Using the EL to Reference Backing Beans

To bind component values and objects to backing bean properties or to reference backing bean methods from component tags, page authors use the unified expression language (EL) syntax. As explained in Overview of EL, the following are some of the features that EL offers:

• Deferred evaluation of expressions

• The ability to use a value expression to both read and write data

• Method expressions

Deferred evaluation of expressions is important because the JavaServer Faces lifecycle is split into several phases where component event handling, data conversion and validation, and data propagation to external objects are all performed in an orderly fashion. The implementation must be able to delay the evaluation of expressions until the proper phase of the life cycle has been reached. Therefore, its tag attributes always use deferred evaluation syntax, which is distinguished by the #{} delimiter.

In order to store data in external objects, almost all JavaServer Faces tag attributes use lvalue value expressions, which are expressions that allow both getting and setting data on external objects.

Finally, some component tag attributes accept method expressions that reference methods that handle component events, or validate or convert component data.

To illustrate a JavaServer Faces tag using EL, let’s suppose that a tag of an application referenced a method to perform the validation of user input:

<h:inputText id="userNo"
     value="#{UserNumberBean.userNumber}"
     validator="#{UserNumberBean.validate}" />

This tag binds the userNo component’s value to the UserNumberBean.userNumber backing bean property using an lvalue expression. It uses a method expression to refer to the UserNumberBean.validate method, which performs validation of the component’s local value. The local value is whatever the user enters into the field corresponding to this tag. This method is invoked when the expression is evaluated, which is during the process validation phase of the life cycle.

Nearly all JavaServer Faces tag attributes accept value expressions. In addition to referencing bean properties, value expressions can also reference lists, maps, arrays, implicit objects, and resource bundles.

Another use of value expressions is binding a component instance to a backing bean property. A page author does this by referencing the property from the binding attribute:

<inputText binding="#{UserNumberBean.userNoComponent}" />

In addition to using expressions with the standard component tags, you can also configure your custom component properties to accept expressions by creating ValueExpression or MethodExpression instances for them.

For information on EL, see Chapter 6, Unified Expression Language.

For information on referencing backing bean methods from component tags, see Referencing a Backing Bean Method.