有条件地渲染< h：commandButton>使用< c：if>

问题描述

我正在使用jstl进行小型测试。并且它不起作用它应该如何工作

I am using jstl for small testing. And it is not working how it is supposed to work

这里是小代码：

<c:set var="id" value="#{mBlog.blog.id}"/>
                    ${id}    //printing  4
                    <c:if test="${id > 0}">
                        <h:commandButton value="Update" action="#{mBlog.update}"/>  //is not rendered
                    </c:if>

                    <c:if test="${id == 0}">
                        <h:commandButton value="Save" action="#{mBlog.save}"/>  //is not rendered
                    </c:if>

我不知道有什么问题。在显示器中我只看到4，没有别的。

I do not know what is wrong. in the display i see only 4, nothing else.

推荐答案

JSTL标签和JSF UI组件不会同步运行期待编码。长话短说：阅读本文小心。

JSTL tags and JSF UI components doesn't run in sync as you'd expect from the coding. Long story short: JSTL in JSF2 Facelets... makes sense? Read this carefully.

在您的特定情况下，JSTL < c：if> 标记条件似乎取决于JSF < f：event> 标记的结果。目前< c：if> 运行时，在视图构建期间，< f：event> 尚未运行，因为它在预渲染视图事件期间运行。因此＃{mBlog.blog.id} 始终是 null 或任何其他默认值。

In your particular case, the JSTL <c:if> tag condition seems to be dependent on the result of the JSF <f:event> tag. At the moment the <c:if> runs, during the view build time, the <f:event> hasn't run yet, because it runs during pre render view event. Hence the #{mBlog.blog.id} is always null or any other default.

您需要使用JSF组件的呈现的属性。它还可以使代码更清晰。

You need to use JSF component's rendered attribute instead. It also keeps your code cleaner.

<h:commandButton value="Update" action="#{mBlog.update}" rendered="#{mBlog.blog.id gt 0}" />
<h:commandButton value="Save" action="#{mBlog.save}" rendered="#{mBlog.blog.id eq 0}" />

然而，在调用操作时，您可能会遇到另一个潜在的未来问题。确保将bean放在视图范围内，而不是放在请求范围内。

You've however another potential future problem when invoking the action. Make sure that the bean is put in view scope, not in the request scope.

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