基于ASP的网站设计 - 图文(16)

The way an application server invokes a JSP page is different from the way ASP pages are invoked. A J2EE application server converts each JSP page into a separate special servlet that compiles and runs when the page is requested. The special servlet remains loaded in memory until the JSP file changes. This minimizes the performance hit that might result if you had to create and compile a class for each JSP page.

Model-View-Controller architecture

J2EE was developed with a particular application structure in mind, called Model-View-Controller (MVC). MVC defines a clear separation among three

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tiers of an application:

The model: The set of data and business rules for the application --

commonly called the application's business logic

The view: The application's user interface

The controller: Definition of the way the application reacts to user input

or to changes in the model tier -- commonly called application logic

Advantages of a MVC architecture

Nothing in J2EE forces you to structure your applications with a MVC architecture, but there are good reasons to do so. By defining a clear separation among the three tiers, MVC allows for loose coupling among the components that comprise each tier. This makes the components more reusable and flexible. For example, suppose one of your requirements is to support different types of views of the same data in a Web application, because different departments need different subsets of the same data in a database. You need to develop new view components specific to each required subset. If the view logic and database-access code were tightly coupled -- as they are an ASP page that interleaves database-access code with HTML -- then each view would contain the database-access code. Maintaining duplicate code not only takes a lot of work, but it's likely to lead to errors. A MVC architecture in this scenario uses the database-access code as part of the model, and the various view components can reuse it.

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J2EE components and MVC

Figure 1 shows how the J2EE components we've discussed so far could map to a MVC architecture. Note that no connection exists between the model and the view. The controller's function is to serve as a conduit between the two.

Figure 1. MVC with J2EE Web applications

In a typical scenario, the user submits an HTML form whose target is a servlet. The servlet parses the input and uses classes in the model to invoke the business logic to satisfy the request. The servlet then passes the results to a JSP page to display them to the user.

JavaServer Faces

The JavaServer Faces (JSF) specification provides the runtime components that let J2EE tool vendors provide drag-and-drop capability for the development of Web-based UIs. It also lets vendors develop custom components their development tools can use. To see JSF in action, take a look at the WebSphere

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