How to use JUnit to test asynchronous processes

Question

How do you test methods that fire asynchronous processes with JUnit?

I don\'t know how to make my test wait for the process to end (it is not exactly a unit test, it

Answer 1

This is what I'm using nowadays if the test result is produced asynchronously.

public class TestUtil {

    public static  R await(Consumer> completer) {
        return await(20, TimeUnit.SECONDS, completer);
    }

    public static  R await(int time, TimeUnit unit, Consumer> completer) {
        CompletableFuture f = new CompletableFuture<>();
        completer.accept(f);
        try {
            return f.get(time, unit);
        } catch (InterruptedException | TimeoutException e) {
            throw new RuntimeException("Future timed out", e);
        } catch (ExecutionException e) {
            throw new RuntimeException("Future failed", e.getCause());
        }
    }
}

Using static imports, the test reads kinda nice. (note, in this example I'm starting a thread to illustrate the idea)

    @Test
    public void testAsync() {
        String result = await(f -> {
            new Thread(() -> f.complete("My Result")).start();
        });
        assertEquals("My Result", result);
    }

If f.complete isn't called, the test will fail after a timeout. You can also use f.completeExceptionally to fail early.