I have a composite component with a MethodExpression attribute changeListener:
<composite:interface>
<composite:attribute name="changeListener" required="false" method-signature="void actionListener(javax.faces.event.ActionEvent)" />
..
</composite:interface>
<composite:implementation>
<p:remoteCommand name="ajaxOnChange"
update="#{cc.attrs.onChangeUpdate}"
oncomplete="#{cc.attrs.onchange}"
actionListener="#{cc.attrs.changeListener}" />
..
</composite:implementation>
This changeListener attribute is an optional method expression used as actionListener in the remoteCommand and I want to render the <p:remoteCommand> ONLY IF the changeListener attribute has been set.
I have tried several ways to check whether the attribute is set or not, especially:
<c:if test="#{! empty cc.attrs.changeListener}">
and
<p:remoteCommand rendered="#{cc.attrs.changeListener != null}" />
But I get a javax.el.PropertyNotFoundException because it tries to evaluate the attribute as a property instead.
How can I evaluate whether the optional method attribute is set or not?
You was in the right direction with <c:if> already. The rendered one is never going to work. You only need to check if the raw EL expression is been set instead of actually evaluating the whole EL expression as a value expression and checking if its result is not empty, which would of course fail if the EL expression represents a method expression.
<c:if test="#{not empty cc.getValueExpression('changeListener')}">
...
</c:if>
This solution is however somewhat scary: you're grabbing the method expression as a value expression here. However, as long as you don't actually evaluate the enclosed EL expression (like as what your initial #{cc.attrs.changeListener} attempt does under the covers), then there's nothing at matter. There's unfortunately no other clean way as there's nothing like UIComponent#getMethodExpression() in Faces API.