Introducing JavaServer Faces
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by Babita Baliarsingha
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JSF Life Cycle

The life cycle of a JavaServer Faces page is similar to that of a JSP page. The life cycle of a JSF web application starts when user makes a request.

When the client submits a page, the JavaServer Faces implementation must perform several tasks, such as validating the data input of components in the view and converting input data to types specified on the server side. So the series of steps that an application follows is called the life cycle.

A JavaServer Faces application supports two kinds of responses and two kinds of requests.

Faces response: A servlet response is one that is created by the execution of the Render Response Phase of the request processing life cycle.

Non-Faces response: A servlet response is not created by the execution of the render response phase. An example is a JSP page that does not incorporate JavaServer Faces components.

Faces request: A servlet request is sent from a previously generated Faces response. An example is a form submission from a JavaServer Faces user interface component, where the request URI identifies the JavaServer Faces component tree to use for processing the request.

Non-Faces request: A servlet request is sent to an application component, such as a servlet or JSP page, rather than directed to a JavaServer Faces component tree.

Figure 1: JavaServer Faces request-response life cycle

The JSF application has six steps in its life cycle.

1.    Restore View Phase

2.    Apply Request Value Phase

3.    Process Validations Phase

4.    Update Model Value Phase

5.    Invoke Application Phase

6.    Render Response Phase

1. Restore View Phase

RestoreView is the first phase in the JSF lifecycle. This phase is used for constructing view to display in the front end. Every view has its own view ID and it is stored in the FacesContext's session object. JSF View is a collection of components associated with its current state. There are two types of state saving mechanism.

·         Server

·         Client

The default value is server. If you specify state saving method as the server, the state of each component will be stored in the server. If it is client, it will be stored in the client side as hidden variables. This value is configured in web.xml as follows.

Listing 1

<context-param>
<param-name>javax.faces.STATE_SAVING_METHOD</param-name>
<param-value>client</param-value>
</context-param>

In this phase a request comes through the FacesServlet controller. The controller examines the request and extracts the view ID, which is determined by the name of the JSP page.

The JSF framework controller uses the view ID to look up the components for the current view. If the view does not already exist, the JSF controller creates it. If the view already exists, the JSF controller uses it. The view contains all the GUI components.

This phase of the lifecycle presents three view instances: new view, initial view, and postback, with each one being handled differently. In the case of a new view, JSF builds the view of the Faces page and wires the event handlers and validators to the components. The view is saved in a FacesContext object.

The FacesContext object contains all the state information JSF needs to manage the GUI component's state for the current request in the current session. The FacesContext stores the view in its viewRoot property; viewRoot contains all the JSF components for the current view ID.

In the case of an initial view (the first time a page is loaded), JSF creates an empty view. The empty view will be populated as the user causes events to occur. From an initial view, JSF advances directly to the render response phase.

In the case of a postback (the user returns to a page he or she has previously accessed), the view corresponding to the page already exists, so it needs only to be restored. In this case, JSF uses the existing view's state information to reconstruct its state. The next phase after a postback is the apply request values.

2. Apply Request Value Phase

The purpose of this phase is for each component to retrieve its current state. After the restoring of component tree in the previous phase each component in the tree retrieves its new value and stores it locally. Component values are typically retrieved from the request parameters. If immediate attribute of a component is set to true, then the validation, conversion, and events associated with the component is processed in this phase.

If a component's immediate event handling property is not set to true, the values are converted. Suppose field is bound to be an Integer property, the value is converted to an Integer. An error message associated with the component is generated if this conversion fails, and queued in the FacesContext. This message will be displayed during the render response phase, along with any validation errors resulting from next process validation phase.

At the end of this phase, the components are set to their new values, and messages and events have been queued.

3. Process Validations Phase

The first event handling of the lifecycle takes place after the apply request values phase. At this stage, each component will have its values validated against the application's validation rules. The validation rules can be pre-defined (shipped with JSF) or defined by the developer. Values entered by the user are compared to the validation rules. If an entered value is invalid, an error message is added to FacesContext, and the component is marked invalid. If a component is marked invalid, JSF advances to the render response phase, which will display the current view with the validation error messages. If there are no validation errors, JSF advances to the update model values phase.

4. Update Model Value Phase

After confirming that data is valid in the previous phase, local values of components can be set to corresponding server side object properties- backing beans. So bean properties will be updated. If the local data cannot be converted to the types specified by the bean properties, the life cycle proceeds directly to the render response phase and errors are displayed.

5. Invoke Application Phase

During this phase, the action method of any command button or link that was activated is called. In addition, any events that were generated during previous phases and that have not yet been handled are passed to the web application so that it can complete any other processing of the request that is required.

6. Render Response Phase

The response UI components are rendered, and the response is sent to the client. The state of the UI components is saved so that the component tree can be restored when the client sends another request. For a JSF-enabled application, the thread of execution for a request/response cycle can flow through each phase, in the order listed here and as shown in Figure below. However, depending on the request and what happens during the processing and response, not every request will flow through all six phases.

In Figure 1, there are a number of optional paths through the life cycle. For example, if errors occur during any of the phases, the flow of execution transfers immediately to the Render Response phase, skipping any remaining phases. One way this might occur is if input data is incorrect or invalid. If data fails validation in either the Apply Request Values or Process Validations phase, information about the error is saved and processing proceeds directly to the Render Response phase. Also, if at any point in the life cycle the request processing is complete and a non-JSF response is to be sent to the client, the flow of execution can exit the life cycle without completing further phases.

Let us start with an example to display "Hello World." The code is given below.

Listing 2

<%@page contentType="text/html"%>
 <%@page pageEncoding="UTF-8"%> 
<%@taglib prefix="f" uri="http://java.sun.com/jsf/core"%>
 <%@taglib prefix="h" uri="http://java.sun.com/jsf/html"%>
<html>
 <head>
 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
 <title>JSP Page</title>
 </head>
 <body>
 <f:view>
 <h1><h:outputText value="Hello World" /></h1>
 </f:view>
 </body>
 </html>

In order to use Java Server Faces Components in JSP pages, we need to have access to the two tag library, the HTML component tag library and the core tag library using html and core tag lib. We will discuss each one with the features in coming sections. The code above has "f:view" as the Core Tag which represents the root of view and all JSF component tags must be inside view tag and "h:outputText" is the HTML Tag which represents the Label.

Output

The following output will get printed on the screen.

Hello World


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User Comments

Title: JSF   
Name: prasannajeet upadhya
Date: 2008-06-27 2:14:30 AM
Comment:
It is a good article.I request the auther to writes about the business logics used in java.
Thank you
prasannajeet upadhya.
Title: java server faces   
Name: Rajiv Srivastav
Date: 2008-06-27 2:10:46 AM
Comment:
hello madam
It is a good article.but u need to focus on the life span of JSF.I really appritiates with ur article.wish u all the best.
Title: Java Server Faces   
Name: Amarnath Singh
Date: 2008-06-26 5:56:03 AM
Comment:
It is a nice article.I go through this article in deep and came to know that it's really the best article.Here the basics of JSF has been clearly mentioned which is very use ful for the learner.
Thank You.
Amarnath Singh.

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