javamultithreadingcountdownlatch

Countdownlatch get count returns inconsistent values


I have developed a sample java program to understand countdownlatch & initialized the countdownlatch with count 4. I expected that after countDown method, the getCount() would return the remaining count for the countdownlatch. But, in the following example:-

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");
}
}

The output is :-

Count:- 2

Count:- 1

Count:- 2

WORKER-3 finished

WORKER-1 finished

WORKER-2 finished

Count:- 0

Final Count:- 0

WORKER-4 finished.

The count is returned as 2 for two times in the output. Is there anything wrong in my code?


Solution

  • It is not a problem. How is CountDownLatch used in Java Multithreading?

    This method only do a volatile read, not a synchronised read.

     /**
     * 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();
    }