Using the Unified EL to Reference Backing Beans

To bind UI component values and objects to backing bean properties or to reference backing bean methods from UI component tags, page authors use the unified expression language (EL) syntax defined by JSP 2.1. As explained in Unified Expression Language, some of the features this language offers are:

• Deferred evaluation of expressions

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

• Method expressions

These features are all especially important for supporting the sophisticated UI component model offered by JavaServer Faces technology.

Deferred evaluation of expressions is important because the JavaServer Faces life cycle is split into separate phases so that 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 #{} delimiters. The Life Cycle of a JavaServer Faces Page describes the life cycle in detail.

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 the unified EL, let’s suppose that the userNo tag of the guessNumber application referenced a method rather than using LongRangeValidator 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}" />

Those component tags that use method expressions are UIInput component tags and UICommand component tags. See sections Using Text Components and Using Command Components for Performing Actions and Navigation for more information on how these component tags use method expressions.

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. See Creating Custom Component Classes and Enabling Component Properties to Accept Expressions for more information on enabling your component’s attributes to support expressions.

To learn more about using expressions to bind to backing bean properties, see Binding Component Values and Instances to External Data Sources.

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