如何对包含 CompletableFuture 的方法进行单元测试？

Question

I tried looking this up, but couldn't quite understand the examples I found. I have a class with a Spring AfterPropertiesSet() method that calls 2 other methods to spin off asynch calls. I'm at a loss how as to to unit test them. The method startProcessingThread() looks like this:

void startProcessingThread(executor) {
    CompletableFuture.runAsync(() -> {
        Path filePath = null;       
        do {
            filePath = retrieveFromFileQueue();
            submitFileForProcessing(filePath);            
        } while (keepRunning);
    }, executor)
    .handle((theVoid, exception) -> {           
        if (exception != null) {
            LOG.error("Error submitting file for processing: " + exception);            
        }
        return null;
    });
}

I don't want to rewrite the CompletableFuture in the test method (do I?). So I figured I need to call startProcessingThread() , using Mocks (from Mockito) inside the methods that need them ( retrieveFromFileQueue() and submitFileForProcessing() ). But what about the CompletableFuture itself? Should I mock that? I'm sorry, but I'm really confused...

Answer 1

You should avoid non deterministic logic in your test and so I would advise to avoid sleep. Instead you can use Mockito's "when" instead of "verify" and wait until the action was performed:

final xyz[] result = new[]{null};
final CountDownLatch latch = new CountDownLatch(1);
when(submitFileForProcessing(...).thenAnswer((Answer<Lwm2mCommand>) invocationOnMock ->
{
    result[0] = invocationOnMock.getArgument(index); // if you need to check the values passed to your method call

    latch.countDown();
}

// Call your method
        assertTrue(latch.await(60L, TimeUnit.SECONDS));
// If needed check the parameters passed to the mocked method

One more tip: I wouldn't mock the actual class under test - instead I would mock one of the dependencies used.