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 Expression Language syntax. As explained in Overview of the 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 in which 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 lifecycle has been reached. Therefore, the implementation’s tag attributes always use deferred-evaluation syntax, which is distinguished by the #{} delimiter.

To store data in external objects, almost all JavaServer Faces tag attributes use lvalue 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 the EL, 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 by using an lvalue expression. The tag 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.

Nearly all JavaServer Faces tag attributes accept value expressions. In addition to referencing bean properties, value expressions can 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 configure your custom component properties to accept expressions by creating javax.el.ValueExpression or javax.el.MethodExpression instances for them.

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

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