Creating a branch for experimenting with some ideas for JMS client.rajith_jms_client

git-svn-id: https://svn.apache.org/repos/asf/qpid/branches/rajith_jms_client@1128369 13f79535-47bb-0310-9956-ffa450edef68

1 files changed, 128 insertions, 0 deletions

diff --git a/qpid/java/common/src/main/java/org/apache/qpid/util/concurrent/BooleanLatch.java b/qpid/java/common/src/main/java/org/apache/qpid/util/concurrent/BooleanLatch.java
new file mode 100644
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+++ b/qpid/java/common/src/main/java/org/apache/qpid/util/concurrent/BooleanLatch.java
@@ -0,0 +1,128 @@
+package org.apache.qpid.util.concurrent;
+/*
+ * 
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ * 
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ * 
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ * 
+ */
+
+
+import java.util.concurrent.locks.AbstractQueuedSynchronizer;
+
+/**
+ * A BooleanLatch is like a set of traffic lights, where threads can wait at a red light until another thread gives
+ * the green light. When threads arrive at the latch it is initially red. They queue up until the green signal is
+ * given, at which point they can all acquire the latch in shared mode and continue to run concurrently. Once the latch
+ * is signalled it cannot be reset to red again.
+ *
+ * <p/> The latch uses a {@link java.util.concurrent.locks.AbstractQueuedSynchronizer} to implement its synchronization.
+ * This has two internal states, 0 which means that the latch is blocked, and 1 which means that the latch is open.
+ *
+ * <p/><table id="crc"><caption>CRC Card</caption>
+ * <tr><th> Responsibilities <th> Collaborations
+ * <tr><td> Block threads until a go signal is given.
+ * </table>
+ *
+ * @todo Might be better to use a countdown latch to count down from 1. Its await method can throw interrupted
+ *       exception which makes the possibility of interruption more explicit, and provides a reminder to recheck the
+ *       latch condition before continuing.
+ */
+public class BooleanLatch
+{
+    /** Holds the synchronizer that provides the thread queueing synchronization. */
+    private final Sync sync = new Sync();
+
+    /**
+     * Tests whether or not the latch has been signalled, that is to say that, the light is green.
+     *
+     * <p/>This method is non-blocking.
+     *
+     * @return <tt>true</tt> if the latch may be acquired; the light is green.
+     */
+    public boolean isSignalled()
+    {
+        return sync.isSignalled();
+    }
+
+    /**
+     * Waits on the latch until the signal is given and the light is green. If the light is already green then the
+     * latch will be acquired and the thread will not have to wait.
+     *
+     * <p/>This method will block until the go signal is given or the thread is otherwise interrupted. Before carrying
+     * out any processing threads that return from this method should confirm that the go signal has really been given
+     * on this latch by calling the {@link #isSignalled()} method.
+     */
+    public void await()
+    {
+        sync.acquireShared(1);
+    }
+
+    /**
+     * Releases any threads currently waiting on the latch. This flips the light to green allowing any threads that
+     * were waiting for this condition to now run.
+     *
+     * <p/>This method is non-blocking.
+     */
+    public void signal()
+    {
+        sync.releaseShared(1);
+    }
+
+    /**
+     * Implements a thread queued synchronizer. The internal state 0 means that the queue is blocked and the internl
+     * state 1 means that the queue is released and that all waiting threads can acquire the synchronizer in shared
+     * mode.
+     */
+    private static class Sync extends AbstractQueuedSynchronizer
+    {
+        /**
+         * Attempts to acquire this synchronizer in shared mode. It may be acquired once it has been released.
+         *
+         * @param ignore This parameter is ignored.
+         *
+         * @return 1 if the shared acquisition succeeds and -1 if it fails.
+         */
+        protected int tryAcquireShared(int ignore)
+        {
+            return isSignalled() ? 1 : -1;
+        }
+
+        /**
+         * Releases the synchronizer, setting its internal state to 1.
+         *
+         * @param ignore This parameter is ignored.
+         *
+         * @return <tt>true</tt> always.
+         */
+        protected boolean tryReleaseShared(int ignore)
+        {
+            setState(1);
+
+            return true;
+        }
+
+        /**
+         * Tests if the synchronizer is signalled. It is signalled when its internal state it 1.
+         *
+         * @return <tt>true</tt> if the internal state is 1, <tt>false</tt> otherwise.
+         */
+        boolean isSignalled()
+        {
+            return getState() != 0;
+        }
+    }
+}