浅析Java中CountDownLatch用法

CountDownLatch如其所写，是一个倒计数的锁存器，当计数减至0时触发特定的事件。利用这种特性，可以让主线程等待子线程的结束。下面以一个模拟运动员比赛的例子加以说明。

import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.Executor;
 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.wait();            //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();
    }
}

接下来是Player类

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
        }
    }
}