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CountDownLatch Usages

22-11-2021 - Shai Zambrovski

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Basically a counter which validate all workers threads are done and the main thread who created those, are no longer block and can continue.

Once we create a CountDownLatch with the number of the parallel threads that will work together we will call to countdown() upon finish the task will decrease the number of remaining threads.

Wait for all threads to complete

We will create simple application that upon each worker thread we will print to console and when all threads are done, we will notify it.

import java.util.List;
import java.util.concurrent.CountDownLatch;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class Main {

    private static final int THREADS_LIMIT = 10;

    public static void main(String[] args) throws InterruptedException {
        CountDownLatch countDownLatch = new CountDownLatch(THREADS_LIMIT);

        List<Thread> threads = Stream
                .generate(() ->
                        new Thread(() -> {
                            System.out.println("in worker thread");
                            countDownLatch.countDown();
                        }))
                .limit(THREADS_LIMIT)
                .collect(Collectors.toList());

        threads.forEach(Thread::start);

        countDownLatch.await();
        System.out.println("finished");
    }
}

We will get the next output:

in worker thread
in worker thread
in worker thread
in worker thread
in worker thread
in worker thread
in worker thread
in worker thread
in worker thread
in worker thread
finished

awaite with timeout

In some scenarios, we would like to terminate the CountDownLatch without wating for all threads to finish and to avoid blocking the caller thread forever.

To do so, we can use the await(long timeout, TimeUnit unit) function.

public class Main {

    private static final int THREADS_LIMIT = 10;

    public static void main(String[] args) throws InterruptedException {
        CountDownLatch countDownLatch = new CountDownLatch(THREADS_LIMIT);

        List<Thread> threads = Stream
                .generate(() ->
                        new Thread(() -> {
                            if(true) {
                                throw new RuntimeException("An Error");
                            } else {
                                countDownLatch.countDown();
                            }
                        }))
                .limit(THREADS_LIMIT)
                .collect(Collectors.toList());

        threads.forEach(Thread::start);

        countDownLatch.await(5L, TimeUnit.SECONDS);
        System.out.println("finished");
    }
}