Java Server Pages

The MVC Model

In addition to the separation of responsibilities into different tiers, J2EE also encourages the use of the Model-View-Controller (MVC) design model, briefly introduced in Chapter 3, when designing applications.

The MVC model was first described by Xerox in a number of papers published in the late 1980s in conjunction with the Smalltalk language. But this model has since been used for GUI applications developed in all popular programming languages. Let’s review: the basic idea is to separate the application data and business logic, the presentation of the data, and the interaction with the data into distinct entities labeled the Model, the View, and the Controller, respectively. This makes for a flexible design, where multiple presentations (Views) can be provided and easily modified, and changes in the business rules or physical representation of the data (the Model) can be made without touching any of the user interface code.

Even though the model was originally developed for standalone GUI applications, it translates fairly well into the multitier application domain of J2EE. The user interacts with the Controller to ask for things to be done, and the Controller relays these requests to the Model in a client-type independent way. Say, for instance, that you have two types of clients: an HTTP client such as a browser, and a GUI client application using IIOP to talk to the server. In this scenario you can have one Controller for each protocol that receives the requests ...