Java CountDownLatch示例主线程不等待latch.await

Question

我正在创建一个简单的Java8 concurrency CountDownLatch示例。我正在创建一个大小为2的闩锁，并启动2个线程，并在闩锁上调用await。请参见下面的示例。

package CountDownLatchExample;

import java.util.concurrent.CountDownLatch;

public class CountDownLatchExample {

    static final CountDownLatch latch = new CountDownLatch(2);

    public static void main(String[] args) throws InterruptedException {
        MyThread thread = new MyThread(latch);
        new Thread(thread).start();
        new Thread(thread).start();
        latch.await();
        System.out.println(Thread.currentThread().getName() + " done.");
    }

}

class MyThread implements Runnable {

    CountDownLatch latch;

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

    @Override
    public void run() {
        latch.countDown();
        System.out.println(Thread.currentThread().getName() + " completed.");
    }
}

我期望Thread-0，Thread-1在main完成之前完成(基于sysout)。然而，当我执行程序时，有时甚至在Thread-1完成之前就会打印出"main done“消息。是我的实现或我的理解不正确吗？请提个建议。

输出：

> Task :CountDownLatchExample.main()
Thread-0 completed.
main done.
Thread-1 completed.

Answer 1

当我执行的时候，我得到了这个输出：

Thread-0 completed.
Thread-1 completed.
main done.

线程之间的上下文切换也可以在run方法中调用latch.countDown()；之后发生。我建议您修改run方法，如下所示，以获得理想和一致的结果。

 @Override
    public void run() {
        System.out.println(Thread.currentThread().getName() + " completed.");
        latch.countDown();

    }