并发工具类：倒计时器-CountDownLatch

Posted 2020-12-01 enjoyitlife

---

title: 并发工具类：倒计时器-CountDownLatch
author: Enjoyitlife.top
date: 2019-10-01 10:51:33
summary: JUC包中的工具类CountDownLatch到底给我们提供了什么功能？
categories: Concurrent
tags:

• Concurrent-Tools

---

并发工具类：倒计时器-CountDownLatch

CountDownLatch是JDK1.5提供一个并发编程辅助类，用于控制一个和多个线程进行等待，直到其他线程完成后在执行任务。

常用方法：

• CountDownLatch latch= new CountDownLatch(10); 构造方法，设定计数值，即要等待的线程数量。
• void countDown() 计数器的值减1，直到为0
• void await() 阻塞等待，直到计数器的值为0
• long getCount() 获取当前计数器的值

应用场景 1：实现多个N个线程并行运行

平常用于在创建N个线程来，验证接口的性能。

public class CountDownLatchTest {
    public static void main(String[] args) {
        CountDownLatch latch= new CountDownLatch(5);
        try {   
            for(int i=0;i<5;i++) {
                new Thread(new Runnable() {
                    @Override
                    public void run() {
                        CountDownLatchTest.sayThreadName();
                    }
                }).start();
            }       
            latch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }   
    }
    /***
    *模拟真正接口
    */
    public static void sayThreadName() {
        System.out.println(System.currentTimeMillis()+"^^"+Thread.currentThread().getName()+"^^^^");
    }   
}

应用场景2：等待N个线程完成任务后在触发其他操作

运动员准备完毕后裁判发令，裁判发令后运动员开始跑步。

public class AthleticMeeting {
    
    public static void main(String[] args) throws InterruptedException {
        //准备队列
        CountDownLatch readLatch = new CountDownLatch(8);
        //结束队列
        CountDownLatch endLatch = new CountDownLatch(8);
        //教练队列
        CountDownLatch  refereeLatch=new CountDownLatch(1); 
        for(int i=0;i<8;i++) {
            final int payerId=i+1;
            Thread thread= new Thread(new Runnable() {
                @Override
                public void run() {
                    try {
                        Long readyCosts=new Double(Math.random()*5000).longValue();
                        Thread.sleep(readyCosts);
                        System.out.println(payerId+"号运动员耗时"+readyCosts+"毫秒准备完毕！");
                        readLatch.countDown();                  
                        refereeLatch.await();                   
                        Long runCost=new Double(Math.random()*5000).longValue();
                        Thread.sleep(runCost);
                        System.out.println(payerId+"号运动员耗时"+runCost+"毫秒完成比赛！");
                        endLatch.countDown();                   
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            });
            thread.start();
        }
        readLatch.await();
        System.out.println("所有运动员准备完毕！");
        refereeLatch.countDown();
        System.out.println("裁判发动发令枪！");
        endLatch.await();
        System.out.println("比赛完成！");    
    }       
}