Testing a @KafkaListener using Spring Embedded Kafka

Question

I am trying to write a unit test for a Kafka listener that I am developing using Spring Boot 2.x. Being a unit test, I don't want to start up a full Kafka server an instance of Zookeeper. So, I decided to use Spring Embedded Kafka.

The definition of my listener is very basic.

@Component
public class Listener {
    private final CountDownLatch latch;

    @Autowired
    public Listener(CountDownLatch latch) {
        this.latch = latch;
    }

    @KafkaListener(topics = "sample-topic")
    public void listen(String message) {
        latch.countDown();
    }
}

Also the test, that verifies the latch counter to be equal to zero after receiving a message, is very easy.

@RunWith(SpringRunner.class)
@SpringBootTest
@DirtiesContext
@EmbeddedKafka(topics = { "sample-topic" })
@TestPropertySource(properties = { "spring.kafka.bootstrap-servers=${spring.embedded.kafka.brokers}" })
public class ListenerTest {

    @Autowired
    private KafkaEmbedded embeddedKafka;

    @Autowired
    private CountDownLatch latch;

    private KafkaTemplate<Integer, String> producer;

    @Before
    public void setUp() {
        this.producer = buildKafkaTemplate();
        this.producer.setDefaultTopic("sample-topic");
    }

    private KafkaTemplate<Integer, String> buildKafkaTemplate() {
        Map<String, Object> senderProps = KafkaTestUtils.producerProps(embeddedKafka);
        ProducerFactory<Integer, String> pf = new DefaultKafkaProducerFactory<>(senderProps);
        return new KafkaTemplate<>(pf);
    }

    @Test
    public void listenerShouldConsumeMessages() throws InterruptedException {
        // Given
        producer.sendDefault(1, "Hello world");
        // Then
        assertThat(latch.await(10L, TimeUnit.SECONDS)).isTrue();
    }
}

Unfortunately, the test fails and I cannot understand why. Is it possible to use an instance of KafkaEmbedded to test a method marked with the annotation @KafkaListener?

All the code is shared in my GitHub repository kafka-listener.

Thanks to all.

Answer 1

You are probably sending the message before the consumer has been assigned the topic/partition. Set property...

spring:
  kafka:
    consumer:
      auto-offset-reset: earliest

...it defaults to latest.

This is like using --from-beginning with the console consumer.

EDIT

Oh; you're not using boot's properties.

Add

props.put(ConsumerConfig.AUTO_OFFSET_RESET_CONFIG, "earliest");

EDIT2

BTW, you should probably also do a get(10L, TimeUnit.SECONDS) on the result of the template.send() (a Future<>) to assert that the send was successful.

EDIT3

To override the offset reset just for the test, you can do the same as what you did for the broker addresses:

@Value("${spring.kafka.consumer.auto-offset-reset:latest}")
private String reset;

...

    props.put(ConsumerConfig.AUTO_OFFSET_RESET_CONFIG, this.reset);

and

@TestPropertySource(properties = { "spring.kafka.bootstrap-servers=${spring.embedded.kafka.brokers}",
        "spring.kafka.consumer.auto-offset-reset=earliest"})

However, bear in mind that this property only applies the first time a group consumes. To always start at the end each time the app starts, you have to seek to the end during startup.

Also, I would recommend setting enable.auto.commit to false so that the container takes care of committing the offsets rather than just relying on the consumer client doing it on a time schedule.