/**

CountDownLatch类是一个同步计数器,构造时传入int参数,该参数就是计数器的初始值，每调用一次countDown()方法，计数器减1,计数器大于0 时，await()方法会阻塞程序继续执行

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.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();
     }
 }

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
         }
     }
 }