多线程之CountDownLatch

下面请看一个应用场景：
有1个driver和5个worker，需要满足以下两点要求：

 当driver完成了全部的工作之后才允许worker们开始工作；

 当所有的worker都完成了自己的工作之后，driver主线程才能结束。

public class Driver {
    public static void main(String[] args) throws InterruptedException {
        CountDownLatch startSignal = new CountDownLatch(1);
        CountDownLatch doneSignal = new CountDownLatch(5);

        // 依次创建并启动5个worker线程
        for (int i = 0; i < 5; ++i) {
            new Thread(new Worker(startSignal, doneSignal)).start();
        }
        
        System.out.println("Driver is doing something...");
        System.out.println("Driver is Finished, start all workers ...");
        startSignal.countDown(); // Driver执行完毕，发出开始信号，使所有的worker线程开始执行
        doneSignal.await(); // 等待所有的worker线程执行结束
        System.out.println("Finished.");
    }
}

class Worker implements Runnable{
    private final CountDownLatch startSignal;
    private final CountDownLatch doneSignal;
    Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
        this.startSignal = startSignal;
        this.doneSignal = doneSignal;
    }
    public void run() {
        try {
            startSignal.await(); // 等待Driver线程执行完毕，获得开始信号
            System.out.println("Working now ...");
            doneSignal.countDown(); // 当前worker执行完毕，释放一个完成信号
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

运行结果：

Driver is doing something...
Driver is Finished, start all workers ...
Working now ...
Working now ...
Working now ...
Working now ...
Working now ...
Finished.

此处修改CountDownLatch个数为一个代码如下：public class Driver {

	public static void main(String[] args) throws Exception {
		
			CountDownLatch latch = new CountDownLatch(5);
			
			 for (int i = 0; i < 5; ++i) {
		            new Thread(new Worker( latch),"t"+i).start();
		        }		
		 latch.await();
		 System.out.println("结束");
	}
	
	
	static class Worker implements  Runnable {
		private final CountDownLatch latch1;
		
		public Worker(CountDownLatch latch1) {
			this.latch1=latch1;
		}
		public void run() {
			
			System.out.println(Thread.currentThread().getName()+"开始干活 work"+"id是"+Thread.currentThread().getId()+" ");
			latch1.countDown();
			
		}
	}
}


结果为：

t4开始干活 workid是14
t1开始干活 workid是11
t2开始干活 workid是12
t3开始干活 workid是13
t0开始干活 workid是10
结束