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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;
}
}
}
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