Mock kafka producer in jUnit to test Future(Java)

Question

I am changing the implementation of how kafka sends events from async to sync

I found the following

 producer.send(producerRecord, (m, e) -> {
    ...some login...
 }).get();
 } catch (InterruptedException | RuntimeException | ExecutionException e) {
       logger.info("", e);
       throw new RuntimeException(e);
 }

I've added the get() method, which makes the code wait for a response before moving to the next one.

I now want to create a jUnit test to test this but I'm struggling to do so I'm mocking the producer using Mockito Then I'm using the following code

KafkaProducer mockProducer = Mockito.mock(KafkaProducer.class);
Future<RecordMetadata> future = Mockito.mock(Future.class);
doReturn(future).when(mockProducer).send(any());
when(future.get()).thenThrow(InterruptedException.class);
communicator.sendEvent(eventValue);//sendEvent then calls the producer and sends an event

I'm getting NullPointerException for some reason but when I evaluate producer.sendEvent and also producer.sendEvent(producerRecord).get() in the debugger. They both have values

So I'm not sure what's wrong.

any advice will be appreciated

Answer 1

You do not mock the method that you call later (as @kiuby_88 already mentioned in a comment).

So you should do it in your test like this:

KafkaProducer mockProducer = Mockito.mock(KafkaProducer.class);
Future<RecordMetadata> future = Mockito.mock(Future.class);

when(mockProducer).send(any(), any()).thenReturn(future);
when(future.get()).thenThrow(InterruptedException.class);

communicator.sendEvent(eventValue);

You mixed the doReturn style of Mockito configurations with the when style, which is confusing a bit. The doReturn style was introduced for setting up reactions on the call of methods returning void but can also be used to return values from other methods - but in my opinion it reduces confusion of the reader of you test-code, if you use one style throughout a whole test. An alternative style would be:

doReturn(future).when(mockProducer).send(any(), any());
doThrow(InterruptedException.class).when(future).get();

See also the docs of Mockito about the doReturn()|doThrow()|doAnswer()|doNothing()|doCallRealMethod() family of methods