visual basic 6.0 barcode generator The JSF Request Processing Lifecycle in Java

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The JSF Request Processing Lifecycle
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When a JSF-enabled XHTML page is requested or when the user invokes an action on a UI component in a JSF-enabled XHTML page, it is important to understand the exact sequence of events that occur on the server in order to fulfill the request to view or submit a JSF page
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FIGURE 1-1 The JSF UI component tree
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Introduction to JavaServer Faces
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Create or restore the view
Apply values from user
Ensure values are valid
PARTIII PART PART
View Web page
Render view
Fetch a new view, if necessary
Update model with valid values
FIGURE 1-2
The JSF request processing lifecycle
The sequence of events that are triggered during requests to JSF pages is known as the JSF request processing lifecycle or sometimes simply as the JSF lifecycle This is shown in Figure 1-2 We ve already touched on what happens when a JSF page is requested for the first time, when the JSF runtime creates a component tree in memory In between requests, when nothing is happening in the application, the component tree is often cached Upon a subsequent request, the tree is quickly reconstituted, and if form input values are sent in the request, they are processed and validations are executed Upon successful validation, the server-side model values of the input fields are updated What follows is continued event processing, and any errors are reported Once all event processing and model updates (if needed) have finished, a completed response is finally rendered back to the client A more detailed review of the JSF request processing lifecycle is presented in 3, but for now it is sufficient to know that the JSF lifecycle is simply the sequence of back-end plumbing events that automatically manage input data so that the Web developer does not need to write code to process the request manually This differs to a certain degree from most other Web technologies, including CGI, PHP, and Struts, where the developer specifically writes code to handle the incoming requests and process the results This is one of the advantages that JSF brings to Web application development It removes the whole notion of having to manually process incoming Web requests Instead, the Web developer can rely on the JSF lifecycle to handle back-end plumbing automatically and can use the JSF event model to jump in and do custom processing only when needed It is important to recognize that the work done by the JSF request processing lifecycle: applying request values, performing validation, interacting with the business logic and application model, performing navigation, and rendering a response, is a fact of life for all Web applications, JSF or not A Web application must deal with at least these concerns in order to be considered production quality A Web application can choose to let a framework, such as JSF, handle these concerns, or it can handle them explicitly The point is, in either case, they must be addressed As a simple example where no custom events are handled, one simply has to bind a UI component such as an input field to a managed bean s property and the lifecycle will automatically update the value of the managed bean s property with the value of the UI component Recall the JSF XHTML example shown earlier where an inputText component is bound to the username property of the managed bean modelBean using the JSF Expression Language (EL)
<h:inputText value="#{modelBeanusername}" />
Part I:
The JavaServer Faces Framework
To allow the user to submit the form and initiate the JSF lifecycle, a command Button UI component is added to the page using
<h:commandButton value="Click Here"/>
Since the JSF lifecycle utilizes the JavaBeans event model, the user simply clicks the rendered command button at runtime and the JSF lifecycle automatically updates the JavaBean s username property with the value provided in the input field! More in-depth coverage of the JSF request processing lifecycle as well as JSF s Expression Language is detailed in later chapters
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