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A quick overview of MVC
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We also assume you are familiar with servlet containers, like Tomcat, Resin, or Jetty. Since the examples in the chapter are packaged as web application archives (WARs), you ll need to get a servlet container in order to run the examples.
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8.1 Defining the application
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Before we delve into the details of each web framework, let s talk about what your new web application is going to do. As we mentioned, it s going to be a fairly simple one-page application. The basic idea is that you ll have a bunch of events every month that you want publicize to your web readers. Here s the list of features your calendar should have:
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Create a visual monthly calendar similar to one you might see in Microsoft Outlook. It should display a single month at time. Each day that has events scheduled should list the names of those events. Include navigation controls so the user can page back and forth between the months. By default, the current month s events should be displayed.
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To give you a sneak preview, figure 8.1 shows what your page will look like. As you see, the current month s events are displayed in a table-based calendar. Back and Next links allow users to go back and forth among the months. Note that the URL is from the WebWork application, but the basic display will be the same for all of the applications.
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8.2 A quick overview of MVC
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Hibernate is commonly used as the persistence service in web applications. This section provides a brief overview of a common web application design pattern and discusses how Hibernate fits into it. Of course, the design we present isn t the only way to build web applications, but
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Web frameworks: WebWork, Struts, and Tapestry
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Figure 8.1 The Event Calendar application in action, showing events for November 2004
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some variation on the theme is fairly common. Later, we discuss how to use Hibernate with specific web application frameworks. We have chosen three popular web application frameworks: WebWork, Struts, and Tapestry. Before we delve into each framework, let s review the common pattern they all share: Model-View-Controller. The MVC architecture, also called Model 2, is a design pattern commonly used to separate an application into three primary concerns: The representation of the data The business logic, or rules, of an application The view the user has of the data representation The MVC pattern isn t limited to web applications. In fact, MVC is used throughout the Swing library. Figure 8.2 shows a diagram of the basic MVC pattern.
A quick overview of MVC
Controller requests Client sees View updates updates Model
Figure 8.2 A diagram of the Model-View-Controller pattern
The MVC pattern can be described by examining the lifecycle of a user request. Your client, defined as a user interacting with a web browser, submits a request to the Controller. The Controller resides on the servlet container and defines the behavior of the application. Application behavior means how the application reacts to user requests. Typically, the Controller performs some update on the state of the Model. Once the Model has been updated, the user is presented with an updated View of the Model. Let s look at the responsibilities of each component in the MVC pattern: The Model does the following: Stores the application state Returns the results of queries to the application state Informs the View of changes to the application state The View is responsible for
Displaying, or rendering, the Model to the user Sending user responses from the Controller The Controller handles the following:
Performs application behavior
Web frameworks: WebWork, Struts, and Tapestry
Updates the Model on behalf of user requests Determines the View that should be displayed to the user Suppose your events management application is web based, and a user wants to make a change to a specific Event instance. Figure 8.3 displays the lifecycle of the user request.
As displayed in figure 8.3, the user submits a request to the controller servlet. The user request contains the changes made to the state of the Event instance. Assume that the user changed the name of the Event. The controller servlet passes the request off to the EventManager class to retrieve the appropriate Event instance and update its name property. Once this task is complete, the controller servlet sends the user a response notifying him or her that the request has been processed successfully. Why did we use the EventManager class to update the state of the Event Why didn t the servlet simply update the Event instance itself instead of delegating to the manager class You ll find out in the next section.
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