Cannot receive messages from the output channel if using mock handler in IntegrationFlow when testing

Question

For testing the following IntegrationFlow:

IntegrationFlows.from("channel.input")
            .enrich(m -> m.header(MessageHeaders.ERROR_CHANNEL, "channel.input.error"))
            .handle("handler", "handle")
            .channel("channel.output")
            .get();

I wrote a configuration class:

@Configuration
@ComponentScan
@EnableIntegration
public class ServiceFlowContext {

    @Bean(name = "handler")
    public Handler handler() {
        return Mockito.mock(Handler.class);
    }

    @Bean("channel.output")
    public QueueChannel outputChannel() {
        return new QueueChannel();
    }
}

and a test class:

@RunWith(SpringJUnit4ClassRunner.class)
@DirtiesContext
@ContextConfiguration(classes = ServiceFlowContext.class)
public class ServiceFlowTest {
    @Autowired
    private Handler handler;

    @Autowired
    @Qualifier("channel.input")
    private MessageChannel inputChannel;

    @Autowired
    @Qualifier("channel.output")
    private QueueChannel outputChannel;

    @Test
    public void shouldGetMessageInErrorChannelIfHandlerFailed() {
        Message<String> message = MessageBuilder.withPayload("empty").build();
        when(handler.handle(message)).thenReturn(message);
        inputChannel.send(message);

        Message result = outputChannel.receive(5000);
        assertThat(result).isNotNull();
    }
}

The test will wait at the receive method for 5 seconds and I will get a null object which causes the test failed. However, if I define a real object instead of the mock object, just as:

public static class Handler1 {
    public Message<String> handle(Message<String> message) {
        return message;
    }
}

@Bean(name = "handler")
public Handler1 handler() {
    return new Handler1();
}

then, I can receive the message from channel.output channel (outputChannel) just as the same as what is sent. Are there any solutions to use mock handler in the test?

Answer 1

You need to stub the handle() method.

Something like:

Handler handler = Mockito.mock(Handler.class);
BDDMockito.willAnswer(invocation -> invocation.getArgument(0))
    .given(handler).handle(any());
return handler;

That will do the same as your Handler1.handle().