CountDownLatch使用例子

CountDownLatch是一个同步辅助类，犹如倒计时计数器，创建对象时通过构造方法设置初始值，调用CountDownLatch对象的await()方法则处于等待状态，调用countDown()方法就将计数器减1，当计数到达0时，则所有等待者或单个等待者开始执行。

简单例子

import java.util.concurrent.CountDownLatch;

/**
* 出发点：等待所有线程执行完成
* @author yinchuan.chen
*
*/
public class CountDownLatchTest {

public static void main(String[] args) throws InterruptedException {
CountDownLatch cdl = new CountDownLatch(4);
for(int i = 0; i < 4; i++) {
final int count = i;
Thread t = new Thread(new Runnable() {

public void run() {
System.out.println(count);
cdl.countDown();
}
});

t.start();
}

cdl.await();
System.out.println("等所有现场执行完成，才打印");

}
}

注：countDown最好是在finally里面调用

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() {
    }
   /**
    * @param args
    */
   public static void main(String[] args) {
       //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,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!");
       try{
           end.await();            //等待end状态变为0，即为比赛结束
       }catch (InterruptedException e) {
           e.printStackTrace();
       }finally{
           System.out.println("Race ends!");
       }
       exe.shutdown();
   }
}

class Player implements Runnable {

    private int id;
    private CountDownLatch end;
    public Player(int i, CountDownLatch end) {
        super();
        this.id = i;
        this.end = end;
    }

    @Override
    public void run() {
        try{
            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) {
            e.printStackTrace();
        }finally{
            end.countDown();    //使end状态减1，最终减至0
        }
    }
}

参考 http://www.cnblogs.com/yezhenhan/archive/2012/01/07/2315652.html