CountDownLatch介绍

CountDownLatch是java.util.concurrent并发包中提供的一个可用于控制多线程同时开始某动作的类。其采用的方式为减计数的方式。当计数减至零时，位于await后的代码才会执行。

CountDownLatch(int)

    创建内部类Sync的对象实例，并将state设置为传入的参数。

  await()

     调用Sync继承的AbstractQueuedSynchronizer 的acquireShareInterruptibly完成。

acquireShareInterruptibly首先调用Sync的tryAcquireShared方法，该方法判断当前的state是否为零。如果为零，则返回1，否则返回-1。如果返回1，则await直接返回，如果返回-1，则将此线程放入队列进行等待，知道tryAcquireSharedfan返回-1或线程被interrupt

countdown()

调用Sync的tryReleaseShared方法，如果state不为零，基于CAS将state的值设置为减1后的值；如果减1后的值为零，则返回true，否则返回false

如果为true，则通知所有在队列等待的线程。

下面是一个例子

package DownTest;

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

public class CountDownLatchDemo {
private static final int PLAYER_AMOUNT = 5;

public CountDownLatchDemo() {
// TODO Auto-generated constructor stub
}

/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
// 对于每位运动员，CountDownLatch减1后即结束比赛
CountDownLatch begin = new CountDownLatch(1);
// 对于整个比赛，所有运动员结束后才算结束
CountDownLatch end = new CountDownLatch(PLAYER_AMOUNT);
Player[] plays = new Player[PLAYER_AMOUNT];

for (int i = 0; i < PLAYER_AMOUNT; i++)
plays[i] = new Player(i + 1, begin, end);

// 设置特定的线程池，大小为5
ExecutorService exe = Executors.newFixedThreadPool(PLAYER_AMOUNT);
for (Player p : plays)
exe.execute(p); // 分配线程
System.out.println("Race begins!");
begin.countDown();

try {
end.await();

// 等待end状态变为0，即为比赛结束
} catch (InterruptedException e) {
// TODO: handle exception
e.printStackTrace();
} finally {
System.out.println("Race ends!");
}
exe.shutdown();

}
}
对于Player类如下
package DownTest;

import java.util.concurrent.CountDownLatch;


public class Player implements Runnable {

     private int id;
     private CountDownLatch begin;
     private CountDownLatch end;
     public Player(int i, CountDownLatch begin, CountDownLatch end) {
         // TODO Auto-generated constructor stub
         super();
         this.id = i;
         this.begin = begin;
         this.end = end;
     }

     @Override
     public void run() {
         // TODO Auto-generated method stub
         try{
             begin.await();        //等待begin的状态为0
             Thread.sleep((long)(Math.random()*100));    //随机分配时间，即运动员完成时间
             System.out.println("Play"+id+" arrived.");
         }catch (InterruptedException e) {
             // TODO: handle exception
             e.printStackTrace();
         }finally{
             end.countDown();    //使end状态减1，最终减至0
         }
     }
}

 执行结果：

Race begins!
Play2 arrived.
Play3 arrived.
Play5 arrived.
Play4 arrived.
Play1 arrived.
Race ends!