Annotation Support

In addition to the XML namespace support for configuring message endpoints, you can also use annotations. First, Spring Integration provides the class-level @MessageEndpoint as a stereotype annotation, meaning that it is itself annotated with Spring’s @Component annotation and is therefore automatically recognized as a bean definition by Spring’s component scanning.

Even more important are the various method-level annotations. They indicate that the annotated method is capable of handling a message. The following example demonstrates both class-level and method-level annotations:

@MessageEndpoint
public class FooService {

    @ServiceActivator
    public void processMessage(Message message) {
        ...
    }
}

Exactly what it means for the method to “handle” the Message depends on the particular annotation. Annotations available in Spring Integration include:

If you use XML configuration in combination with annotations, the @MessageEndpoint annotation is not required. If you want to configure a POJO reference from the ref attribute of a <service-activator/> element, you can provide only the method-level annotations. In that case, the annotation prevents ambiguity even when no method-level attribute exists on the <service-activator/> element.

In most cases, the annotated handler method should not require the Message type as its parameter. Instead, the method parameter type can match the message’s payload type, as the following example shows:

public class ThingService {

    @ServiceActivator
    public void bar(Thing thing) {
        ...
    }

}

When the method parameter should be mapped from a value in the MessageHeaders, another option is to use the parameter-level @Header annotation. In general, methods annotated with the Spring Integration annotations can accept the Message itself, the message payload, or a header value (with @Header) as the parameter. In fact, the method can accept a combination, as the following example shows:

public class ThingService {

    @ServiceActivator
    public void otherThing(String payload, @Header("x") int valueX, @Header("y") int valueY) {
        ...
    }

}

You can also use the @Headers annotation to provide all the message headers as a Map, as the following example shows:

public class ThingService {

    @ServiceActivator
    public void otherThing(String payload, @Headers Map<String, Object> headerMap) {
        ...
    }

}
The value of the annotation can also be a SpEL expression (for example, someHeader.toUpperCase()), which is useful when you wish to manipulate the header value before injecting it. It also provides an optional required property, which specifies whether the attribute value must be available within the headers. The default value for the required property is true.

For several of these annotations, when a message-handling method returns a non-null value, the endpoint tries to send a reply. This is consistent across both configuration options (namespace and annotations) in that such an endpoint’s output channel is used (if available), and the REPLY_CHANNEL message header value is used as a fallback.

The combination of output channels on endpoints and the reply channel message header enables a pipeline approach, where multiple components have an output channel and the final component allows the reply message to be forwarded to the reply channel (as specified in the original request message). In other words, the final component depends on the information provided by the original sender and can dynamically support any number of clients as a result. This is an example of the return address pattern.

In addition to the examples shown here, these annotations also support the inputChannel and outputChannel properties, as the following example shows:

@Service
public class ThingService {

    @ServiceActivator(inputChannel="input", outputChannel="output")
    public void otherThing(String payload, @Headers Map<String, Object> headerMap) {
        ...
    }

}

The processing of these annotations creates the same beans as the corresponding XML components — AbstractEndpoint instances and MessageHandler instances (or MessageSource instances for the inbound channel adapter). See Annotations on @Bean Methods. The bean names are generated from the following pattern: [componentName].[methodName].[decapitalizedAnnotationClassShortName]. In the preceding example the bean name is thingService.otherThing.serviceActivator for the AbstractEndpoint and the same name with an additional .handler (.source) suffix for the MessageHandler (MessageSource) bean. Such a name can be customized using an @EndpointId annotation alongside with these messaging annotations. The MessageHandler instances (MessageSource instances) are also eligible to be tracked by the message history.

Starting with version 4.0, all messaging annotations provide SmartLifecycle options (autoStartup and phase) to allow endpoint lifecycle control on application context initialization. They default to true and 0, respectively. To change the state of an endpoint (such as start() or stop()), you can obtain a reference to the endpoint bean by using the BeanFactory (or autowiring) and invoke the methods. Alternatively, you can send a command message to the Control Bus. For these purposes, you should use the beanName mentioned earlier in the preceding paragraph.

Channels automatically created after parsing the mentioned annotations (when no specific channel bean is configured), and the corresponding consumer endpoints, are declared as beans near the end of the context initialization. These beans can be autowired in other services, but they have to be marked with the @Lazy annotation because the definitions, typically, won’t yet be available during normal autowiring processing.

@Autowired
@Lazy
@Qualifier("someChannel")
MessageChannel someChannel;
...

@Bean
Thing1 dependsOnSPCA(@Qualifier("someInboundAdapter") @Lazy SourcePollingChannelAdapter someInboundAdapter) {
    ...
}

Starting with version 6.0, all the messaging annotations are @Repeatable now, so several of the same type can be declared on the same service method with the meaning to create as many endpoints as those annotations are repeated:

@Transformer(inputChannel = "inputChannel1", outputChannel = "outputChannel1")
@Transformer(inputChannel = "inputChannel2", outputChannel = "outputChannel2")
public String transform(String input) {
    return input.toUpperCase();
}

Using the @Poller Annotation

Before Spring Integration 4.0, messaging annotations required that the inputChannel be a reference to a SubscribableChannel. For PollableChannel instances, an <int:bridge/> element was needed to configure an <int:poller/> and make the composite endpoint be a PollingConsumer. Version 4.0 introduced the @Poller annotation to allow the configuration of poller attributes directly on the messaging annotations, as the following example shows:

public class AnnotationService {

    @Transformer(inputChannel = "input", outputChannel = "output",
        poller = @Poller(maxMessagesPerPoll = "${poller.maxMessagesPerPoll}", fixedDelay = "${poller.fixedDelay}"))
    public String handle(String payload) {
        ...
    }
}

The @Poller annotation provides only simple PollerMetadata options. You can configure the @Poller annotation’s attributes (maxMessagesPerPoll, fixedDelay, fixedRate, and cron) with property placeholders. Also, starting with version 5.1, the receiveTimeout option for PollingConsumer s is also provided. If it is necessary to provide more polling options (for example, transaction, advice-chain, error-handler, and others), you should configure the PollerMetadata as a generic bean and use its bean name as the @Poller 's value attribute. In this case, no other attributes are allowed (they must be specified on the PollerMetadata bean). Note, if inputChannel is a PollableChannel and no @Poller is configured, the default PollerMetadata is used (if it is present in the application context). To declare the default poller by using a @Configuration annotation, use code similar to the following example:

@Bean(name = PollerMetadata.DEFAULT_POLLER)
public PollerMetadata defaultPoller() {
    PollerMetadata pollerMetadata = new PollerMetadata();
    pollerMetadata.setTrigger(new PeriodicTrigger(10));
    return pollerMetadata;
}

The following example shows how to use the default poller:

public class AnnotationService {

    @Transformer(inputChannel = "aPollableChannel", outputChannel = "output")
    public String handle(String payload) {
        ...
    }
}

The following example shows how to use a named poller:

@Bean
public PollerMetadata myPoller() {
    PollerMetadata pollerMetadata = new PollerMetadata();
    pollerMetadata.setTrigger(new PeriodicTrigger(1000));
    return pollerMetadata;
}

The following example shows an endpoint that uses the default poller:

public class AnnotationService {

    @Transformer(inputChannel = "aPollableChannel", outputChannel = "output"
                           poller = @Poller("myPoller"))
    public String handle(String payload) {
         ...
    }
}

Starting with version 4.3.3, the @Poller annotation has the errorChannel attribute for easier configuration of the underlying MessagePublishingErrorHandler. This attribute plays the same role as error-channel in the <poller> XML component. See Endpoint Namespace Support for more information.

The poller() attribute on the messaging annotations is mutually exclusive with the reactive() attribute. See next section for more information.

Using @Reactive Annotation

The ReactiveStreamsConsumer has been around since version 5.0, but it was applied only when an input channel for the endpoint is a FluxMessageChannel (or any org.reactivestreams.Publisher implementation). Starting with version 5.3, its instance is also created by the framework when the target message handler is a ReactiveMessageHandler independently of the input channel type. The @Reactive sub-annotation (similar to mentioned above @Poller) has been introduced for all the messaging annotations starting with version 5.5. It accepts an optional Function<? super Flux<Message<?>>, ? extends Publisher<Message<?>>> bean reference and, independently of the input channel type and message handler, turns the target endpoint into the ReactiveStreamsConsumer instance. The function is used from the Flux.transform() operator to apply some customization (publishOn(), doOnNext(), log(), retry() etc.) on a reactive stream source from the input channel.

The following example demonstrates how to change the publishing thread from the input channel independently of the final subscriber and producer to that DirectChannel:

@Bean
public Function<Flux<?>, Flux<?>> publishOnCustomizer() {
    return flux -> flux.publishOn(Schedulers.parallel());
}

@ServiceActivator(inputChannel = "directChannel", reactive = @Reactive("publishOnCustomizer"))
public void handleReactive(String payload) {
    ...
}

The reactive() attribute on the messaging annotations is mutually exclusive with the poller() attribute. See Using the @Poller Annotation and Reactive Streams Support for more information.

Using the @InboundChannelAdapter Annotation

Version 4.0 introduced the @InboundChannelAdapter method-level annotation. It produces a SourcePollingChannelAdapter integration component based on a MethodInvokingMessageSource for the annotated method. This annotation is an analogue of the <int:inbound-channel-adapter> XML component and has the same restrictions: The method cannot have parameters, and the return type must not be void. It has two attributes: value (the required MessageChannel bean name) and poller (an optional @Poller annotation, as described earlier). If you need to provide some MessageHeaders, use a Message<?> return type and use a MessageBuilder to build the Message<?>. Using a MessageBuilder lets you configure the MessageHeaders. The following example shows how to use an @InboundChannelAdapter annotation:

@InboundChannelAdapter("counterChannel")
public Integer count() {
    return this.counter.incrementAndGet();
}

@InboundChannelAdapter(value = "fooChannel", poller = @Poller(fixed-rate = "5000"))
public String foo() {
    return "foo";
}

Version 4.3 introduced the channel alias for the value annotation attribute, to provide better source code readability. Also, the target MessageChannel bean is resolved in the SourcePollingChannelAdapter by the provided name (set by the outputChannelName option) on the first receive() call, not during the initialization phase. It allows “late binding” logic: The target MessageChannel bean from the consumer perspective is created and registered a bit later than the @InboundChannelAdapter parsing phase.

The first example requires that the default poller has been declared elsewhere in the application context.

Using the @MessagingGateway Annotation

See @MessagingGateway Annotation.

Using the @IntegrationComponentScan Annotation

The standard Spring Framework @ComponentScan annotation does not scan interfaces for stereotype @Component annotations. To overcome this limitation and allow the configuration of @MessagingGateway (see @MessagingGateway Annotation), we introduced the @IntegrationComponentScan mechanism. This annotation must be placed with a @Configuration annotation and be customized to define its scanning options, such as basePackages and basePackageClasses. In this case, all discovered interfaces annotated with @MessagingGateway are parsed and registered as GatewayProxyFactoryBean instances. All other class-based components are parsed by the standard @ComponentScan.