java并发：线程协作机制之CountDownLatch
一、初识CountDownLatch
（1）概述
（2）应用场景
Application’s main thread wants to wait, till other service threads which are responsible for starting framework services have completed started all services.
二、详述CountDownLatch
CountDownLatch是通过一个计数器来实现的。
每当一个线程完成了自己的任务后，计数器的值就会减1，当计数器值到达0时，它表示所有的线程已经完成了任务，然后处于等待的线程就可以恢复执行任务。
下图展示了CountDownLatch中的方法：
- public CountDownLatch(int count)
构造函数，count（计数器）实际上就是需要等待的线程数量，这个值只能被设置一次（即：CountDownLatch没有提供任何机制去重新设置这个值）。
- public void countDown()
每调用一次这个方法，在构造函数中初始化的count值就减1。
- public void await() throws InterruptedException
调用此方法的线程会一直阻塞，直到计时器的值为0。
下图展示了CountDownLatch中内部类Sync的类图：
解释：
根据类图即可知道Sync继承了AbstractQueuedSynchronizer，进而可知CountDownLatch是使用 AQS 实现的。
构造函数
CountDownLatch的构造函数的定义如下：
/** * 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); }
解读：
构造函数创建了内部类Sync的实例。
Sync的定义如下：
/** * 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; } } }
解读：
通过上述代码可以发现，计数器count实际上是赋值给了 AQS 的状态变量 state。
await方法
其定义如下：
/** * 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); }
解读：
上述方法实际上是调用了父类AbstractQueuedSynchronizer中的acquireSharedInterruptibly方法，其定义如下：
/** * Acquires in shared mode, aborting if interrupted. Implemented * by first checking interrupt status, then invoking at least once * {@link #tryAcquireShared}, returning on success. Otherwise the * thread is queued, possibly repeatedly blocking and unblocking, * invoking {@link #tryAcquireShared} until success or the thread * is interrupted. * @param arg the acquire argument. * This value is conveyed to {@link #tryAcquireShared} but is * otherwise uninterpreted and can represent anything * you like. * @throws InterruptedException if the current thread is interrupted */ public final void acquireSharedInterruptibly(int arg) throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); if (tryAcquireShared(arg) < 0) doAcquireSharedInterruptibly(arg); }
解读：
根据AQS的实现原理，Sync需要实现tryAcquireShared方法。
请参见本文前面Sync的定义，Sync 中的 tryAcquireShared 方法查看当前状态值(计数器值)是否为 0，是则直接返回，否则调用 AQS 的 doAcquireSharedlnterruptibly 方法让当前线程阻塞。
countDown方法
/** * 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); }
解读：
上述方法实际上是调用了父类AbstractQueuedSynchronizer中的releaseShared方法，其定义如下：
/** * 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; }
解读：
根据AQS的实现原理，Sync需要实现tryReleaseShared方法。
请参见本文前面Sync的定义，Sync 中的 tryReleaseShared 方法首先获取当前状态值(计数器值) ，如果当前状态值为 0 则直接返回 false；否则使用 CAS 将计数器值减 1（在此过程中，CAS 如果失败则循环重试），如果计数器值被减为 0 则返回 true 以调用 AQS 的 doReleaseShared 方法来激活阻塞的线程。
三、示例
package com.test; import java.util.concurrent.CountDownLatch; public class CountDownLatchDemo{ public static void main(String args[]) throws Exception{ CountDownLatch latch = new CountDownLatch(3); Worker worker1 = new Worker("Jack 程序员1",latch); Worker worker2 = new Worker("Rose 程序员2",latch); Worker worker3 = new Worker("Json 程序员3",latch); worker1.start(); worker2.start(); worker3.start(); latch.await(); System.out.println("Main thread end!"); } static class Worker extends Thread { private String workerName; private CountDownLatch latch; public Worker(String workerName,CountDownLatch latch) { this.workerName = workerName; this.latch = latch; } @Override public void run() { try { System.out.println("Worker:"+workerName +" is begin."); Thread.sleep(1000L); System.out.println("Worker:"+workerName +" is end."); } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); }//模仿干活； latch.countDown(); } } }
上述程序运行结果如下：
Worker:Rose 程序员2 is begin.
Worker:Json 程序员3 is begin.
Worker:Jack 程序员1 is begin.
Worker:Jack 程序员1 is end.
Worker:Json 程序员3 is end.
Worker:Rose 程序员2 is end.
Main thread end!
从结果上可以看出，MainThread执行到latch.await();处会阻塞在该处，直到三个线程均完成的时候MainThread才会继续往下执行
Note：
Other N threads must have a reference of the latch object because they will need to notify the CountDownLatch object that they have completed their task.
四、分析总结
问题：CountDownLatch与join方法的区别
在线程中调用某个子线程的join() 方法后，该线程将一直被阻塞直到子线程运行完毕，而 CountDownLatch通过使用计数器来使得子线程在运行完或者运行中递减计数，所以 CountDownLatch可以在子线程运行的任何时候让await方法返回，并不需要等到线程结束。
另外，使用线程池来管理线程时一般都是直接添加 Runable 到线程池，这时没有办法调用线程的 join 方法，所以 countDownLatch 相比 join 方法更灵活。
（1）https://howtodoinjava.com/java/multi-threading/when-to-use-countdownlatch-java-concurrency-example-tutorial/
