CountDownLatch，一个同步辅助类，在完成一组正在其他线程中执行的操作之前，它允许一个或多个线程一直等待。

主要方法

 public CountDownLatch(int count);

 public void countDown();

 public void await() throws InterruptedException
 

构造方法参数指定了计数的次数

countDown方法，当前线程调用此方法，则计数减一

awaint方法，调用此方法会一直阻塞当前线程，直到计时器的值为0

例子

Java代码  

1. public class CountDownLatchDemo {  
2.     final static SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");  
3.     public static void main(String[] args) throws InterruptedException {  
4.         CountDownLatch latch=new CountDownLatch(2);//两个工人的协作  
5.         Worker worker1=new Worker("zhang san", 5000, latch);  
6.         Worker worker2=new Worker("li si", 8000, latch);  
7.         worker1.start();//  
8.         worker2.start();//  
9.         latch.await();//等待所有工人完成工作  
10.         System.out.println("all work done at "+sdf.format(new Date()));  
11.     }  
12.       
13.       
14.     static class Worker extends Thread{  
15.         String workerName;   
16.         int workTime;  
17.         CountDownLatch latch;  
18.         public Worker(String workerName ,int workTime ,CountDownLatch latch){  
19.              this.workerName=workerName;  
20.              this.workTime=workTime;  
21.              this.latch=latch;  
22.         }  
23.         public void run(){  
24.             System.out.println("Worker "+workerName+" do work begin at "+sdf.format(new Date()));  
25.             doWork();//工作了  
26.             System.out.println("Worker "+workerName+" do work complete at "+sdf.format(new Date()));  
27.             latch.countDown();//工人完成工作，计数器减一  
28.   
29.         }  
30.           
31.         private void doWork(){  
32.             try {  
33.                 Thread.sleep(workTime);  
34.             } catch (InterruptedException e) {  
35.                 e.printStackTrace();  
36.             }  
37.         }  
38.     }  
39.       
40.        
41. }  

输出：

Worker zhang san do work begin at 2011-04-14 11:05:11
Worker li si do work begin at 2011-04-14 11:05:11
Worker zhang san do work complete at 2011-04-14 11:05:16
Worker li si do work complete at 2011-04-14 11:05:19
all work done at 2011-04-14 11:05:19

CountDownLatch