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

await()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

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