若有不正之处请多多谅解，并欢迎批评指正。
请尊重作者劳动成果，转载请标明原文链接：
http://www.cnblogs.com/go2sea/p/5623218.html
我们已经了解了AQS的大致工作流程，接下来看下AQS的一个应用——CountDownLatch。
我们已经知道，AQS提供了两种模式：独占模式&共享模式。CountDownLatch就是一个使用共享模式的自定义同步器实现的共享锁。
源代码：
/* * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. * * * * * * * * * * * * * * * * * * * * */ /* * * * * * * 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/publicdomain/zero/1.0/ */ 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: Until the count reaches * zero, 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(); } protected int tryAcquireShared(int acquires) { return (getState() == 0) ? 1 : -1; } protected 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() + "]"; } }
View Code
可以看到，CountDownLatch只有一个成员变量，是它自定义的同步器：
private final Sync sync;
一、await方法
public void await() throws InterruptedException { sync.acquireSharedInterruptibly(1); } public boolean await(long timeout, TimeUnit unit) throws InterruptedException { return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout)); }
CountDownLatch提供了两种await方法：有等待时长限制和一直等待。两种均能响应中断（归根到底是UNSAFE.park可响应中断。但是如果是定时的park，则不能判断被唤醒的原因是超时还是被中断，因此需要isInterrupted判断下，而此方法会清除中断标志，因此如果是延迟处理要“补上”）。
await（）方法调用了同步器的acquireSharedInterruptibly方法，这个方法由上层AQS提供，它调用了我们重写的tryAcquireShared方法而封装了排队等待、唤醒、响应中断的细节，我们只关注自定义同步器中的tryAcquireShared方法即可：
protected int tryAcquireShared(int acquires) { return (getState() == 0) ? 1 : -1; }
注意，tryAcquireShared方法的返回值的意义在AQS是这样规定的：负值代表获取资源失败，非负值代表成功获取资源后剩余资源的数量。而这里当getState返回值为0的时候，我们却总是返回1，表示仍有剩余资源。这看上去并不合理，但这确实是正确的：因为可能有多个线程调用了await，同时在队列中等待资源，CountDownLatch的语义要求我们在倒计时结束有唤醒所有等待线程。因此我们在成功获取资源后，总是要告诉AQS“还有剩余”，这样AQS便会继续唤醒队列中的其他等待线程（由AQS中的setHeadAndPropagate方法调用doReleaseShared来唤醒）。一句话：成功获取总返回1是为了保证唤醒的“延续性”。
有等待时长限制的await(long, TimeUnit)方法调用了同步器的tryAcquireSharedNanos方法：
public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout) throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); return tryAcquireShared(arg) >= 0 || doAcquireSharedNanos(arg, nanosTimeout); }
这个方法首先检测中断，然后试图获取，失败后进入“自旋-等待”阶段，直到成功获取或被中断。这是AQS的内容，不再赘述。
二、countDown方法
public void countDown() { sync.releaseShared(1); }
countDown方法调用releaseShared释放资源：
public final boolean releaseShared(int arg) { if (tryReleaseShared(arg)) { doReleaseShared(); return true; } return false; }
releaseShared会调用tryReleaseShared方法：
protected 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; } }
方法一直自旋，直到成功释放或倒计时完毕。因为可能有超过count的线程调用countDown，因此releaseShared是可能失败的。当然在释放过程中也可能发生竞争，CAS自旋保证竞争发生时的正确执行。
三、总结
CountDownLatch是一个共享锁，但有些特别：他在初始化的时候锁住了所有共享资源，任何线程都可以调用countDown方法释放一个资源，当所有资源都被释放后，所有等待线程被唤醒。从而实现了倒计时的效果。
CountDownLatch是一次性的，计数值不可恢复。
