By: Emiley J. Emailed: 1728 times Printed: 2331 times
Web applications based on JavaServer Pages sometimes commingle database code, page design code, and control flow code. In practice, we find that unless these concerns are separated, larger applications become difficult to maintain.
One way to separate concerns in a software application is to use a Model-View-Controller (MVC) architecture. The Model represents the business or database code, the View represents the page design code, and the Controller represents the navigational code.
The term "MVC" originated with the SmallTalk Model-View-Controller framework. In Smalltalk MVC, the View updates itself from the Model, via the "Observer" pattern. The original MVC pattern is like a closed loop: The View talks to the Controller, which talks to the Model, which talks to the View.
But, a direct link between the Model and the View is not practical for web applications, and we modify the classic MVC arrangement so that it would look less like a loop and more like a horseshoe with the controller in the middle.
In the MVC/Model 2 design pattern, application flow is mediated by a central Controller. The Controller delegates requests - in our case, HTTP requests - to an appropriate handler. The handlers are tied to a Model, and each handler acts as an adapter between the request and the Model. The Model represents, or encapsulates, an application's business logic or state. Control is usually then forwarded back through the Controller to the appropriate View. The forwarding can be determined by consulting a set of mappings, usually loaded from a database or configuration file. This provides a loose coupling between the View and Model, which can make applications significantly easier to create and maintain.
While MVC is a convenient paradigm, many workers find that applcations may utilize more than three layers. For example, within the Model, there is often distinct business logic and data access layers.
The framework provides the control layer for a Model 2 web applications. Developers can use this layer with other standard technologies to build the business, data access, and presentation layers.
For more about MVC, see
- Web-Tier Application Framework Design (Sun J2EE Blueprints)
- Smalltalk MVC framework.
- Wikipedia - MVC
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