JSF 2.1 ValueExpression in action-attribute

Question

Section 3.1.4 of the JSF 2.1 Specification says that all attributes of standard components are value expression enabled.

I want to assign a value expression to the action attribute of a commandButton:

<h:commandButton value="OK" action="#{myBean.valExp}" />

I also defined corresponding getValExp and setValExp methods in the bean's class.

However my JSF implementation (JBoss 6) takes that expression to be a method expression and thus yields a "method not found" error because there's no valExp()-method.

Am I doing something wrong or is the specification just too sloppy and actually doesn't mean all, but only the non method expression attributes? Or am I misunderstanding the spec?

[Remark: The reason for this question is no actual technical problem but me trying to understand the difference of value and method expressions.]

Answer 1

Value expressions are bound to properties which are exposed by public getter/setter methods.

<h:inputText value="#{bean.value}" />

This requires public T getValue() and public void setValue(T value) methods. Note that the presence of a private T value; property with literally exactly the same name is not mandatory. In pure output components like <h:outputText>, <h:dataTable>, <f:selectItems>, etc, the setter method is also not mandatory.

Method expressions are bound to non-getter/setter methods ("action" methods).

<h:commandButton value="submit" action="#{bean.submit}" />

This requires a public T submit() method where T can eventually be void and the method can eventually take additional arguments, depending on the attribute's method expression signature. You can read the exact details in the view declaration language documentation, for example the <h:inputText>, <h:commandButton> and <f:ajax> ones. Here's an extract of the action and actionListener attribute definitions of the <h:commandButton>:

Name:        action
Type:        javax.el.MethodExpression (signature must match java.lang.Object
             action())
Description: MethodExpression representing the application action to invoke when 
             this component is activated by the user. The expression must 
             evaluate to a public method that takes no parameters, and returns an 
             Object (the toString() of which is called to derive the logical 
             outcome) which is passed to the NavigationHandler for this 
             application.

Name:        actionListener
Type:        javax.el.MethodExpression (signature must match void 
             actionListener(javax.faces.event.ActionEvent))     
Description: MethodExpression representing an action listener method that will be
             notified when this component is activated by the user. The
             expression must evaluate to a public method that takes an 
             ActionEvent parameter, with a return type of void, or to a public 
             method that takes no arguments with a return type of void. In the 
             latter case, the method has no way of easily knowing where the event
             came from, but this can be useful in cases where a notification is 
             needed that "some action happened". 

Yes, I agree that the spec is somewhat sloppy in stating that all attributes support value expressions. Generally, they actually mean that all attributes support expression language as in #{}. From the other hand on, you can also interpret method expressions as if they are just "special" value expressions, although they are not exactly that. I've posted a spec issue report about this with the request to clear up some confusion: issue 1036.