Introduction to JavaServer Faces, Part 1

A JSF application looks like any other servlet/JSP application. It has a deployment descriptor, JSP pages, custom tag libraries, static resources, and so on. The user interface of a JSF application is one or many JSP pages that host JSF components such as forms, input boxes, and buttons. These components are represented by JSF custom tags and can hold data. A component can be nested inside another component, and it is possible to draw a tree of components. In fact, a JSP page in a JSF application is represented by a component tree. Just as in normal servlet/JSP applications, you use JavaBeans to store the data the user entered.

In this chapter, you will get an overview of how JSF works and the steps to writing a simple JSF application. Then you will put this knowledge to use by working through three examples. Finally, you will be introduced to the JSF Application Programming Interface (API).

How Does JSF Work?

A JSF application works by processing events triggered by the JSF components on the pages. These events are caused by user actions. For example, when the user clicks a button,the button triggers an event. You, the JSF programmer, decide what the JSF application will do when a particular event is fired. You do this by writing event listeners. In other words, a JSF application is event-driven. Figure 1 illustrates the processing of a JSF application.

When an event occurs (say, when the user clicks a button), the event notification is sent via HTTP to the server. On the server is a special servlet called the FacesServlet. Each JSF application in the Web container has its own FacesServlet.

In the background, three things happen for each JSF request, as illustrated in Figure 2.

For JSF requests to be processed, they must be directed to a servlet called FacesServlet. The redirection is accomplished by using the following servlet and servlet-mapping tags in the deployment descriptor:


<!-- Faces Servlet -->
<servlet>
  <servlet-name>Faces Servlet</servlet-name>
  <servlet-class>javax.faces.webapp.FacesServlet</servlet-class>
  <load-on-startup>1</load-on-startup>
</servlet>
<!-- Faces Servlet Mapping -->
<servlet-mapping>
  <servlet-name>Faces Servlet</servlet-name>
  <url-pattern>/faces/*</url-pattern>
</servlet-mapping>
<!-- Faces Servlet --><SERVLET><SERVLET-NAME></SERVLET-NAME><SERVLET-CLASS></SERVLET-CLASS><LOAD-ON-STARTUP></LOAD-ON-STARTUP></SERVLET><!-- Faces Servlet Mapping --><SERVLET-MAPPING><SERVLET-NAME></SERVLET-NAME><URL-PATTERN></URL-PATTERN></SERVLET-MAPPING>


This means that the URL of every request must contain the /faces/ pattern, as specified in the url-pattern element under the servlet-mapping element. 

Figure 1 JSF applications are event-driven

NOTE You can specify a context parameter saveStateInClient with a value of true to force JSF to save state in the client as opposed to saving it in the server. If you choose to do so, you must add the following context-param element before the servlet element in your deployment descriptor.


<context-param>
  <param-name>saveStateInClient</param-name>
  <param-value>false</param-value>
</context-param>



FacesServlet creates an object called FacesContext, which contains information necessary for request processing. To be more precise, FacesContext contains the ServletContext, ServletRequest, and ServletResponse objects that are passed to the service method of FacesServlet by the Web container. During processing, FacesContext is the object that is modified. 

Figure 2 How JSF works in a nutshell 

Next is the processing. The processor is an object called Lifecycle. The FacesServlet servlet hands over control to the Lifecycle object. The Lifecycle object processes the FacesContext object in six phases, which we will look at next.

NOTE The series of actions necessary for JSF request processing by the Lifecycle object is referred to as the request processing lifecycle. You will encounter this term throughout this book.

JSF also allows you to configure a JSF application via an application configuration file. After discussing the Lifecycle object phases, we will discuss how to use this configuration file to register JavaBeans.

Remember: This is part one of the second chapter of JavaServer Faces Programming, by Budi Kurniawan (McGraw-Hill/Osborne, ISBN 0-07-222983). Stay tuned for part 2 of "Introduction to JavaServer Faces (JSF)," where we learn about JSP, JavaBeans, and Model 2. Buy this book!