How to evaluate a dynamic/nested Spring property placeholder expression?

Question

I'm working on a JSP tag. Here is the old line that starts looping through items in a model:

<c:forEach var="toc" items="${requestScope[formKey].model.sharingTocs}">

But the code has been refactored so the model path (model.sharingTocs above) is now dynamic rather than fixed. It is now passed into the tag via a JSP @attribute:

<%@attribute name="path" required="true"%>

So ${path} now evaluates to "model.sharingTocs".

How can items now be assigned?

Answer 1

Well. Good question.

This is a solution: writing a custom jstl tag to Evaluate a property expression of a bean:

<mytag:eval bean="${requestScope['formKey']}" propertyExpression = "${path}" var="items" />

And ForEach:

<c:forEach var="toc" items="${items}">
</c:forEach>

Sample code of mytag:eval JSTL tag ( Classic model )

public class EvalTag extends TagSupport
{

    private Object bean;
    private String propertyExpression; //Ex: 'model.sharingTocs'
    private String var;

    //............


    @Override
    public int doEndTag() throws JspException {
        try {

            // Use reflection to eval propertyExpression ('model.sharingTocs') on the given bean

            Object propObject = SomeLibs.eval ( this.bean, this.propertyExpression);

            this.pageContext.getRequest().setAttribute(this.var, propObject);

            // You can add propObject into Other scopes too.

        } catch (Exception ex) {
            throw new JspTagException(ex.getMessage(), ex);
        }
        return EVAL_PAGE;
    }

      //............
      // SETTERS here
 }

A lib you can use to eval propertyExpression of a bean is Apache bean utils.

http://commons.apache.org/proper/commons-beanutils/apidocs/org/apache/commons/beanutils/package-summary.html#standard.nested

Answer 2

If you are using spring you can utilise the spring tag library, it also assumes you are inside a form:form tag.

<%@ taglib prefix="spring" uri="http://www.springframework.org/tags" %>

<%@ attribute name="path" required="false" %>

<spring:bind path="${path}">
    <c:forEach var="item" items="${status.value}">
          ${item}
    </c:forEach>
</spring:bind>

Answer 3

It's been quite a while since I asked this question but (with new knowledge gained since) I think this ought to work:

<c:set var="itemsPath" value="requestScope[formKey].${path}"/>
<c:forEach var="toc" items="${itemsPath}">

i.e. set up an intermediary JSTL variable with the full path to the items and evaluate that one instead.