@Async prevent a thread to continue until other thread have finished

Question

I have an application where for a certain number of times something needs to be calculated. This calculation function has the annotation @Async (from the Spring Framework), that

Answer 1

If you need to wait for the executions to finish, then you can return a Future as a return value, e.g.

@Async
public Future<Void> executeBla() {
    System.out.println("Bla!");
    return new AsyncResult<Void>(null);
}

This is slightly artificial, since there's no actual value being returned, but it will still allow the calling code to wait for all executions to finish:

public void executeBlaALotOfTimes() {
    long before = System.currentTimeMillis();

    Collection<Future<Void>> futures = new ArrayList<Future<Void>>();

    for (int i = 0; i<40000; i++) {
        futures.add(executeBla());
    }

    for (Future<Void> future : futures) {
        future.get();
    }

    long after = System.currentTimeMillis(); 

    System.out.println("Time it took for a lot of bla to execute: " + (after - before) / 1000.0 + " seconds.");
}

Here, the first loop fires off the async tasks and stores the futures in a list. The seconds loop then iterates over the futures, waiting for each one to finish.

Answer 2

An alternative is to return a ListenableFuture and to use a CountDownLatch.

@Async
public ListenableFuture<Void> executeBla() {
    try {
        System.out.println("Bla!");
        return AsyncResult.forValue(null);
    } catch (Throwable t) {
        return AsyncResult.forExecutionException(t);
    }
}

This scenario allows you to avoid explicitly calling future.get() for each future. You accomplish this by adding success and failure callbacks which in turn decrement the CountDownLatch, which was created exactly for this purpose.

public void executeBlaALotOfTimes() {
    long before = System.currentTimeMillis();

    int numExecutions = 40000;
    CountDownLatch countDownLatch = new CountDownLatch(numExecutions);

    for (int i = 0; i<numExecutions; i++) {
        ListenableFuture<Void> future = executeBla();
        future.addCallback(
            aVoid -> countDownLatch.countDown(), 
            throwable -> countDownLatch.countDown()
        );
    }

    try {
        countDownLatch.await();
    } catch (InterruptedException e) {
        // Handle exception
    } finally {
        long after = System.currentTimeMillis();
        System.out.println("Time it took for a lot of bla to execute: " + (after - before) / 1000.0 + " seconds.");
    }

}