Java CountDownLatch

    concurrent包里面的CountDownLatch其实可以把它看作一个计数器，只不过这个计数器的操作是原子操作，同时只能有一个线程去操作这个计数器，也就是同时只能有一个线程去减这个计数器里面的值。 
CountDownLatch的一个非常典型的应用场景是：有一个任务想要往下执行，但必须要等到其他的任务执行完毕后才可以继续往下执行。假如我们这个想要继续往下执行的任务调用一个CountDownLatch对象的await()方法，其他的任务执行完自己的任务后调用同一个CountDownLatch对象上的countDown()方法，这个调用await()方法的任务将一直阻塞等待，直到这个CountDownLatch对象的计数值减到0为止。 

      举个例子，有三个工人在为老板干活，这个老板有一个习惯，就是当三个工人把一天的活都干完了的时候，他就来检查所有工人所干的活。记住这个条件：三个工人先全部干完活，老板才检查。所以在这里用Java代码设计两个类，Worker代表工人，Boss代表老板，具体的代码实现如下：

Worker类：

import java.util.Random;import java.util.concurrent.CountDownLatch;import java.util.concurrent.TimeUnit;public class Worker implements Runnable {private CountDownLatch downLatch;private String name;public Worker(CountDownLatch downLatch, String name) {this.downLatch = downLatch;this.name = name;} @Overridepublic void run() {// TODO Auto-generated method stubthis.doWork();try {TimeUnit.SECONDS.sleep(new Random().nextInt(10));}catch(InterruptedException ie) {   }}  public void doWork() { System.out.println(this.name + "活干完了！"); this.downLatch.countDown(); }}

Boss类：

import java.util.Random;import java.util.concurrent.CountDownLatch;import java.util.concurrent.TimeUnit;public class Worker implements Runnable {private CountDownLatch downLatch;private String name;public Worker(CountDownLatch downLatch, String name) {this.downLatch = downLatch;this.name = name;} @Overridepublic void run() {// TODO Auto-generated method stubthis.doWork();try {TimeUnit.SECONDS.sleep(new Random().nextInt(10));}catch(InterruptedException ie) {   }}  public void doWork() { System.out.println(this.name + "活干完了！"); this.downLatch.countDown(); }}

CountDownLatchDemo类：

import java.util.concurrent.CountDownLatch;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;public class CountDownLatchDemo { public static void main(String[] args) {    ExecutorService executor = Executors.newCachedThreadPool();                    CountDownLatch latch = new CountDownLatch(3);                   Worker w1 = new Worker(latch,"张三");           Worker w2 = new Worker(latch,"李四");           Worker w3 = new Worker(latch,"王二");                      Boss boss = new Boss(latch);                   executor.execute(boss);executor.execute(w3);   executor.execute(w2);   executor.execute(w1);                         executor.shutdown();       }   }