How to compare two object variables in EL expression language?

问题

I am creating a drop down list of all languages. The default language selection for the list will be determined by information added by the user:

<select>
    <c:forEach items=\"${languages}\" var=\"lang\">
        <c:choose>
            <c:when test=\"${lang}.equals(${pageLang})\">
                <option value=\"${lang}\" selected>${lang}</option>
            </c:when>
            <c:otherwise>
                <option value=\"${lang}\">${lang}</option>
            </c:otherwise>
        </c:choose>
    </c:forEach>
</select>

.equals doesn\'t appear to exist in EL. Having had a look here it\'s suggested I write my own function and then import and use that. As this is a one off tiny thing just for this page I don\'t want to have to start creating libraries etc just for this. Nor do I want to start creating specialist objects for the servlet to return with this extra info in them.

Only thing I can think to do is to return the actual html for the whole option line from the servlet rather than just the language string, but that strikes me as ugly so I\'m hoping there\'s a more elegant solution.

What is the best plan for a quick fix to comparing two strings in EL?

The J2EE 1.4 Tutorial

回答1:

In Expression Language you can just use the == or eq operator to compare object values. Behind the scenes they will actually use the Object#equals(). This way is done so, because until with the current EL 2.1 version you cannot invoke methods with other signatures than standard getter (and setter) methods (in the upcoming EL 2.2 it would be possible).

So the particular line

<c:when test="${lang}.equals(${pageLang})">

should be written as (note that the whole expression is inside the { and })

<c:when test="${lang == pageLang}">

or, equivalently

<c:when test="${lang eq pageLang}">

Both are behind the scenes roughly interpreted as

jspContext.findAttribute("lang").equals(jspContext.findAttribute("pageLang"))

If you want to compare constant String values, then you need to quote it

<c:when test="${lang == 'en'}">

or, equivalently

<c:when test="${lang eq 'en'}">

which is behind the scenes roughly interpreted as

jspContext.findAttribute("lang").equals("en")

回答2:

Not sure if I get you right, but the simplest way would be something like:

<c:if test="${languageBean.locale == 'en'">
  <f:selectItems value="#{customerBean.selectableCommands_limited_en}" />
</c:if>

Just a quick copy and paste from an app of mine...

HTH

来源：https://stackoverflow.com/questions/1900843/how-to-compare-two-object-variables-in-el-expression-language

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jsp

operators

jstl

object-comparison