概述
本文详细介绍CountDownLatch的两种使用场景，分别是倒数（等待所有的线程处理完成）和唤醒所有线程同时运行（适用于性能测试中触发所有并发同时运行）。
下文从这个两个方面进行介绍
CountDownLatch倒数实例
实例代码利用CountDownLatch的downLatch方法让子线程在运行结束了减2；利用await方法，让主线程阻塞，见下实例代码。
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package com.yang.concurrent;import java.util.concurrent.CountDownLatch;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;public class CountDownLatchDemo01 { public static void main(String[] args) { final CountDownLatch countDownLatch=new CountDownLatch(5); ExecutorService executorService = Executors.newFixedThreadPool(5); for (int i = 0; i < 5; i++) { Runnable runnable = new Runnable() { @Override public void run() { try { System.out.println(Thread.currentThread().getName() + "等待运行中"); Thread.sleep(1000); System.out.println(Thread.currentThread().getName() + "运行结束"); countDownLatch.countDown(); } catch (InterruptedException e) { e.printStackTrace(); } } }; executorService.submit(runnable); } System.out.println("主线程等待子线程运行结束"); try { countDownLatch.await(); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("主线程运行结束"); }} |
运行结果如下图所示：
CountDownLatch同时运行实例
本实例利用CountDownLatch的await方法让所以的子线程都等待，待相关资源准备好后，主线程通知线程运行。如下实例代码所示：
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package com.yang.concurrent;import java.util.concurrent.CountDownLatch;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;/** * 本实例演示子线程都等待 */public class CountDownLatchDemo02 { public static void main(String[] args) { final CountDownLatch countDownLatch=new CountDownLatch(1); ExecutorService executorService = Executors.newFixedThreadPool(5); for (int i = 0; i < 5; i++) { Runnable runnable = new Runnable() { @Override public void run() { try { System.out.println(Thread.currentThread().getName() + "等待运行中"); countDownLatch.await(); System.out.println(Thread.currentThread().getName() + "运行结束"); } catch (InterruptedException e) { e.printStackTrace(); } } }; executorService.submit(runnable); } System.out.println("主线程准备相关资源"); try { Thread.sleep(1000); System.out.println("主线程资源准备就绪，子线程可以运行了"); countDownLatch.countDown(); } catch (InterruptedException e) { e.printStackTrace(); } }} |
运行结果如下图所示：
CountDownLatch赛跑实例
下实例代码我们模拟百米赛跑的故事，赛跑先要进行相关的准备工作，待准备工作完成后，明枪起跑，待所有人都到终点后，起跑结束，相关实例代码如下图所示。
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package com.yang.concurrent;import java.util.Random;import java.util.concurrent.CountDownLatch;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;/** * 本实例演示子线程都等待 */public class CountDownLatchDemo03 { public static void main(String[] args) { final CountDownLatch beginCountDownLatch=new CountDownLatch(1); final CountDownLatch endCountDownLatch=new CountDownLatch(5); ExecutorService executorService = Executors.newFixedThreadPool(5); System.out.println("百米赛跑准备中"); for (int i = 0; i < 5; i++) { Runnable runnable = new Runnable() { @Override public void run() { try { System.out.println(Thread.currentThread().getName() + "准备起跑！"); beginCountDownLatch.await(); Thread.sleep((long)new Random(1000).nextLong()); System.out.println(Thread.currentThread().getName() + "到达终点"); endCountDownLatch.countDown(); } catch (InterruptedException e) { e.printStackTrace(); } } }; executorService.submit(runnable); } try { Thread.sleep(1000); System.out.println("名枪起跑"); beginCountDownLatch.countDown(); endCountDownLatch.await(); System.out.println("所有人到达终点，赛跑结束"); } catch (InterruptedException e) { e.printStackTrace(); } }} |
运行结果如下：
