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Reputation: 2731

Java CountDownLatch example main thread doesn't wait on latch.await

I am creating a simple Java8 concurrency CountDownLatch example. I am creating a latch of size 2 and start 2 threads and call await on the latch. See the below example.

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.");
    }
}

I am expecting Thread-0, Thread-1 to complete before main completes (based on sysout) consistently. However when I execute the program sometimes the "main done" message prints before even the Thread-1 completes. Is my implementation or my understanding is incorrect? Please suggest.

Output:

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

Upvotes: 0

Views: 1186

Answers (1)

akshaya pandey
akshaya pandey

Reputation: 997

When I executed, I am getting this output:

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

The context switch between thread can also happend after the call to latch.countDown(); in the run method. I suggest you modify the run method as below to get desired and consistent result.

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

    }

Upvotes: 2

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