Section 18.104.22.168 in the EL specification mentions that a method expression can consist of a single variable:
As it appears, it's not entirely clear how such single variable should be evaluated.
In the reference implementation for example, we see that in this case a MethodExpressionImpl will reference an AstIdentifier, which will use the context's variable mapper to obtain a ValueExpression. The value is then obtained from this value expression, and it's expected to be a MethodExpression, which is then invoked.
This can be seen at http://java.net/projects/el-spec/sources/source-code/content/trunk/impl/src/main/java/com/sun/el/parser/AstIdentifier.java?rev=198 where the method getMethodExpression at line 198 demonstrates the "value expression wrapping a method expression" assumption. invoke at line 183 then shows this obtained method expression is simply invoked.
Comments on line 198 and 209 explicitly mention 2 cases:
case A: ValueExpression exists, getValue which must
be a MethodExpression
case B: evaluate the identity against the ELResolver, again, must be
a MethodExpression to be able to invoke
These cases however are not outlined in the specification. As a result, alternative EL implementations (like JUEL) have taken a completely different approach. In the case of JUEL, it also expects a ValueExpression, but then guessed that this value expression should be wrapping a Method. This on its turn leads to unexpected behavior and bugs such as reported here: http://code.google.com/p/omnifaces/issues/detail?id=100
In order to ensure portability between EL implementations, I would like to request this specific case to be clarified in the specification.