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：

import java.util.Random;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

/**
 * taobao.com Inc. Copyright (c) 2009-2012 All Rights Reserved.
 */

/**
 * CountDownLatchDemo1 class
 * 
 * @author liugen.lilg 2012-7-24
 */
public class CountDownLatchDemo1 {
	public static void main(String[] args) {
		ExecutorService executor = Executors.newCachedThreadPool();
		CountDownLatch latch = new CountDownLatch(3);
		Worker w1 = new Worker(latch, "李四");
		Worker w2 = new Worker(latch, "李四");
		Worker w3 = new Worker(latch, "王二");

		Boss boss = new Boss(latch);

		executor.execute(w3);
		executor.execute(w2);
		executor.execute(w1);
		executor.execute(boss);

		executor.shutdown();
	}

	static class Boss implements Runnable {

		private CountDownLatch downLatch;

		public Boss(CountDownLatch downLatch) {
			this.downLatch = downLatch;
		}

		public void run() {
			System.out.println("老板正在等所有的工人干完活......");
			try {
				this.downLatch.await();
			} catch (InterruptedException e) {
			}
			System.out.println("工人活都干完了，老板开始检查了！");
		}

	}

	static class Worker implements Runnable {

		private CountDownLatch downLatch;
		private String name;

		public Worker(CountDownLatch downLatch, String name) {
			this.downLatch = downLatch;
			this.name = name;
		}

		public void run() {
			this.doWork();
			try {
				TimeUnit.SECONDS.sleep(new Random().nextInt(10));
			} catch (InterruptedException ie) {
			}
			System.out.println(this.name + "活干完了！");
			this.downLatch.countDown();

		}

		private void doWork() {
			System.out.println(this.name + "正在干活!");
		}

	}
}