You can turn a Spring application into a Spring Cloud Stream application by applying the @EnableBinding annotation to one of the application’s configuration classes. The @EnableBinding annotation itself is meta-annotated with @Configuration and triggers the configuration of Spring Cloud Stream infrastructure:

...
@Import(...)
@Configuration
@EnableIntegration
public @interface EnableBinding {
    ...
    Class<?>[] value() default {};
}

The @EnableBinding annotation can take as parameters one or more interface classes that contain methods which represent bindable components (typically message channels).

	Note
	In Spring Cloud Stream 1.0, the only supported bindable components are the Spring Messaging MessageChannel and its extensions SubscribableChannel and PollableChannel. Future versions should extend this support to other types of components, using the same mechanism. In this documentation, we will continue to refer to channels.

A Spring Cloud Stream application can have an arbitrary number of input and output channels defined in an interface as @Input and @Output methods:

public interface Barista {

    @Input
    SubscribableChannel orders();

    @Output
    MessageChannel hotDrinks();

    @Output
    MessageChannel coldDrinks();
}

Using this interface as a parameter to @EnableBinding will trigger the creation of three bound channels named orders, hotDrinks, and coldDrinks, respectively.

@EnableBinding(Barista.class)
public class CafeConfiguration {

   ...
}

public interface Barista {
    ...
    @Input("inboundOrders")
    SubscribableChannel orders();
}

public interface Source {

  String OUTPUT = "output";

  @Output(Source.OUTPUT)
  MessageChannel output();

}

public interface Sink {

  String INPUT = "input";

  @Input(Sink.INPUT)
  SubscribableChannel input();

}

public interface Processor extends Source, Sink {
}

You can write a Spring Cloud Stream application using either Spring Integration annotations or Spring Cloud Stream’s @StreamListener annotation. The @StreamListener annotation is modeled after other Spring Messaging annotations (such as @MessageMapping, @JmsListener, @RabbitListener, etc.) but adds content type management and type coercion features.

@EnableBinding(Source.class)
public class TimerSource {

  @Value("${format}")
  private String format;

  @Bean
  @InboundChannelAdapter(value = Source.OUTPUT, poller = @Poller(fixedDelay = "${fixedDelay}", maxMessagesPerPoll = "1"))
  public MessageSource<String> timerMessageSource() {
    return () -> new GenericMessage<>(new SimpleDateFormat(format).format(new Date()));
  }
}

@EnableBinding(Processor.class)
public class TransformProcessor {
  @Transformer(inputChannel = Processor.INPUT, outputChannel = Processor.OUTPUT)
  public Object transform(String message) {
    return message.toUpper();
  }
}

Using @StreamListener for Automatic Content Type Handling

Complementary to its Spring Integration support, Spring Cloud Stream provides its own @StreamListener annotation, modeled after other Spring Messaging annotations (e.g. @MessageMapping, @JmsListener, @RabbitListener, etc.). The @StreamListener annotation provides a simpler model for handling inbound messages, especially when dealing with use cases that involve content type management and type coercion.

Spring Cloud Stream provides an extensible MessageConverter mechanism for handling data conversion by bound channels and for, in this case, dispatching to methods annotated with @StreamListener. The following is an example of an application which processes external Vote events:

@EnableBinding(Sink.class)
public class VoteHandler {

  @Autowired
  VotingService votingService;

  @StreamListener(Sink.INPUT)
  public void handle(Vote vote) {
    votingService.record(vote);
  }
}

The distinction between @StreamListener and a Spring Integration @ServiceActivator is seen when considering an inbound Message that has a String payload and a contentType header of application/json. In the case of @StreamListener, the MessageConverter mechanism will use the contentType header to parse the String payload into a Vote object.

As with other Spring Messaging methods, method arguments can be annotated with @Payload, @Headers and @Header.

	Note
	For methods which return data, you must use the @SendTo annotation to specify the output binding destination for data returned by the method: @EnableBinding(Processor.class) public class TransformProcessor { @Autowired VotingService votingService; @StreamListener(Processor.INPUT) @SendTo(Processor.OUTPUT) public VoteResult handle(Vote vote) { return votingService.record(vote); } }

	Note
	In the case of RabbitMQ, content type headers can be set by external applications. Spring Cloud Stream supports them as part of an extended internal protocol used for any type of transport (including transports, such as Kafka, that do not normally support headers).

Spring Cloud Stream provides support for aggregating multiple applications together, connecting their input and output channels directly and avoiding the additional cost of exchanging messages via a broker. As of version 1.0 of Spring Cloud Stream, aggregation is supported only for the following types of applications:

They can be aggregated together by creating a sequence of interconnected applications, in which the output channel of an element in the sequence is connected to the input channel of the next element, if it exists. A sequence can start with either a source or a processor, it can contain an arbitrary number of processors and must end with either a processor or a sink.

Depending on the nature of the starting and ending element, the sequence may have one or more bindable channels, as follows:

Aggregation is performed using the AggregateApplicationBuilder utility class, as in the following example. Let’s consider a project in which we have source, processor and a sink, which may be defined in the project, or may be contained in one of the project’s dependencies.

@SpringBootApplication
@EnableBinding(Sink.class)
public class SinkApplication {

	private static Logger logger = LoggerFactory.getLogger(SinkModuleDefinition.class);

	@ServiceActivator(inputChannel=Sink.INPUT)
	public void loggerSink(Object payload) {
		logger.info("Received: " + payload);
	}
}

@SpringBootApplication
@EnableBinding(Processor.class)
public class ProcessorApplication {

	@Transformer
	public String loggerSink(String payload) {
		return payload.toUpperCase();
	}
}

@SpringBootApplication
@EnableBinding(Source.class)
public class SourceApplication {

	@Bean
	@InboundChannelAdapter(value = Source.OUTPUT)
	public String timerMessageSource() {
		return new SimpleDateFormat().format(new Date());
	}
}

Each configuration can be used for running a separate component, but in this case they can be aggregated together as follows:

@SpringBootApplication
public class SampleAggregateApplication {

	public static void main(String[] args) {
		new AggregateApplicationBuilder()
			.from(SourceApplication.class).args("--fixedDelay=5000")
			.via(ProcessorApplication.class)
			.to(SinkApplication.class).args("--debug=true").run(args);
	}
}

The starting component of the sequence is provided as argument to the from() method. The ending component of the sequence is provided as argument to the to() method. Intermediate processors are provided as argument to the via() method. Multiple processors of the same type can be chained together (e.g. for pipelining transformations with different configurations). For each component, the builder can provide runtime arguments for Spring Boot configuration.

Spring Cloud Stream provides support for RxJava-based processors through the RxJavaProcessor available in spring-cloud-stream-rxjava.

public interface RxJavaProcessor<I, O> {
	Observable<O> process(Observable<I> input);
}

An implementation of RxJavaProcessor will receive Observable as an input that represents the flow of inbound message payloads. The process method is invoked once at startup for setting up the data flow.

You can enable the use of RxJava-based processors and use them in your processor application by using the @EnableRxJavaProcessor annotation. @EnableRxJavaProcessor is meta-annotated with @EnableBinding(Processor.class) and will create the Processor binding. Here is an example of an RxJava-based processor:

@EnableRxJavaProcessor
public class RxJavaTransformer {

	private static Logger logger = LoggerFactory.getLogger(RxJavaTransformer.class);

	@Bean
	public RxJavaProcessor<String,String> processor() {
		return inputStream -> inputStream.map(data -> {
			logger.info("Got data = " + data);
			return data;
		})
		.buffer(5)
		.map(data -> String.valueOf(avg(data)));
	}

	private static Double avg(List<String> data) {
		double sum = 0;
		double count = 0;
		for(String d : data) {
			count++;
			sum += Double.valueOf(d);
		}
		return sum/count;
	}
}

	Note
	When implementing an RxJava processor, it is important to handle exceptions as part of your processing flow. Uncaught exceptions will be treated as errors by RxJava and will cause the Observable to complete, disrupting the flow.