评估是否设置了MethodExpression属性(获取PropertyNotFoundException) [英] Evaluating if MethodExpression attribute is set (getting PropertyNotFoundException)

问题描述

我有一个具有MethodExpression属性changeListener的UI组件:

I have a UI 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>

此changeListener属性是在remoteCommand中用作 actionListener 的可选方法表达式，并且仅在设置了changeListener属性的情况下，我才想呈现<p:remoteCommand>.

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.

我尝试了几种检查属性是否设置的方法，尤其是:

I have tried several ways to check whether the attribute is set or not, especially:

<c:if test="#{! empty cc.attrs.changeListener}">

和

<p:remoteCommand rendered="#{cc.attrs.changeListener != null}" />

但是我得到了javax.el.PropertyNotFoundException，因为它试图将属性评估为属性.

But I get a javax.el.PropertyNotFoundException because it tries to evaluate the attribute as a property instead.

如何评估是否设置了可选方法属性?

How can I evaluate whether the optional method attribute is set or not ?

谢谢

推荐答案

您已经正确地使用了<c:if>. rendered永远无法工作.您只需要检查 EL设置了表达式，而不是将整个EL表达式实际评估为值表达式并检查其结果是否为空，这当然会失败，如果EL表达式表示方法表达式.

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 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>

然而，这种解决方案有些令人恐惧:您在这里将方法表达式作为值表达式.但是，只要您实际上不评估包含的EL表达式(就像您最初的#{cc.attrs.changeListener}尝试在幕后所做的一样)，那么就没有任何问题了.没有其他干净的方法，因为JSF API中没有像UIComponent#getMethodExpression()这样的东西.

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 no other clean way as there's nothing like UIComponent#getMethodExpression() in JSF API.

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