How to use JUnit to test asynchronous processes

You can try using the Awaitility library. It makes it easy to test the systems you're talking about.

TL;DR; Unfortunately, there is no built-in solution yet (at time of writting, 2022), hence you are free to use and/or implement whatever fits your situation.

Example

An alternative is to use the CountDownLatch class.

public class DatabaseTest {

    /**
     * Data limit
     */
    private static final int DATA_LIMIT = 5;

    /**
     * Countdown latch
     */
    private CountDownLatch lock = new CountDownLatch(1);

    /**
     * Received data
     */
    private List<Data> receiveddata;

    @Test
    public void testDataRetrieval() throws Exception {
        Database db = new MockDatabaseImpl();
        db.getData(DATA_LIMIT, new DataCallback() {
            @Override
            public void onSuccess(List<Data> data) {
                receiveddata = data;
                lock.countDown();
            }
        });

        lock.await(2000, TimeUnit.MILLISECONDS);

        assertNotNull(receiveddata);
        assertEquals(DATA_LIMIT, receiveddata.size());
    }
}

NOTE you can't just used syncronized with a regular object as a lock, as fast callbacks can release the lock before the lock's wait method is called. See this blog post by Joe Walnes.

EDIT Removed syncronized blocks around CountDownLatch thanks to comments from @jtahlborn and @Ring

If you use a CompletableFuture (introduced in Java 8) or a SettableFuture (from Google Guava), you can make your test finish as soon as it's done, rather than waiting a pre-set amount of time. Your test would look something like this:

CompletableFuture<String> future = new CompletableFuture<>();
executorService.submit(new Runnable() {         
    @Override
    public void run() {
        future.complete("Hello World!");                
    }
});
assertEquals("Hello World!", future.get());

Note that there is a library which Provides CompletableFuture for pre Java-8, which even uses the same names (and provides all related Java-8 classes), like:

net.sourceforge.streamsupport:streamsupport-minifuture:1.7.4

This is useful for Android development, where even if we build with JDK-v11, we want to keep codes compatible with pre Android-7 devices.

IMHO it's bad practice to have unit tests create or wait on threads, etc. You'd like these tests to run in split seconds. That's why I'd like to propose a 2-step approach to testing async processes.

1. Test that your async process is submitted properly. You can mock the object that accepts your async requests and make sure that the submitted job has correct properties, etc.
2. Test that your async callbacks are doing the right things. Here you can mock out the originally submitted job and assume it's initialized properly and verify that your callbacks are correct.