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

• 
  public class CountDownLatchDemo {  
•     final static SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");  
•     public static void main(String[] args) throws InterruptedException {  
•         CountDownLatch latch=new CountDownLatch(2);//两个工人的协作  
•         Worker worker1=new Worker("zhang san", 5000, latch);  
•         Worker worker2=new Worker("li si", 8000, latch);  
•         worker1.start();//  
•         worker2.start();//  
•         latch.await();//等待所有工人完成工作  
•         System.out.println("all work done at "+sdf.format(new Date()));  
•     }  
•       
•       
•     static class Worker extends Thread{  
•         String workerName;   
•         int workTime;  
•         CountDownLatch latch;  
•         public Worker(String workerName ,int workTime ,CountDownLatch latch){  
•              this.workerName=workerName;  
•              this.workTime=workTime;  
•              this.latch=latch;  
•         }  
•         public void run(){  
•             System.out.println("Worker "+workerName+" do work begin at "+sdf.format(new Date()));  
•             doWork();//工作了  
•             System.out.println("Worker "+workerName+" do work complete at "+sdf.format(new Date()));  
•             latch.countDown();//工人完成工作，计数器减一  
•   
•         }  
•           
•         private void doWork(){  
•             try {  
•                 Thread.sleep(workTime);  
•             } catch (InterruptedException e) {  
•                 e.printStackTrace();  
•             }  
•         }  
•     }  
•       
•        
• } 

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