Combining a string with the value of a variable to be the name of another variable in EL

Question

I want to do something like this:

<display:table name="${summary${index}}">

But it does not work throws exception: "${summary${selChrm}}" contains invalid expression(s). I would like the user to use a drop down list to choose a selection and put in the variable index. Then the table will display the corresponding list of javabean objects, ie. named summary01, summary02, etc. How can I do that? Thanks in advance.

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Update: Thanks guys. The problem is with the nested EL. I tried to use

<display:table name="summary${index}">

But nothing shows in the table since summary01 is the variable name. If I hard code the variable name, it will work:

<display:table name="${summary01}">

Therefore, how can I do nested EL? If it is not possibly in JSTL, how can I achieve the behavior that user can use a dropdown list to determine what contents to be display in the table? Thanks again.

Affined

Please don't confuse JSTL with EL. JSTL is this taglib. EL are those ${} things: Expression Language – Gneiss

Answer 1

You cannot nest EL expressions. You can however just concatenate them.

<display:table name="${summary}${index}">

Or, if ${summary} is actually not an object in any of the scopes, do

<display:table name="summary${index}">

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Update: OK, you actually need to access ${summary01}. If you know the scope beforehand, then you can access it using the brace notation. Imagine that it's request scoped, you could access it "plain" as follows:

<display:table name="${requestScope['summary01']}">

But you can actually also use another EL variable in the braces (just unquote it):

<display:table name="${requestScope[summary]}">

So, we'd like to set it somewhere. The JSTL c:set tag is perfectly suitable for this:

<c:set var="summary" value="summary${index}" />

Gneiss

Answer 2

Step one:

<c:set var="summary" value="${requestScope['summary'.concat(index)]}" />

Step two:

<display:table name="${summary}">

Simple as that :)

UPDATE as BalusC mentioned in comments, you need 3.0 Servlets to use this.

Waechter

Note that this requires EL 2.2 (Servlet 3.0) which was relatively new at the moment this question was asked. In older EL versions you cannot call methods like that. – Gneiss

Answer 3

This is a thing that really sucks about JSTL: there's no direct way to do that if you want to do it indide a JSTL expression. There's no string concatenation operator, in other words.

You can always do

 <something foo='${something} ${something_else}'/>

but that's not always useful.

In my world, I've implemented my own "concat" function, so I can say

$(foo:bar(mystuff:concat("something", something.else))}

Lindberg

Answer 4

It worked for me.

${not empty marginLeft ? 'margin-left:'.concat(marginLeft) : ''}

Messier