What is messaging and how is it different from RMI?

Messaging occurs between applications or software components which may or may not be distributive in nature.In enterprise applications where the possibility of distributed components across the geographies are too high, messaging comes as an obvious choice to make them communicate with each other.

In Java , middleware(which acts as an infrastructural support for messaging) level messaging is supported with Java Message Service(JMS) APIs.JMS is a specification (java.sun.com/products/jms ) that describes the properties and behavior of an information pipe for Java software. It also describes how Java client applications interact with the information pipe.

JMS helps business applications asynchronously send and receive critical business data and events.It supports both message queueing and publish-subscribe styles of messaging.

The constituents of JMS are producers,consumers and messages themselves through interfaces' abstraction.The beauty lies here in loose coupling and infact message broker systems need not go in details of message-contents, they just have to deliver them between systems reliably.There is not dependency on APIs of systems involved in message communications.

In case of RMI, the message passing is tightly coupled with APIs of remote application or component.That is what makes JMS as an obvious choice in enterprise applications for reliable messaging.