Question

I’m using an API that makes heavy use of Iterables (java.lang.Iterable), but I can’t use them directly on my JSP pages because JSTL tags and EL can not process them. Right now, I’m transforming each iterable to a list prior to rendering them.

What would be the cleanest and simplest approach to make it work without previous transformations?
Do newest JSTL-Jasper-EL-taglibs-etc. versions support it? Where can I found that information? I don’t find anything about it googling…

I know I can use Iterable.iterator(), but I can’t call that method inside the JSP, only in my controller class, and this is very limiting.

Answer 1

You can implement your own JSP tags in Java, so you could write a tag that accepted an Iterable and then use that in the same way as you’d use the JSTL forEach tag.

This documentation has an example on how to create a custom tag that iterates through an arbitrary custom object – your implementation would likely be simpler. The key is in returning EVAL_BODY_AGAIN to allow you to process the body of the tag multiple times.

http://java.sun.com/j2ee/tutorial/1_3-fcs/doc/JSPTags6.html#68297

I haven’t tried running this, but it might be something to start with:

public class IterableForEachTag extends BodyTagSupport {
    private static final long serialVersionUID = 1L;
    private Iterable<?> iterable = null;
    private Iterator<?> iterator = null;
    private String var = null;

    public int doStartTag() throws JspException {
        if (iterator == null) {
            iterator = iterable.iterator();
        }
        if (iterator.hasNext()) {
            Object element = iterator.next();
            pageContext.setAttribute(var, element);
            return (EVAL_BODY_AGAIN);
        } else
            return (SKIP_BODY);
    }

    public int doAfterBody() throws JspException {
        if (bodyContent != null) {
            try {
                JspWriter out = getPreviousOut();
                out.print(bodyContent.getString());
                bodyContent.clearBody();
            } catch (IOException e) {
                throw new JspException(e);
            }
        }
        if (iterator.hasNext()) {
            Object element = iterator.next();
            pageContext.setAttribute(var, element);
            return (EVAL_BODY_AGAIN);
        } else {
            return (SKIP_BODY);
        }
    }

    public void setIterable(Iterable i) {
        iterable = i;
        iterator = null;
    }

    public void setVar(String v) {
        var = v;
    }
}