JSTL/EL中的三元运算符 [英] Ternary operator in JSTL/EL

问题描述

以下JSTL标记可用于在请求范围内为变量设置值.

The following tag of JSTL can be used to set a value to a variable in a request scope.

<c:set var="value" scope="request" value="someValue"/>

我想有条件地检查设置的变量value是否为空，并使用<c:when>...</c:when>相应地显示结果，如下所示.

I want to check conditionally, if the variable value being set is empty or not and display the result accordingly something like the following, using <c:when>...</c:when>.

<c:choose>
    <c:when test="${not empty value}">
        <c:out default="None" value="${value}"/>
    </c:when>
    <c:otherwise>
        <c:out default="None" value="None"/>
    </c:otherwise>
</c:choose>

我想使用三元表达式来减少代码行，

I want to reduce the line of code using a ternary expression like,

<c:out default="None" value="${not empty value ? value : 'None'}"/>

它实际上是被评估的，但是如果我互换像这样的表达式的顺序，

It is evaluated as it actually means but if I interchange the order of the expressions like,

<c:out default="None" value="${empty value ? 'None' : value}"/>

那是一个语法错误，表明，

then it is a syntax error indicating,

"$ {空值?'无':值}"包含无效的表达式: javax.el.E​​LException:错误解析:$ {空值?'None':value}

"${empty value?'None':value}" contains invalid expression(s): javax.el.ELException: Error Parsing: ${empty value?'None':value}

那为什么会发生这种情况?

So why does this happen?

我正在使用JSTL 1.1库，其中包含以下标记库，

I'm using the JSTL 1.1 library and the following taglib is included,

<%@taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"%>

推荐答案

我在Tomcat 5.59， JSP 2.0和 JSTL 1.1中测试了以下页面.它运行没有任何错误.

I tested the following page in Tomcat 5.59, JSP 2.0 and JSTL 1.1. It ran without any errors.

<%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%> 
<c:set var="value" scope="request" value="someValue"/>
<c:out default="None" escapeXml="true" value="${not empty value ? value : 'None'}" />
<c:out default="None" escapeXml="true" value="${empty value ? 'None' : value}" />
<c:set var="value" scope="request" value="" />
<br/>
<c:out default="None" escapeXml="true" value="${not empty value ? value : 'None'}" />
<c:out default="None" escapeXml="true" value="${empty value ? 'None' : value}" />

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