c：选择中的EL表达式，无法使其正常工作 [英] EL expression in c:choose, can't get it working

问题描述

我正在尝试类似这样的东西（JSF2）：

I'm trying something (JSF2) like this:

<p>#{projectPageBean.availableMethods}</p>
<c:choose>
  <c:when test="${projectPageBean.availableMethods == true}">
    <p>Abc</p>
  </c:when>
  <c:otherwise>
    <p>Xyz</p>
  </c:otherwise>
</c:choose> 

但这似乎不起作用，尽管顶部的EL表达式从false变为是的，下一段总是显示Xyz吗？

But this doesn't seem to work, although the EL expression in the top paragraph changes from false to true, the next paragraph always shows Xyz?

我还尝试将测试更改为：

I also tried to change the test to:

${projectPageBean.availableMethods}

但仍然是同样的问题！

But still the same problem!

推荐答案

首要的是： JSTL标记在视图构建期间运行，而不是在视图呈现期间运行 。

First and foremost: JSTL tags runs during view build time, not during view render time.

您的具体问题表明＃{projectPageBean} 是在视图渲染期间设置的，例如，将其定义为< ui：repeat var> ，< h：dataTable var> ，< p：tabView var> 等，因此为 null 在视图构建期间。

Your concrete problem suggests that #{projectPageBean} is been set during view render time, such as would happen when definied as <ui:repeat var>, <h:dataTable var>, <p:tabView var>, etc. It's thus null during view build time.

在这种情况下，您不应使用视图构建时间标记来控制按传统方式呈现HTML。您应该改为使用视图渲染时间组件来有条件地渲染HTML。首选使用< ui：fragment> ：

In that case, you should not be using a view build time tag to conditionally render HTML. You should instead use a view render time component to conditionally render HTML. As first choice, use <ui:fragment>:

<p>#{projectPageBean.availableMethods}</p>
<ui:fragment rendered="#{projectPageBean.availableMethods}">
    <p>Abc</p>
</ui:fragment>
<ui:fragment rendered="#{not projectPageBean.availableMethods}">
    <p>Xyz</p>
</ui:fragment> 

在Facelets中，无需在＃{} 和 $ {} 。与JSP相反，在Facelets中， $ {} 的行为与＃{} 完全相同。为了避免潜在的混乱和维护麻烦，我建议始终坚持使用＃{} 。

By the way, there's in Facelets no need to switch between #{} and ${}. In contrary to JSP, in Facelets the ${} behaves exactly the same as #{}. To avoid potential confusion and maintenance trouble, I recommend to stick to #{} all the time.

• 非JSF组件的条件渲染（普通的HTML和模板文本）

• Conditional rendering of non-JSF components (plain vanilla HTML and template text)

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