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

```java 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       }   }  

} ```