Guards a method with a condition. The method cannot be called by a web request if the condition is not met.
Additionally, if the method is exposed as a button, this annotation will cause the button
to be hidden or disabled if the condition is not met, depending on the type parameter.
The test condition, an OGNL expression that must return a boolean. The root object for the expression
is the action bean to which the method belongs; it is therefore easy to place the condition in a method
and call it in the expression.