HttpInvoker is a Spring-specific remoting option that essentially enables Remote Procedure Calls (RPC) over HTTP. In order to accomplish this, an outbound representation of a method invocation is serialized using standard Java serialization and then passed within an HTTP POST request. After being invoked on the target system, the method’s return value is then serialized and written to the HTTP response. There are two main requirements. First, you must be using Spring on both sides since the marshalling to and from HTTP requests and responses is handled by the client-side invoker and server-side exporter. Second, the Objects that you are passing must implement Serializable and be available on both the client and server.
While traditional RPC provides physical decoupling, it does not offer nearly the same degree of logical decoupling as a messaging-based system. In other words, both participants in an RPC-based invocation must be aware of a specific interface and specific argument types. Interestingly, in Spring Integration, the “parameter” being sent is a Spring Integration Message, and the interface is an internal detail of Spring Integration’s implementation. Therefore, the RPC mechanism is being used as a transport so that from the end user’s perspective, it is not necessary to consider the interface and argument types. It’s just another adapter to enable messaging between two systems.