How to properly test kafkaTemplate.send() within a function in Junit5? - apache-kafka

Question

I'm learning how to write tests and especially tests that have a producer in it. I cannot post all the classes because it's HUGE (and not mine, I should just practice by changing the test to work with KafkaTemplate). I'm lost as to how a call like this should be tested.
I'm getting a NPE because of a producer.send("topic", JsonObject) that is in the function I'm testing. The functions is built like so:
#Autowired
private KafkaTemplate<String,EventDto> kafkaTemplate;
public EventDto sendEvent(Event event) {
EventDto eventToSend = this.dtoMapper.mapToDto(event, SomeEvent.class);
this.kafkaTemplate.send("topic",eventToSend);
return eventToSend;
}
in the unit test it's like this (irrelevant parts omitted):
#Test
void testSendEvent() {
//omitted lines regarding second assert that works
EventProducer producer = new EventProducer(something);
EventDto dto = producer.sendEvent(Event.newBuilder().build());
assertThat(dto).isNotNull();
//there is a second assert here that passes, nothing to do with kafka
}
We have Mockito and I assume I need to mock the KafkaTemplate somehow. But I'm not quite getting how I can "direct" the sendEvent to use the KafkaTemplate within the producer.sendEvent() call?
Solution edit: I changed the #Autowired to injecting it with the constructor instead. Works well! Here is the full class and method now
#Service
public class EventProducer implements EventProducerInterface {
private final DtoMapper dtoMapper;
private KafkaTemplate<String,EventDto> kafkaTemplate;
#Autowired
public EventProducer (KafkaTemplate<String,EventDto> kafkaTemplate, IDtoMapper dtoMapper) {
Assert.notNull(dtoMapper, "dtoMapper must not be null");
this.dtoMapper = dtoMapper;
this.kafkaTemplate=kafkaTemplate;
}
public EventDto sendEvent(Event event) {
EventDto eventToSend = this.dtoMapper.mapToDto(event, EventDto.class);
this.kafkaTemplate.send("output-topic",eventToSend);
return eventToSend;
}
}

Answer 1

You should use constructor injection instead of #Autowired:
private KafkaTemplate<String,EventDto> kafkaTemplate;
public EventProducer(KafkaTemplate<String,EventDto> kafkaTemplate, something) {
this.kafkaTemplate = kafkaTemplate;
}
public EventDto sendEvent(Event event) {
EventDto eventToSend = this.dtoMapper.mapToDto(event, SomeEvent.class);
this.kafkaTemplate.send("topic",eventToSend);
return eventToSend;
}
This way you can inject a mock in your tests:
#Test
void testSendEvent() {
//omitted lines regarding second assert that works
KafkaTemplate<<String,EventDto>> templateMock = mock(KafkaTemplate.class);
EventProducer producer = new EventProducer(templateMock, something);
EventDto dto = producer.sendEvent(Event.newBuilder().build());
assertThat(dto).isNotNull();
//there is a second assert here that passes, nothing to do with kafka
}
If you can't change the class' constructor, you can provide a mock using #MockBean:
#MockBean
KafkaTemplate<String,EventDto> kafkaTemplate;
#Test
void testSendEvent() {
//omitted lines regarding second assert that works
EventProducer producer = new EventProducer(something);
EventDto dto = producer.sendEvent(Event.newBuilder().build());
assertThat(dto).isNotNull();
//there is a second assert here that passes, nothing to do with kafka
}
But there's something odd with this design - does the EventProducer class have #Autowired and constructor arguments? Autowiring only works on beans, and usually either the class has a default constructor and #Autowired dependencies, or injects everything through the constructor.
If those options I present do not work for you, please add more details on the class' constructor and overall design.