深入理解JUC：第四章：CountDownLatch倒计时器

Java廖志伟

发布于 2021-01-29 07:13:38

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发布于 2021-01-29 07:13:38

文章被收录于专栏：高级开发进阶

理论：

CountDownLatch是一个非常实用的多线程控制工具类。常用的就下面几个方法：

CountDownLatch(int count) //实例化一个倒计数器，count指定计数个数
countDown() // 计数减一
await() //等待，当计数减到0时，所有线程并行执行

未使用CountDownLatch的代码：

public class Demo {

    public static void main(String[] args) {
        for (int i = 0; i < 5; i++) {
            new Thread(()->{
                System.out.println(Thread.currentThread().getName()+"\t 工作任务完成，离开公司");
            },String.valueOf(i)).start();
        }
        System.out.println(Thread.currentThread().getName()+"\t 最后将公司门锁死，离开公司");
    }
}

控制台：

使用CountDownLatch的代码：

public class Demo {

    public static void main(String[] args) throws Exception{
        CountDownLatch countDownLatch = new CountDownLatch(5);
        for (int i = 0; i < 5; i++) {
            new Thread(()->{
                System.out.println(Thread.currentThread().getName()+"\t 工作任务完成，离开公司");
                countDownLatch.countDown();
            },String.valueOf(i)).start();
        }
        countDownLatch.await();
        System.out.println(Thread.currentThread().getName()+"\t 最后将公司门锁死，离开公司");
    }
}

控制台：

案例二：

package tst.project;

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

public class Test {

    public static void main(String[] args) {
        int tasks = 3;
        final CountDownLatch latch = new CountDownLatch(tasks);
        ExecutorService executorService = Executors.newFixedThreadPool(tasks);
        executorService.submit(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(5000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("5秒完成："+latch);
                latch.countDown();
            }
        });
        executorService.submit(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(3000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("3秒完成："+latch);
                latch.countDown();
            }
        });
        executorService.submit(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("1秒完成："+latch);
                latch.countDown();
            }
        });
        executorService.shutdown();
        try {
            latch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

控制台：