Correct syntax to compare values in JSTL 2019-01-10 23:55发布 站内文章 / 前沿技术 39 0 0

问题:

This question already has an answer here:

• How to compare two object variables in EL expression language? 2 answers

I have an if statement that I am trying to perform with JSTL.

My code is below (the variables values is an ArrayList that contains a user defined object and type is a private property of that object, with public getter/setter methods):

<c:forEach items="${list}" var="values">
    <c:if test="${values.type}=='object'">
        <!-- code here -->
    </c:if>
</c:forEeach>

What would be the correct syntax of the part within the test attribute. The docs don't really help with that part http://download.oracle.com/javaee/5/jstl/1.1/docs/tlddocs/index.html

Thanks.

回答1:

The comparison needs to be evaluated fully inside EL ${ ... }, not outside.

<c:if test="${values.type eq 'object'}">

As to the docs, those ${} things are not JSTL, but EL (Expression Language) which is a whole subject at its own. JSTL (as every other JSP taglib) is just utilizing it. You can find some more EL examples here.

<c:if test="#{bean.booleanValue}" />
<c:if test="#{bean.intValue gt 10}" />
<c:if test="#{bean.objectValue eq null}" />
<c:if test="#{bean.stringValue ne 'someValue'}" />
<c:if test="#{not empty bean.collectionValue}" />
<c:if test="#{not bean.booleanValue and bean.intValue ne 0}" />
<c:if test="#{bean.enumValue eq 'ONE' or bean.enumValue eq 'TWO'}" />

See also:

• Our EL wiki page

---

By the way, unrelated to the concrete problem, if I guess your intent right, you could also just call Object#getClass() and then Class#getSimpleName() instead of adding a custom getter.

<c:forEach items="${list}" var="value">
    <c:if test="${value['class'].simpleName eq 'Object'}">
        <!-- code here -->
    </c:if>
</c:forEeach>

See also:

• instanceof check in EL expression language