The Java EE 5 Tutorial

Method Expressions

Another feature of the unified expression language is its support of deferred method expressions. A method expression is used to invoke an arbitrary public method, which can return a result. A similar feature of the unified EL is functions. Method expressions differ from functions in many ways. Functions explains more about the differences between functions and method expressions.

Method expressions primarily benefit JavaServer Faces technology, but they are available to any technology that can support the unified expression language. Let’s take a look at how JavaServer Faces technology employs method expressions.

In JavaServer Faces technology, a component tag represents a UI component on a page. The component tag uses method expressions to invoke methods that perform some processing for the component. These methods are necessary for handling events that the components generate and validating component data, as shown in this example:

        action="#{customer.submit}" />

The inputText tag displays a UIInput component as a text field. The validator attribute of this inputText tag references a method, called validateName, in the bean, called customer. The TLD (see Tag Library Descriptors) that defines the inputText tag specifies what signature the method referred to by the validator attribute must have. The same is true of the customer.submit method referenced by the action attribute of the commandButton tag. The TLD specifies that the submit method must return an Object instance that specifies which page to navigate to next after the button represented by the commandButton tag is clicked.

The validation method is invoked during the process validation phase of the life cycle, whereas the submit method is invoked during the invoke application phase of the life cycle. Because a method can be invoked during different phases of the life cycle, method expressions must always use the deferred evaluation syntax.

Similarly to lvalue expressions, method expressions can use the . and [] operators. For example, #{object.method} is equivalent to #{object["method"]}. The literal inside the [] is coerced to String and is used to find the name of the method that matches it. Once the method is found, it is invoked or information about the method is returned.

Method expressions can be used only in tag attributes and only in the following ways: