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// $Id$
// This is a queue class for use by multiple threads, with at least one
// thread enqueueing objects and another dequeueing them. The dequeue_*
// methods block the thread if there are no more objects in the queue,
// while the try_dequeue_* methods do not block but rather return a
// null reference if there is nothing in the queue.
public class MTQueue
{
// Initially, the head and tail of the queue are null
MTQueue_Node head_ = null;
MTQueue_Node tail_ = null;
// Constructor - does nothing.
public MTQueue ()
{
}
// Places a passed Object at the end of the queue.
public synchronized void enqueue_tail (Object new_data)
{
// Create a new node to hold the object.
MTQueue_Node new_node = new MTQueue_Node(new_data);
// Insert the node into the queue.
if (tail_ == null)
{
tail_ = new_node;
head_ = new_node;
}
else
{
new_node.prev_ = tail_;
tail_.next_ = new_node;
tail_ = new_node;
}
// Wake up any waiting threads
notifyAll ();
}
// Places a passed Object at the front of the queue.
public synchronized void enqueue_head(Object new_data)
{
// Create a new node to hold the object.
MTQueue_Node new_node = new MTQueue_Node(new_data);
// Insert the node into the queue.
if (head_ == null)
{
tail_ = new_node;
head_ = new_node;
}
else
{
new_node.next_ = head_;
head_.prev_ = new_node;
head_ = new_node;
}
// Wake up any waiting threads
notifyAll ();
}
// Try to remove an object from the head of the queue - nonblocking.
public synchronized Object try_dequeue_head()
{
// Start with a null reference.
Object return_value = null;
// If there's anything there, dequeue it.
if (head_ != null)
{
return_value = dequeue_head ();
}
// Return what we found, if anything.
return return_value;
}
// Remove an object from the head of the queue - blocking.
public synchronized Object dequeue_head()
{
// Start with a null reference.
Object return_value = null;
// Wait until there's something to dequeue.
while (head_ == null)
{
try
{
wait ();
}
catch (InterruptedException e)
{
return return_value;
}
}
// Dequeue the object at the head of the queue. Make sure
// to null out references within dequeued nodes to prevent
// out of memory errors.
if (tail_ == head_)
{
return_value = head_.data_;
head_.next_ = null;
head_.prev_ = null;
head_.data_ = null;
tail_ = null;
head_ = null;
}
else
{
return_value = head_.data_;
head_ = head_.next_;
head_.prev_.next_ = null;
head_.prev_.prev_ = null;
head_.prev_.data_ = null;
head_.prev_ = null;
}
// Return the object we dequeued.
return return_value;
}
// Try to remove an object from the tail of the queue - nonblocking.
public synchronized Object try_dequeue_tail ()
{
// Start with a null reference.
Object return_value = null;
// If there's anything there, dequeue it.
if (tail_ != null)
{
return_value = dequeue_tail ();
}
// Return what we found, if anything.
return return_value;
}
// Remove an object from the tail of the queue - blocking.
public synchronized Object dequeue_tail ()
{
// Start with a null reference.
Object return_value = null;
// Wait until there's something to dequeue.
while (tail_ == null)
{
try
{
wait ();
}
catch (InterruptedException e)
{
return return_value;
}
}
// Dequeue the object at the back of the queue. Make sure
// to null out references within dequeued nodes to prevent
// out of memory errors.
if (tail_ == head_)
{
return_value = tail_.data_;
tail_.data_ = null;
tail_.next_ = null;
tail_.prev_ = null;
tail_ = null;
head_ = null;
}
else
{
return_value = tail_.data_;
tail_ = tail_.prev_;
tail_.next_.data_ = null;
tail_.next_.next_ = null;
tail_.next_.prev_ = null;
tail_.next_ = null;
}
// Return the object we dequeued.
return return_value;
}
}
class MTQueue_Node
{
public MTQueue_Node prev_ = null;
public MTQueue_Node next_ = null;;
public Object data_;
public MTQueue_Node(Object data)
{
data_ = data;
}
}
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