[FIXED] Can't print the Java List object in JSP, with JSTL

Issue

I have a servlet

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException, ServletException {
    List<String> topics = new ArrayList<>();
    ServletConfig config = getServletConfig();
    
    topics.add(config.getInitParameter("first"));
    System.out.println(config.getInitParameter("first")); //prints proper value, not null;

    topics.add(config.getInitParameter("second"));
    System.out.println(config.getInitParameter("second")); //prints proper value, not null;

    topics.add(config.getInitParameter("third"));
    System.out.println(config.getInitParameter("third")); //prints proper value, not null;

    req.setAttribute("params", topics); //doesn't show up
    req.setAttribute("name", config.getInitParameter("name")); //works good
    req.getRequestDispatcher("index.jsp").forward(req, resp);
}

and

index.jsp

...
<ol>
    <c:forEach var="param" items="${params}">
        <li>${param}</li>
    </c:forEach>
</ol>
...

Servlet configuration is ok, initialization is ok, mapping and naming is also ok, that's why when I access respective URL, I do print the parameters in output console stream and they are there. However, for some strange reason, JSP displays:

 1. {}
 2. {}
 3. {}

N.B. I don't want to use Scriptlet Java code, I'm trying to use JSTL. I have seen a lot of projects working in this way.. what's wrong here? just got tired of figuring out.

Solution

I've spent on this half of my day, and at the end, it really got me tired and anxious, because it seems so obvious and simple code - what should be going wrong? As some may be searching for the solution of same kind of problem, I think, it's better to have this answered here - what was the problem.

The crucial point here is the name of the iteration variable - the identifier param.

At the beginning of [probably] all .jsp files, we have the statement for importing core tags and we give it some prefix

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>

The reason, why

...
<ol>
    <c:forEach var="param" items="${params}">
        <li>${param}</li>
    </c:forEach>
</ol>
...

was not working and was showing

 1. {}
 2. {}
 3. {}

is that, param is the keyword identifying one of the core tags, from jstl/core.

The <c:param> fetches/gets the Request Parameter(s) array.
So, each time forEach loop was iterating, param variable was assigned to request parameter(s) from query string rather than iteration value from ${params} variable/placeholder, and as I was passing nothing - empty array was appearing.

P. S. Be cautious not to use JSTL tags as variables/identifiers in your code.

Hope some of you will find this information useful.

Answered By - Giorgi Tsiklauri