CountDownLatch，一個同步輔助類，在完成一組正在其他線程中執行的操作之前，它允許一個或多個線程一直等待。

主要方法

public CountDownLatch(int count);

public void countDown();

public void await() throws InterruptedException

構造方法參數指定了計數的次數

countDown方法，當前線程調用此方法，則計數減一

awaint方法，調用此方法會一直阻塞當前線程，直到計時器的值為0

例子

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);//兩個工人的協作
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. }
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，計數器減一
29. }
31. private void doWork(){
32. try {
33. Thread.sleep(workTime);
34. } catch (InterruptedException e) {
35. e.printStackTrace();
36. }
37. }
38. }
41. }

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);//兩個工人的協作
    	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，計數器減一

    	}
    	
    	private void doWork(){
    		try {
				Thread.sleep(workTime);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
    	}
    }
    
     
}

輸出：

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

Java之CountDownLatch使用