JUC系列之（五）CountDownLatch闭锁

CountDownLatch闭锁

闭锁：延迟当前线程的进度，直到其他线程都执行完成当前线程才继续执行。

示例：计算多线程操作耗费时间

以下操作时无法正常计算多线程操作耗时的

package com.atguigu.juc;

public class TestCountDownLatch {
    public static void main(String[] args) {
        LatchDemo latchDemo = new LatchDemo();

        long start = System.currentTimeMillis();
        for (int i = 0; i < 10; i++) {
            new Thread(latchDemo).start();
        }

        long end = System.currentTimeMillis();

        System.out.println(end - start);
    }
}

class LatchDemo implements Runnable{

    @Override
    public void run() {
        for (int i = 0; i < 50000; i++) {
            if (i % 2 == 0){
                System.out.println(i);
            }
        }
    }
}

使用闭锁改造为如下即可

package com.atguigu.juc;

import java.util.concurrent.CountDownLatch;

public class TestCountDownLatch {
    public static void main(String[] args) {
        // 这里的10要和线程个数一致
        CountDownLatch countDownLatch = new CountDownLatch(10);

        LatchDemo latchDemo = new LatchDemo(countDownLatch);

        long start = System.currentTimeMillis();
        for (int i = 0; i < 10; i++) {
            new Thread(latchDemo).start();
        }

        try {
            // countDownLatch的值为 0 时主线程会继续执行
            countDownLatch.await();
        } catch (InterruptedException e) {
        }

        long end = System.currentTimeMillis();

        System.out.println("耗时：" + (end - start));
    }
}

class LatchDemo implements Runnable{

    private CountDownLatch latch;

    LatchDemo(CountDownLatch latch){
        this.latch = latch;
    }

    @Override
    public void run() {
        synchronized (this) {
            try {
                for (int i = 0; i < 50000; i++) {
                    if (i % 2 == 0){
                        System.out.println(i);
                    }
                }
            }finally {
                // 每个线程执行完都会减1
                latch.countDown();
            }
        }
    }
}

注：汇总类的业务中可能会用到