Spring Integration’s MessageHandler interface is implemented by many of the components within the framework. In other words, this is not part of the public API, and you would not typically implement MessageHandler directly. Nevertheless, it is used by a message consumer for actually handling the consumed messages, so being aware of this strategy interface does help in terms of understanding the overall role of a consumer. The interface is defined as follows:

Despite its simplicity, this interface provides the foundation for most of the components (routers, transformers, splitters, aggregators, service activators, and others) covered in the following chapters. Those components each perform very different functionality with the messages they handle, but the requirements for actually receiving a message are the same, and the choice between polling and event-driven behavior is also the same. Spring Integration provides two endpoint implementations that host these callback-based handlers and let them be connected to message channels.

Because it is the simpler of the two, we cover the event-driven consumer endpoint first. You may recall that the SubscribableChannel interface provides a subscribe() method and that the method accepts a MessageHandler parameter (as shown in SubscribableChannel). The following listing shows the definition of the subscribe method:

Since a handler that is subscribed to a channel does not have to actively poll that channel, this is an event-driven consumer, and the implementation provided by Spring Integration accepts a SubscribableChannel and a MessageHandler, as the following example shows:

Spring Integration also provides a PollingConsumer, and it can be instantiated in the same way except that the channel must implement PollableChannel, as the following example shows:

There are many other configuration options for the polling consumer. For example, the trigger is a required property. The following example shows how to set the trigger:

Spring Integration currently provides two implementations of the Trigger interface: IntervalTrigger and CronTrigger. The IntervalTrigger is typically defined with a simple interval (in milliseconds) but also supports an initialDelay property and a boolean fixedRate property (the default is false — that is, no fixed delay). The following example sets both properties:

The result of the three settings in the preceding example is a trigger that waits five seconds and then triggers every second.

The CronTrigger requires a valid cron expression. See the Javadoc for details. The following example sets a new CronTrigger:

The result of the trigger defined in the previous example is a trigger that triggers every ten seconds, Monday through Friday.

In addition to the trigger, you can specify two other polling-related configuration properties: maxMessagesPerPoll and receiveTimeout. The following example shows how to set these two properties:

The maxMessagesPerPoll property specifies the maximum number of messages to receive within a given poll operation. This means that the poller continues calling receive() without waiting, until either null is returned or the maximum value is reached. For example, if a poller has a ten-second interval trigger and a maxMessagesPerPoll setting of 25, and it is polling a channel that has 100 messages in its queue, all 100 messages can be retrieved within 40 seconds. It grabs 25, waits ten seconds, grabs the next 25, and so on.

The receiveTimeout property specifies the amount of time the poller should wait if no messages are available when it invokes the receive operation. For example, consider two options that seem similar on the surface but are actually quite different: The first has an interval trigger of 5 seconds and a receive timeout of 50 milliseconds, while the second has an interval trigger of 50 milliseconds and a receive timeout of 5 seconds. The first one may receive a message up to 4950 milliseconds later than it arrived on the channel (if that message arrived immediately after one of its poll calls returned). On the other hand, the second configuration never misses a message by more than 50 milliseconds. The difference is that the second option requires a thread to wait. However, as a result, it can respond much more quickly to arriving messages. This technique, known as “long polling”, can be used to emulate event-driven behavior on a polled source.

A polling consumer can also delegate to a Spring TaskExecutor, as the following example shows:

Furthermore, a PollingConsumer has a property called adviceChain. This property lets you to specify a List of AOP advices for handling additional cross cutting concerns including transactions. These advices are applied around the doPoll() method. For more in-depth information, see the sections on AOP advice chains and transaction support under Endpoint Namespace Support.

The earlier examples show dependency lookups. However, keep in mind that these consumers are most often configured as Spring bean definitions. In fact, Spring Integration also provides a FactoryBean called ConsumerEndpointFactoryBean that creates the appropriate consumer type based on the type of channel. Also, Spring Integration has full XML namespace support to even further hide those details. The namespace-based configuration is in this guide featured as each component type is introduced.

Throughout this reference manual, you can find specific configuration examples for endpoint elements, such as router, transformer, service-activator, and so on. Most of these support an input-channel attribute and many support an output-channel attribute. After being parsed, these endpoint elements produce an instance of either the PollingConsumer or the EventDrivenConsumer, depending on the type of the input-channel that is referenced: PollableChannel or SubscribableChannel, respectively. When the channel is pollable, the polling behavior is based on the endpoint element’s poller sub-element and its attributes.

The following listing lists all of the available configuration options for a poller:

When configuring a poller with a fixed-delay or a fixed-rate attribute, the default implementation uses a PeriodicTrigger instance. The PeriodicTrigger is part of the core Spring Framework. It accepts the interval only as a constructor argument. Therefore, it cannot be changed at runtime.

However, you can define your own implementation of the org.springframework.scheduling.Trigger interface. You could even use the PeriodicTrigger as a starting point. Then you can add a setter for the interval (period), or you can even embed your own throttling logic within the trigger itself. The period property is used with each call to nextExecutionTime to schedule the next poll. To use this custom trigger within pollers, declare the bean definition of the custom trigger in your application context and inject the dependency into your poller configuration by using the trigger attribute, which references the custom trigger bean instance. You can now obtain a reference to the trigger bean and change the polling interval between polls.

For an example, see the Spring Integration Samples project. It contains a sample called dynamic-poller, which uses a custom trigger and demonstrates the ability to change the polling interval at runtime.

The sample provides a custom trigger that implements the org.springframework.scheduling.Trigger interface. The sample’s trigger is based on Spring’s PeriodicTrigger implementation. However, the fields of the custom trigger are not final, and the properties have explicit getters and setters, letting you dynamically change the polling period at runtime.

Throughout this reference manual, you can also see specific configuration and implementation examples of various endpoints that accept a message or any arbitrary Object as an input parameter. In the case of an Object, such a parameter is mapped to a message payload or part of the payload or header (when using the Spring Expression Language). However, the type of input parameter of the endpoint method sometimes does not match the type of the payload or its part. In this scenario, we need to perform type conversion. Spring Integration provides a convenient way for registering type converters (by using the Spring ConversionService) within its own instance of a conversion service bean named integrationConversionService. That bean is automatically created as soon as the first converter is defined by using the Spring Integration infrastructure. To register a converter, you can implement org.springframework.core.convert.converter.Converter, org.springframework.core.convert.converter.GenericConverter, or org.springframework.core.convert.converter.ConverterFactory.

The Converter implementation is the simplest and converts from a single type to another. For more sophistication, such as converting to a class hierarchy, you can implement a GenericConverter and possibly a ConditionalConverter. These give you complete access to the from and to type descriptors, enabling complex conversions. For example, if you have an abstract class called Something that is the target of your conversion (parameter type, channel data type, and so on), you have two concrete implementations called Thing1 and Thing, and you wish to convert to one or the other based on the input type, the GenericConverter would be a good fit. For more information, see the Javadoc for these interfaces:

When you have implemented your converter, you can register it with convenient namespace support, as the following example shows:

Alternately, you can use an inner bean, as the following example shows:

Starting with Spring Integration 4.0, you can use annotations to create the preceding configuration, as the following example shows:

Alternately, you can use the @Configuration annotation, as the following example shows:

Starting with version 5.0, by default, the method invocation mechanism is based on the org.springframework.messaging.handler.invocation.InvocableHandlerMethod infrastructure. Its HandlerMethodArgumentResolver implementations (such as PayloadArgumentResolver and MessageMethodArgumentResolver) can use the MessageConverter abstraction to convert an incoming payload to the target method argument type. The conversion can be based on the contentType message header. For this purpose, Spring Integration provides the ConfigurableCompositeMessageConverter, which delegates to a list of registered converters to be invoked until one of them returns a non-null result. By default, this converter provides (in strict order):

See the Javadoc (linked in the preceding list) for more information about their purpose and appropriate contentType values for conversion. The ConfigurableCompositeMessageConverter is used because it can be be supplied with any other MessageConverter implementations, including or excluding the previously mentioned default converters. It can also be registered as an appropriate bean in the application context, overriding the default converter, as the following example shows:

Those two new converters are registered in the composite before the defaults. You can also not use a ConfigurableCompositeMessageConverter but provide your own MessageConverter by registering a bean with the name, integrationArgumentResolverMessageConverter (by setting the IntegrationContextUtils.ARGUMENT_RESOLVER_MESSAGE_CONVERTER_BEAN_NAME property).

If you want the polling to be asynchronous, a poller can optionally specify a task-executor attribute that points to an existing instance of any TaskExecutor bean (Spring 3.0 provides a convenient namespace configuration through the task namespace). However, there are certain things you must understand when configuring a poller with a TaskExecutor. 

The problem is that there are two configurations in place, the poller and the TaskExecutor. They must be in tune with each other. Otherwise, you might end up creating an artificial memory leak.

Consider the following configuration:

The preceding configuration demonstrates an out-of-tune configuration.

By default, the task executor has an unbounded task queue. The poller keeps scheduling new tasks even though all the threads are blocked, waiting for either a new message to arrive or the timeout to expire. Given that there are 20 threads executing tasks with a five-second timeout, they aree executed at a rate of 4 per second. However, new tasks are being scheduled at a rate of 20 per second, so the internal queue in the task executor grows at a rate of 16 per second (while the process is idle), so we have a memory leak.

One of the ways to handle this is to set the queue-capacity attribute of the task executor. Even 0 is a reasonable value. You can also manage it by specifying what to do with messages that can not be queued by setting the rejection-policy attribute of the Task Executor (for example, to DISCARD). In other words, there are certain details you must understand when configuring TaskExecutor. See “Task Execution and Scheduling” in the Spring reference manual for more detail on the subject.

Many endpoints are composite beans. This includes all consumers and all polled inbound channel adapters. Consumers (polled or event-driven) delegate to a MessageHandler. Polled adapters obtain messages by delegating to a MessageSource. Often, it is useful to obtain a reference to the delegate bean, perhaps to change configuration at runtime or for testing. These beans can be obtained from the ApplicationContext with well known names. MessageHandler instances are registered with the application context with bean IDs similar to someConsumer.handler (where 'consumer' is the value of the endpoint’s id attribute). MessageSource instances are registered with bean IDs similar to somePolledAdapter.source, where 'somePolledAdapter' is the ID of the adapter.

The preceding only applies to the framework component itself. You can instead use an inner bean definition, as the following example shows:

The bean is treated like any inner bean declared and is not registered with the application context. If you wish to access this bean in some other manner, declare it at the top level with an id and use the ref attribute instead. See the Spring Documentation for more information.

As mentioned earlier, it would be great to have no dependency on the Spring Integration API — including the gateway class. For that reason, Spring Integration provides the GatewayProxyFactoryBean, which generates a proxy for any interface and internally invokes the gateway methods shown below. By using dependency injection, you can then expose the interface to your business methods.

The following example shows an interface that can be used to interact with Spring Integration:

Namespace support is also provided. It lets you configure an interface as a service, as the following example shows:

With this configuration defined, the cafeService can now be injected into other beans, and the code that invokes the methods on that proxied instance of the Cafe interface has no awareness of the Spring Integration API. The general approach is similar to that of Spring Remoting (RMI, HttpInvoker, and so on). See the “Samples” Appendix for an example that uses the gateway element (in the Cafe demo).

The defaults in the preceding configuration are applied to all methods on the gateway interface. If a reply timeout is not specified, the calling thread waits indefinitely for a reply. See Gateway Behavior When No response Arrives.

The defaults can be overridden for individual methods. See Gateway Configuration with Annotations and XML.

Typically, you need not specify the default-reply-channel, since a Gateway auto-creates a temporary, anonymous reply channel, where it listens for the reply. However, some cases may prompt you to define a default-reply-channel (or reply-channel with adapter gateways, such as HTTP, JMS, and others).

For some background, we briefly discuss some of the inner workings of the gateway. A gateway creates a temporary point-to-point reply channel. It is anonymous and is added to the message headers with the name, replyChannel. When providing an explicit default-reply-channel (reply-channel with remote adapter gateways), you can point to a publish-subscribe channel, which is so named because you can add more than one subscriber to it. Internally, Spring Integration creates a bridge between the temporary replyChannel and the explicitly defined default-reply-channel.

Suppose you want your reply to go not only to the gateway but also to some other consumer. In this case, you want two things:

The default strategy used by the gateway does not satisfy those needs, because the reply channel added to the header is anonymous and point-to-point. This means that no other subscriber can get a handle to it and, even if it could, the channel has point-to-point behavior such that only one subscriber would get the message. By defining a default-reply-channel you can point to a channel of your choosing. In this case, that is a publish-subscribe-channel. The gateway creates a bridge from it to the temporary, anonymous reply channel that is stored in the header.

You might also want to explicitly provide a reply channel for monitoring or auditing through an interceptor (for example, wiretap). To configure a channel interceptor, you need a named channel.

Consider the following example, which expands on the previous Cafe interface example by adding a @Gateway annotation:

The @Header annotation lets you add values that are interpreted as message headers, as the following example shows:

If you prefer the XML approach to configuring gateway methods, you can add method elements to the gateway configuration, as the following example shows:

You can also use XML to provide individual headers for each method invocation. This could be useful if the headers you want to set are static in nature and you do not want to embed them in the gateway’s method signature by using @Header annotations. For example, in the loan broker example, we want to influence how aggregation of the loan quotes is done, based on what type of request was initiated (single quote or all quotes). Determining the type of the request by evaluating which gateway method was invoked, although possible, would violate the separation of concerns paradigm (the method is a Java artifact). However, expressing your intention (meta information) in message headers is natural in a messaging architecture. The following example shows how to add a different message header for each of two methods:

In the preceding example a different value is set for the 'RESPONSE_TYPE' header, based on the gateway’s method.

Using the configuration techniques in the previous section allows control of how method arguments are mapped to message elements (payload and headers). When no explicit configuration is used, certain conventions are used to perform the mapping. In some cases, these conventions cannot determine which argument is the payload and which should be mapped to headers. Consider the following example:

In the first case, the convention is to map the first argument to the payload (as long as it is not a Map) and the contents of the second argument become headers.

In the second case (or the first when the argument for parameter thing1 is a Map), the framework cannot determine which argument should be the payload. Consequently, mapping fails. This can generally be resolved using a payload-expression, a @Payload annotation, or a @Headers annotation.

Alternatively (and whenever the conventions break down), you can take the entire responsibility for mapping the method calls to messages. To do so, implement an MethodArgsMessageMapper and provide it to the <gateway/> by using the mapper attribute. The mapper maps a MethodArgsHolder, which is a simple class that wraps the java.reflect.Method instance and an Object[] containing the arguments. When providing a custom mapper, the default-payload-expression attribute and <default-header/> elements are not allowed on the gateway. Similarly, the payload-expression attribute and <header/> elements are not allowed on any <method/> elements.

Starting with version 4.0, gateway service interfaces can be marked with a @MessagingGateway annotation instead of requiring the definition of a <gateway /> xml element for configuration. The following pair of examples compares the two approaches for configuring the same gateway:

Along with the @MessagingGateway annotation you can mark a service interface with the @Profile annotation to avoid the bean creation, if such a profile is not active.

When invoking methods on a Gateway interface that do not have any arguments, the default behavior is to receive a Message from a PollableChannel.

Sometimes, however, you may want to trigger no-argument methods so that you can interact with other components downstream that do not require user-provided parameters, such as triggering no-argument SQL calls or stored procedures.

To achieve send-and-receive semantics, you must provide a payload. To generate a payload, method parameters on the interface are not necessary. You can either use the @Payload annotation or the payload-expression attribute in XML on the method element. The following list includes a few examples of what the payloads could be:

The following example shows how to use the @Payload annotation:

If a method has no argument and no return value but does contain a payload expression, it is treated as a send-only operation.

The gateway invocation can result in errors. By default, any error that occurs downstream is re-thrown “as is” upon the gateway’s method invocation. For example, consider the following simple flow:

If the service invoked by the service activator throws a MyException (for example), the framework wraps it in a MessagingException and attaches the message passed to the service activator in the failedMessage property. Consequently, any logging performed by the framework has full the context of the failure. By default, when the exception is caught by the gateway, the MyException is unwrapped and thrown to the caller. You can configure a throws clause on the gateway method declaration to match the particular exception type in the cause chain. For example, if you want to catch a whole MessagingException with all the messaging information of the reason of downstream error, you should have a gateway method similar to the following:

Since we encourage POJO programming, you may not want to expose the caller to messaging infrastructure.

If your gateway method does not have a throws clause, the gateway traverses the cause tree, looking for a RuntimeException that is not a MessagingException. If none is found, the framework throws the MessagingException. If the MyException in the preceding discussion has a cause of SomeOtherException and your method throws SomeOtherException, the gateway further unwraps that and throws it to the caller.

When a gateway is declared with no service-interface, an internal framework interface RequestReplyExchanger is used.

Consider the following example:

Before version 5.0, this exchange method did not have a throws clause and, as a result, the exception was unwrapped. If you use this interface and want to restore the previous unwrap behavior, use a custom service-interface instead or access the cause of the MessagingException yourself.

However, you may want to log the error rather than propagating it or you may want to treat an exception as a valid reply (by mapping it to a message that conforms to some "error message" contract that the caller understands). To accomplish this, the gateway provides support for a message channel dedicated to the errors by including support for the error-channel attribute. In the following example, a 'transformer' creates a reply Message from the Exception:

The exceptionTransformer could be a simple POJO that knows how to create the expected error response objects. That becomes the payload that is sent back to the caller. You could do many more elaborate things in such an “error flow”, if necessary. It might involve routers (including Spring Integration’s ErrorMessageExceptionTypeRouter), filters, and so on. Most of the time, a simple 'transformer' should be sufficient, however.

Alternatively, you might want to only log the exception (or send it somewhere asynchronously). If you provide a one-way flow, nothing would be sent back to the caller. If you want to completely suppress exceptions, you can provide a reference to the global nullChannel (essentially a /dev/null approach). Finally, as mentioned above, if no error-channel is defined, then the exceptions propagate as usual.

When you use the @MessagingGateway annotation (see @MessagingGateway Annotation), you can use use the errorChannel attribute.

Starting with version 5.0, when you use a gateway method with a void return type (one-way flow), the error-channel reference (if provided) is populated in the standard errorChannel header of each sent message. This feature allows a downstream asynchronous flow, based on the standard ExecutorChannel configuration (or a QueueChannel), to override a default global errorChannel exceptions sending behavior. Previously you had to manually specify an errorChannel header with the @GatewayHeader annotation or the <header> element. The error-channel property was ignored for void methods with an asynchronous flow. Instead, error messages were sent to the default errorChannel.

Gateways have two timeout properties: requestTimeout and replyTimeout. The request timeout applies only if the channel can block (for example, a bounded QueueChannel that is full). The replyTimeout value is how long the gateway waits for a reply or returns null. It defaults to infinity.

The timeouts can be set as defaults for all methods on the gateway (defaultRequestTimeout and defaultReplyTimeout) or on the MessagingGateway interface annotation. Individual methods can override these defaults (in <method/> child elements) or on the @Gateway annotation.

Starting with version 5.0, the timeouts can be defined as expressions, as the following example shows:

The evaluation context has a BeanResolver (use @someBean to reference other beans), and the #args array variable is available.

When configuring with XML, the timeout attributes can be a long value or a SpEL expression, as the following example shows:

As a pattern, the messaging gateway offers a nice way to hide messaging-specific code while still exposing the full capabilities of the messaging system. As described earlier, the GatewayProxyFactoryBean provides a convenient way to expose a proxy over a service-interface giving you POJO-based access to a messaging system (based on objects in your own domain, primitives/Strings, or other objects). However, when a gateway is exposed through simple POJO methods that return values, it implies that, for each request message (generated when the method is invoked), there must be a reply message (generated when the method has returned). Since messaging systems are naturally asynchronous, you may not always be able to guarantee the contract where “for each request, there will always be be a reply”. Spring Integration 2.0 introduced support for an asynchronous gateway, which offers a convenient way to initiate flows when you may not know if a reply is expected or how long it takes for replies to arrive.

To handle these types of scenarios, Spring Integration uses java.util.concurrent.Future instances to support an asynchronous gateway.

From the XML configuration, nothing changes, and you still define asynchronous gateway the same way as you define a regular gateway, as the following example shows:

However, the gateway interface (a service interface) is a little different, as follows:

As the preceding example shows, the return type for the gateway method is a Future. When GatewayProxyFactoryBean sees that the return type of the gateway method is a Future, it immediately switches to the asynchronous mode by using an AsyncTaskExecutor. That is the extent of the differences. The call to such a method always returns immediately with a Future instance. Then you can interact with the Future at your own pace to get the result, cancel, and so on. Also, as with any other use of Future instances, calling get() may reveal a timeout, an execution exception, and so on. The following example shows how to use a Future that returns from an asynchronous gateway:

For a more detailed example, see the async-gateway sample in the Spring Integration samples.

As explained earlier, the gateway provides a convenient way of interacting with a messaging system through POJO method invocations. However, a typical method invocation, which is generally expected to always return (even with an Exception), might not always map one-to-one to message exchanges (for example, a reply message might not arrive — the equivalent to a method not returning).

The rest of this section covers various scenarios and how to make the gateway behave more predictably. Certain attributes can be configured to make synchronous gateway behavior more predictable, but some of them might not always work as you might expect. One of them is reply-timeout (at the method level or default-reply-timeout at the gateway level). We examine the reply-timeout attribute to see how it can and cannot influence the behavior of the synchronous gateway in various scenarios. We examine a single-threaded scenario (all components downstream are connected through a direct channel) and multi-threaded scenarios (for example, somewhere downstream you may have a pollable or executor channel that breaks the single-thread boundary).

To create a service activator, use the 'service-activator' element with the 'input-channel' and 'ref' attributes, as the following example shows:

The preceding configuration selects all the methods from the exampleHandler that meet one of the messaging requirements, which are as follows:

The target method for invocation at runtime is selected for each request message by their payload type or as a fallback to the Message<?> type if such a method is present on target class.

Starting with version 5.0, one service method can be marked with the @org.springframework.integration.annotation.Default as a fallback for all non-matching cases. This can be useful when using content-type conversion with the target method being invoked after conversion.

To delegate to an explicitly defined method of any object, you can add the method attribute, as the following example shows:

In either case, when the service method returns a non-null value, the endpoint tries to send the reply message to an appropriate reply channel. To determine the reply channel, it first checks whether an output-channel was provided in the endpoint configuration, as the following example shows:

If the method returns a result and no output-channel is defined, the framework then checks the request message’s replyChannel header value. If that value is available, it then checks its type. If it is a MessageChannel, the reply message is sent to that channel. If it is a String, the endpoint tries to resolve the channel name to a channel instance. If the channel cannot be resolved, a DestinationResolutionException is thrown. It it can be resolved, the message is sent there. If the request message does not have a replyChannel header and the reply object is a Message, its replyChannel header is consulted for a target destination. This is the technique used for request-reply messaging in Spring Integration, and it is also an example of the return address pattern.

If your method returns a result and you want to discard it and end the flow, you should configure the output-channel to send to a NullChannel. For convenience, the framework registers one with the name, nullChannel. See Special Channels for more information.

The service activator is one of those components that is not required to produce a reply message. If your method returns null or has a void return type, the service activator exits after the method invocation, without any signals. This behavior can be controlled by the AbstractReplyProducingMessageHandler.requiresReply option, which is also exposed as requires-reply when configuring with the XML namespace. If the flag is set to true and the method returns null, a ReplyRequiredException is thrown.

The argument in the service method could be either a message or an arbitrary type. If the latter, then it is assumed to be a message payload, which is extracted from the message and injected into the service method. We generally recommend this approach, as it follows and promotes a POJO model when working with Spring Integration. Arguments may also have @Header or @Headers annotations, as described in Annotation Support.

Starting with version 4.1, the framework correctly converts message properties (payload and headers) to the Java 8 Optional POJO method parameters, as the following example shows:

We generally recommend using a ref attribute if the custom service activator handler implementation can be reused in other <service-activator> definitions. However, if the custom service activator handler implementation is only used within a single definition of the <service-activator>, you can provide an inner bean definition, as the following example shows:

The service activator is invoked by the calling thread. This is an upstream thread if the input channel is a SubscribableChannel or a poller thread for a PollableChannel. If the service returns a ListenableFuture<?>, the default action is to send that as the payload of the message sent to the output (or reply) channel. Starting with version 4.3, you can now set the async attribute to true (by using setAsync(true) when using Java configuration). If the service returns a ListenableFuture<?> when this the async attribute is set to true, the calling thread is released immediately and the reply message is sent on the thread (from within your service) that completes the future. This is particularly advantageous for long-running services that use a PollableChannel, because the poller thread is released to perform other services within the framework.

If the service completes the future with an Exception, normal error processing occurs. An ErrorMessage is sent to the errorChannel message header, if present. Otherwise, an ErrorMessage is sent to the default errorChannel (if available).

The service method can return any type which becomes reply message payload. In this case a new Message<?> object is created and all the headers from a request message are copied. This works the same way for most Spring Integration MessageHandler implementations, when interaction is based on a POJO method invocation.

A complete Message<?> object can also be returned from the method. However keep in mind that, unlike transformers, for a Service Activator this message will be modified by copying the headers from the request message if they are not already present in the returned message. So, if your method parameter is a Message<?> and you copy some, but not all, existing headers in your service method, they will reappear in the reply message. It is not a Service Activator responsibility to remove headers from a reply message and, pursuing the loosely-coupled principle, it is better to add a HeaderFilter in the integration flow. Alternatively, a Transformer can be used instead of a Service Activator but, in that case, when returning a full Message<?> the method is completely responsible for the message, including copying request message headers (if needed). You must ensure that important framework headers (e.g. replyChannel, errorChannel), if present, have to be preserved.

The <delayer> element is used to delay the message flow between two message channels. As with the other endpoints, you can provide the 'input-channel' and 'output-channel' attributes, but the delayer also has 'default-delay' and 'expression' attributes (and the 'expression' element) that determine the number of milliseconds by which each message should be delayed. The following example delays all messages by three seconds:

If you need to determine the delay for each message, you can also provide the SpEL expression by using the 'expression' attribute, as the following expression shows:

In the preceding example, the three-second delay applies only when the expression evaluates to null for a given inbound message. If you want to apply a delay only to messages that have a valid result of the expression evaluation, you can use a 'default-delay' of 0 (the default). For any message that has a delay of 0 (or less), the message is sent immediately, on the calling thread.

The following example shows the Java configuration equivalent of the preceding example:

The following example shows the Java DSL equivalent of the preceding example:

The delayer delegates to an instance of Spring’s TaskScheduler abstraction. The default scheduler used by the delayer is the ThreadPoolTaskScheduler instance provided by Spring Integration on startup. See Configuring the Task Scheduler. If you want to delegate to a different scheduler, you can provide a reference through the delayer element’s 'scheduler' attribute, as the following example shows:

The DelayHandler persists delayed messages into the message group in the provided MessageStore. (The 'groupId' is based on the required 'id' attribute of the <delayer> element.) A delayed message is removed from the MessageStore by the scheduled task immediately before the DelayHandler sends the message to the output-channel. If the provided MessageStore is persistent (such as JdbcMessageStore), it provides the ability to not lose messages on the application shutdown. After application startup, the DelayHandler reads messages from its message group in the MessageStore and reschedules them with a delay based on the original arrival time of the message (if the delay is numeric). For messages where the delay header was a Date, that Date is used when rescheduling. If a delayed message remains in the MessageStore more than its 'delay', it is sent immediately after startup.

The <delayer> can be enriched with either of two mutually exclusive elements: <transactional> and <advice-chain>. The List of these AOP advices is applied to the proxied internal DelayHandler.ReleaseMessageHandler, which has the responsibility to release the message, after the delay, on a Thread of the scheduled task. It might be used, for example, when the downstream message flow throws an exception and the transaction of the ReleaseMessageHandler is rolled back. In this case, the delayed message remains in the persistent MessageStore. You can use any custom org.aopalliance.aop.Advice implementation within the <advice-chain>. The <transactional> element defines a simple advice chain that has only the transactional advice. The following example shows an advice-chain within a <delayer>:

The DelayHandler can be exported as a JMX MBean with managed operations (getDelayedMessageCount and reschedulePersistedMessages), which allows the rescheduling of delayed persisted messages at runtime — for example, if the TaskScheduler has previously been stopped. These operations can be invoked through a Control Bus command, as the following example shows:

Starting with version 5.0.8, there are two new properties on the delayer:

When a message is released, if the downstream flow fails, the release will be attempted after the retryDelay. If the maxAttempts is reached, the message is discarded (unless the release is transactional, in which case the message will remain in the store, but will no longer be scheduled for release, until the application is restarted, or the reschedulePersistedMessages() method is invoked, as discussed above).

In addition, you can configure a delayedMessageErrorChannel; when a release fails, an ErrorMessage is sent to that channel with the exception as the payload and has the originalMessage property. The ErrorMessage contains a header IntegrationMessageHeaderAccessor.DELIVERY_ATTEMPT containing the current count.

If the error flow consumes the error message and exits normally, no further action is taken; if the release is transactional, the transaction will commit and the message deleted from the store. If the error flow throws an exception, the release will be retried up to maxAttempts as discussed above.

Depending on the complexity of your integration requirements, scripts may be provided inline as CDATA in XML configuration or as a reference to a Spring resource that contains the script. To enable scripting support, Spring Integration defines a ScriptExecutingMessageProcessor, which binds the message payload to a variable named payload and the message headers to a headers variable, both accessible within the script execution context. All you need to do is write a script that uses these variables. The following pair of examples show sample configurations that create filters:

As the preceding examples show, the script can be included inline or can be included by reference to a resource location (by using the location attribute). Additionally, the lang attribute corresponds to the language name (or its JSR223 alias)

Other Spring Integration endpoint elements that support scripting include router, service-activator, transformer, and splitter. The scripting configuration in each case would be identical to the above (besides the endpoint element).

Another useful feature of scripting support is the ability to update (reload) scripts without having to restart the application context. To do so, specify the refresh-check-delay attribute on the script element, as the following example shows:

In the preceding example, the script location is checked for updates every 5 seconds. If the script is updated, any invocation that occurs later than 5 seconds since the update results in running the new script.

Consider the following example:

In the preceding example, the context is updated with any script modifications as soon as such modification occurs, providing a simple mechanism for 'real-time' configuration. Any negative value means the script is not reloaded after initialization of the application context. This is the default behavior. The following example shows a script that never updates:

With Spring Integration 2.1, the configuration namespace for the Groovy support is an extension of Spring Integration’s scripting support and shares the core configuration and behavior described in detail in the Scripting Support section. Even though Groovy scripts are well supported by generic scripting support, the Groovy support provides the Groovy configuration namespace, which is backed by the Spring Framework’s org.springframework.scripting.groovy.GroovyScriptFactory and related components, offering extended capabilities for using Groovy. The following listing shows two sample configurations:

As the preceding examples show, the configuration looks identical to the general scripting support configuration. The only difference is the use of the Groovy namespace, as indicated by the int-groovy namespace prefix. Also note that the lang attribute on the <script> tag is not valid in this namespace.

If you need to customize the Groovy object itself (beyond setting variables) you can reference a bean that implements GroovyObjectCustomizer by using the customizer attribute. For example, this might be useful if you want to implement a domain-specific language (DSL) by modifying the MetaClass and registering functions to be available within the script. The following example shows how to do so:

Setting a custom GroovyObjectCustomizer is not mutually exclusive with <variable> elements or the script-variable-generator attribute. It can also be provided when defining an inline script.

Spring Integration 3.0 introduced the variables attribute, which works in conjunction with the variable element. Also, groovy scripts have the ability to resolve a variable to a bean in the BeanFactory, if a binding variable was not provided with the name. The following example shows how to use a variable (entityManager):

entityManager must be an appropriate bean in the application context.

For more information regarding the <variable> element, the variables attribute, and the script-variable-generator attribute, see Script Variable Bindings.

The @CompileStatic hint is the most popular Groovy compiler customization option. It can be used on the class or method level. For more information, see the Groovy Reference Manual and, specifically, @CompileStatic. To utilize this feature for short scripts (in integration scenarios), we are forced to change simple scripts to more Java-like code. Consider the following <filter> script:

The preceding script becomes the following method in Spring Integration:

With that, the filter() method is transformed and compiled to static Java code, bypassing the Groovy dynamic phases of invocation, such as getProperty() factories and CallSite proxies.

Starting with version 4.3, you can configure the Spring Integration Groovy components with the compile-static boolean option, specifying that ASTTransformationCustomizer for @CompileStatic should be added to the internal CompilerConfiguration. With that in place, you can omit the method declaration with @CompileStatic in our script code and still get compiled plain Java code. In this case, the preceding script can be short but still needs to be a little more verbose than interpreted script, as the following example shows:

You must access the headers and payload (or any other) variables through the groovy.lang.Script binding property because, with @CompileStatic, we do not have the dynamic GroovyObject.getProperty() capability.

In addition, we introduced the compiler-configuration bean reference. With this attribute, you can provide any other required Groovy compiler customizations, such as ImportCustomizer. For more information about this feature, see the Groovy Documentation for advanced compiler configuration.

As described in (Enterprise Integration Patterns), the idea behind the control bus is that you can use the same messaging system for monitoring and managing the components within the framework as is used for “application-level” messaging. In Spring Integration, we build upon the adapters described earlier so that you can send Messages as a means of invoking exposed operations. One option for those operations is Groovy scripts. The following example configures a Groovy script for the control bus:

The control bus has an input channel that can be accessed to invoke operations on the beans in the application context.

The Groovy control bus runs messages on the input channel as Groovy scripts. It takes a message, compiles the body to a script, customizes it with a GroovyObjectCustomizer, and runs it. The control bus' MessageProcessor exposes all beans in the application context that are annotated with @ManagedResource and implement Spring’s Lifecycle interface or extend Spring’s CustomizableThreadCreator base class (for example, several of the TaskExecutor and TaskScheduler implementations).

If you need to further customize the Groovy objects, you can also provide a reference to a bean that implements GroovyObjectCustomizer through the customizer attribute, as the following example shows:

In addition to providing the general mechanism to apply AOP advice classes, Spring Integration provides these out-of-the-box advice implementations:

In addition to the provided advice classes described earlier, you can implement your own advice classes. While you can provide any implementation of org.aopalliance.aop.Advice (usually org.aopalliance.intercept.MethodInterceptor), we generally recommend that you subclass o.s.i.handler.advice.AbstractRequestHandlerAdvice. This has the benefit of avoiding the writing of low-level aspect-oriented programming code as well as providing a starting point that is specifically tailored for use in this environment.

Subclasses need to implement the doInvoke() method, the definition of which follows:

The callback parameter is a convenience to avoid subclasses that deal with AOP directly. Invoking the callback.execute() method invokes the message handler.

The target parameter is provided for those subclasses that need to maintain state for a specific handler, perhaps by maintaining that state in a Map keyed by the target. This feature allows the same advice to be applied to multiple handlers. The RequestHandlerCircuitBreakerAdvice uses advice this to keep circuit breaker state for each handler.

The message parameter is the message sent to the handler. While the advice cannot modify the message before invoking the handler, it can modify the payload (if it has mutable properties). Typically, an advice would use the message for logging or to send a copy of the message somewhere before or after invoking the handler.

The return value would normally be the value returned by callback.execute(). However, the advice does have the ability to modify the return value. Note that only AbstractReplyProducingMessageHandler instances return values. The following example shows a custom advice class that extends AbstractRequestHandlerAdvice:

While the abstract class mentioned above is a convenience, you can add any Advice, including a transaction advice, to the chain.

As discussed in the introduction to this section, advice objects in a request handler advice chain are applied to just the current endpoint, not the downstream flow (if any). For MessageHandler objects that produce a reply (such as those that extend AbstractReplyProducingMessageHandler), the advice is applied to an internal method: handleRequestMessage() (called from MessageHandler.handleMessage()). For other message handlers, the advice is applied to MessageHandler.handleMessage().

There are some circumstances where, even if a message handler is an AbstractReplyProducingMessageHandler, the advice must be applied to the handleMessage method. For example, the idempotent receiver might return null, which would cause an exception if the handler’s replyRequired property is set to true. Another example is the BoundRabbitChannelAdvice — see Strict Message Ordering.

Starting with version 4.3.1, a new HandleMessageAdvice interface and its base implementation (AbstractHandleMessageAdvice) have been introduced. Advice objects that implement HandleMessageAdvice are always applied to the handleMessage() method, regardless of the handler type.

It is important to understand that HandleMessageAdvice implementations (such as idempotent receiver), when applied to a handlers that return responses, are dissociated from the adviceChain and properly applied to the MessageHandler.handleMessage() method.

Consider the following configuration:

In the preceding example, the <tx:advice> is applied to the AbstractReplyProducingMessageHandler.handleRequestMessage(). However, myHandleMessageAdvice is applied for to MessageHandler.handleMessage(). Therefore, it is invoked before the <tx:advice>. To retain the order, you should follow the standard Spring AOP configuration approach and use an endpoint id together with the .handler suffix to obtain the target MessageHandler bean. Note that, in that case, the entire downstream flow is within the transaction scope.

In the case of a MessageHandler that does not return a response, the advice chain order is retained.

Starting with version 5.0, a new TransactionHandleMessageAdvice has been introduced to make the whole downstream flow transactional, thanks to the HandleMessageAdvice implementation. When a regular TransactionInterceptor is used in the <request-handler-advice-chain> element (for example, through configuring <tx:advice>), a started transaction is only applied only for an internal AbstractReplyProducingMessageHandler.handleRequestMessage() and is not propagated to the downstream flow.

To simplify XML configuration, along with the <request-handler-advice-chain>, a <transactional> element has been added to all <outbound-gateway> and <service-activator> and related components. The following example shows <transactional> in use:

If you are familiar with the JPA integration components, such a configuration is not new, but now we can start a transaction from any point in our flow — not only from the <poller> or a message-driven channel adapter such as JMS.

Java configuration can be simplified by using the TransactionInterceptorBuilder, and the result bean name can be used in the messaging annotations adviceChain attribute, as the following example shows:

Note the true parameter on the TransactionInterceptorBuilder constructor. It causes the creation of a TransactionHandleMessageAdvice, not a regular TransactionInterceptor.

Java DSL supports an Advice through the .transactional() options on the endpoint configuration, as the following example shows:

There is an additional consideration when advising Filter advices. By default, any discard actions (when the filter returns false) are performed within the scope of the advice chain. This could include all the flow downstream of the discard channel. So, for example, if an element downstream of the discard channel throws an exception and there is a retry advice, the process is retried. Also, if throwExceptionOnRejection is set to true (the exception is thrown within the scope of the advice).

Setting discard-within-advice to false modifies this behavior and the discard (or exception) occurs after the advice chain is called.

When configuring certain endpoints by using annotations (@Filter, @ServiceActivator, @Splitter, and @Transformer), you can supply a bean name for the advice chain in the adviceChain attribute. In addition, the @Filter annotation also has the discardWithinAdvice attribute, which can be used to configure the discard behavior, as discussed in Advising Filters. The following example causes the discard to be performed after the advice:

Advice classes are “around” advices and are applied in a nested fashion. The first advice is the outermost, while the last advice is the innermost (that is, closest to the handler being advised). It is important to put the advice classes in the correct order to achieve the functionality you desire.

For example, suppose you want to add a retry advice and a transaction advice. You may want to place the retry advice advice first, followed by the transaction advice. Consequently, each retry is performed in a new transaction. On the other hand, if you want all the attempts and any recovery operations (in the retry RecoveryCallback) to be scoped within the transaction, you could put the transaction advice first.

Sometimes, it is useful to access handler properties from within the advice. For example, most handlers implement NamedComponent to let you access the component name.

The target object can be accessed through the target argument (when subclassing AbstractRequestHandlerAdvice) or invocation.getThis() (when implementing org.aopalliance.intercept.MethodInterceptor).

When the entire handler is advised (such as when the handler does not produce replies or the advice implements HandleMessageAdvice), you can cast the target object to an interface, such as NamedComponent, as shown in the following example:

When you implement MethodInterceptor directly, you could cast the target object as follows:

When only the handleRequestMessage() method is advised (in a reply-producing handler), you need to access the full handler, which is an AbstractReplyProducingMessageHandler. The following example shows how to do so:

Starting with version 4.1, Spring Integration provides an implementation of the Idempotent Receiver Enterprise Integration Pattern. It is a functional pattern and the whole idempotency logic should be implemented in the application. However, to simplify the decision-making, the IdempotentReceiverInterceptor component is provided. This is an AOP Advice that is applied to the MessageHandler.handleMessage() method and that can filter a request message or mark it as a duplicate, according to its configuration.

Previously, you could have implemented this pattern by using a custom MessageSelector in a <filter/> (see Filter), for example. However, since this pattern really defines the behavior of an endpoint rather than being an endpoint itself, the idempotent receiver implementation does not provide an endpoint component. Rather, it is applied to endpoints declared in the application.

The logic of the IdempotentReceiverInterceptor is based on the provided MessageSelector and, if the message is not accepted by that selector, it is enriched with the duplicateMessage header set to true. The target MessageHandler (or downstream flow) can consult this header to implement the correct idempotency logic. If the IdempotentReceiverInterceptor is configured with a discardChannel or throwExceptionOnRejection = true, the duplicate message is not sent to the target MessageHandler.handleMessage(). Rather, it is discarded. If you want to discard (do nothing with) the duplicate message, the discardChannel should be configured with a NullChannel, such as the default nullChannel bean.

To maintain state between messages and provide the ability to compare messages for the idempotency, we provide the MetadataStoreSelector. It accepts a MessageProcessor implementation (which creates a lookup key based on the Message) and an optional ConcurrentMetadataStore (Metadata Store). See the MetadataStoreSelector Javadoc for more information. You can also customize the value for ConcurrentMetadataStore by using an additional MessageProcessor. By default, MetadataStoreSelector uses the timestamp message header.

For convenience, the MetadataStoreSelector options are configurable directly on the <idempotent-receiver> component. The following listing shows all the possible attributes:

For Java configuration, Spring Integration provides the method-level @IdempotentReceiver annotation. It is used to mark a method that has a messaging annotation (@ServiceActivator, @Router, and others) to specify which `IdempotentReceiverInterceptor objects are applied to this endpoint. The following example shows how to use the @IdempotentReceiver annotation:

When you use the Java DSL, you can add the interceptor to the endpoint’s advice chain, as the following example shows:

The following Spring Boot application shows an example of configuring the LoggingHandler by using Java configuration:

The following Spring Boot application shows an example of configuring the logging channel adapter by using the Java DSL:

The Framework also has been improved to support Kotlin lambdas for functions so now you can use a combination of the Kotlin language and Spring Integration flow definitions: