Spring Cloud Stream provides support for testing your microservice applications without connecting to a messaging system. You can do that by using the TestSupportBinder provided by the spring-cloud-stream-test-support library, which can be added as a test dependency to the application, as shown in the following example:

   <dependency>
       <groupId>org.springframework.cloud</groupId>
       <artifactId>spring-cloud-stream-test-support</artifactId>
       <scope>test</scope>
   </dependency>

	Note
	The TestSupportBinder uses the Spring Boot autoconfiguration mechanism to supersede the other binders found on the classpath. Therefore, when adding a binder as a dependency, you must make sure that the test scope is being used.

The TestSupportBinder lets you interact with the bound channels and inspect any messages sent and received by the application.

For outbound message channels, the TestSupportBinder registers a single subscriber and retains the messages emitted by the application in a MessageCollector. They can be retrieved during tests and have assertions made against them.

You can also send messages to inbound message channels so that the consumer application can consume the messages. The following example shows how to test both input and output channels on a processor:

@RunWith(SpringRunner.class)
@SpringBootTest(webEnvironment= SpringBootTest.WebEnvironment.RANDOM_PORT)
public class ExampleTest {

  @Autowired
  private Processor processor;

  @Autowired
  private MessageCollector messageCollector;

  @Test
  @SuppressWarnings("unchecked")
  public void testWiring() {
    Message<String> message = new GenericMessage<>("hello");
    processor.input().send(message);
    Message<String> received = (Message<String>) messageCollector.forChannel(processor.output()).poll();
    assertThat(received.getPayload(), equalTo("hello world"));
  }


  @SpringBootApplication
  @EnableBinding(Processor.class)
  public static class MyProcessor {

    @Autowired
    private Processor channels;

    @Transformer(inputChannel = Processor.INPUT, outputChannel = Processor.OUTPUT)
    public String transform(String in) {
      return in + " world";
    }
  }
}

In the preceding example, we create an application that has an input channel and an output channel, both bound through the Processor interface. The bound interface is injected into the test so that we can have access to both channels. We send a message on the input channel, and we use the MessageCollector provided by Spring Cloud Stream’s test support to capture that the message has been sent to the output channel as a result. Once we have received the message, we can validate that the component functions correctly.

    @SpringBootApplication(exclude = TestSupportBinderAutoConfiguration.class)
    @EnableBinding(Processor.class)
    public static class MyProcessor {

        @Transformer(inputChannel = Processor.INPUT, outputChannel = Processor.OUTPUT)
        public String transform(String in) {
            return in + " world";
        }
    }