How to signal the caller thread that ExecutorService has finished a task

Question

From the main thread:

executorService.submit(new Runnable() { ... });

Now when Runnable has finished executing, is there the Java's standard way of signaling the caller thread that it has finished executing, without making a new interface/listener class?

Bonus points if the signal can be emitted from the caller thread.

Answer 1

submit returns a Future on which the submitting thread can call get to block until the task completes.

Future<?> future = executor.submit(new Runnable() { ... });
future.get();

Answer 2

You can block on get() of the Future object that is returned, or poll the isDone() method on it. Alternatively, you could use the google guava library that has ListenableFuture .

Answer 3

I am not sure of your implementation and how many threads will be running simultaneously, so this may not work. One thing you could take a look at is to have the runnable itsself signal it is completed by calling a method that knows how to update the UI component that is needed.