CountDownLatch是一个线程的同步计数器，主要有2个使用场景。1、作为一个开关，N个子线程准备就绪，等待主线程的放行，然后子线程一起开始执行。Task

package com.jane;import java.util.concurrent.CountDownLatch;public class Task implements Runnable { private CountDownLatch countDownLatch; private String name; public Task(CountDownLatch countDownLatch, String name) { this.countDownLatch = countDownLatch; this.name = name; } @Override public void run() { try { System.out.println(name + "开始等待开关......"); countDownLatch.await(); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println(name + "执行任务完成"); }}主线程

public class Main { public static void main(String[] args) throws InterruptedException { //主线程创建，和子线程共享 CountDownLatch countDownLatch = new CountDownLatch(1); System.out.println("主线程启动......"); //创建3个子线程 Thread t1 = new Thread(new Task(countDownLatch, "线程-1")); Thread t2 = new Thread(new Task(countDownLatch, "线程-2")); Thread t3 = new Thread(new Task(countDownLatch, "线程-3")); t1.start(); t2.start(); t3.start(); Thread.sleep(2000); System.out.println("主线程执行任务完毕，放行......"); countDownLatch.countDown(); System.out.println("主线程执行完毕"); }}执行结果

2、主线程等待N个子线程的处理结果，子线程都完成后，主线程继续执行。Task

package com.jane;import java.util.concurrent.CountDownLatch;public class Task1 implements Runnable { private CountDownLatch countDownLatch; private String name; public Task1(CountDownLatch countDownLatch, String name) { this.countDownLatch = countDownLatch; this.name = name; } @Override public void run() { try { System.out.println(name + "开始开始执行任务......"); Thread.sleep(2000); System.out.println(name + "执行任务完成"); countDownLatch.countDown(); } catch (Exception e) { e.printStackTrace(); } }}主线程

package com.jane;import java.util.concurrent.CountDownLatch;public class Main1 { public static void main(String[] args) throws InterruptedException { //主线程创建，和子线程共享 CountDownLatch countDownLatch = new CountDownLatch(3); System.out.println("主线程启动......"); //创建3个子线程 Thread t1 = new Thread(new Task1(countDownLatch, "线程-1")); Thread t2 = new Thread(new Task1(countDownLatch, "线程-2")); Thread t3 = new Thread(new Task1(countDownLatch, "线程-3")); t1.start(); t2.start(); t3.start(); System.out.println("主线程等待子线程执行结果......"); countDownLatch.await(); System.out.println("主线程执行完毕"); }}执行结果

注意：CountDownLatch的计数只能使用一次，不会自动重新开始。