| 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; |
| } |
| } |
| } |