Sunday, April 29, 2007

Making a simple JSF application

Here's how we build a simple JSF application:

  1. Configure web.xml for FacesServlvet
  2. Create faces-context.xml (This is the file from where FacesServlet reads navigation rules, details of managed beans and other JSF specific configuration details)
  3. Create a controller or controllers for the application. Here is where JSF differs from Struts. Struts had a front controller, the ActioServlet which delegated requests to Action classes. JSF does not have the notion of a front controller. A JSF application uses an event model. The UI is tied to the backend with events. Controller classes (also known as backing beans in JSF) accept these events and also hold properties that accept user supplied values from a form. These properties will also be used to display data on pages. For example if we have a simple application that allows employees to put in a leave request, a backing bean will have properties that will be populated with the request that the user enters on the form. The same backing bean may also be responsible for displaying the leave status to the user. Essentially we will have one backing bean class for a related set of UI views. The view pages for adding, editing, viewing, and deleting "leave requests" will be mnaged by the same backing bean class. A JSF application can have many backing beans, thus resulting in many controllers. Even though we can code business logic in these backing beans, we should desist from doing so. The business logic must in other application classes which are invoked from the backing bean. This way the backing bean acts as a glue between the view and the model (which is what a controller should be doing anyways).
  4. Create the view (JSP) files.
    1. All the JSF tags should be put within an f:view tag. Components placed within this tag are managed by the JSF system. If we not use f:view, then the JSF system will remain oblivious of the components.
    2. Fields in a JSF form can be associated with beacking bean properties using JSF - EL. For example #{BeanName.PropertyName}. JSF-EL is similar to JSTL-EL, however, it directly associates a field (within which it is used) with a backing bean property (or a properties property). These properties are used to hold values submitted by users as well as hold values that will be displayed to the user on a form.
    3. We can associate commandButtons to methods in the backing beans, using JSF-EL. I believe we can associate them with events, in which case appropriate event handlers will be invoked[verify this].
    4. The JSF system can also run simple validations on the form fields automatically. Things like required fields, or fields that represent numbers can be validated automatically. More complex validations can be defined in custom validators.

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