Configuration

Spring Integration offers a number of configuration options. Which option you choose depends upon your particular needs and at what level you prefer to work. As with the Spring framework in general, you can mix and match the various techniques to suit the problem at hand. For example, you can choose the XSD-based namespace for the majority of configuration and combine it with a handful of objects that you configure with annotations. As much as possible, the two provide consistent naming. The XML elements defined by the XSD schema match the names of the annotations, and the attributes of those XML elements match the names of annotation properties. You can also use the API directly, but we expect most developers to choose one of the higher-level options or a combination of the namespace-based and annotation-driven configuration.

Namespace Support

You can configure Spring Integration components with XML elements that map directly to the terminology and concepts of enterprise integration. In many cases, the element names match those of the Enterprise Integration Patterns book.

To enable Spring Integration’s core namespace support within your Spring configuration files, add the following namespace reference and schema mapping in your top-level 'beans' element:

<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:int="http://www.springframework.org/schema/integration"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
           https://www.springframework.org/schema/beans/spring-beans.xsd
           http://www.springframework.org/schema/integration
           https://www.springframework.org/schema/integration/spring-integration.xsd">

(We have emphasized the lines that are particular to Spring Integration.)

You can choose any name after "xmlns:". We use int (short for Integration) for clarity, but you might prefer another abbreviation. On the other hand, if you use an XML editor or IDE support, the availability of auto-completion may convince you to keep the longer name for clarity. Alternatively, you can create configuration files that use the Spring Integration schema as the primary namespace, as the following example shows:

<beans:beans xmlns="http://www.springframework.org/schema/integration"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:beans="http://www.springframework.org/schema/beans"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
           https://www.springframework.org/schema/beans/spring-beans.xsd
           http://www.springframework.org/schema/integration
           https://www.springframework.org/schema/integration/spring-integration.xsd">

(We have emphasized the lines that are particular to Spring Integration.)

When using this alternative, no prefix is necessary for the Spring Integration elements. On the other hand, if you define a generic Spring bean within the same configuration file, the bean element requires a prefix (<beans:bean …​/>). Since it is generally a good idea to modularize the configuration files themselves (based on responsibility or architectural layer), you may find it appropriate to use the latter approach in the integration-focused configuration files, since generic beans are seldom necessary within those files. For the purposes of this documentation, we assume the integration namespace is the primary.

Spring Integration provides many other namespaces. In fact, each adapter type (JMS, file, and so on) that provides namespace support defines its elements within a separate schema. In order to use these elements, add the necessary namespaces with an xmlns entry and the corresponding schemaLocation mapping. For example, the following root element shows several of these namespace declarations:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:int="http://www.springframework.org/schema/integration"
  xmlns:int-file="http://www.springframework.org/schema/integration/file"
  xmlns:int-jms="http://www.springframework.org/schema/integration/jms"
  xmlns:int-mail="http://www.springframework.org/schema/integration/mail"
  xmlns:int-rmi="http://www.springframework.org/schema/integration/rmi"
  xmlns:int-ws="http://www.springframework.org/schema/integration/ws"
  xsi:schemaLocation="http://www.springframework.org/schema/beans
    https://www.springframework.org/schema/beans/spring-beans.xsd
    http://www.springframework.org/schema/integration
    https://www.springframework.org/schema/integration/spring-integration.xsd
    http://www.springframework.org/schema/integration/file
    https://www.springframework.org/schema/integration/file/spring-integration-file.xsd
    http://www.springframework.org/schema/integration/jms
    https://www.springframework.org/schema/integration/jms/spring-integration-jms.xsd
    http://www.springframework.org/schema/integration/mail
    https://www.springframework.org/schema/integration/mail/spring-integration-mail.xsd
    http://www.springframework.org/schema/integration/rmi
    https://www.springframework.org/schema/integration/rmi/spring-integration-rmi.xsd
    http://www.springframework.org/schema/integration/ws
    https://www.springframework.org/schema/integration/ws/spring-integration-ws.xsd">
 ...
</beans>

This reference manual provides specific examples of the various elements in their corresponding chapters. Here, the main thing to recognize is the consistency of the naming for each namespace URI and schema location.

Configuring the Task Scheduler

In Spring Integration, the ApplicationContext plays the central role of a message bus, and you need to consider only a couple of configuration options. First, you may want to control the central TaskScheduler instance. You can do so by providing a single bean named taskScheduler. This is also defined as a constant, as follows:

IntegrationContextUtils.TASK_SCHEDULER_BEAN_NAME

By default, Spring Integration relies on an instance of ThreadPoolTaskScheduler, as described in the Task Execution and Scheduling section of the Spring Framework reference manual. That default TaskScheduler starts up automatically with a pool of ten threads, but see Global Properties. If you provide your own TaskScheduler instance instead, you can set the 'autoStartup' property to false or provide your own pool size value.

When polling consumers provide an explicit task executor reference in their configuration, the invocation of the handler methods happens within that executor’s thread pool and not the main scheduler pool. However, when no task executor is provided for an endpoint’s poller, it is invoked by one of the main scheduler’s threads.

Do not run long-running tasks on poller threads. Use a task executor instead. If you have a lot of polling endpoints, you can cause thread starvation, unless you increase the pool size. Also, polling consumers have a default receiveTimeout of one second. Since the poller thread blocks for this time, we recommend that you use a task executor when many such endpoints exist, again to avoid starvation. Alternatively, you can reduce the receiveTimeout.
An endpoint is a Polling Consumer if its input channel is one of the queue-based (that is, pollable) channels. Event-driven consumers are those having input channels that have dispatchers instead of queues (in other words, they are subscribable). Such endpoints have no poller configuration, since their handlers are invoked directly.

When running in a JEE container, you may need to use Spring’s TimerManagerTaskScheduler, as described here, instead of the default taskScheduler. To do so, define a bean with the appropriate JNDI name for your environment, as the following example shows:

<bean id="taskScheduler" class="o.s.scheduling.commonj.TimerManagerTaskScheduler">
    <property name="timerManagerName" value="tm/MyTimerManager" />
    <property name="resourceRef" value="true" />
</bean>

The next section describes what happens if exceptions occur within the asynchronous invocations.

Global Properties

Certain global framework properties can be overridden by providing a properties file on the classpath.

The default properties can be found in /META-INF/spring.integration.default.properties in the spring-integration-core jar. You can see them on GitHub here. The following listing shows the default values:

spring.integration.channels.autoCreate=true (1)
spring.integration.channels.maxUnicastSubscribers=0x7fffffff (2)
spring.integration.channels.maxBroadcastSubscribers=0x7fffffff (3)
spring.integration.taskScheduler.poolSize=10 (4)
spring.integration.messagingTemplate.throwExceptionOnLateReply=false (5)
spring.integration.readOnly.headers= (6)
spring.integration.endpoints.noAutoStartup= (7)
spring.integration.channels.error.requireSubscribers=true (8)
1 When true, input-channel instances are automatically declared as DirectChannel instances when not explicitly found in the application context.
2 Sets the default number of subscribers allowed on, for example, a DirectChannel. It can be used to avoid inadvertently subscribing multiple endpoints to the same channel. You can override it on individual channels by setting the max-subscribers attribute.
3 This property provides the default number of subscribers allowed on, for example, a PublishSubscribeChannel. It can be used to avoid inadvertently subscribing more than the expected number of endpoints to the same channel. You can override it on individual channels by setting the max-subscribers attribute.
4 The number of threads available in the default taskScheduler bean. See Configuring the Task Scheduler.
5 When true, messages that arrive at a gateway reply channel throw an exception when the gateway is not expecting a reply (because the sending thread has timed out or already received a reply).
6 A comma-separated list of message header names that should not be populated into Message instances during a header copying operation. The list is used by the DefaultMessageBuilderFactory bean and propagated to the IntegrationMessageHeaderAccessor instances (see MessageHeaderAccessor API) used to build messages via MessageBuilder (see The MessageBuilder Helper Class). By default, only MessageHeaders.ID and MessageHeaders.TIMESTAMP are not copied during message building. Since version 4.3.2.
7 A comma-separated list of AbstractEndpoint bean names patterns (xxx*, xxx, *xxx or xxx*yyy) that should not be started automatically during application startup. You can manually start these endpoints later by their bean name through a Control Bus (see Control Bus), by their role with the SmartLifecycleRoleController (see Endpoint Roles), or by Lifecycle bean injection. You can explicitly override the effect of this global property by specifying auto-startup XML annotation or the autoStartup annotation attribute or by calling AbstractEndpoint.setAutoStartup() in the bean definition. Since version 4.3.12.
8 A boolean flag to indicate that default global errorChannel must be configured with the requireSubscribers option. Since version 5.4.3. See Error Handling for more information.

These properties can be overridden by adding a /META-INF/spring.integration.properties file to the classpath. You need not provide all the properties — only those that you want to override.

Starting with version 5.1, all the merged global properties are printed in the logs after application context startup when a DEBUG logic level is turned on for the org.springframework.integration category. The output looks like this:

Spring Integration global properties:

spring.integration.endpoints.noAutoStartup=fooService*
spring.integration.taskScheduler.poolSize=20
spring.integration.channels.maxUnicastSubscribers=0x7fffffff
spring.integration.channels.autoCreate=true
spring.integration.channels.maxBroadcastSubscribers=0x7fffffff
spring.integration.readOnly.headers=
spring.integration.messagingTemplate.throwExceptionOnLateReply=true

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 of 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 (see 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) {
    ...
}

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.

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.

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:

@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.

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:

@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 MessageHandler 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 followig example shows how to use that combination:

@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).

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.

  • The AbstractEndpoint bean name is generated with the following pattern: [configurationComponentName].[methodName].[decapitalizedAnnotationClassShortName]. For example, the SourcePollingChannelAdapter endpoint for the consoleSource() definition shown earlier gets a bean name of myFlowConfiguration.consoleSource.inboundChannelAdapter. See also Endpoint Bean Names.

When using these annotations on @Bean definitions, the inputChannel must reference a declared bean. Channels are not automatically declared in this case.

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:

@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.

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:

@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.

Advising Annotated Endpoints

See Advising Endpoints Using Annotations.

Message Mapping Rules and Conventions

Spring Integration implements a flexible facility to map messages to methods and their arguments without providing extra configuration, by relying on some default rules and defining certain conventions. The examples in the following sections articulate the rules.

Sample Scenarios

The following example shows a single un-annotated parameter (object or primitive) that is not a Map or a Properties object with a non-void return type:

public String doSomething(Object o);

The input parameter is a message payload. If the parameter type is not compatible with a message payload, an attempt is made to convert it by using a conversion service provided by Spring 3.0. The return value is incorporated as a payload of the returned message.

The following example shows a single un-annotated parameter (object or primitive)that is not a Map or a Properties with a Message return type:

public Message doSomething(Object o);

The input parameter is a message payload. If the parameter type is not compatible with a message payload, an attempt is made to convert it by using a conversion service provided by Spring 3.0. The return value is a newly constructed message that is sent to the next destination.

The followig example shows a single parameter that is a message (or one of its subclasses) with an arbitrary object or primitive return type:

public int doSomething(Message  msg);

The input parameter is itself a Message. The return value becomes a payload of the Message that is sent to the next destination.

The following example shows a single parameter that is a Message (or one of its subclasses) with a Message (or one of its subclasses) as the return type:

public Message doSomething(Message msg);

The input parameter is itself a Message. The return value is a newly constructed Message that is sent to the next destination.

The following example shows a single parameter of type Map or Properties with a Message as the return type:

public Message doSomething(Map m);

This one is a bit interesting. Although, at first, it might seem like an easy mapping straight to message headers, preference is always given to a Message payload. This means that if a Message payload is of type Map, this input argument represents a Message payload. However, if the Message payload is not of type Map, the conversion service does not try to convert the payload, and the input argument is mapped to message headers.

The following example shows two parameters, where one of them is an arbitrary type (an object or a primitive) that is not a Map or a Properties object and the other is of type Map or Properties type (regardless of the return):

public Message doSomething(Map h, <T> t);

This combination contains two input parameters where one of them is of type Map. The non-Map parameters (regardless of the order) are mapped to a Message payload and the Map or Properties (regardless of the order) is mapped to message headers, giving you a nice POJO way of interacting with Message structure.

The following example shows no parameters (regardless of the return):

public String doSomething();

This message handler method is invoked based on the Message sent to the input channel to which this handler is connected. However no Message data is mapped, thus making the Message act as event or trigger to invoke the handler. The output is mapped according to the rules described earlier.

The following example shows no parameters and a void return:

public void soSomething();

This example is the same as the previous example, but it produces no output.

Annotation-based Mapping

Annotation-based mapping is the safest and least ambiguous approach to map messages to methods. The following example shows how to explicitly map a method to a header:

public String doSomething(@Payload String s, @Header("someheader") String b)

As you can see later on, without an annotation this signature would result in an ambiguous condition. However, by explicitly mapping the first argument to a Message payload and the second argument to a value of the someheader message header, we avoid any ambiguity.

The following example is nearly identical to the preceding example:

public String doSomething(@Payload String s, @RequestParam("something") String b)

@RequestMapping or any other non-Spring Integration mapping annotation is irrelevant and is therefore ignored, leaving the second parameter unmapped. Although the second parameter could easily be mapped to a payload, there can only be one payload. Therefore, the annotations keep this method from being ambiguous.

The following example shows another similar method that would be ambiguous were it not for annotations to clarify the intent:

public String foo(String s, @Header("foo") String b)

The only difference is that the first argument is implicitly mapped to the message payload.

The following example shows yet another signature that would definitely be treated as ambiguous without annotations, because it has more than two arguments:

public String soSomething(@Headers Map m, @Header("something") Map f, @Header("someotherthing") String bar)

This example would be especially problematic, because two of its arguments are Map instances. However, with annotation-based mapping, the ambiguity is easily avoided. In this example the first argument is mapped to all the message headers, while the second and third argument map to the values of the message headers named 'something' and 'someotherthing'. The payload is not being mapped to any argument.

Complex Scenarios

The following example uses multiple parameters:

Multiple parameters can create a lot of ambiguity with regards to determining the appropriate mappings. The general advice is to annotate your method parameters with @Payload, @Header, and @Headers. The examples in this section show ambiguous conditions that result in an exception being raised.

public String doSomething(String s, int i)

The two parameters are equal in weight. Therefore, there is no way to determine which one is a payload.

The following example shows a similar problem, only with three parameters:

public String foo(String s, Map m, String b)

Although the Map could be easily mapped to message headers, there is no way to determine what to do with the two String parameters.

The following example shows another ambiguous method:

public String foo(Map m, Map f)

Although one might argue that one Map could be mapped to the message payload and the other one to the message headers, we cannot rely on the order.

Any method signature with more than one method argument that is not (Map, <T>) and with unannotated parameters results in an ambiguous condition and triggers an exception.

The next set of examples each show mutliple methods that result in ambiguity.

Message handlers with multiple methods are mapped based on the same rules that are described earlier (in the examples). However, some scenarios might still look confusing.

The following example shows multiple methods with legal (mappable and unambiguous) signatures:

public class Something {
    public String doSomething(String str, Map m);

    public String doSomething(Map m);
}

(Whether the methods have the same name or different names makes no difference). The Message could be mapped to either method. The first method would be invoked when the message payload could be mapped to str and the message headers could be mapped to m. The second method could also be a candidate by mapping only the message headers to m. To make matters worse, both methods have the same name. At first, that might look ambiguous because of the following configuration:

<int:service-activator input-channel="input" output-channel="output" method="doSomething">
    <bean class="org.things.Something"/>
</int:service-activator>

It works because mappings are based on the payload first and everything else next. In other words, the method whose first argument can be mapped to a payload takes precedence over all other methods.

Now consider an alternate example, which produces a truly ambiguous condition:

public class Something {
    public String doSomething(String str, Map m);

    public String doSomething(String str);
}

Both methods have signatures that could be mapped to a message payload. They also have the same name. Such handler methods will trigger an exception. However, if the method names were different, you could influence the mapping with a method attribute (shown in the next example). The following example shows the same example with two different method names:

public class Something {
    public String doSomething(String str, Map m);

    public String doSomethingElse(String str);
}

The following example shows how to use the method attribute to dictate the mapping:

<int:service-activator input-channel="input" output-channel="output" method="doSomethingElse">
    <bean class="org.bar.Foo"/>
</int:service-activator>

Because the configuration explicitly maps the doSomethingElse method, we have eliminated the ambiguity.