The Service Activator is the endpoint type for connecting any Spring-managed Object to an input channel so that it may play the role of a service. If the service produces output, it may also be connected to an output channel. Alternatively, an output producing service may be located at the end of a processing pipeline or message flow in which case, the inbound Message's "replyChannel" header can be used. This is the default behavior if no output channel is defined, and as with most of the configuration options you'll see here, the same behavior actually applies for most of the other components we have seen.

To create a Service Activator, use the 'service-activator' element with the 'input-channel' and 'ref' attributes:

<service-activator input-channel="exampleChannel" ref="exampleHandler"/>

The configuration above assumes that "exampleHandler" either contains a single method annotated with the @ServiceActivator annotation or that it contains only one public method at all. To delegate to an explicitly defined method of any object, simply add the "method" attribute.

<service-activator input-channel="exampleChannel" ref="somePojo" method="someMethod"/>

In either case, when the service method returns a non-null value, the endpoint will attempt to send the reply message to an appropriate reply channel. To determine the reply channel, it will first check if an "output-channel" was provided in the endpoint configuration:

<service-activator input-channel="exampleChannel" output-channel="replyChannel"
                   ref="somePojo" method="someMethod"/>

If no "output-channel" is available, it will then check the Message's REPLY_CHANNEL header value. If that value is available, it will then check its type. If it is a MessageChannel, the reply message will be sent to that channel. If it is a String, then the endpoint will attempt to resolve the channel name to a channel instance. If the channel cannot be resolved, then a ChannelResolutionException will be thrown.

The argument in the service method could be either a Message or an arbitrary type. If the latter, then it will be assumed that it is a Message payload, which will be extracted from the message and injected into such service method. This is generally the recommended approach as it follows and promotes a POJO model when working with Spring Integration. Arguments may also have @Header, @Headers or @MessageMapping annotations as described in Section B.5, “Annotation Support”

Note

Since v1.0.3 of Spring Integration, the service method is not required to have an argument at all, which means you can now implement event-style Service Activators, where all you care about is an invocation of the service method, not worrying about the contents of the message. Think of it as a NULL JMS message. An example use-case for such an implementation could be a simple counter/monitor of messages deposited on the input channel.

Using a "ref" attribute is generally recommended if the custom Service Activator handler implementation can be reused in other <service-activator> definitions. However if the custom Service Activator handler implementation should be scoped to a single definition of the <service-activator>, you can use an inner bean definition:

<service-activator id="exampleServiceActivator" input-channel="inChannel" 
						output-channel = "outChannel" method="foo">
		<beans:bean class="org.foo.ExampleServiceActivator"/>
</service-activator>

	Note
	Using both the "ref" attribute and an inner handler definition in the same <service-activator> configuration is not allowed, as it creates an ambiguous condition and will result in an Exception being thrown.