CountDownLatch的用法

       CountDownLatch，是线程同步辅助类，在一组执行线程中有未完成的线程时，可以让已完成的线程一直等待其他线程完成。设计理念有点类似于团队爬山，前队等后队，人到齐后才算爬山任务完成。

       下面的例子模仿有两个人abtest和unittest，两个人跑步，只有两个人都跑完后，才是真正的跑步结束，才打印跑步结束语句，所以打印跑步结束语句的动物要在两个人（线程）跑完步动作之后。

/**
 * Description
 * 
 * @author usr1999 2015-1-13
 */
public class CountDownLatchDemo2 {

	/**
	 * 
	 */
	public static void main(String[] args) {
		CountDownLatch latch = new CountDownLatch(2);

		Runner run1 = new Runner("abtest", 3000, latch);
		Runner run2 = new Runner("unittest", 5000, latch);

		ExecutorService exec = Executors.newFixedThreadPool(2);
		exec.submit(run1);
		exec.submit(run2);

		try {
			latch.await();
		} catch (InterruptedException e) {
			e.printStackTrace();
		}

		System.out.println("abtest and unittest completed running");
		exec.shutdown();
	}
}

class Runner implements Runnable {
	SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

	String name;
	int time;
	CountDownLatch latch;

	public Runner(String name, int time, CountDownLatch latch) {
		this.name = name;
		this.time = time;
		this.latch = latch;
	}

	@Override
	public void run() {
		System.out.println(name + " is ready to run at "
				+ sdf.format(new Date()));
		doRun();
		System.out.println(name + " has runed at " + sdf.format(new Date()));
		latch.countDown(); // 计数递减
	}

	private void doRun() {
		try {
			TimeUnit.MILLISECONDS.sleep(time);
			// Thread.sleep(time);
		} catch (InterruptedException e) {
			System.out.println(e.getStackTrace());
		}
	}
}

      

 CountDownLatch主要有三个方法：

       CountDownLatch(int count)，构造函数，参数为计数的数目。
       countDown(), 计数递减，如果计数减到0，则释放所有等待状态的线程。
       await(), 线程阻塞方法，使当前线程在计数变为0之前一直等待，直到count值为0才继续执行。