JSTL forEach to iterate over user defined class implementing Iterable

Question

my ask is similar to the one asked here: JSTL - Using forEach to iterate over a user-defined class However, the (obvious and simple) solution does not work for me, so I am asking again looking for any advice.

I have a simple class implementing Iterable interface, like:

public class Lizt implements Iterable<String> {

  private ArrayList<String> inner = new ArrayList<>();

  public Lizt(){
    this.inner.add("A");
    this.inner.add("B");
  }

  @Override
  public Iterator<String> iterator() {
    return inner.iterator();
  }

  public Iterable<String> getInner(){
    return this.inner;
  }
}

(Ignore "getInner()" function for now). Then I have a simple JSP page:

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<jsp:useBean id="lst" scope="session" class="model.Lizt"/>
<html>
<body>
<ul>
<c:forEach var="item" items="${lst}">
    <li>${item}</li>
</c:forEach>
</ul></body></html>

So, I am trying to iterate over an instance implementing Iterable. However, the server returns me (among the others)

javax.servlet.ServletException: org.apache.jasper.JasperException: An exception occurred processing JSP page [/forEachTest.jsp] at line [7]
...
Caused by: javax.servlet.ServletException: javax.servlet.jsp.JspTagException: Don't know how to iterate over supplied "items" in &lt;forEach&gt;

If I replace the JSP expression to access directly the internal inner field, like

<c:forEach var="item" items="${lst.inner}">

... everything works fine. Where is the difference between "my" Iterable and ArrayList's Iterable?

Thanks for any advice in advance.

score 3 · Answer 1 · answered Oct 19 '18 at 16:39

The difference is that ArrayList is a Collection, and your class is not. Here's what the specification of the JSTL says:

Collections Supported & Current Item The data types listed below must be supported for items [...]

Arrays [...]

Implementation of java.util.Collection [...]

Implementation of java.util.Iterator [...]

Implementation of java.util.Enumeration [...]

Implementation of java.util.Map [...]

String

As you can see, Iterable is not listed.

score 3 · Answer 2 · answered Oct 19 '18 at 16:42

As the comment by Adam Gent to the accepted answer there states:

Caveat to that is that JSTL forEach does not support iterable (extremely annoying) as forEach type. So you need to do ${projectSet.iterator()}

The forEach simply does not support Iterable, as counter-intuitive as it may be. It does support iterator though, which I assume should be easy for you to use instead.

You can find evidence for this in source code of ForEachSupport, in method supportedTypeForEachIterator().

JSTL forEach to iterate over user defined class implementing Iterable

2 Answers2