@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 executeBla() {
    System.out.println("Bla!");
    return new AsyncResult(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> futures = new ArrayList>();

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

    for (Future 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.