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

我有一个带有 MethodExpression 属性的 UI 组件 changeListener:

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>

这个 changeListener 属性是一个可选的方法表达式，在 remoteCommand 中用作 actionListener，我想渲染 仅当 changeListener 属性已设置时.

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.

我尝试了几种方法来检查属性是否设置，特别是:

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}" />

但是我得到一个 javax.el.PropertyNotFoundException 因为它试图将属性作为一个属性来评估.

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

如何判断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

<c:if> 你的方向是正确的.rendered 永远不会工作.您只需要检查 EL设置表达式，而不是实际将整个 EL 表达式评估为值表达式并检查其结果是否不为空，如果 EL 表达式表示方法表达式，这当然会失败.

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，这个解决方案有点吓人:你在这里将方法表达式作为值表达式来获取.但是，只要您不实际评估封闭的 EL 表达式(就像您最初的 #{cc.attrs.changeListener} 尝试在幕后所做的那样)，那么就没有任何问题.没有其他干净的方法，因为在 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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