J.U.C并发框架源码阅读（四）CountDownLatch
基于版本jdk1.7.0_80
java.util.concurrent.CountDownLatch
代码如下
/* * 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() + "]"; } }
只有320行，其中大部分是注释，纯代码应该在100行以下
主要功能由AQS完成，这再一次说明了AQS的强大之处。
0. CountDownLatch简介
CountDownLatch，一个同步辅助类，在完成一组正在其他线程中执行的操作之前，它允许一个或多个线程一直等待。
主要方法
public CountDownLatch(int count);
public void countDown();
public void await() throws InterruptedException
构造方法参数指定了计数的次数
countDown方法，当前线程调用此方法，则计数减一
await方法，调用此方法会一直阻塞当前线程，直到计时器的值为0
1. CountDownLatch原理概述
CountDownLatch利用AQS的state变量维护了计数的次数，每次调用countDown方法会对state做自减操作，当state减少到0时，释放所有在CountDownLatch上等待的线程。
2. CountDownLatch构造方法
CountDownLatch public CountDownLatch(int count) { if (count < 0) throw new IllegalArgumentException("count < 0"); this.sync = new Sync(count); } CountDownLatch.Sync Sync(int count) { setState(count);//初始化时，将AQS的state变量设置为count }
3. CountDownLatch.await方法的执行轨迹
CountDownLatch.await public void await() throws InterruptedException { sync.acquireSharedInterruptibly(1); } AbstractQueuedSynchronizer.acquireSharedInterruptibly public final void acquireSharedInterruptibly(int arg) throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); if (tryAcquireShared(arg) < 0) doAcquireSharedInterruptibly(arg); } CountDownLatch.Sync.tryAcquireShared protected int tryAcquireShared(int acquires) { return (getState() == 0) ? 1 : -1;//如果state为0，说明此CountDownLatch已失效，返回1，acquireInterruptibly函数立即返回。否则后续会调用下面的AbstractQueuedSynchronizer.doAcquireSharedInterruptibly方法 } AbstractQueuedSynchronizer.doAcquireSharedInterruptibly /** * Acquires in shared interruptible mode. * @param arg the acquire argument */ private void doAcquireSharedInterruptibly(int arg) throws InterruptedException { final Node node = addWaiter(Node.SHARED);//创建共享类型的节点并插入AQS的等待队列中 boolean failed = true; try { for (;;) { final Node p = node.predecessor(); if (p == head) { int r = tryAcquireShared(arg);//state != 0 (CountDownLatch仍处于锁定状态) -> r == -1 -> 继续自旋；state == 0 (CountDownLatch已经解锁) -> r == 1 -> 函数返回 -> 工作线程不再阻塞 if (r >= 0) { setHeadAndPropagate(node, r);//头结点的状态位被标记为Propagate，后续节点的状态一般都是Shared，也可能是Canceled p.next = null; // help GC failed = false; return; } } if (shouldParkAfterFailedAcquire(p, node) &&//park当前线程 parkAndCheckInterrupt()) throw new InterruptedException(); } } finally { if (failed) cancelAcquire(node); } }
大概意思是凡是调用CountDownLatch.await方法的线程，只要CountDownLatch未失效（state不等于0），都会去AQS的等待队列里创建节点，然后将自己park住
CountDownLatch还有带超时参数重载的await方法，逻辑比较接近，在此就不赘述了。
4. CountDownLatch.countDown方法的执行轨迹
CountDownLatch.countDown /** * 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); } AbstractQueuedSynchronizer.releaseShared /** * Releases in shared mode. Implemented by unblocking one or more * threads if {@link #tryReleaseShared} returns true. * * @param arg the release argument. This value is conveyed to * {@link #tryReleaseShared} but is otherwise uninterpreted * and can represent anything you like. * @return the value returned from {@link #tryReleaseShared} */ public final boolean releaseShared(int arg) { if (tryReleaseShared(arg)) { doReleaseShared(); return true; } return false; } CountDownLatch.Sync.tryReleaseShared protected boolean tryReleaseShared(int releases) { // Decrement count; signal when transition to zero for (;;) { int c = getState();//获取AQS的state变量 if (c == 0)//如果state已经为0了，那么CountDownLatch已经没有资源可以释放了，上面的releaseShared方法也会直接返回 return false; int nextc = c-1;//利用cas操作原子性的将state自减1，如果state刚好被减到0，那么返回true，触发后续的doReleaseShared调用 if (compareAndSetState(c, nextc)) return nextc == 0; } } AbstractQueuedSynchronizer.doReleaseShared /** * Release action for shared mode -- signal successor and ensure * propagation. (Note: For exclusive mode, release just amounts * to calling unparkSuccessor of head if it needs signal.) */ private void doReleaseShared() { /* * Ensure that a release propagates, even if there are other * in-progress acquires/releases. This proceeds in the usual * way of trying to unparkSuccessor of head if it needs * signal. But if it does not, status is set to PROPAGATE to * ensure that upon release, propagation continues. * Additionally, we must loop in case a new node is added * while we are doing this. Also, unlike other uses of * unparkSuccessor, we need to know if CAS to reset status * fails, if so rechecking. */ for (;;) { Node h = head; if (h != null && h != tail) { int ws = h.waitStatus; if (ws == Node.SIGNAL) { if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0)) continue; // loop to recheck cases unparkSuccessor(h); } else if (ws == 0 && !compareAndSetWaitStatus(h, 0, Node.PROPAGATE)) continue; // loop on failed CAS } if (h == head) // loop if head changed break; } }
大意就是每次调用CountDownLatch.countDown方法都会使AQS维护的state变量自减1
state减到0的时候，会触发对AbstractQueuedSynchronizer.doReleaseShared方法的调用，它会唤醒AQS等待队列中的所有等待线程
这些线程被唤醒之后，发现state变量已经变成0了，于是结束自旋并返回
这样就实现了CountDownLatch的语义
