Java中CountDownLatch怎么用

简述

用来干嘛的？当你在方法中调用了多个线程，对数据库进行了一些不为人知的操作后，还有一个操作需要留到前者都执行完的重头戏，就需要用到 CountDownLatch 了

实践代码

package com.github.gleans;

import java.util.concurrent.CountDownLatch;

public class TestCountDownLatch {

    public static void main(String[] args) throws InterruptedException {
        CountDownLatch latch = new CountDownLatch(3);
        new KeyPass(1000L, "thin jack", latch).start();
        new KeyPass(2000L, "noral jack", latch).start();
        new KeyPass(3000L, "fat jack", latch).start();
        latch.await();
        System.out.println("此处对数据库进行最后的插入操作~");
    }

    static class KeyPass extends Thread {

        private long times;

        private CountDownLatch countDownLatch;

        public KeyPass(long times, String name, CountDownLatch countDownLatch) {
            super(name);
            this.times = times;
            this.countDownLatch = countDownLatch;
        }

        @Override
        public void run() {
            try {
                System.out.println("操作人：" + Thread.currentThread().getName()
                        + "对数据库进行插入，持续时间：" + this.times / 1000 + "秒");
                Thread.sleep(times);
                countDownLatch.countDown();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

图解

使用await()提前结束操作

package com.github.gleans;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;

public class TestCountDownLatch {

    public static void main(String[] args) throws InterruptedException {
        CountDownLatch latch = new CountDownLatch(3);
        new KeyPass(2000L, "公司一", latch).start();
        new KeyPass(3000L, "公司二", latch).start();
        new KeyPass(5000L, "公司三", latch).start();
        latch.await(2, TimeUnit.SECONDS);
        System.out.println("~~~贾总PPT巡演~~~~");
        System.out.println("~~~~融资完成，撒花~~~~");
    }

    static class KeyPass extends Thread {

        private long times;

        private CountDownLatch countDownLatch;

        public KeyPass(long times, String name, CountDownLatch countDownLatch) {
            super(name);
            this.times = times;
            this.countDownLatch = countDownLatch;
        }

        @Override
        public void run() {
            try {
                Thread.sleep(times);
                System.out.println("负责人：" + Thread.currentThread().getName()
                        + "开始工作，持续时间：" + this.times / 1000 + "秒");
                countDownLatch.countDown();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

假设公司一、公司二、公司三各需要2s、3s、5s来完成工作，贾总等不了，只能等2s，那么就设置await的超时时间

latch.await(2, TimeUnit.SECONDS);

执行结果

负责人：公司一开始工作，持续时间：2秒
~~~贾总PPT巡演~~~~
~~~~融资完成，撒花~~~~
负责人：公司二开始工作，持续时间：3秒
负责人：公司三开始工作，持续时间：5秒

方法描述