CountDownLatch是JDK 5+里面闭锁的一个实现，允许一个或者多个线程等待某个事件的发生。今天我们通过一些实例来学习一下它的用法。
CountDownLatch的简单使用
CountDownLatch的简单介绍：
CountDownLatch类是一个同步计数器,构造时传入int参数,该参数就是计数器的初始值，每调用一次countDown()方法，计数器减1,计数器大于0 时，await()方法会阻塞程序继续执行
CountDownLatch是一个倒计数的锁存器，当计数减至0时触发特定的事件。
CountDownLatch被用来同步一个或多个任务，强制它们等待由其他任务执行的一组操作完成。你可以向CountDownLatch对象设置一个初始计数值，任何在这个对象上调用wait()方法都将阻塞，直至这个计数值达到0。其他任务在结束工作时，可以在该对象上调用凑你countDown()来减小这个计数值。CountDownLatch被设计为只触发一次，计数值不能被重置。如果你需要能够重置计数值的版本，则可以使用CyclicBarrier。
以下是jdk文档上的两个例子，我们可以学习一下。
一、开始信号，阻止任何工人进行，直到司机准备好他们继续进行
package com.linux.huhx.concurreny; import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit; public class Driver1 {
public static void main(String[] args) throws Exception {
CountDownLatch startSignal = new CountDownLatch(1);
CountDownLatch doneSignal = new CountDownLatch(5);
for (int i = 0; i < 5; ++i) // create and start threads
new Thread(new TaskWorker(startSignal, doneSignal)).start();
System.out.println("countDown before."); // don't let run yet
startSignal.countDown(); // let all threads proceed
TimeUnit.SECONDS.sleep(1);
System.out.println("await before");
/**
* 1、由于doneSignal里面的初始count=5，所以主线程在此会阻塞。
* 2、当所有的子线程执行完（doneSignal.countDown()）之后，此时的count=0。所以后面代码可以继续执行。
*/
doneSignal.await();
System.out.println("await after");
} private static class TaskWorker implements Runnable {
private final CountDownLatch startSignal;
private final CountDownLatch doneSignal; public TaskWorker(CountDownLatch startSignal, CountDownLatch doneSignal) {
this.startSignal = startSignal;
this.doneSignal = doneSignal;
} @Override
public void run() {
try {
/**
* 1、刚进来的时候，由于startSignal的count=1。所以await()方法阻塞于此。
* 2、当主线程执行了startSignal.countDown();此时的startSignal的count=0。所以子线程可以正常运行。
*/
startSignal.await();
doSomeWork();
doneSignal.countDown();
} catch (InterruptedException e) {
e.printStackTrace();
}
} private void doSomeWork() {
System.out.println("in run method: " + Thread.currentThread().getName());
}
}
}
执行的结果如下：子线程的执行顺序不固定
countDown before.
in run method: Thread-
in run method: Thread-
in run method: Thread-
in run method: Thread-
in run method: Thread-
await before
await after
调用countDown()的任务在产生这个调用时并没有阻塞，只有对await()的调用会被阻塞，直至计数值到达0。
二、完成信号，允许司机等到所有工人完成为止
package com.linux.huhx.concurreny; import java.util.Random;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors; public class Driver2 {
public static void main(String[] args) throws Exception {
CountDownLatch doneSignal = new CountDownLatch(5);
ExecutorService service = Executors.newFixedThreadPool(3);
for (int i = 0; i < 5; ++i) // create and start threads
service.execute(new WorkerRunnable(doneSignal, i));
System.out.println("await before");
doneSignal.await();
System.out.println("await after");
service.shutdown();
} private static class WorkerRunnable implements Runnable {
private final CountDownLatch doneSignal;
private final int i; public WorkerRunnable(CountDownLatch doneSignal, int i) {
this.doneSignal = doneSignal;
this.i = i;
} @Override
public void run() {
doWork(i);
try {
Thread.sleep(new Random().nextInt(1000));
doneSignal.countDown();
} catch (InterruptedException e) {
e.printStackTrace();
}
} private void doWork(int i) {
System.out.println("in run method: " + i + " " + Thread.currentThread().getName());
}
}
}
运行的结果如下：子线程的执行顺序不固定
await before
in run method: pool--thread-
in run method: pool--thread-
in run method: pool--thread-
in run method: pool--thread-
in run method: pool--thread-
await after