount}.
+ *
+ * <p>The constructor for this class accepts an optional
+ * <em>fairness</em> parameter.  When set {@code true}, under
+ * contention, locks favor granting access to the longest-waiting
+ * thread.  Otherwise this lock does not guarantee any particular
+ * access order.  Programs using fair locks accessed by many threads
+ * may display lower overall throughput (i.e., are slower; often much
+ * slower) than those using the default setting, but have smaller
+ * variances in times to obtain locks and guarantee lack of
+ * starvation. Note however, that fairness of locks does not guarantee
+ * fairness of thread scheduling. Thus, one of many threads using a
+ * fair lock may obtain it multiple times in succession while other
+ * active threads are not progressing and not currently holding the
+ * lock.
+ * Also note that the untimed {@link #tryLock() tryLock} method does not
+ * honor the fairness setting. It will succeed if the lock
+ * is available even if other threads are waiting.
+ *
+ * <p>It is recommended practice to <em>always</em> immediately
+ * follow a call to {@code lock} with a {@code try} block, most
+ * typically in a before/after construction such as:
+ *
+ * <pre>
+ * class X {
+ *   private final ReentrantLock lock = new ReentrantLock();
+ *   // ...
+ *
+ *   public void m() {
+ *     lock.lock();  // block until condition holds
+ *     try {
+ *       // ... method body
+ *     } finally {
+ *       lock.unlock()
+ *     }
+ *   }
+ * }
+ * </pre>
+ *
+ * <p>In addition to implementing the {@link Lock} interface, this
+ * class defines methods {@code isLocked} and
+ * {@code getLockQueueLength}, as well as some associated
+ * {@code protected} access methods that may be useful for
+ * instrumentation and monitoring.
+ *
+ * <p>Serialization of this class behaves in the same way as built-in
+ * locks: a deserialized lock is in the unlocked state, regardless of
+ * its state when serialized.
+ *
+ * <p>This lock supports a maximum of 2147483647 recursive locks by
+ * the same thread. Attempts to exceed this limit result in
+ * {@link Error} throws from locking methods.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ */
+public class ReentrantLock implements Lock, java.io.Serializable {
+    private static final long serialVersionUID = 7373984872572414699L;
+    /** Synchronizer providing all implementation mechanics */
+    private final Sync sync;
+
+    /**
+     * Base of synchronization control for this lock. Subclassed
+     * into fair and nonfair versions below. Uses AQS state to
+     * represent the number of holds on the lock.
+     */
+    static abstract class Sync extends AbstractQueuedSynchronizer {
+        private static final long serialVersionUID = -5179523762034025860L;
+
+        /**
+         * Performs {@link Lock#lock}. The main reason for subclassing
+         * is to allow fast path for nonfair version.
+         */
+        abstract void lock();
+
+        /**
+         * Performs non-fair tryLock.  tryAcquire is
+         * implemented in subclasses, but both need nonfair
+         * try for trylock method.
+         */
+        final boolean nonfairTryAcquire(int acquires) {
+            final Thread current = Thread.currentThread();
+            int c = getState();
+            if (c == 0) {
+                if (compareAndSetState(0, acquires)) {
+                    setExclusiveOwnerThread(current);
+                    return true;
+                }
+            }
+            else if (current == getExclusiveOwnerThread()) {
+                int nextc = c + acquires;
+                if (nextc < 0) // overflow
+                    throw new Error("Maximum lock count exceeded");
+                setState(nextc);
+                return true;
+            }
+            return false;
+        }
+
+        protected final boolean tryRelease(int releases) {
+            int c = getState() - releases;
+            if (Thread.currentThread() != getExclusiveOwnerThread())
+                throw new IllegalMonitorStateException();
+            boolean free = false;
+            if (c == 0) {
+                free = true;
+                setExclusiveOwnerThread(null);
+            }
+            setState(c);
+            return free;
+        }
+
+        protected final boolean isHeldExclusively() {
+            // While we must in general read state before owner,
+            // we don't need to do so to check if current thread is owner
+            return getExclusiveOwnerThread() == Thread.currentThread();
+        }
+
+        final ConditionObject newCondition() {
+            return new ConditionObject();
+        }
+
+        // Methods relayed from outer class
+
+        final Thread getOwner() {
+            return getState() == 0 ? null : getExclusiveOwnerThread();
+        }
+
+        final int getHoldCount() {
+            return isHeldExclusively() ? getState() : 0;
+        }
+
+        final boolean isLocked() {
+            return getState() != 0;
+        }
+
+        /**
+         * Reconstitutes this lock instance from a stream.
+         * @param s the stream
+         */
+        private void readObject(java.io.ObjectInputStream s)
+            throws java.io.IOException, ClassNotFoundException {
+            s.defaultReadObject();
+            setState(0); // reset to unlocked state
+        }
+    }
+
+    /**
+     * Sync object for non-fair locks
+     */
+    final static class NonfairSync extends Sync {
+        private static final long serialVersionUID = 7316153563782823691L;
+
+        /**
+         * Performs lock.  Try immediate barge, backing up to normal
+         * acquire on failure.
+         */
+        final void lock() {
+            if (compareAndSetState(0, 1))
+                setExclusiveOwnerThread(Thread.currentThread());
+            else
+                acquire(1);
+        }
+
+        protected final boolean tryAcquire(int acquires) {
+            return nonfairTryAcquire(acquires);
+        }
+    }
+
+    /**
+     * Sync object for fair locks
+     */
+    final static class FairSync extends Sync {
+        private static final long serialVersionUID = -3000897897090466540L;
+
+        final void lock() {
+            acquire(1);
+        }
+
+        /**
+         * Fair version of tryAcquire.  Don't grant access unless
+         * recursive call or no waiters or is first.
+         */
+        protected final boolean tryAcquire(int acquires) {
+            final Thread current = Thread.currentThread();
+            int c = getState();
+            if (c == 0) {
+                if (isFirst(current) &&
+                    compareAndSetState(0, acquires)) {
+                    setExclusiveOwnerThread(current);
+                    return true;
+                }
+            }
+            else if (current == getExclusiveOwnerThread()) {
+                int nextc = c + acquires;
+                if (nextc < 0)
+                    throw new Error("Maximum lock count exceeded");
+                setState(nextc);
+                return true;
+            }
+            return false;
+        }
+    }
+
+    /**
+     * Creates an instance of {@code ReentrantLock}.
+     * This is equivalent to using {@code ReentrantLock(false)}.
+     */
+    public ReentrantLock() {
+        sync = new NonfairSync();
+    }
+
+    /**
+     * Creates an instance of {@code ReentrantLock} with the
+     * given fairness policy.
+     *
+     * @param fair {@code true} if this lock should use a fair ordering policy
+     */
+    public ReentrantLock(boolean fair) {
+        sync = (fair)? new FairSync() : new NonfairSync();
+    }
+
+    /**
+     * Acquires the lock.
+     *
+     * <p>Acquires the lock if it is not held by another thread and returns
+     * immediately, setting the lock hold count to one.
+     *
+     * <p>If the current thread already holds the lock then the hold
+     * count is incremented by one and the method returns immediately.
+     *
+     * <p>If the lock is held by another thread then the
+     * current thread becomes disabled for thread scheduling
+     * purposes and lies dormant until the lock has been acquired,
+     * at which time the lock hold count is set to one.
+     */
+    public void lock() {
+        sync.lock();
+    }
+
+    /**
+     * Acquires the lock unless the current thread is
+     * {@linkplain Thread#interrupt interrupted}.
+     *
+     * <p>Acquires the lock if it is not held by another thread and returns
+     * immediately, setting the lock hold count to one.
+     *
+     * <p>If the current thread already holds this lock then the hold count
+     * is incremented by one and the method returns immediately.
+     *
+     * <p>If the lock is held by another thread then the
+     * current thread becomes disabled for thread scheduling
+     * purposes and lies dormant until one of two things happens:
+     *
+     * <ul>
+     *
+     * <li>The lock is acquired by the current thread; or
+     *
+     * <li>Some other thread {@linkplain Thread#interrupt interrupts} the
+     * current thread.
+     *
+     * </ul>
+     *
+     * <p>If the lock is acquired by the current thread then the lock hold
+     * count is set to one.
+     *
+     * <p>If the current thread:
+     *
+     * <ul>
+     *
+     * <li>has its interrupted status set on entry to this method; or
+     *
+     * <li>is {@linkplain Thread#interrupt interrupted} while acquiring
+     * the lock,
+     *
+     * </ul>
+     *
+     * then {@link InterruptedException} is thrown and the current thread's
+     * interrupted status is cleared.
+     *
+     * <p>In this implementation, as this method is an explicit
+     * interruption point, preference is given to responding to the
+     * interrupt over normal or reentrant acquisition of the lock.
+     *
+     * @throws InterruptedException if the current thread is interrupted
+     */
+    public void lockInterruptibly() throws InterruptedException {
+        sync.acquireInterruptibly(1);
+    }
+
+    /**
+     * Acquires the lock only if it is not held by another thread at the time
+     * of invocation.
+     *
+     * <p>Acquires the lock if it is not held by another thread and
+     * returns immediately with the value {@code true}, setting the
+     * lock hold count to one. Even when this lock has been set to use a
+     * fair ordering policy, a call to {@code tryLock()} <em>will</em>
+     * immediately acquire the lock if it is available, whether or not
+     * other threads are currently waiting for the lock.
+     * This &quot;barging&quot; behavior can be useful in certain
+     * circumstances, even though it breaks fairness. If you want to honor
+     * the fairness setting for this lock, then use
+     * {@link #tryLock(long, TimeUnit) tryLock(0, TimeUnit.SECONDS) }
+     * which is almost equivalent (it also detects interruption).
+     *
+     * <p> If the current thread already holds this lock then the hold
+     * count is incremented by one and the method returns {@code true}.
+     *
+     * <p>If the lock is held by another thread then this method will return
+     * immediately with the value {@code false}.
+     *
+     * @return {@code true} if the lock was free and was acquired by the
+     *         current thread, or the lock was already held by the current
+     *         thread; and {@code false} otherwise
+     */
+    public boolean tryLock() {
+        return sync.nonfairTryAcquire(1);
+    }
+
+    /**
+     * Acquires the lock if it is not held by another thread within the given
+     * waiting time and the current thread has not been
+     * {@linkplain Thread#interrupt interrupted}.
+     *
+     * <p>Acquires the lock if it is not held by another thread and returns
+     * immediately with the value {@code true}, setting the lock hold count
+     * to one. If this lock has been set to use a fair ordering policy then
+     * an available lock <em>will not</em> be acquired if any other threads
+     * are waiting for the lock. This is in contrast to the {@link #tryLock()}
+     * method. If you want a timed {@code tryLock} that does permit barging on
+     * a fair lock then combine the timed and un-timed forms together:
+     *
+     * <pre>if (lock.tryLock() || lock.tryLock(timeout, unit) ) { ... }
+     * </pre>
+     *
+     * <p>If the current thread
+     * already holds this lock then the hold count is incremented by one and
+     * the method returns {@code true}.
+     *
+     * <p>If the lock is held by another thread then the
+     * current thread becomes disabled for thread scheduling
+     * purposes and lies dormant until one of three things happens:
+     *
+     * <ul>
+     *
+     * <li>The lock is acquired by the current thread; or
+     *
+     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
+     * the current thread; or
+     *
+     * <li>The specified waiting time elapses
+     *
+     * </ul>
+     *
+     * <p>If the lock is acquired then the value {@code true} is returned and
+     * the lock hold count is set to one.
+     *
+     * <p>If the current thread:
+     *
+     * <ul>
+     *
+     * <li>has its interrupted status set on entry to this method; or
+     *
+     * <li>is {@linkplain Thread#interrupt interrupted} while
+     * acquiring the lock,
+     *
+     * </ul>
+     * then {@link InterruptedException} is thrown and the current thread's
+     * interrupted status is cleared.
+     *
+     * <p>If the specified waiting time elapses then the value {@code false}
+     * is returned.  If the time is less than or equal to zero, the method
+     * will not wait at all.
+     *
+     * <p>In this implementation, as this method is an explicit
+     * interruption point, preference is given to responding to the
+     * interrupt over normal or reentrant acquisition of the lock, and
+     * over reporting the elapse of the waiting time.
+     *
+     * @param timeout the time to wait for the lock
+     * @param unit the time unit of the timeout argument
+     * @return {@code true} if the lock was free and was acquired by the
+     *         current thread, or the lock was already held by the current
+     *         thread; and {@code false} if the waiting time elapsed before
+     *         the lock could be acquired
+     * @throws InterruptedException if the current thread is interrupted
+     * @throws NullPointerException if the time unit is null
+     *
+     */
+    public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException {
+        return sync.tryAcquireNanos(1, unit.toNanos(timeout));
+    }
+
+    /**
+     * Attempts to release this lock.
+     *
+     * <p>If the current thread is the holder of this lock then the hold
+     * count is decremented.  If the hold count is now zero then the lock
+     * is released.  If the current thread is not the holder of this
+     * lock then {@link IllegalMonitorStateException} is thrown.
+     *
+     * @throws IllegalMonitorStateException if the current thread does not
+     *         hold this lock
+     */
+    public void unlock() {
+        sync.release(1);
+    }
+
+    /**
+     * Returns a {@link Condition} instance for use with this
+     * {@link Lock} instance.
+     *
+     * <p>The returned {@link Condition} instance supports the same
+     * usages as do the {@link Object} monitor methods ({@link
+     * Object#wait() wait}, {@link Object#notify notify}, and {@link
+     * Object#notifyAll notifyAll}) when used with the built-in
+     * monitor lock.
+     *
+     * <ul>
+     *
+     * <li>If this lock is not held when any of the {@link Condition}
+     * {@linkplain Condition#await() waiting} or {@linkplain
+     * Condition#signal signalling} methods are called, then an {@link
+     * IllegalMonitorStateException} is thrown.
+     *
+     * <li>When the condition {@linkplain Condition#await() waiting}
+     * methods are called the lock is released and, before they
+     * return, the lock is reacquired and the lock hold count restored
+     * to what it was when the method was called.
+     *
+     * <li>If a thread is {@linkplain Thread#interrupt interrupted}
+     * while waiting then the wait will terminate, an {@link
+     * InterruptedException} will be thrown, and the thread's
+     * interrupted status will be cleared.
+     *
+     * <li> Waiting threads are signalled in FIFO order.
+     *
+     * <li>The ordering of lock reacquisition for threads returning
+     * from waiting methods is the same as for threads initially
+     * acquiring the lock, which is in the default case not specified,
+     * but for <em>fair</em> locks favors those threads that have been
+     * waiting the longest.
+     *
+     * </ul>
+     *
+     * @return the Condition object
+     */
+    public Condition newCondition() {
+        return sync.newCondition();
+    }
+
+    /**
+     * Queries the number of holds on this lock by the current thread.
+     *
+     * <p>A thread has a hold on a lock for each lock action that is not
+     * matched by an unlock action.
+     *
+     * <p>The hold count information is typically only used for testing and
+     * debugging purposes. For example, if a certain section of code should
+     * not be entered with the lock already held then we can assert that
+     * fact:
+     *
+     * <pre>
+     * class X {
+     *   ReentrantLock lock = new ReentrantLock();
+     *   // ...
+     *   public void m() {
+     *     assert lock.getHoldCount() == 0;
+     *     lock.lock();
+     *     try {
+     *       // ... method body
+     *     } finally {
+     *       lock.unlock();
+     *     }
+     *   }
+     * }
+     * </pre>
+     *
+     * @return the number of holds on this lock by the current thread,
+     *         or zero if this lock is not held by the current thread
+     */
+    public int getHoldCount() {
+        return sync.getHoldCount();
+    }
+
+    /**
+     * Queries if this lock is held by the current thread.
+     *
+     * <p>Analogous to the {@link Thread#holdsLock} method for built-in
+     * monitor locks, this method is typically used for debugging and
+     * testing. For example, a method that should only be called while
+     * a lock is held can assert that this is the case:
+     *
+     * <pre>
+     * class X {
+     *   ReentrantLock lock = new ReentrantLock();
+     *   // ...
+     *
+     *   public void m() {
+     *       assert lock.isHeldByCurrentThread();
+     *       // ... method body
+     *   }
+     * }
+     * </pre>
+     *
+     * <p>It can also be used to ensure that a reentrant lock is used
+     * in a non-reentrant manner, for example:
+     *
+     * <pre>
+     * class X {
+     *   ReentrantLock lock = new ReentrantLock();
+     *   // ...
+     *
+     *   public void m() {
+     *       assert !lock.isHeldByCurrentThread();
+     *       lock.lock();
+     *       try {
+     *           // ... method body
+     *       } finally {
+     *           lock.unlock();
+     *       }
+     *   }
+     * }
+     * </pre>
+     *
+     * @return {@code true} if current thread holds this lock and
+     *         {@code false} otherwise
+     */
+    public boolean isHeldByCurrentThread() {
+        return sync.isHeldExclusively();
+    }
+
+    /**
+     * Queries if this lock is held by any thread. This method is
+     * designed for use in monitoring of the system state,
+     * not for synchronization control.
+     *
+     * @return {@code true} if any thread holds this lock and
+     *         {@code false} otherwise
+     */
+    public boolean isLocked() {
+        return sync.isLocked();
+    }
+
+    /**
+     * Returns {@code true} if this lock has fairness set true.
+     *
+     * @return {@code true} if this lock has fairness set true
+     */
+    public final boolean isFair() {
+        return sync instanceof FairSync;
+    }
+
+    /**
+     * Returns the thread that currently owns this lock, or
+     * {@code null} if not owned. When this method is called by a
+     * thread that is not the owner, the return value reflects a
+     * best-effort approximation of current lock status. For example,
+     * the owner may be momentarily {@code null} even if there are
+     * threads trying to acquire the lock but have not yet done so.
+     * This method is designed to facilitate construction of
+     * subclasses that provide more extensive lock monitoring
+     * facilities.
+     *
+     * @return the owner, or {@code null} if not owned
+     */
+    protected Thread getOwner() {
+        return sync.getOwner();
+    }
+
+    /**
+     * Queries whether any threads are waiting to acquire this lock. Note that
+     * because cancellations may occur at any time, a {@code true}
+     * return does not guarantee that any other thread will ever
+     * acquire this lock.  This method is designed primarily for use in
+     * monitoring of the system state.
+     *
+     * @return {@code true} if there may be other threads waiting to
+     *         acquire the lock
+     */
+    public final boolean hasQueuedThreads() {
+        return sync.hasQueuedThreads();
+    }
+
+
+    /**
+     * Queries whether the given thread is waiting to acquire this
+     * lock. Note that because cancellations may occur at any time, a
+     * {@code true} return does not guarantee that this thread
+     * will ever acquire this lock.  This method is designed primarily for use
+     * in monitoring of the system state.
+     *
+     * @param thread the thread
+     * @return {@code true} if the given thread is queued waiting for this lock
+     * @throws NullPointerException if the thread is null
+     */
+    public final boolean hasQueuedThread(Thread thread) {
+        return sync.isQueued(thread);
+    }
+
+
+    /**
+     * Returns an estimate of the number of threads waiting to
+     * acquire this lock.  The value is only an estimate because the number of
+     * threads may change dynamically while this method traverses
+     * internal data structures.  This method is designed for use in
+     * monitoring of the system state, not for synchronization
+     * control.
+     *
+     * @return the estimated number of threads waiting for this lock
+     */
+    public final int getQueueLength() {
+        return sync.getQueueLength();
+    }
+
+    /**
+     * Returns a collection containing threads that may be waiting to
+     * acquire this lock.  Because the actual set of threads may change
+     * dynamically while constructing this result, the returned
+     * collection is only a best-effort estimate.  The elements of the
+     * returned collection are in no particular order.  This method is
+     * designed to facilitate construction of subclasses that provide
+     * more extensive monitoring facilities.
+     *
+     * @return the collection of threads
+     */
+    protected Collection<Thread> getQueuedThreads() {
+        return sync.getQueuedThreads();
+    }
+
+    /**
+     * Queries whether any threads are waiting on the given condition
+     * associated with this lock. Note that because timeouts and
+     * interrupts may occur at any time, a {@code true} return does
+     * not guarantee that a future {@code signal} will awaken any
+     * threads.  This method is designed primarily for use in
+     * monitoring of the system state.
+     *
+     * @param condition the condition
+     * @return {@code true} if there are any waiting threads
+     * @throws IllegalMonitorStateException if this lock is not held
+     * @throws IllegalArgumentException if the given condition is
+     *         not associated with this lock
+     * @throws NullPointerException if the condition is null
+     */
+    public boolean hasWaiters(Condition condition) {
+        if (condition == null)
+            throw new NullPointerException();
+        if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
+            throw new IllegalArgumentException("not owner");
+        return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject)condition);
+    }
+
+    /**
+     * Returns an estimate of the number of threads waiting on the
+     * given condition associated with this lock. Note that because
+     * timeouts and interrupts may occur at any time, the estimate
+     * serves only as an upper bound on the actual number of waiters.
+     * This method is designed for use in monitoring of the system
+     * state, not for synchronization control.
+     *
+     * @param condition the condition
+     * @return the estimated number of waiting threads
+     * @throws IllegalMonitorStateException if this lock is not held
+     * @throws IllegalArgumentException if the given condition is
+     *         not associated with this lock
+     * @throws NullPointerException if the condition is null
+     */
+    public int getWaitQueueLength(Condition condition) {
+        if (condition == null)
+            throw new NullPointerException();
+        if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
+            throw new IllegalArgumentException("not owner");
+        return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject)condition);
+    }
+
+    /**
+     * Returns a collection containing those threads that may be
+     * waiting on the given condition associated with this lock.
+     * Because the actual set of threads may change dynamically while
+     * constructing this result, the returned collection is only a
+     * best-effort estimate. The elements of the returned collection
+     * are in no particular order.  This method is designed to
+     * facilitate construction of subclasses that provide more
+     * extensive condition monitoring facilities.
+     *
+     * @param condition the condition
+     * @return the collection of threads
+     * @throws IllegalMonitorStateException if this lock is not held
+     * @throws IllegalArgumentException if the given condition is
+     *         not associated with this lock
+     * @throws NullPointerException if the condition is null
+     */
+    protected Collection<Thread> getWaitingThreads(Condition condition) {
+        if (condition == null)
+            throw new NullPointerException();
+        if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
+            throw new IllegalArgumentException("not owner");
+        return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject)condition);
+    }
+
+    /**
+     * Returns a string identifying this lock, as well as its lock state.
+     * The state, in brackets, includes either the String {@code "Unlocked"}
+     * or the String {@code "Locked by"} followed by the
+     * {@linkplain Thread#getName name} of the owning thread.
+     *
+     * @return a string identifying this lock, as well as its lock state
+     */
+    public String toString() {
+        Thread o = sync.getOwner();
+        return super.toString() + ((o == null) ?
+                                   "[Unlocked]" :
+                                   "[Locked by thread " + o.getName() + "]");
+    }
+}
diff --git a/external/jsr166/java/util/concurrent/locks/ReentrantReadWriteLock.java b/external/jsr166/java/util/concurrent/locks/ReentrantReadWriteLock.java
new file mode 100644
index 000000000..df18a7c08
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/locks/ReentrantReadWriteLock.java
@@ -0,0 +1,1346 @@
+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/licenses/publicdomain
+ */
+
+package java.util.concurrent.locks;
+import java.util.concurrent.*;
+import java.util.concurrent.atomic.*;
+import java.util.*;
+
+/**
+ * An implementation of {@link ReadWriteLock} supporting similar
+ * semantics to {@link ReentrantLock}.
+ * <p>This class has the following properties:
+ *
+ * <ul>
+ * <li><b>Acquisition order</b>
+ *
+ * <p> This class does not impose a reader or writer preference
+ * ordering for lock access.  However, it does support an optional
+ * <em>fairness</em> policy.
+ *
+ * <dl>
+ * <dt><b><i>Non-fair mode (default)</i></b>
+ * <dd>When constructed as non-fair (the default), the order of entry
+ * to the read and write lock is unspecified, subject to reentrancy
+ * constraints.  A nonfair lock that is continously contended may
+ * indefinitely postpone one or more reader or writer threads, but
+ * will normally have higher throughput than a fair lock.
+ * <p>
+ *
+ * <dt><b><i>Fair mode</i></b>
+ * <dd> When constructed as fair, threads contend for entry using an
+ * approximately arrival-order policy. When the currently held lock
+ * is released either the longest-waiting single writer thread will
+ * be assigned the write lock, or if there is a group of reader threads
+ * waiting longer than all waiting writer threads, that group will be
+ * assigned the read lock.
+ *
+ * <p>A thread that tries to acquire a fair read lock (non-reentrantly)
+ * will block if either the write lock is held, or there is a waiting
+ * writer thread. The thread will not acquire the read lock until
+ * after the oldest currently waiting writer thread has acquired and
+ * released the write lock. Of course, if a waiting writer abandons
+ * its wait, leaving one or more reader threads as the longest waiters
+ * in the queue with the write lock free, then those readers will be
+ * assigned the read lock.
+ *
+ * <p>A thread that tries to acquire a fair write lock (non-reentrantly)
+ * will block unless both the read lock and write lock are free (which
+ * implies there are no waiting threads).  (Note that the non-blocking
+ * {@link ReadLock#tryLock()} and {@link WriteLock#tryLock()} methods
+ * do not honor this fair setting and will acquire the lock if it is
+ * possible, regardless of waiting threads.)
+ * <p>
+ * </dl>
+ *
+ * <li><b>Reentrancy</b>
+ *
+ * <p>This lock allows both readers and writers to reacquire read or
+ * write locks in the style of a {@link ReentrantLock}. Non-reentrant
+ * readers are not allowed until all write locks held by the writing
+ * thread have been released.
+ *
+ * <p>Additionally, a writer can acquire the read lock, but not
+ * vice-versa.  Among other applications, reentrancy can be useful
+ * when write locks are held during calls or callbacks to methods that
+ * perform reads under read locks.  If a reader tries to acquire the
+ * write lock it will never succeed.
+ *
+ * <li><b>Lock downgrading</b>
+ * <p>Reentrancy also allows downgrading from the write lock to a read lock,
+ * by acquiring the write lock, then the read lock and then releasing the
+ * write lock. However, upgrading from a read lock to the write lock is
+ * <b>not</b> possible.
+ *
+ * <li><b>Interruption of lock acquisition</b>
+ * <p>The read lock and write lock both support interruption during lock
+ * acquisition.
+ *
+ * <li><b>{@link Condition} support</b>
+ * <p>The write lock provides a {@link Condition} implementation that
+ * behaves in the same way, with respect to the write lock, as the
+ * {@link Condition} implementation provided by
+ * {@link ReentrantLock#newCondition} does for {@link ReentrantLock}.
+ * This {@link Condition} can, of course, only be used with the write lock.
+ *
+ * <p>The read lock does not support a {@link Condition} and
+ * {@code readLock().newCondition()} throws
+ * {@code UnsupportedOperationException}.
+ *
+ * <li><b>Instrumentation</b>
+ * <p>This class supports methods to determine whether locks
+ * are held or contended. These methods are designed for monitoring
+ * system state, not for synchronization control.
+ * </ul>
+ *
+ * <p>Serialization of this class behaves in the same way as built-in
+ * locks: a deserialized lock is in the unlocked state, regardless of
+ * its state when serialized.
+ *
+ * <p><b>Sample usages</b>. Here is a code sketch showing how to exploit
+ * reentrancy to perform lock downgrading after updating a cache (exception
+ * handling is elided for simplicity):
+ * <pre>
+ * class CachedData {
+ *   Object data;
+ *   volatile boolean cacheValid;
+ *   ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
+ *
+ *   void processCachedData() {
+ *     rwl.readLock().lock();
+ *     if (!cacheValid) {
+ *        // Must release read lock before acquiring write lock
+ *        rwl.readLock().unlock();
+ *        rwl.writeLock().lock();
+ *        // Recheck state because another thread might have acquired
+ *        //   write lock and changed state before we did.
+ *        if (!cacheValid) {
+ *          data = ...
+ *          cacheValid = true;
+ *        }
+ *        // Downgrade by acquiring read lock before releasing write lock
+ *        rwl.readLock().lock();
+ *        rwl.writeLock().unlock(); // Unlock write, still hold read
+ *     }
+ *
+ *     use(data);
+ *     rwl.readLock().unlock();
+ *   }
+ * }
+ * </pre>
+ *
+ * ReentrantReadWriteLocks can be used to improve concurrency in some
+ * uses of some kinds of Collections. This is typically worthwhile
+ * only when the collections are expected to be large, accessed by
+ * more reader threads than writer threads, and entail operations with
+ * overhead that outweighs synchronization overhead. For example, here
+ * is a class using a TreeMap that is expected to be large and
+ * concurrently accessed.
+ *
+ * <pre>{@code
+ * class RWDictionary {
+ *    private final Map<String, Data> m = new TreeMap<String, Data>();
+ *    private final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
+ *    private final Lock r = rwl.readLock();
+ *    private final Lock w = rwl.writeLock();
+ *
+ *    public Data get(String key) {
+ *        r.lock();
+ *        try { return m.get(key); }
+ *        finally { r.unlock(); }
+ *    }
+ *    public String[] allKeys() {
+ *        r.lock();
+ *        try { return m.keySet().toArray(); }
+ *        finally { r.unlock(); }
+ *    }
+ *    public Data put(String key, Data value) {
+ *        w.lock();
+ *        try { return m.put(key, value); }
+ *        finally { w.unlock(); }
+ *    }
+ *    public void clear() {
+ *        w.lock();
+ *        try { m.clear(); }
+ *        finally { w.unlock(); }
+ *    }
+ * }}</pre>
+ *
+ * <h3>Implementation Notes</h3>
+ *
+ * <p>This lock supports a maximum of 65535 recursive write locks
+ * and 65535 read locks. Attempts to exceed these limits result in
+ * {@link Error} throws from locking methods.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ *
+ */
+public class ReentrantReadWriteLock implements ReadWriteLock, java.io.Serializable  {
+    private static final long serialVersionUID = -6992448646407690164L;
+    /** Inner class providing readlock */
+    private final ReentrantReadWriteLock.ReadLock readerLock;
+    /** Inner class providing writelock */
+    private final ReentrantReadWriteLock.WriteLock writerLock;
+    /** Performs all synchronization mechanics */
+    private final Sync sync;
+
+    /**
+     * Creates a new {@code ReentrantReadWriteLock} with
+     * default (nonfair) ordering properties.
+     */
+    public ReentrantReadWriteLock() {
+        this(false);
+    }
+
+    /**
+     * Creates a new {@code ReentrantReadWriteLock} with
+     * the given fairness policy.
+     *
+     * @param fair {@code true} if this lock should use a fair ordering policy
+     */
+    public ReentrantReadWriteLock(boolean fair) {
+        sync = (fair)? new FairSync() : new NonfairSync();
+        readerLock = new ReadLock(this);
+        writerLock = new WriteLock(this);
+    }
+
+    public ReentrantReadWriteLock.WriteLock writeLock() { return writerLock; }
+    public ReentrantReadWriteLock.ReadLock  readLock()  { return readerLock; }
+
+    /**
+     * Synchronization implementation for ReentrantReadWriteLock.
+     * Subclassed into fair and nonfair versions.
+     */
+    static abstract class Sync extends AbstractQueuedSynchronizer {
+        private static final long serialVersionUID = 6317671515068378041L;
+
+        /*
+         * Read vs write count extraction constants and functions.
+         * Lock state is logically divided into two shorts: The lower
+         * one representing the exclusive (writer) lock hold count,
+         * and the upper the shared (reader) hold count.
+         */
+
+        static final int SHARED_SHIFT   = 16;
+        static final int SHARED_UNIT    = (1 << SHARED_SHIFT);
+        static final int MAX_COUNT      = (1 << SHARED_SHIFT) - 1;
+        static final int EXCLUSIVE_MASK = (1 << SHARED_SHIFT) - 1;
+
+        /** Returns the number of shared holds represented in count  */
+        static int sharedCount(int c)    { return c >>> SHARED_SHIFT; }
+        /** Returns the number of exclusive holds represented in count  */
+        static int exclusiveCount(int c) { return c & EXCLUSIVE_MASK; }
+
+        /**
+         * A counter for per-thread read hold counts.
+         * Maintained as a ThreadLocal; cached in cachedHoldCounter
+         */
+        static final class HoldCounter {
+            int count;
+            // Use id, not reference, to avoid garbage retention
+            final long tid = Thread.currentThread().getId();
+            /** Decrement if positive; return previous value */
+            int tryDecrement() {
+                int c = count;
+                if (c > 0)
+                    count = c - 1;
+                return c;
+            }
+        }
+
+        /**
+         * ThreadLocal subclass. Easiest to explicitly define for sake
+         * of deserialization mechanics.
+         */
+        static final class ThreadLocalHoldCounter
+            extends ThreadLocal<HoldCounter> {
+            public HoldCounter initialValue() {
+                return new HoldCounter();
+            }
+        }
+
+        /**
+         * The number of read locks held by current thread.
+         * Initialized only in constructor and readObject.
+         */
+        transient ThreadLocalHoldCounter readHolds;
+
+        /**
+         * The hold count of the last thread to successfully acquire
+         * readLock. This saves ThreadLocal lookup in the common case
+         * where the next thread to release is the last one to
+         * acquire. This is non-volatile since it is just used
+         * as a heuristic, and would be great for threads to cache.
+         */
+        transient HoldCounter cachedHoldCounter;
+
+        Sync() {
+            readHolds = new ThreadLocalHoldCounter();
+            setState(getState()); // ensures visibility of readHolds
+        }
+
+        /*
+         * Acquires and releases use the same code for fair and
+         * nonfair locks, but differ in whether/how they allow barging
+         * when queues are non-empty.
+         */
+
+        /**
+         * Return true if a reader thread that is otherwise
+         * eligible for lock should block because of policy
+         * for overtaking other waiting threads.
+         */
+        abstract boolean readerShouldBlock(Thread current);
+
+        /**
+         * Return true if a writer thread that is otherwise
+         * eligible for lock should block because of policy
+         * for overtaking other waiting threads.
+         */
+        abstract boolean writerShouldBlock(Thread current);
+
+        /*
+         * Note that tryRelease and tryAcquire can be called by
+         * Conditions. So it is possible that their arguments contain
+         * both read and write holds that are all released during a
+         * condition wait and re-established in tryAcquire.
+         */
+
+        protected final boolean tryRelease(int releases) {
+            int nextc = getState() - releases;
+            if (Thread.currentThread() != getExclusiveOwnerThread())
+                throw new IllegalMonitorStateException();
+            if (exclusiveCount(nextc) == 0) {
+                setExclusiveOwnerThread(null);
+                setState(nextc);
+                return true;
+            } else {
+                setState(nextc);
+                return false;
+            }
+        }
+
+        protected final boolean tryAcquire(int acquires) {
+            /*
+             * Walkthrough:
+             * 1. if read count nonzero or write count nonzero
+             *     and owner is a different thread, fail.
+             * 2. If count would saturate, fail. (This can only
+             *    happen if count is already nonzero.)
+             * 3. Otherwise, this thread is eligible for lock if
+             *    it is either a reentrant acquire or
+             *    queue policy allows it. If so, update state
+             *    and set owner.
+             */
+            Thread current = Thread.currentThread();
+            int c = getState();
+            int w = exclusiveCount(c);
+            if (c != 0) {
+                // (Note: if c != 0 and w == 0 then shared count != 0)
+                if (w == 0 || current != getExclusiveOwnerThread())
+                    return false;
+                if (w + exclusiveCount(acquires) > MAX_COUNT)
+                    throw new Error("Maximum lock count exceeded");
+            }
+            if ((w == 0 && writerShouldBlock(current)) ||
+                !compareAndSetState(c, c + acquires))
+                return false;
+            setExclusiveOwnerThread(current);
+            return true;
+        }
+
+        protected final boolean tryReleaseShared(int unused) {
+            HoldCounter rh = cachedHoldCounter;
+            Thread current = Thread.currentThread();
+            if (rh == null || rh.tid != current.getId())
+                rh = readHolds.get();
+            if (rh.tryDecrement() <= 0)
+                throw new IllegalMonitorStateException();
+            for (;;) {
+                int c = getState();
+                int nextc = c - SHARED_UNIT;
+                if (compareAndSetState(c, nextc))
+                    return nextc == 0;
+            }
+        }
+
+        protected final int tryAcquireShared(int unused) {
+            /*
+             * Walkthrough:
+             * 1. If write lock held by another thread, fail
+             * 2. If count saturated, throw error
+             * 3. Otherwise, this thread is eligible for
+             *    lock wrt state, so ask if it should block
+             *    because of queue policy. If not, try
+             *    to grant by CASing state and updating count.
+             *    Note that step does not check for reentrant
+             *    acquires, which is postponed to full version
+             *    to avoid having to check hold count in
+             *    the more typical non-reentrant case.
+             * 4. If step 3 fails either because thread
+             *    apparently not eligible or CAS fails,
+             *    chain to version with full retry loop.
+             */
+            Thread current = Thread.currentThread();
+            int c = getState();
+            if (exclusiveCount(c) != 0 &&
+                getExclusiveOwnerThread() != current)
+                return -1;
+            if (sharedCount(c) == MAX_COUNT)
+                throw new Error("Maximum lock count exceeded");
+            if (!readerShouldBlock(current) &&
+                compareAndSetState(c, c + SHARED_UNIT)) {
+                HoldCounter rh = cachedHoldCounter;
+                if (rh == null || rh.tid != current.getId())
+                    cachedHoldCounter = rh = readHolds.get();
+                rh.count++;
+                return 1;
+            }
+            return fullTryAcquireShared(current);
+        }
+
+        /**
+         * Full version of acquire for reads, that handles CAS misses
+         * and reentrant reads not dealt with in tryAcquireShared.
+         */
+        final int fullTryAcquireShared(Thread current) {
+            /*
+             * This code is in part redundant with that in
+             * tryAcquireShared but is simpler overall by not
+             * complicating tryAcquireShared with interactions between
+             * retries and lazily reading hold counts.
+             */
+            HoldCounter rh = cachedHoldCounter;
+            if (rh == null || rh.tid != current.getId())
+                rh = readHolds.get();
+            for (;;) {
+                int c = getState();
+                int w = exclusiveCount(c);
+                if ((w != 0 && getExclusiveOwnerThread() != current) ||
+                    ((rh.count | w) == 0 && readerShouldBlock(current)))
+                    return -1;
+                if (sharedCount(c) == MAX_COUNT)
+                    throw new Error("Maximum lock count exceeded");
+                if (compareAndSetState(c, c + SHARED_UNIT)) {
+                    cachedHoldCounter = rh; // cache for release
+                    rh.count++;
+                    return 1;
+                }
+            }
+        }
+
+        /**
+         * Performs tryLock for write, enabling barging in both modes.
+         * This is identical in effect to tryAcquire except for lack
+         * of calls to writerShouldBlock
+         */
+        final boolean tryWriteLock() {
+            Thread current = Thread.currentThread();
+            int c = getState();
+            if (c != 0) {
+                int w = exclusiveCount(c);
+                if (w == 0 ||current != getExclusiveOwnerThread())
+                    return false;
+                if (w == MAX_COUNT)
+                    throw new Error("Maximum lock count exceeded");
+            }
+            if (!compareAndSetState(c, c + 1))
+                return false;
+            setExclusiveOwnerThread(current);
+            return true;
+        }
+
+        /**
+         * Performs tryLock for read, enabling barging in both modes.
+         * This is identical in effect to tryAcquireShared except for
+         * lack of calls to readerShouldBlock
+         */
+        final boolean tryReadLock() {
+            Thread current = Thread.currentThread();
+            for (;;) {
+                int c = getState();
+                if (exclusiveCount(c) != 0 &&
+                    getExclusiveOwnerThread() != current)
+                    return false;
+                if (sharedCount(c) == MAX_COUNT)
+                    throw new Error("Maximum lock count exceeded");
+                if (compareAndSetState(c, c + SHARED_UNIT)) {
+                    HoldCounter rh = cachedHoldCounter;
+                    if (rh == null || rh.tid != current.getId())
+                        cachedHoldCounter = rh = readHolds.get();
+                    rh.count++;
+                    return true;
+                }
+            }
+        }
+
+        protected final boolean isHeldExclusively() {
+            // While we must in general read state before owner,
+            // we don't need to do so to check if current thread is owner
+            return getExclusiveOwnerThread() == Thread.currentThread();
+        }
+
+        // Methods relayed to outer class
+
+        final ConditionObject newCondition() {
+            return new ConditionObject();
+        }
+
+        final Thread getOwner() {
+            // Must read state before owner to ensure memory consistency
+            return ((exclusiveCount(getState()) == 0)?
+                    null :
+                    getExclusiveOwnerThread());
+        }
+
+        final int getReadLockCount() {
+            return sharedCount(getState());
+        }
+
+        final boolean isWriteLocked() {
+            return exclusiveCount(getState()) != 0;
+        }
+
+        final int getWriteHoldCount() {
+            return isHeldExclusively() ? exclusiveCount(getState()) : 0;
+        }
+
+        final int getReadHoldCount() {
+            return getReadLockCount() == 0? 0 : readHolds.get().count;
+        }
+
+        /**
+         * Reconstitute this lock instance from a stream
+         * @param s the stream
+         */
+        private void readObject(java.io.ObjectInputStream s)
+            throws java.io.IOException, ClassNotFoundException {
+            s.defaultReadObject();
+            readHolds = new ThreadLocalHoldCounter();
+            setState(0); // reset to unlocked state
+        }
+
+        final int getCount() { return getState(); }
+    }
+
+    /**
+     * Nonfair version of Sync
+     */
+    final static class NonfairSync extends Sync {
+        private static final long serialVersionUID = -8159625535654395037L;
+        final boolean writerShouldBlock(Thread current) {
+            return false; // writers can always barge
+        }
+        final boolean readerShouldBlock(Thread current) {
+            /* As a heuristic to avoid indefinite writer starvation,
+             * block if the thread that momentarily appears to be head
+             * of queue, if one exists, is a waiting writer. This is
+             * only a probablistic effect since a new reader will not
+             * block if there is a waiting writer behind other enabled
+             * readers that have not yet drained from the queue.
+             */
+            return apparentlyFirstQueuedIsExclusive();
+        }
+    }
+
+    /**
+     * Fair version of Sync
+     */
+    final static class FairSync extends Sync {
+        private static final long serialVersionUID = -2274990926593161451L;
+        final boolean writerShouldBlock(Thread current) {
+            // only proceed if queue is empty or current thread at head
+            return !isFirst(current);
+        }
+        final boolean readerShouldBlock(Thread current) {
+            // only proceed if queue is empty or current thread at head
+            return !isFirst(current);
+        }
+    }
+
+    /**
+     * The lock returned by method {@link ReentrantReadWriteLock#readLock}.
+     */
+    public static class ReadLock implements Lock, java.io.Serializable  {
+        private static final long serialVersionUID = -5992448646407690164L;
+        private final Sync sync;
+
+        /**
+         * Constructor for use by subclasses
+         *
+         * @param lock the outer lock object
+         * @throws NullPointerException if the lock is null
+         */
+        protected ReadLock(ReentrantReadWriteLock lock) {
+            sync = lock.sync;
+        }
+
+        /**
+         * Acquires the read lock.
+         *
+         * <p>Acquires the read lock if the write lock is not held by
+         * another thread and returns immediately.
+         *
+         * <p>If the write lock is held by another thread then
+         * the current thread becomes disabled for thread scheduling
+         * purposes and lies dormant until the read lock has been acquired.
+         */
+        public void lock() {
+            sync.acquireShared(1);
+        }
+
+        /**
+         * Acquires the read lock unless the current thread is
+         * {@linkplain Thread#interrupt interrupted}.
+         *
+         * <p>Acquires the read lock if the write lock is not held
+         * by another thread and returns immediately.
+         *
+         * <p>If the write lock is held by another thread then the
+         * current thread becomes disabled for thread scheduling
+         * purposes and lies dormant until one of two things happens:
+         *
+         * <ul>
+         *
+         * <li>The read lock is acquired by the current thread; or
+         *
+         * <li>Some other thread {@linkplain Thread#interrupt interrupts}
+         * the current thread.
+         *
+         * </ul>
+         *
+         * <p>If the current thread:
+         *
+         * <ul>
+         *
+         * <li>has its interrupted status set on entry to this method; or
+         *
+         * <li>is {@linkplain Thread#interrupt interrupted} while
+         * acquiring the read lock,
+         *
+         * </ul>
+         *
+         * then {@link InterruptedException} is thrown and the current
+         * thread's interrupted status is cleared.
+         *
+         * <p>In this implementation, as this method is an explicit
+         * interruption point, preference is given to responding to
+         * the interrupt over normal or reentrant acquisition of the
+         * lock.
+         *
+         * @throws InterruptedException if the current thread is interrupted
+         */
+        public void lockInterruptibly() throws InterruptedException {
+            sync.acquireSharedInterruptibly(1);
+        }
+
+        /**
+         * Acquires the read lock only if the write lock is not held by
+         * another thread at the time of invocation.
+         *
+         * <p>Acquires the read lock if the write lock is not held by
+         * another thread and returns immediately with the value
+         * {@code true}. Even when this lock has been set to use a
+         * fair ordering policy, a call to {@code tryLock()}
+         * <em>will</em> immediately acquire the read lock if it is
+         * available, whether or not other threads are currently
+         * waiting for the read lock.  This &quot;barging&quot; behavior
+         * can be useful in certain circumstances, even though it
+         * breaks fairness. If you want to honor the fairness setting
+         * for this lock, then use {@link #tryLock(long, TimeUnit)
+         * tryLock(0, TimeUnit.SECONDS) } which is almost equivalent
+         * (it also detects interruption).
+         *
+         * <p>If the write lock is held by another thread then
+         * this method will return immediately with the value
+         * {@code false}.
+         *
+         * @return {@code true} if the read lock was acquired
+         */
+        public  boolean tryLock() {
+            return sync.tryReadLock();
+        }
+
+        /**
+         * Acquires the read lock if the write lock is not held by
+         * another thread within the given waiting time and the
+         * current thread has not been {@linkplain Thread#interrupt
+         * interrupted}.
+         *
+         * <p>Acquires the read lock if the write lock is not held by
+         * another thread and returns immediately with the value
+         * {@code true}. If this lock has been set to use a fair
+         * ordering policy then an available lock <em>will not</em> be
+         * acquired if any other threads are waiting for the
+         * lock. This is in contrast to the {@link #tryLock()}
+         * method. If you want a timed {@code tryLock} that does
+         * permit barging on a fair lock then combine the timed and
+         * un-timed forms together:
+         *
+         * <pre>if (lock.tryLock() || lock.tryLock(timeout, unit) ) { ... }
+         * </pre>
+         *
+         * <p>If the write lock is held by another thread then the
+         * current thread becomes disabled for thread scheduling
+         * purposes and lies dormant until one of three things happens:
+         *
+         * <ul>
+         *
+         * <li>The read lock is acquired by the current thread; or
+         *
+         * <li>Some other thread {@linkplain Thread#interrupt interrupts}
+         * the current thread; or
+         *
+         * <li>The specified waiting time elapses.
+         *
+         * </ul>
+         *
+         * <p>If the read lock is acquired then the value {@code true} is
+         * returned.
+         *
+         * <p>If the current thread:
+         *
+         * <ul>
+         *
+         * <li>has its interrupted status set on entry to this method; or
+         *
+         * <li>is {@linkplain Thread#interrupt interrupted} while
+         * acquiring the read lock,
+         *
+         * </ul> then {@link InterruptedException} is thrown and the
+         * current thread's interrupted status is cleared.
+         *
+         * <p>If the specified waiting time elapses then the value
+         * {@code false} is returned.  If the time is less than or
+         * equal to zero, the method will not wait at all.
+         *
+         * <p>In this implementation, as this method is an explicit
+         * interruption point, preference is given to responding to
+         * the interrupt over normal or reentrant acquisition of the
+         * lock, and over reporting the elapse of the waiting time.
+         *
+         * @param timeout the time to wait for the read lock
+         * @param unit the time unit of the timeout argument
+         * @return {@code true} if the read lock was acquired
+         * @throws InterruptedException if the current thread is interrupted
+         * @throws NullPointerException if the time unit is null
+         *
+         */
+        public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException {
+            return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
+        }
+
+        /**
+         * Attempts to release this lock.
+         *
+         * <p> If the number of readers is now zero then the lock
+         * is made available for write lock attempts.
+         */
+        public  void unlock() {
+            sync.releaseShared(1);
+        }
+
+        /**
+         * Throws {@code UnsupportedOperationException} because
+	 * {@code ReadLocks} do not support conditions.
+         *
+         * @throws UnsupportedOperationException always
+         */
+        public Condition newCondition() {
+            throw new UnsupportedOperationException();
+        }
+
+        /**
+         * Returns a string identifying this lock, as well as its lock state.
+         * The state, in brackets, includes the String {@code "Read locks ="}
+         * followed by the number of held read locks.
+         *
+         * @return a string identifying this lock, as well as its lock state
+         */
+        public String toString() {
+            int r = sync.getReadLockCount();
+            return super.toString() +
+                "[Read locks = " + r + "]";
+        }
+    }
+
+    /**
+     * The lock returned by method {@link ReentrantReadWriteLock#writeLock}.
+     */
+    public static class WriteLock implements Lock, java.io.Serializable  {
+        private static final long serialVersionUID = -4992448646407690164L;
+	private final Sync sync;
+
+        /**
+         * Constructor for use by subclasses
+         *
+         * @param lock the outer lock object
+         * @throws NullPointerException if the lock is null
+         */
+        protected WriteLock(ReentrantReadWriteLock lock) {
+            sync = lock.sync;
+        }
+
+        /**
+         * Acquires the write lock.
+         *
+         * <p>Acquires the write lock if neither the read nor write lock
+	 * are held by another thread
+         * and returns immediately, setting the write lock hold count to
+         * one.
+         *
+         * <p>If the current thread already holds the write lock then the
+         * hold count is incremented by one and the method returns
+         * immediately.
+         *
+         * <p>If the lock is held by another thread then the current
+         * thread becomes disabled for thread scheduling purposes and
+         * lies dormant until the write lock has been acquired, at which
+         * time the write lock hold count is set to one.
+         */
+        public void lock() {
+            sync.acquire(1);
+        }
+
+        /**
+         * Acquires the write lock unless the current thread is
+         * {@linkplain Thread#interrupt interrupted}.
+         *
+         * <p>Acquires the write lock if neither the read nor write lock
+	 * are held by another thread
+         * and returns immediately, setting the write lock hold count to
+         * one.
+         *
+         * <p>If the current thread already holds this lock then the
+         * hold count is incremented by one and the method returns
+         * immediately.
+         *
+         * <p>If the lock is held by another thread then the current
+         * thread becomes disabled for thread scheduling purposes and
+         * lies dormant until one of two things happens:
+         *
+         * <ul>
+         *
+         * <li>The write lock is acquired by the current thread; or
+         *
+         * <li>Some other thread {@linkplain Thread#interrupt interrupts}
+         * the current thread.
+         *
+         * </ul>
+         *
+         * <p>If the write lock is acquired by the current thread then the
+         * lock hold count is set to one.
+         *
+         * <p>If the current thread:
+         *
+         * <ul>
+         *
+         * <li>has its interrupted status set on entry to this method;
+         * or
+         *
+         * <li>is {@linkplain Thread#interrupt interrupted} while
+         * acquiring the write lock,
+         *
+         * </ul>
+         *
+         * then {@link InterruptedException} is thrown and the current
+         * thread's interrupted status is cleared.
+         *
+         * <p>In this implementation, as this method is an explicit
+         * interruption point, preference is given to responding to
+         * the interrupt over normal or reentrant acquisition of the
+         * lock.
+         *
+         * @throws InterruptedException if the current thread is interrupted
+         */
+        public void lockInterruptibly() throws InterruptedException {
+            sync.acquireInterruptibly(1);
+        }
+
+        /**
+         * Acquires the write lock only if it is not held by another thread
+         * at the time of invocation.
+         *
+         * <p>Acquires the write lock if neither the read nor write lock
+	 * are held by another thread
+         * and returns immediately with the value {@code true},
+         * setting the write lock hold count to one. Even when this lock has
+         * been set to use a fair ordering policy, a call to
+         * {@code tryLock()} <em>will</em> immediately acquire the
+         * lock if it is available, whether or not other threads are
+         * currently waiting for the write lock.  This &quot;barging&quot;
+         * behavior can be useful in certain circumstances, even
+         * though it breaks fairness. If you want to honor the
+         * fairness setting for this lock, then use {@link
+         * #tryLock(long, TimeUnit) tryLock(0, TimeUnit.SECONDS) }
+         * which is almost equivalent (it also detects interruption).
+         *
+         * <p> If the current thread already holds this lock then the
+         * hold count is incremented by one and the method returns
+         * {@code true}.
+         *
+         * <p>If the lock is held by another thread then this method
+         * will return immediately with the value {@code false}.
+         *
+         * @return {@code true} if the lock was free and was acquired
+         * by the current thread, or the write lock was already held
+         * by the current thread; and {@code false} otherwise.
+         */
+        public boolean tryLock( ) {
+            return sync.tryWriteLock();
+        }
+
+        /**
+         * Acquires the write lock if it is not held by another thread
+         * within the given waiting time and the current thread has
+         * not been {@linkplain Thread#interrupt interrupted}.
+         *
+         * <p>Acquires the write lock if neither the read nor write lock
+	 * are held by another thread
+         * and returns immediately with the value {@code true},
+         * setting the write lock hold count to one. If this lock has been
+         * set to use a fair ordering policy then an available lock
+         * <em>will not</em> be acquired if any other threads are
+         * waiting for the write lock. This is in contrast to the {@link
+         * #tryLock()} method. If you want a timed {@code tryLock}
+         * that does permit barging on a fair lock then combine the
+         * timed and un-timed forms together:
+         *
+         * <pre>if (lock.tryLock() || lock.tryLock(timeout, unit) ) { ... }
+         * </pre>
+         *
+         * <p>If the current thread already holds this lock then the
+         * hold count is incremented by one and the method returns
+         * {@code true}.
+         *
+         * <p>If the lock is held by another thread then the current
+         * thread becomes disabled for thread scheduling purposes and
+         * lies dormant until one of three things happens:
+         *
+         * <ul>
+         *
+         * <li>The write lock is acquired by the current thread; or
+         *
+         * <li>Some other thread {@linkplain Thread#interrupt interrupts}
+         * the current thread; or
+         *
+         * <li>The specified waiting time elapses
+         *
+         * </ul>
+         *
+         * <p>If the write lock is acquired then the value {@code true} is
+         * returned and the write lock hold count is set to one.
+         *
+         * <p>If the current thread:
+         *
+         * <ul>
+         *
+         * <li>has its interrupted status set on entry to this method;
+         * or
+         *
+         * <li>is {@linkplain Thread#interrupt interrupted} while
+         * acquiring the write lock,
+         *
+         * </ul>
+         *
+         * then {@link InterruptedException} is thrown and the current
+         * thread's interrupted status is cleared.
+         *
+         * <p>If the specified waiting time elapses then the value
+         * {@code false} is returned.  If the time is less than or
+         * equal to zero, the method will not wait at all.
+         *
+         * <p>In this implementation, as this method is an explicit
+         * interruption point, preference is given to responding to
+         * the interrupt over normal or reentrant acquisition of the
+         * lock, and over reporting the elapse of the waiting time.
+         *
+         * @param timeout the time to wait for the write lock
+         * @param unit the time unit of the timeout argument
+         *
+         * @return {@code true} if the lock was free and was acquired
+         * by the current thread, or the write lock was already held by the
+         * current thread; and {@code false} if the waiting time
+         * elapsed before the lock could be acquired.
+         *
+         * @throws InterruptedException if the current thread is interrupted
+         * @throws NullPointerException if the time unit is null
+         *
+         */
+        public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException {
+            return sync.tryAcquireNanos(1, unit.toNanos(timeout));
+        }
+
+        /**
+         * Attempts to release this lock.
+         *
+         * <p>If the current thread is the holder of this lock then
+         * the hold count is decremented. If the hold count is now
+         * zero then the lock is released.  If the current thread is
+         * not the holder of this lock then {@link
+         * IllegalMonitorStateException} is thrown.
+         *
+         * @throws IllegalMonitorStateException if the current thread does not
+         * hold this lock.
+         */
+        public void unlock() {
+            sync.release(1);
+        }
+
+        /**
+         * Returns a {@link Condition} instance for use with this
+         * {@link Lock} instance.
+         * <p>The returned {@link Condition} instance supports the same
+         * usages as do the {@link Object} monitor methods ({@link
+         * Object#wait() wait}, {@link Object#notify notify}, and {@link
+         * Object#notifyAll notifyAll}) when used with the built-in
+         * monitor lock.
+         *
+         * <ul>
+         *
+         * <li>If this write lock is not held when any {@link
+         * Condition} method is called then an {@link
+         * IllegalMonitorStateException} is thrown.  (Read locks are
+         * held independently of write locks, so are not checked or
+         * affected. However it is essentially always an error to
+         * invoke a condition waiting method when the current thread
+         * has also acquired read locks, since other threads that
+         * could unblock it will not be able to acquire the write
+         * lock.)
+         *
+         * <li>When the condition {@linkplain Condition#await() waiting}
+         * methods are called the write lock is released and, before
+         * they return, the write lock is reacquired and the lock hold
+         * count restored to what it was when the method was called.
+         *
+         * <li>If a thread is {@linkplain Thread#interrupt interrupted} while
+         * waiting then the wait will terminate, an {@link
+         * InterruptedException} will be thrown, and the thread's
+         * interrupted status will be cleared.
+         *
+         * <li> Waiting threads are signalled in FIFO order.
+         *
+         * <li>The ordering of lock reacquisition for threads returning
+         * from waiting methods is the same as for threads initially
+         * acquiring the lock, which is in the default case not specified,
+         * but for <em>fair</em> locks favors those threads that have been
+         * waiting the longest.
+         *
+         * </ul>
+         *
+         * @return the Condition object
+         */
+        public Condition newCondition() {
+            return sync.newCondition();
+        }
+
+        /**
+         * Returns a string identifying this lock, as well as its lock
+         * state.  The state, in brackets includes either the String
+         * {@code "Unlocked"} or the String {@code "Locked by"}
+         * followed by the {@linkplain Thread#getName name} of the owning thread.
+         *
+         * @return a string identifying this lock, as well as its lock state
+         */
+        public String toString() {
+            Thread o = sync.getOwner();
+            return super.toString() + ((o == null) ?
+                                       "[Unlocked]" :
+                                       "[Locked by thread " + o.getName() + "]");
+        }
+
+	/**
+	 * Queries if this write lock is held by the current thread.
+	 * Identical in effect to {@link
+	 * ReentrantReadWriteLock#isWriteLockedByCurrentThread}.
+	 *
+	 * @return {@code true} if the current thread holds this lock and
+	 *	   {@code false} otherwise
+	 * @since 1.6
+	 */
+	public boolean isHeldByCurrentThread() {
+	    return sync.isHeldExclusively();
+	}
+
+	/**
+	 * Queries the number of holds on this write lock by the current
+	 * thread.  A thread has a hold on a lock for each lock action
+	 * that is not matched by an unlock action.  Identical in effect
+	 * to {@link ReentrantReadWriteLock#getWriteHoldCount}.
+	 *
+	 * @return the number of holds on this lock by the current thread,
+	 *	   or zero if this lock is not held by the current thread
+	 * @since 1.6
+	 */
+	public int getHoldCount() {
+	    return sync.getWriteHoldCount();
+	}
+    }
+
+    // Instrumentation and status
+
+    /**
+     * Returns {@code true} if this lock has fairness set true.
+     *
+     * @return {@code true} if this lock has fairness set true
+     */
+    public final boolean isFair() {
+        return sync instanceof FairSync;
+    }
+
+    /**
+     * Returns the thread that currently owns the write lock, or
+     * {@code null} if not owned. When this method is called by a
+     * thread that is not the owner, the return value reflects a
+     * best-effort approximation of current lock status. For example,
+     * the owner may be momentarily {@code null} even if there are
+     * threads trying to acquire the lock but have not yet done so.
+     * This method is designed to facilitate construction of
+     * subclasses that provide more extensive lock monitoring
+     * facilities.
+     *
+     * @return the owner, or {@code null} if not owned
+     */
+    protected Thread getOwner() {
+        return sync.getOwner();
+    }
+
+    /**
+     * Queries the number of read locks held for this lock. This
+     * method is designed for use in monitoring system state, not for
+     * synchronization control.
+     * @return the number of read locks held.
+     */
+    public int getReadLockCount() {
+        return sync.getReadLockCount();
+    }
+
+    /**
+     * Queries if the write lock is held by any thread. This method is
+     * designed for use in monitoring system state, not for
+     * synchronization control.
+     *
+     * @return {@code true} if any thread holds the write lock and
+     *         {@code false} otherwise
+     */
+    public boolean isWriteLocked() {
+        return sync.isWriteLocked();
+    }
+
+    /**
+     * Queries if the write lock is held by the current thread.
+     *
+     * @return {@code true} if the current thread holds the write lock and
+     *         {@code false} otherwise
+     */
+    public boolean isWriteLockedByCurrentThread() {
+        return sync.isHeldExclusively();
+    }
+
+    /**
+     * Queries the number of reentrant write holds on this lock by the
+     * current thread.  A writer thread has a hold on a lock for
+     * each lock action that is not matched by an unlock action.
+     *
+     * @return the number of holds on the write lock by the current thread,
+     *         or zero if the write lock is not held by the current thread
+     */
+    public int getWriteHoldCount() {
+        return sync.getWriteHoldCount();
+    }
+
+    /**
+     * Queries the number of reentrant read holds on this lock by the
+     * current thread.  A reader thread has a hold on a lock for
+     * each lock action that is not matched by an unlock action.
+     *
+     * @return the number of holds on the read lock by the current thread,
+     *         or zero if the read lock is not held by the current thread
+     * @since 1.6
+     */
+    public int getReadHoldCount() {
+        return sync.getReadHoldCount();
+    }
+
+    /**
+     * Returns a collection containing threads that may be waiting to
+     * acquire the write lock.  Because the actual set of threads may
+     * change dynamically while constructing this result, the returned
+     * collection is only a best-effort estimate.  The elements of the
+     * returned collection are in no particular order.  This method is
+     * designed to facilitate construction of subclasses that provide
+     * more extensive lock monitoring facilities.
+     *
+     * @return the collection of threads
+     */
+    protected Collection<Thread> getQueuedWriterThreads() {
+        return sync.getExclusiveQueuedThreads();
+    }
+
+    /**
+     * Returns a collection containing threads that may be waiting to
+     * acquire the read lock.  Because the actual set of threads may
+     * change dynamically while constructing this result, the returned
+     * collection is only a best-effort estimate.  The elements of the
+     * returned collection are in no particular order.  This method is
+     * designed to facilitate construction of subclasses that provide
+     * more extensive lock monitoring facilities.
+     *
+     * @return the collection of threads
+     */
+    protected Collection<Thread> getQueuedReaderThreads() {
+        return sync.getSharedQueuedThreads();
+    }
+
+    /**
+     * Queries whether any threads are waiting to acquire the read or
+     * write lock. Note that because cancellations may occur at any
+     * time, a {@code true} return does not guarantee that any other
+     * thread will ever acquire a lock.  This method is designed
+     * primarily for use in monitoring of the system state.
+     *
+     * @return {@code true} if there may be other threads waiting to
+     *         acquire the lock
+     */
+    public final boolean hasQueuedThreads() {
+        return sync.hasQueuedThreads();
+    }
+
+    /**
+     * Queries whether the given thread is waiting to acquire either
+     * the read or write lock. Note that because cancellations may
+     * occur at any time, a {@code true} return does not guarantee
+     * that this thread will ever acquire a lock.  This method is
+     * designed primarily for use in monitoring of the system state.
+     *
+     * @param thread the thread
+     * @return {@code true} if the given thread is queued waiting for this lock
+     * @throws NullPointerException if the thread is null
+     */
+    public final boolean hasQueuedThread(Thread thread) {
+        return sync.isQueued(thread);
+    }
+
+    /**
+     * Returns an estimate of the number of threads waiting to acquire
+     * either the read or write lock.  The value is only an estimate
+     * because the number of threads may change dynamically while this
+     * method traverses internal data structures.  This method is
+     * designed for use in monitoring of the system state, not for
+     * synchronization control.
+     *
+     * @return the estimated number of threads waiting for this lock
+     */
+    public final int getQueueLength() {
+        return sync.getQueueLength();
+    }
+
+    /**
+     * Returns a collection containing threads that may be waiting to
+     * acquire either the read or write lock.  Because the actual set
+     * of threads may change dynamically while constructing this
+     * result, the returned collection is only a best-effort estimate.
+     * The elements of the returned collection are in no particular
+     * order.  This method is designed to facilitate construction of
+     * subclasses that provide more extensive monitoring facilities.
+     *
+     * @return the collection of threads
+     */
+    protected Collection<Thread> getQueuedThreads() {
+        return sync.getQueuedThreads();
+    }
+
+    /**
+     * Queries whether any threads are waiting on the given condition
+     * associated with the write lock. Note that because timeouts and
+     * interrupts may occur at any time, a {@code true} return does
+     * not guarantee that a future {@code signal} will awaken any
+     * threads.  This method is designed primarily for use in
+     * monitoring of the system state.
+     *
+     * @param condition the condition
+     * @return {@code true} if there are any waiting threads
+     * @throws IllegalMonitorStateException if this lock is not held
+     * @throws IllegalArgumentException if the given condition is
+     *         not associated with this lock
+     * @throws NullPointerException if the condition is null
+     */
+    public boolean hasWaiters(Condition condition) {
+        if (condition == null)
+            throw new NullPointerException();
+        if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
+            throw new IllegalArgumentException("not owner");
+        return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject)condition);
+    }
+
+    /**
+     * Returns an estimate of the number of threads waiting on the
+     * given condition associated with the write lock. Note that because
+     * timeouts and interrupts may occur at any time, the estimate
+     * serves only as an upper bound on the actual number of waiters.
+     * This method is designed for use in monitoring of the system
+     * state, not for synchronization control.
+     *
+     * @param condition the condition
+     * @return the estimated number of waiting threads
+     * @throws IllegalMonitorStateException if this lock is not held
+     * @throws IllegalArgumentException if the given condition is
+     *         not associated with this lock
+     * @throws NullPointerException if the condition is null
+     */
+    public int getWaitQueueLength(Condition condition) {
+        if (condition == null)
+            throw new NullPointerException();
+        if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
+            throw new IllegalArgumentException("not owner");
+        return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject)condition);
+    }
+
+    /**
+     * Returns a collection containing those threads that may be
+     * waiting on the given condition associated with the write lock.
+     * Because the actual set of threads may change dynamically while
+     * constructing this result, the returned collection is only a
+     * best-effort estimate. The elements of the returned collection
+     * are in no particular order.  This method is designed to
+     * facilitate construction of subclasses that provide more
+     * extensive condition monitoring facilities.
+     *
+     * @param condition the condition
+     * @return the collection of threads
+     * @throws IllegalMonitorStateException if this lock is not held
+     * @throws IllegalArgumentException if the given condition is
+     *         not associated with this lock
+     * @throws NullPointerException if the condition is null
+     */
+    protected Collection<Thread> getWaitingThreads(Condition condition) {
+        if (condition == null)
+            throw new NullPointerException();
+        if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
+            throw new IllegalArgumentException("not owner");
+        return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject)condition);
+    }
+
+    /**
+     * Returns a string identifying this lock, as well as its lock state.
+     * The state, in brackets, includes the String {@code "Write locks ="}
+     * followed by the number of reentrantly held write locks, and the
+     * String {@code "Read locks ="} followed by the number of held
+     * read locks.
+     *
+     * @return a string identifying this lock, as well as its lock state
+     */
+    public String toString() {
+        int c = sync.getCount();
+        int w = Sync.exclusiveCount(c);
+        int r = Sync.sharedCount(c);
+
+        return super.toString() +
+            "[Write locks = " + w + ", Read locks = " + r + "]";
+    }
+
+}
diff --git a/external/jsr166/java/util/concurrent/locks/package.html b/external/jsr166/java/util/concurrent/locks/package.html
new file mode 100644
index 000000000..4b945258b
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/locks/package.html
@@ -0,0 +1,50 @@
+<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
+<html> <head>
+<title>Locks</title>
+</head>
+
+<body>
+
+Interfaces and classes providing a framework for locking and waiting
+for conditions that is distinct from built-in synchronization and
+monitors. The framework permits much greater flexibility in the use of
+locks and conditions, at the expense of more awkward syntax.
+
+<p> The {@link java.util.concurrent.locks.Lock} interface supports
+locking disciplines that differ in semantics (reentrant, fair, etc),
+and that can be used in non-block-structured contexts including
+hand-over-hand and lock reordering algorithms. The main implementation
+is {@link java.util.concurrent.locks.ReentrantLock}.
+
+<p> The {@link java.util.concurrent.locks.ReadWriteLock} interface
+similarly defines locks that may be shared among readers but are
+exclusive to writers.  Only a single implementation, {@link
+java.util.concurrent.locks.ReentrantReadWriteLock}, is provided, since
+it covers most standard usage contexts. But programmers may create
+their own implementations to cover nonstandard requirements.
+
+<p> The {@link java.util.concurrent.locks.Condition} interface
+describes condition variables that may be associated with Locks.
+These are similar in usage to the implicit monitors accessed using
+<tt>Object.wait</tt>, but offer extended capabilities.  In particular,
+multiple <tt>Condition</tt> objects may be associated with a single
+<tt>Lock</tt>.  To avoid compatibility issues, the names of
+<tt>Condition</tt> methods are different than the corresponding
+<tt>Object</tt> versions.
+
+<p> The {@link java.util.concurrent.locks.AbstractQueuedSynchronizer}
+class serves as a useful superclass for defining locks and other
+synchronizers that rely on queuing blocked threads.  The {@link
+java.util.concurrent.locks.AbstractQueuedLongSynchronizer} class
+provides the same functionality but extends support to 64 bits of
+synchronization state. Both extend class {@link
+java.util.concurrent.locks.AbstractOwnableSynchronizer}, a simple
+class that helps record the thread currently holding exclusive
+synchronization.  The {@link java.util.concurrent.locks.LockSupport}
+class provides lower-level blocking and unblocking support that is
+useful for those developers implementing their own customized lock
+classes.
+
+@since 1.5
+
+</body> </html>
diff --git a/external/jsr166/java/util/concurrent/package.html b/external/jsr166/java/util/concurrent/package.html
new file mode 100644
index 000000000..20227e1fe
--- /dev/null
+++ b/external/jsr166/java/util/concurrent/package.html
@@ -0,0 +1,222 @@
+<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
+<html> <head>
+<title>Concurrency Utilities</title>
+</head>
+
+<body>
+
+<p> Utility classes commonly useful in concurrent programming.  This
+package includes a few small standardized extensible frameworks, as
+well as some classes that provide useful functionality and are
+otherwise tedious or difficult to implement.  Here are brief
+descriptions of the main components. See also the <tt>locks</tt> and
+<tt>atomic</tt> packages.
+
+<h2>Executors</h2>
+
+<b>Interfaces.</b> {@link java.util.concurrent.Executor} is a simple
+standardized interface for defining custom thread-like subsystems,
+including thread pools, asynchronous IO, and lightweight task
+frameworks.  Depending on which concrete Executor class is being used,
+tasks may execute in a newly created thread, an existing
+task-execution thread, or the thread calling <tt>execute()</tt>, and
+may execute sequentially or concurrently.  {@link
+java.util.concurrent.ExecutorService} provides a more complete
+asynchronous task execution framework.  An ExecutorService manages
+queuing and scheduling of tasks, and allows controlled shutdown.  The
+{@link java.util.concurrent.ScheduledExecutorService} subinterface
+and associated interfaces add support for delayed and periodic task execution.
+ExecutorServices provide methods arranging asynchronous execution of
+any function expressed as {@link java.util.concurrent.Callable}, the
+result-bearing analog of {@link java.lang.Runnable}.  A {@link
+java.util.concurrent.Future} returns the results of a function, allows
+determination of whether execution has completed, and provides a means to
+cancel execution.  A {@link java.util.concurrent.RunnableFuture} is
+a Future that possesses a <tt>run</tt> method that upon execution,
+sets its results.
+
+<p>
+
+<b>Implementations.</b> Classes {@link
+java.util.concurrent.ThreadPoolExecutor} and {@link
+java.util.concurrent.ScheduledThreadPoolExecutor} provide tunable,
+flexible thread pools. The {@link java.util.concurrent.Executors}
+class provides factory methods for the most common kinds and
+configurations of Executors, as well as a few utility methods for
+using them. Other utilities based on Executors include the concrete
+class {@link java.util.concurrent.FutureTask} providing a common
+extensible implementation of Futures, and {@link
+java.util.concurrent.ExecutorCompletionService}, that assists in
+coordinating the processing of groups of asynchronous tasks.
+
+<h2>Queues</h2>
+
+The java.util.concurrent {@link
+java.util.concurrent.ConcurrentLinkedQueue} class supplies an
+efficient scalable thread-safe non-blocking FIFO queue.  Five
+implementations in java.util.concurrent support the extended {@link
+java.util.concurrent.BlockingQueue} interface, that defines blocking
+versions of put and take: {@link
+java.util.concurrent.LinkedBlockingQueue}, {@link
+java.util.concurrent.ArrayBlockingQueue}, {@link
+java.util.concurrent.SynchronousQueue}, {@link
+java.util.concurrent.PriorityBlockingQueue}, and {@link
+java.util.concurrent.DelayQueue}. The different classes cover the most
+common usage contexts for producer-consumer, messaging, parallel
+tasking, and related concurrent designs. The {@link
+java.util.concurrent.BlockingDeque} interface extends
+<tt>BlockingQueue</tt> to support both FIFO and LIFO (stack-based)
+operations. Class {@link java.util.concurrent.LinkedBlockingDeque}
+provides an implementation.
+
+
+<h2>Timing</h2>
+
+The {@link java.util.concurrent.TimeUnit} class provides multiple
+granularities (including nanoseconds) for specifying and controlling
+time-out based operations.  Most classes in the package contain
+operations based on time-outs in addition to indefinite waits.  In all
+cases that time-outs are used, the time-out specifies the minimum time
+that the method should wait before indicating that it
+timed-out.  Implementations make a &quot;best effort&quot; to detect
+time-outs as soon as possible after they occur.  However, an indefinite
+amount of time may elapse between a time-out being detected and a
+thread actually executing again after that time-out.  All methods
+that accept timeout parameters treat values less than or equal to
+zero to mean not to wait at all.  To wait "forever", you can use
+a value of <tt>Long.MAX_VALUE</tt>.
+
+<h2>Synchronizers</h2>
+
+Four classes aid common special-purpose synchronization idioms.
+{@link java.util.concurrent.Semaphore} is a classic concurrency tool.
+{@link java.util.concurrent.CountDownLatch} is a very simple yet very
+common utility for blocking until a given number of signals, events,
+or conditions hold.  A {@link java.util.concurrent.CyclicBarrier} is a
+resettable multiway synchronization point useful in some styles of
+parallel programming. An {@link java.util.concurrent.Exchanger} allows
+two threads to exchange objects at a rendezvous point, and is useful
+in several pipeline designs.
+
+<h2>Concurrent Collections</h2>
+
+Besides Queues, this package supplies Collection implementations
+designed for use in multithreaded contexts:
+{@link java.util.concurrent.ConcurrentHashMap},
+{@link java.util.concurrent.ConcurrentSkipListMap},
+{@link java.util.concurrent.ConcurrentSkipListSet},
+{@link java.util.concurrent.CopyOnWriteArrayList}, and
+{@link java.util.concurrent.CopyOnWriteArraySet}.
+When many threads are expected to access a given collection,
+a <tt>ConcurrentHashMap</tt> is normally preferable to
+a synchronized <tt>HashMap</tt>, and a
+<tt>ConcurrentSkipListMap</tt> is normally preferable
+to a synchronized <tt>TreeMap</tt>. A
+<tt>CopyOnWriteArrayList</tt> is preferable to
+a synchronized <tt>ArrayList</tt> when the expected number of reads
+and traversals greatly outnumber the number of updates to a list.
+
+<p>The "Concurrent" prefix used with some classes in this package is a
+shorthand indicating several differences from similar "synchronized"
+classes. For example <tt>java.util.Hashtable</tt> and
+<tt>Collections.synchronizedMap(new HashMap())</tt> are
+synchronized. But {@link java.util.concurrent.ConcurrentHashMap} is
+"concurrent".  A concurrent collection is thread-safe, but not
+governed by a single exclusion lock. In the particular case of
+ConcurrentHashMap, it safely permits any number of concurrent reads as
+well as a tunable number of concurrent writes.  "Synchronized" classes
+can be useful when you need to prevent all access to a collection via
+a single lock, at the expense of poorer scalability. In other cases in
+which multiple threads are expected to access a common collection,
+"concurrent" versions are normally preferable. And unsynchronized
+collections are preferable when either collections are unshared, or
+are accessible only when holding other locks.
+
+<p> Most concurrent Collection implementations (including most Queues)
+also differ from the usual java.util conventions in that their Iterators
+provide <em>weakly consistent</em> rather than fast-fail traversal. A
+weakly consistent iterator is thread-safe, but does not necessarily
+freeze the collection while iterating, so it may (or may not) reflect
+any updates since the iterator was created.
+
+<a name="MemoryVisibility">
+<h2> Memory Consistency Properties </h2>
+
+<a href="http://java.sun.com/docs/books/jls/third_edition/html/memory.html">
+Chapter 17 of the Java Language Specification</a> defines the
+<i>happens-before</i> relation on memory operations such as reads and
+writes of shared variables.  The results of a write by one thread are
+guaranteed to be visible to a read by another thread only if the write
+operation <i>happens-before</i> the read operation.  The
+{@code synchronized} and {@code volatile} constructs, as well as the
+{@code Thread.start()} and {@code Thread.join()} methods, can form
+<i>happens-before</i> relationships. In particular:
+
+<ul>
+  <li>Each action in a thread <i>happens-before</i> every action in that
+  thread that comes later in the program's order.
+
+  <li>An unlock ({@code synchronized} block or method exit) of a
+  monitor <i>happens-before</i> every subsequent lock ({@code synchronized}
+  block or method entry) of that same monitor. And because
+  the <i>happens-before</i> relation is transitive, all actions
+  of a thread prior to unlocking <i>happen-before</i> all actions
+  subsequent to any thread locking that monitor.
+
+  <li>A write to a {@code volatile} field <i>happens-before</i> every
+  subsequent read of that same field. Writes and reads of
+  {@code volatile} fields have similar memory consistency effects
+  as entering and exiting monitors, but do <em>not</em> entail
+  mutual exclusion locking.
+
+  <li>A call to {@code start} on a thread <i>happens-before</i> any action in the
+  started thread.
+
+  <li>All actions in a thread <i>happen-before</i> any other thread
+  successfully returns from a {@code join} on that thread.
+
+</ul>
+
+
+The methods of all classes in {@code java.util.concurrent} and its
+subpackages extend these guarantees to higher-level
+synchronization. In particular:
+
+<ul>
+
+  <li>Actions in a thread prior to placing an object into any concurrent
+  collection <i>happen-before</i> actions subsequent to the access or
+  removal of that element from the collection in another thread.
+
+  <li>Actions in a thread prior to the submission of a {@code Runnable}
+  to an {@code Executor} <i>happen-before</i> its execution begins.
+  Similarly for {@code Callables} submitted to an {@code ExecutorService}.
+
+  <li>Actions taken by the asynchronous computation represented by a
+  {@code Future} <i>happen-before</i> actions subsequent to the
+  retrieval of the result via {@code Future.get()} in another thread.
+
+  <li>Actions prior to "releasing" synchronizer methods such as
+  {@code Lock.unlock}, {@code Semaphore.release}, and
+  {@code CountDownLatch.countDown} <i>happen-before</i> actions
+  subsequent to a successful "acquiring" method such as
+  {@code Lock.lock}, {@code Semaphore.acquire},
+  {@code Condition.await}, and {@code CountDownLatch.await} on the
+  same synchronizer object in another thread.
+
+  <li>For each pair of threads that successfully exchange objects via
+  an {@code Exchanger}, actions prior to the {@code exchange()}
+  in each thread <i>happen-before</i> those subsequent to the
+  corresponding {@code exchange()} in another thread.
+
+  <li>Actions prior to calling {@code CyclicBarrier.await}
+  <i>happen-before</i> actions performed by the barrier action, and
+  actions performed by the barrier action <i>happen-before</i> actions
+  subsequent to a successful return from the corresponding {@code await}
+  in other threads.
+
+</ul>
+
+@since 1.5
+
+</body> </html>
diff --git a/external/jsr166/readme b/external/jsr166/readme
new file mode 100644
index 000000000..eef3c9169
--- /dev/null
+++ b/external/jsr166/readme
@@ -0,0 +1,45 @@
+The software comprising JSR166 was written by Doug Lea with assistance
+from members of JCP JSR-166 Expert roup and released to the public
+domain, as explained at:
+http://creativecommons.org/licenses/publicdomain, excepting portions
+of the class java.util.concurrent.CopyOnWriteArrayList, which were
+adapted from class java.util.ArrayList, written by Sun Microsystems,
+Inc, which are used with kind permission, and subject to the
+following:
+
+Copyright 2002-2004 Sun Microsystems, Inc. All rights reserved. Use is
+subject to the following license terms.
+
+  "Sun hereby grants you a non-exclusive, worldwide, non-transferrable
+  license to use and distribute the Java Software technologies as part
+  of a larger work in source and binary forms, with or without
+  modification, provided that the following conditions are met:
+
+   -Neither the name of or trademarks of Sun may be used to endorse or
+    promote products derived from the Java Software technology without
+    specific prior written permission.
+
+   -Redistributions of source or binary code must be accompanied by the
+    following notice and disclaimers:
+
+    Portions copyright Sun Microsystems, Inc. Used with kind permission.
+
+    This software is provided AS IS, without a warranty of any kind.  ALL
+    EXPRESS OR IMPLIED CONDITIONS, REPRESENTATIONS AND
+    WARRANTIES, INCLUDING ANY IMPLIED WARRANTY OF
+    MERCHANTABILITY, FITNESS FOR A PARTICULAR PUPOSE OR
+    NON-INFRINGEMENT, ARE HEREBY EXCLUDED. SUN
+    MICROSYSTEMS, INC. AND ITS LICENSORS SHALL NOT BE LIABLE
+    FOR ANY DAMAGES SUFFERED BY LICENSEE AS A RESULT OF
+    USING, MODIFYING OR DISTRIBUTING THE SOFTWARE OR ITS
+    DERIVATIVES. IN NO EVENT WILL SUN MICROSYSTEMS, INC. OR
+    ITS LICENSORS BE LIABLE FOR ANY LOST REVENUE, PROFIT OR
+    DATA, OR FOR DIRECT, INDIRECT,CONSQUENTIAL, INCIDENTAL
+    OR PUNITIVE DAMAGES, HOWEVER CAUSED AND REGARDLESS OF
+    THE THEORY OR LIABILITY, ARISING OUT OF THE USE OF OR
+    INABILITY TO USE SOFTWARE, EVEN IF SUN MICROSYSTEMS, INC.
+    HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.
+
+    You acknowledge that Software is not designed, licensed or intended for
+    use in the design, construction, operation or maintenance of any nuclear
+    facility."