JUC并发编程 共享模式之工具 JUC CountdownLatch（倒计时锁） -- CountdownLatch（使用CountdownLatch原理改进: 配合线程池使用）

Posted 2021-10-23 Z && Y

1. CountdownLatch

• 用来进行线程同步协作，等待所有线程完成倒计时。
• 其中构造参数用来初始化等待计数值，await() 用来等待计数归零，countDown() 用来让计数减一

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1.1 使用

示例代码:

package com.tian;

import lombok.extern.slf4j.Slf4j;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutionException;

@Slf4j(topic = "c.TestCountDownLatch")
public class TestCountDownLatch {
    public static void main(String[] args) throws InterruptedException, ExecutionException {
        test();
    }


    private static void test() throws InterruptedException {
        // 初始倒计时为3
        CountDownLatch latch = new CountDownLatch(3);
        System.out.println("初始倒计时为3 主线程开始等待");

        new Thread(() -> {
            log.debug("begin...");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            // 倒计时减一
            latch.countDown();
            System.out.println("倒计时减1");
            log.debug("end...{}", latch.getCount());
        }, "线程01").start();

        new Thread(() -> {
            log.debug("begin...");
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            // 倒计时减二
            latch.countDown();
            System.out.println("倒计时减1");
            log.debug("end...{}", latch.getCount());
        }, "线程02").start();

        new Thread(() -> {
            log.debug("begin...");
            try {
                Thread.sleep(1500);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            // 倒计时减三
            latch.countDown();
            System.out.println("倒计时减1");
            log.debug("end...{}", latch.getCount());
        }, "线程03").start();

        log.debug("waiting...");
        latch.await();
        log.debug("wait end...");
        System.out.println("倒计时为0 主线程结束等待");
    }
}

运行结果:

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1.2 CountdownLatch原理

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1.3 改进: 配合线程池使用

package com.tian;

import lombok.extern.slf4j.Slf4j;

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

@Slf4j(topic = "c.TestCountDownLatch")
public class TestCountDownLatch {
    public static void main(String[] args) throws InterruptedException, ExecutionException {
        test();
    }

    private static void test() {
        // 初始化倒计时为3
        CountDownLatch latch = new CountDownLatch(3);
        // 线程池固定大小为4
        ExecutorService service = Executors.newFixedThreadPool(4);
        System.out.println("初始化倒计时为3");
        service.submit(() -> {
            log.debug("begin...");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            // 倒计时减一
            latch.countDown();
            System.out.println("倒计时减1");
            log.debug("end...{}", latch.getCount());
        });
        service.submit(() -> {
            log.debug("begin...");
            try {
                Thread.sleep(1500);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            // 倒计时减一
            latch.countDown();
            System.out.println("倒计时减1");
            log.debug("end...{}", latch.getCount());
        });
        service.submit(() -> {
            log.debug("begin...");
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            // 倒计时减一
            latch.countDown();
            System.out.println("倒计时减1");
            log.debug("end...{}", latch.getCount());
        });
        service.submit(() -> {
            try {
                log.debug("waiting......");
                // 等待倒计时为0 这个不用join方法的原因时,在线程池里面,线程不会轻易的结束
                // 所以不适合使用join方法
                latch.await();
                log.debug("倒计时为0 wait end......");
            } catch (Exception e) {
                e.printStackTrace();
            }
        });
    }
}

运行结果:

说明:

latch.await();
等待倒计时为0 这个不用join方法的原因时,在线程池里面,线程不会轻易的结束，所以不适合使用join方法。

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