java CountDownLatch

CountDownLatch 介绍

CountDownLatch 类位于java.util.concurrent包下，利用它可以实现类似计数器的功能。比如有一个任务A，它要等待其他几个任务执行完毕之后才能执行，此时就可以利用CountDownLatch来实现这种功能了。CountDownLatch是通过一个计数器来实现的，计数器的初始值为线程的数量。每当一个线程完成了自己的任务后，计数器的值就会减1。当计数器值到达0时，它表示所有的线程已经完成了任务，然后在闭锁上等待的线程就可以恢复执行任务。

定义两个线程

• 线程1

public class Thread1 extends Thread {
    private CountDownLatch countDownLatch;
    public Thread1(CountDownLatch countDownLatch) {
        this.countDownLatch = countDownLatch;
    }

    @Override
    public void run() {
        try {
            Thread.sleep(5000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("thread-1");
        countDownLatch.countDown();
    }
}

• 线程2

public class Thread2 extends Thread {
    private CountDownLatch countDownLatch;

    public Thread2(CountDownLatch countDownLatch) {
        this.countDownLatch = countDownLatch;
    }

    @Override
    public void run() {
        try {
            Thread.sleep(5000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("thread-2");
        countDownLatch.countDown();
    }
}

请求

@RequestMapping("test-countdownlatch")
    public void testCountDownLatch() {
        CountDownLatch countDownLatch = new CountDownLatch(2);
        Thread1 thread1 = new Thread1(countDownLatch);
        Thread2 thread2 = new Thread2(countDownLatch);
        thread1.start();
        thread2.start();
        try {
            countDownLatch.await();
            System.out.println("main...");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

结果

"main..."总是等待两个线程完成后打印

CountDownLatch代码