Messaging Meta-Annotations

Starting with version 4.0, all messaging annotations can be configured as meta-annotations and all user-defined messaging annotations can define the same attributes to override their default values. In addition, meta-annotations can be configured hierarchically, as the following example shows:spring-doc.cn

@Target({ElementType.METHOD, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@ServiceActivator(inputChannel = "annInput", outputChannel = "annOutput")
public @interface MyServiceActivator {

    String[] adviceChain = { "annAdvice" };
}

@Target({ElementType.METHOD, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@MyServiceActivator
public @interface MyServiceActivator1 {

    String inputChannel();

    String outputChannel();
}
...

@MyServiceActivator1(inputChannel = "inputChannel", outputChannel = "outputChannel")
public Object service(Object payload) {
   ...
}

Configuring meta-annotations hierarchically lets users set defaults for various attributes and enables isolation of framework Java dependencies to user annotations, avoiding their use in user classes. If the framework finds a method with a user annotation that has a framework meta-annotation, it is treated as if the method were annotated directly with the framework annotation.spring-doc.cn

Annotations on @Bean Methods

Starting with version 4.0, you can configure messaging annotations on @Bean method definitions in @Configuration classes, to produce message endpoints based on the beans, not the methods. It is useful when @Bean definitions are “out-of-the-box” MessageHandler instances (AggregatingMessageHandler, DefaultMessageSplitter, and others), Transformer instances (JsonToObjectTransformer, ClaimCheckOutTransformer, and others), and MessageSource instances (FileReadingMessageSource, RedisStoreMessageSource, and others). The following example shows how to use messaging annotations with @Bean annotations:spring-doc.cn

@Configuration
@EnableIntegration
public class MyFlowConfiguration {

    @Bean
    @InboundChannelAdapter(value = "inputChannel", poller = @Poller(fixedDelay = "1000"))
    public MessageSource<String> consoleSource() {
        return CharacterStreamReadingMessageSource.stdin();
    }

    @Bean
    @Transformer(inputChannel = "inputChannel", outputChannel = "httpChannel")
    public ObjectToMapTransformer toMapTransformer() {
        return new ObjectToMapTransformer();
    }

    @Bean
    @ServiceActivator(inputChannel = "httpChannel")
    public HttpRequestExecutingMessageHandler httpHandler() {
    HttpRequestExecutingMessageHandler handler = new HttpRequestExecutingMessageHandler("https://foo/service");
        handler.setExpectedResponseType(String.class);
        handler.setOutputChannelName("outputChannel");
        return handler;
    }

    @Bean
    @ServiceActivator(inputChannel = "outputChannel")
    public LoggingHandler loggingHandler() {
        return new LoggingHandler("info");
    }

}

Version 5.0 introduced support for a @Bean annotated with @InboundChannelAdapter that returns java.util.function.Supplier, which can produce either a POJO or a Message. The following example shows how to use that combination:spring-doc.cn

@Configuration
@EnableIntegration
public class MyFlowConfiguration {

    @Bean
    @InboundChannelAdapter(value = "inputChannel", poller = @Poller(fixedDelay = "1000"))
    public Supplier<String> pojoSupplier() {
        return () -> "foo";
    }

    @Bean
    @InboundChannelAdapter(value = "inputChannel", poller = @Poller(fixedDelay = "1000"))
    public Supplier<Message<String>> messageSupplier() {
        return () -> new GenericMessage<>("foo");
    }
}

The meta-annotation rules work on @Bean methods as well (the @MyServiceActivator annotation described earlier can be applied to a @Bean definition).spring-doc.cn

When you use these annotations on consumer @Bean definitions, if the bean definition returns an appropriate MessageHandler (depending on the annotation type), you must set attributes (such as outputChannel, requiresReply, order, and others), on the MessageHandler @Bean definition itself. Only the following annotation attributes are used: adviceChain, autoStartup, inputChannel, phase, and poller. All other attributes are for the handler.
The bean names are generated with the following algorithm:

  • The MessageHandler (MessageSource) @Bean gets its own standard name from the method name or name attribute on the @Bean. This works as though there were no messaging annotation on the @Bean method.spring-doc.cn

  • The AbstractEndpoint bean name is generated with the following pattern: [@Bean name].[decapitalizedAnnotationClassShortName]. For example, the SourcePollingChannelAdapter endpoint for the consoleSource() definition shown earlier gets a bean name of consoleSource.inboundChannelAdapter. Unlike with POJO methods, the bean method name is not included in the endpoint bean name. See also Endpoint Bean Names.spring-doc.cn

  • If @Bean cannot be used directly in the target endpoint (not an instance of a MessageSource, AbstractReplyProducingMessageHandler or AbstractMessageRouter), a respective AbstractStandardMessageHandlerFactoryBean is registered to delegate to this @Bean. The bean name for this wrapper is generated with the following pattern: [@Bean name].[decapitalizedAnnotationClassShortName].[handler (or source)].spring-doc.cn

When using these annotations on @Bean definitions, the inputChannel must reference a declared bean. Channels are automatically declared if not present in the application context yet.

With Java configuration, you can use any @Conditional (for example, @Profile) definition on the @Bean method level to skip the bean registration for some conditional reason. The following example shows how to do so:spring-doc.cn

@Bean
@ServiceActivator(inputChannel = "skippedChannel")
@Profile("thing")
public MessageHandler skipped() {
    return System.out::println;
}

Together with the existing Spring container logic, the messaging endpoint bean (based on the @ServiceActivator annotation), is also not registered.spring-doc.cn

Creating a Bridge with Annotations

Starting with version 4.0, Java configuration provides the @BridgeFrom and @BridgeTo @Bean method annotations to mark MessageChannel beans in @Configuration classes. These really exists for completeness, providing a convenient mechanism to declare a BridgeHandler and its message endpoint configuration:spring-doc.cn

@Bean
public PollableChannel bridgeFromInput() {
    return new QueueChannel();
}

@Bean
@BridgeFrom(value = "bridgeFromInput", poller = @Poller(fixedDelay = "1000"))
public MessageChannel bridgeFromOutput() {
    return new DirectChannel();
}
@Bean
public QueueChannel bridgeToOutput() {
    return new QueueChannel();
}

@Bean
@BridgeTo("bridgeToOutput")
public MessageChannel bridgeToInput() {
    return new DirectChannel();
}

You can use these annotations as meta-annotations as well.spring-doc.cn

Advising Annotated Endpoints

See Advising Endpoints Using Annotations.spring-doc.cn