CountDownLatch

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本文介绍了 CountDownLatch 的基本概念与使用方法，并通过一个示例程序详细展示了如何利用 CountDownLatch 控制主线程等待子线程完成任务。

CountDownLatch 用于阻塞一条或多条线程，直至其它线程把工作完成。

构那建器：

CountDownLatch(int count);count为计数器

方法：await()

调用await方法的线程在计数器=0前一直阻塞，除非线程被中断。

countDown()

将计数器count-1，如果计数到达零，则唤醒因为调用await方法而阻塞的所有的线程。

调用完countDown()方法后线程会继续执行。

一般用于一个线程或多个线程，等待其它线程完成count个工作，才继续执行。

示例：

public class TestCountDownLatch  implements Runnable{
	CountDownLatch countDownLatch = null;
	String threadName = null;
	public TestCountDownLatch(String threadName,CountDownLatch countDownLatch){
		this.countDownLatch = countDownLatch;
		this.threadName = threadName;
	}
	static SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
	@Override
	public void run() {
		// TODO Auto-generated method stub
		try{
			System.out.println(sdf.format(new Date())+": "+this.threadName +" start work");
			Thread.sleep(5000);
			System.out.println(sdf.format(new Date())+": "+this.threadName +" finish work");
			countDownLatch.countDown();//每次调用countDown 计算器-1  如果不调用该方法，countDownLatch.await()会一直阻塞
			System.out.println(sdf.format(new Date())+": "+this.threadName +" to exit");//线程退出
		}catch(Exception e){
			e.printStackTrace();
		}finally{
			countDownLatch.countDown();//计数减一
		}
	}
	public static void main(String args []){
		
		CountDownLatch countDownLatch = new CountDownLatch(5);//5个计数，每次调用countDown -1
		  ExecutorService es=Executors.newCachedThreadPool();  
          
	        for (int i = 0; i < 5; i++) {  
	            es.execute(new TestCountDownLatch("Thread_"+i,countDownLatch));  
	        }  
	        es.shutdown(); 
		
		try{
			System.out.println("main thread blocking");
			countDownLatch.await();//等待5次countDown方法调用，即计数器为0时， await方法阻塞才结束，执行后续线程,  CountDownLatch类计数器清0后不能再使用，重复使用可以使用CyclicBarrier
			System.out.println("main thread wake up");
			Thread.sleep(1000);
		}catch(Exception e){
			e.printStackTrace();
		}
		System.out.println(sdf.format(new Date())+": all thread run finish");
	}
}

执行结果：

2013-11-14 09:31:31: Thread_0 start work
2013-11-14 09:31:31: Thread_1 start work
main thread blocking
2013-11-14 09:31:31: Thread_3 start work
2013-11-14 09:31:31: Thread_2 start work
2013-11-14 09:31:31: Thread_4 start work
2013-11-14 09:31:36: Thread_1 finish work
2013-11-14 09:31:36: Thread_1 to exit
2013-11-14 09:31:36: Thread_0 finish work
2013-11-14 09:31:36: Thread_0 to exit
2013-11-14 09:31:36: Thread_2 finish work
2013-11-14 09:31:36: Thread_3 finish work
main thread wake up
2013-11-14 09:31:36: Thread_2 to exit
2013-11-14 09:31:36: Thread_4 finish work
2013-11-14 09:31:36: Thread_3 to exit
2013-11-14 09:31:36: Thread_4 to exit
2013-11-14 09:31:37: all thread run finish