Creating an annotation for JUnit 4/5 to initialize and inject an object in tests

Question

I am developing a testing library for Kafka, Kafkaesque . The library lets you develop integration tests for Kafka using a fluid and elegant (?!) API. For now, I develop the version for Spring Kafka.

The library needs to be initialized in every test:

 @Test
 void consumeShouldConsumeMessagesProducesFromOutsideProducer() {
   kafkaTemplate.sendDefault(1, "data1");
   kafkaTemplate.sendDefault(2, "data2");
   new SpringKafkaesque(broker)
       .<Integer, String>consume()
       .fromTopic(CONSUMER_TEST_TOPIC)
       .waitingAtMost(1L, TimeUnit.SECONDS)
       .waitingEmptyPolls(5, 100L, TimeUnit.MILLISECONDS)
       .withDeserializers(new IntegerDeserializer(), new StringDeserializer())
       .expecting()
       .havingRecordsSize(2)
       .assertingThatPayloads(Matchers.containsInAnyOrder("data1", "data2"))
       .andCloseConsumer();
 }

Instead of manually initializing the SpringKafkaesque object, I want to create an annotation that does the magic for me. Something like the @EmbeddedKafka annotation of Spring Kafka.

@SpringBootTest(classes = {TestConfiguration.class})
@Kafkaesque(
    topics = {SpringKafkaesqueTest.CONSUMER_TEST_TOPIC, SpringKafkaesqueTest.PRODUCER_TEST_TOPIC})
class SpringKafkaesqueTest {
  @Autowired
  private Kafkaesque kafkaesque;

  @Test
  void consumeShouldConsumeMessagesProducesFromOutsideProducer() {
    kafkaTemplate.sendDefault(1, "data1");
    kafkaTemplate.sendDefault(2, "data2");
    kafkaesque
        .<Integer, String>consume()
        .fromTopic(CONSUMER_TEST_TOPIC)
        .waitingAtMost(1L, TimeUnit.SECONDS)
        .waitingEmptyPolls(5, 100L, TimeUnit.MILLISECONDS)
        .withDeserializers(new IntegerDeserializer(), new StringDeserializer())
        .expecting()
        .havingRecordsSize(2)
        .assertingThatPayloads(Matchers.containsInAnyOrder("data1", "data2"))
        .andCloseConsumer();
   }

Is it possible? Any suggestion?

Answer 1

JUnit 4

One possible solution is to create a custom annotation processing using reflection. You can get the test method name with @Rule , so for example:

public class CustomAnnotationTest {
    
    private SpringKafkaesque kafkaesqueInstance;

    @Rule
    public TestName testName = new TestName();

    @Before
    public void init() {
        Method method = null;
        try {
            method = this.getClass().getMethod(testName.getMethodName());
        } catch (Exception ex) {
            // handle exceptions
        }
        if (method.isAnnotationPresent(EmbeddedKafka.class)) {
            // Init your SpringKafkaesque instance here
            // kafkaesqueInstance = new SpringKafkaesque(broker)
            //
        }
    }

    @EmbeddedKafka
    @Test
    public void testCustomAnnotated() {
        // your test here
    }

    @Retention(RetentionPolicy.RUNTIME)
    @Target(ElementType.METHOD)
    @interface EmbeddedKafka {
    }
}

You need to store this instance in the class-level variable. For the methods with no @EmbeddedKafka annotation, this variable will be null .

JUnit 5

With JUnit 5 you may consider using parameter injection with ParameterResolver . First of all, you need to implement this interface:

public class KafkaesqueResolver implements ParameterResolver {
    @Override
    public boolean supportsParameter(ParameterContext parameterContext,
                                     ExtensionContext extensionContext) throws ParameterResolutionException {
        return parameterContext.getParameter().getType() == SpringKafkaesque.class;
    }

    @Override
    public Object resolveParameter(ParameterContext parameterContext,
                                   ExtensionContext extensionContext) throws ParameterResolutionException {
        // Create an instance of SpringKafkaesque here and return it
        return new SpringKafkaesque();
    }
}

Next, add @ExtendWith(KafkaesqueResolver.class) annotation to your test class, and add a parameter to your test method, where you need the instance of SpringKafkaesque :

@ExtendWith(KafkaesqueResolver.class)
public class ParamInjectionTest {

    @Test
    public void testNoParams() {
        // nothing to inject
    }

    @Test
    public void testWithParam(SpringKafkaesque instance) {
        // do what you need with your instance
    }
}

No custom annotation required in this case.