CountDownLatch

关于java的CountDownLatch。Latch：门闩之意。

CountDownLatch经常用来在多线程环境下，主线程协调多个子线程的步调。生活中最相似的场景就是运动员比赛，裁判员（主线程）控制比赛的开始和结束，运动员（子线程）完成自己的比赛，当且仅当所有运动员都完成比赛时，裁判员就可以下令整场比赛结束。下面转载网友针对上述场景的模拟代码，以便更好地理解CountDownLatch的使用。

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  publicclass CountDownLatchDemo {
 7 privatestaticfinalint PLAYER_AMOUNT =5;
 8 public CountDownLatchDemo() {
10      }
11 /**
12      * @param args
13 */
14 publicstaticvoid main(String[] args) {
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();//相当于裁判员下令比赛开始
31 try{
32             end.wait();            //等待end状态变为0，即为比赛结束
33         }catch (InterruptedException e) {
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 publicclass Player implements Runnable {
 5 
 6 privateint id;
 7 private CountDownLatch begin;
 8 private CountDownLatch end;
 9 public Player(int i, CountDownLatch begin, CountDownLatch end) {
11 super();
12 this.id = i;
13 this.begin = begin;
14 this.end = end;
15     }
16 
17     @Override
18 publicvoid run() {
20 try{
21             begin.await();        //等待begin的状态为0，相当于运动员已做好准备，等待裁判员宣布比赛开始。
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) {
26             e.printStackTrace();
27         }finally{
28             end.countDown();    //使end状态减1，最终减至0
29         }
30     }
31 }