CountDownLatch -了解await和countDown

Question

根据Javadoc的说法: CountDownLatch是用给定的计数初始化的。await方法会一直阻塞，直到当前计数为零。

这意味着在下面的代码中，因为我将CountDownLatch初始化为1。只要锁存器调用倒计时，所有线程都应该从它的await方法中解锁。

但是，主线程正在等待所有线程完成。而且，我没有将主线程连接到其他线程的末尾。为什么主线程在等待？

    import java.util.concurrent.CountDownLatch;
    import java.util.concurrent.atomic.AtomicLong;

public class Sample implements Runnable {

    private CountDownLatch latch;

    public Sample(CountDownLatch latch)
    {
        this.latch = latch;
    }
    private static AtomicLong number = new AtomicLong(0);

    public long next() {
         return number.getAndIncrement();
    }

    public static void main(String[] args) {

        CountDownLatch latch = new CountDownLatch(1);
        for (int threadNo = 0; threadNo < 4000; threadNo++) {
          Runnable t = new Sample(latch);
          new Thread(t).start();
        }
        try {
            latch.countDown();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    @Override
    public void run() {
        try {
            latch.await();
            Thread.sleep(100);
            System.out.println("Count:"+next());
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

}

Answer 1

尝试运行以下修改后的代码版本：

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.atomic.AtomicLong;

public class Test implements Runnable {

    private CountDownLatch latch;

    public Test(CountDownLatch latch)
    {
        this.latch = latch;
    }
    private static AtomicLong number = new AtomicLong(0);

    public long next() {
         return number.getAndIncrement();
    }

    public static void main(String[] args) {

        CountDownLatch latch = new CountDownLatch(1);
        for (int threadNo = 0; threadNo < 1000; threadNo++) {
          Runnable t = new Test(latch);
          new Thread(t).start();
        }
        try {
            latch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println( "done" );
    }

    @Override
    public void run() {
        try {
            Thread.sleep(1000 + (int) ( Math.random() * 3000 ));
            System.out.println(next());
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            latch.countDown();
        }
    }

}

您应该会看到类似这样的内容：

%0

完成

1

2

3.

4.

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19个

这表明在第一个线程latch.countDown()之后，主线程确实解除了对latch.await()调用的阻塞

Answer 2

您正在启动4000个线程，它们只等待100毫秒。你很可能会压倒整个盒子(所有的线程都会在大致相同的时间结束)。在您的线程启动外观中添加睡眠，并尝试增加超时以查看它是否如您所期望的那样工作。