CountDownLatch类在java.util.concurrent(俗称juc)包下,一般用于一个任务需要等待多个线程执行的结果的情况下,是一个同步辅助类。
主要用到它的方法是
①构造器:设置计数器的数量
public CountDownLatch(int count) { if (count < 0) throw new IllegalArgumentException("count < 0"); this.sync = new Sync(count); }
②await():阻塞当前线程,等待计数器的数量变为0
public void await() throws InterruptedException { sync.acquireSharedInterruptibly(1); }
③countDown():计数器减一
public void countDown() { sync.releaseShared(1); }
demo
package juc; import java.text.SimpleDateFormat; import java.util.Date; import java.util.concurrent.CountDownLatch; public class CountDownLatchDemo { final static SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); public static void main(String[] args) throws InterruptedException { // 两个人比赛跑步 CountDownLatch latch = new CountDownLatch(2); Runner runner1 = new Runner("博尔特", latch, 7000); Runner runner2 = new Runner("苏炳添", latch, 8000); runner1.start(); runner2.start(); // 等待两个运动员都跑完 latch.await(); System.out.println("运动员都跑完了,时间"+ sdf.format(new Date())); } static class Runner extends Thread{ String runnerName; CountDownLatch latch; int runTime; public Runner(String runnerName,CountDownLatch latch,int runTime) { this.runnerName = runnerName; this.latch = latch; this.runTime = runTime; } public void run() { System.out.println(runnerName+" run begin at " +sdf.format(new Date())); // 开始跑步 doRun(); System.out.println(runnerName+" run end at " + sdf.format(new Date())); // 运动员跑步完成,计数器减一 latch.countDown(); } private void doRun() { try { Thread.sleep(runTime); } catch (InterruptedException e) { e.printStackTrace(); } } } }
输出结果:
博尔特 run begin at 2018-04-16 12:56:15 苏炳添 run begin at 2018-04-16 12:56:15 博尔特 run end at 2018-04-16 12:56:22 苏炳添 run end at 2018-04-16 12:56:23 运动员都跑完了,时间2018-04-16 12:56:23