Countdownlatch get count returns 个不一致的值

Question

我已经开发了一个示例 java 程序来理解倒计时闩锁并用计数 4 初始化倒计时闩锁。我预计在 countDown 方法之后，getCount() 将 return 倒计时闩锁的剩余计数.但是，在下面的例子中：-

public static void main(String args[]) throws InterruptedException {
    CountDownLatch latch = new CountDownLatch(4);
    Worker first = new Worker(latch, "WORKER-1");
    Worker second = new Worker(latch, "WORKER-2");
    Worker third = new Worker(latch, "WORKER-3");
    Worker fourth = new Worker(latch, "WORKER-4");
    first.start();
    second.start();
    third.start();
    fourth.start();

    latch.await();
    System.out.println("Final Count:- " + latch.getCount());
}
}


class Worker extends Thread {
private CountDownLatch latch;

public Worker(CountDownLatch latch, String name) {
    super(name);
    this.latch = latch;
}

@Override
public void run() {
    latch.countDown();
    System.out.println("Count:- " + latch.getCount());
    System.out.println(Thread.currentThread().getName() + " finished");
}
}

输出为：-

计数：- 2

计数：- 1

计数：- 2

WORKER-3 完成

WORKER-1 完成

WORKER-2 完成

计数：- 0

最终计数：- 0

WORKER-4 完成。

计数在输出中两次被 returned 为 2。我的代码有什么问题吗？

Answer 1

没问题。 How is CountDownLatch used in Java Multithreading?

此方法只进行 volatile 读取，而不进行 synchronised 读取。

 /**
 * Returns the current count.
 *
 * <p>This method is typically used for debugging and testing purposes.
 *
 * @return the current count
 */
public long getCount() {
    return sync.getCount();
}

Answer 2

四个Worker同时是运行。由于latch.countDown();和latch.getCount()不是原子的，执行顺序可能是：

thread1 latch.countDown();
thread2 latch.countDown();
thread1 System.out.println("Count:- " + latch.getCount()); // Count:- 2
thread2 System.out.println("Count:- " + latch.getCount()); // Count:- 2

Countdownlatch get count returns 个不一致的值

Countdownlatch get count returns inconsistent values

java

multithreading

countdownlatch