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。

 1 import java.util.concurrent.CountDownLatch;
 2  import java.util.concurrent.Executor;
 3  import java.util.concurrent.ExecutorService;
 4  import java.util.concurrent.Executors;
 5 
 6  public class CountDownLatchDemo {
 7     private static final int PLAYER_AMOUNT = 5;
 8     public CountDownLatchDemo() {
 9         // TODO Auto-generated constructor stub    
10      }
11     /**
12      * @param args
13      */
14     public static void main(String[] args) {
15         // TODO Auto-generated method stub
16         //对于每位运动员，CountDownLatch减1后即结束比赛
17         CountDownLatch begin = new CountDownLatch(1);
18         //对于整个比赛，所有运动员结束后才算结束
19         CountDownLatch end = new CountDownLatch(PLAYER_AMOUNT);
20         Player[] plays = new Player[PLAYER_AMOUNT];
21         
22         for(int i=0;i<PLAYER_AMOUNT;i++)
23             plays[i] = new Player(i+1,begin,end);
24         
25         //设置特定的线程池，大小为5
26         ExecutorService exe = Executors.newFixedThreadPool(PLAYER_AMOUNT);
27         for(Player p:plays)
28             exe.execute(p);            //分配线程
29         System.out.println("Race begins!");
30         begin.countDown();
31         try{
32             end.wait();            //等待end状态变为0，即为比赛结束
33         }catch (InterruptedException e) {
34             // TODO: handle exception
35             e.printStackTrace();
36         }finally{
37             System.out.println("Race ends!");
38         }
39         exe.shutdown();
40     }
41 }

接下来是Player类

 1 import java.util.concurrent.CountDownLatch;
 2 
 3 
 4 public class Player implements Runnable {
 5 
 6     private int id;
 7     private CountDownLatch begin;
 8     private CountDownLatch end;
 9     public Player(int i, CountDownLatch begin, CountDownLatch end) {
10         // TODO Auto-generated constructor stub
11         super();
12         this.id = i;
13         this.begin = begin;
14         this.end = end;
15     }
16 
17     @Override
18     public void run() {
19         // TODO Auto-generated method stub
20         try{
21             begin.await();        //等待begin的状态为0
22             Thread.sleep((long)(Math.random()*100));    //随机分配时间，即运动员完成时间
23             System.out.println("Play"+id+" arrived.");
24         }catch (InterruptedException e) {
25             // TODO: handle exception
26             e.printStackTrace();
27         }finally{
28             end.countDown();    //使end状态减1，最终减至0
29         }
30     }
31 }

转自http://en.wikipedia.org/wiki/Double-checked_locking