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.