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CountDownLatch介绍

原创

HLee

修改于 2021-10-15 09:44:11

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代码可运行

文章被收录于专栏：房东的猫房东的猫

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代码可运行

简介

CountDownLatch中count down是倒数的意思，latch则是门闩的含义。整体含义可以理解为倒数的门栓，似乎有一点“三二一，芝麻开门”的感觉。CountDownLatch的作用也是如此，在构造CountDownLatch的时候需要传入一个整数n，在这个整数“倒数”到0之前，主线程需要等待在门口，而这个“倒数”过程则是由各个执行线程驱动的，每个线程执行完一个任务“倒数”一次。总结来说，CountDownLatch的作用就是等待其他的线程都执行完任务，必要时可以对各个任务的执行结果进行汇总，然后主线程才继续往下执行，存在于java.util.cucurrent包下。

CountDownLatch这个类使一个线程等待其他线程各自执行完毕后再执行。是通过一个计数器来实现的，计数器的初始值是线程的数量。每当一个线程执行完毕后，计数器的值就-1，当计数器的值为0时，表示所有线程都执行完毕，然后在闭锁上等待的线程就可以恢复工作了。

countDownLatch是一个计数器，线程完成一个记录一个，计数器递减且只能只用一次。

//构造函数：参数count为计数值
public CountDownLatch(int count) { };  

//调用await()方法的线程会被挂起，它会等待直到count值为0才继续执行
public void await() throws InterruptedException { };   
//和await()类似，只不过等待一定的时间后count值还没变为0的话就会继续执行
public boolean await(long timeout, TimeUnit unit) throws InterruptedException { };  
//将count值减1
public void countDown() { };

使用示例

普通示例

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class CountDownLatchTest {

    public static void main(String[] args) {

        // 计数器的初始值是线程的数量
        final CountDownLatch latch = new CountDownLatch(2);
        System.out.println("主线程开始执行");
        //第一个子线程执行
        ExecutorService es1 = Executors.newSingleThreadExecutor();
        es1.execute(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(3000);
                    System.out.println("子线程：" + Thread.currentThread().getName() + "执行");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                latch.countDown();
            }
        });
        es1.shutdown();

        //第二个子线程执行
        ExecutorService es2 = Executors.newSingleThreadExecutor();
        es2.execute(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(3000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("子线程：" + Thread.currentThread().getName() + "执行");
                latch.countDown();
            }
        });
        es2.shutdown();
        System.out.println("等待两个线程执行完毕");
        try {
            latch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("两个子线程都执行完毕，继续执行主线程");
    }
}
执行结果：
主线程开始执行
等待两个线程执行完毕
子线程：pool-1-thread-1执行
子线程：pool-2-thread-1执行
两个子线程都执行完毕，继续执行主线程

并发示例

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class CountDownLatchTest2 {
    public static void main(String[] args) {
        ExecutorService pool = Executors.newCachedThreadPool();
        CountDownLatch cdl = new CountDownLatch(100);
        for (int i = 0; i < 100; i++) {
            CountRunnable runnable = new CountRunnable(cdl);
            pool.execute(runnable);
        }
    }
}

class CountRunnable implements Runnable {

    private CountDownLatch countDownLatch;

    public CountRunnable(CountDownLatch countDownLatch) {
        this.countDownLatch = countDownLatch;
    }

    @Override
    public void run() {
        try {
            synchronized (countDownLatch) {
                // 每次减少一个容量
                countDownLatch.countDown();
                System.out.println("thread counts = " + (countDownLatch.getCount()));
            }
            countDownLatch.await();
            System.out.println("concurrency counts = " + (100 - countDownLatch.getCount()));
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}
执行结果；
thread counts = 99
thread counts = 98
thread counts = 97
thread counts = 96
......
hread counts = 3
thread counts = 2
thread counts = 1
thread counts = 0
concurrency counts = 100
concurrency counts = 100
concurrency counts = 100
......