从名字可以看出,CountDownLatch是一个倒数计数的锁,当倒数到0时触发事件,也就是开锁,其他人就可以进入了。在一些应用场合中,需要等待某个条件达到要求后才能做后面的事情;同时当线程都完成后也会触发事件,以便进行后面的操作。
CountDownLatch最重要的方法是countDown()和await(),前者主要是倒数一次,后者是等待倒数到0,如果没有到达0,就只有阻塞等待了。
一个CountDouwnLatch实例是不能重复使用的,也就是说它是一次性的,锁一经被打开就不能再关闭使用了,如果想重复使用,请考虑使用CyclicBarrier。
下面的例子简单的说明了CountDownLatch的使用方法,模拟了100米赛跑,10名选手已经准备就绪,只等裁判一声令下。当所有人都到达终点时,比赛结束。
同样,线程池需要显式shutdown。
package concurrent;
import java.util.concurrent.CountDownLatch; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors;
public class TestCountDownLatch { public static void main(String[] args) throws InterruptedException { // 开始的倒数锁 final CountDownLatch begin = new CountDownLatch(1); // 结束的倒数锁 final CountDownLatch end = new CountDownLatch(10); // 十名选手 final ExecutorService exec = Executors.newFixedThreadPool(10); for(int index = 0; index < 10; index++) { final int NO = index + 1; Runnable run = new Runnable(){ public void run() { try { begin.await(); Thread.sleep((long) (Math.random() * 10000)); System.out.println("No." + NO + " arrived"); } catch (InterruptedException e) { } finally { end.countDown(); } } }; exec.submit(run); } System.out.println("Game Start"); begin.countDown(); end.await(); System.out.println("Game Over"); exec.shutdown(); } } |
运行结果:
Game Start
No.4 arrived
No.1 arrived
No.7 arrived
No.9 arrived
No.3 arrived
No.2 arrived
No.8 arrived
No.10 arrived
No.6 arrived
No.5 arrived
Game Over