Spring Integration provides a number of implementations of the MessageSource and MessageTarget interfaces that serve as adapters for interacting with external systems or components that are not part of the messaging system. These source and target implementations can be configured within the same channel-adapter element that we have already discussed. Essentially, the external system or component sends-to and/or receives-from a MessageChannel. In the 1.0 Milestone 6 release, Spring Integration includes source and target implementations for JMS, Files, FTP, Streams, and Spring ApplicationEvents.

Adapters that allow an external system to perform request-reply operations across Spring Integration MessageChannels are actually examples of the Messaging Gateway pattern. Therefore, those implementations are typically called "gateways" (whereas "source" and "target" are in-only and out-only interactions respectively). For example, Spring Integration provides a JmsSource that is polled by the bus-managed scheduler, but also provides a JmsGateway. The gateway differs from the source in that it is an event-driven consumer rather than a polling consumer, and it is capable of waiting for reply messages. Spring Integration also provides gateways for RMI and Spring's HttpInvoker.

Finally, adapters that enable interaction with external systems by invoking them for request/reply interactions (the response is sent back on a Message Channel) are typically called handlers in Spring Integration, since they implement the MessageHandler interface. Basically, these types of adapters can be configured exactly like any POJO with the <service-activator> element. Spring Integration provides RMI, HttpInvoker, and Web Service handler implementations.

All of these adapters are discussed in this section. However, namespace support is provided for many of them and is typically the most convenient option for configuration. For examples, see Section 4.2.4, “Configuring Adapters”.