J2EE is an open, standard-based, development and deployment platform for building n-tier, web-based and server-centric, and component-based enterprise applications. The evolution of enterprise application framework starts from single-tier mainframe based model in the beginning, moved to first phase of distributed framework, two-tier client server model, then three tier model. Within the three-tier model, there are two communication models.  First one is  based on RPC (Remote Procedure Call) model in which non-object-based procedures are invoked by the client.  The other model  incorporates the concept of object in which remote object is accessed by the client. When the Internet took off, the three tier model takes a very radical transformation in which HTML browser talks to a Web server, which then talks to backend database or enterprise information systems.

The next two phases are advent of the concept of application server, first, app server based on proprietary architecture and API and the next phase is where we are today with open standard based app server architecture, which is what J2EE is all about. Now if you think about the things that make up a typical enterprise application, they include presentation logic, business logic, data access logic, and system services.

The presentation logic deals with how to present information to the end-users. The business logic includes the business logic of the application.  And the data access logic deals with accessing data in the back-end database. The system services includes security, caching, logging, transaction, and other services that typical enterprise application would use.

Now the way the enterprise application framework evolves reflects how flexibly you want to make changes in the presentation logic, business logic and data access and data models.  The other factor reflects how the system services are provided by the framework.

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