Populating cascading dropdown lists in JSP/Servlet

Judging by your question, you're really not using a web framework but using servlets to render html.

I'll be nice and say that you're about a decade behind the times :), people use JSPs (and a web framework like struts) for this sort of thing. However, having said that, here goes:

1. Create a hidden field in your form and set the value to '1', '2' or '3' depending on which drop down is to be populated;
2. In your servlet, capture this value (request.getParamter()) and use it a 'case'/if/else statement to return the appropriate dropdown values.

I'll say it again, just use a web-framework, or atleast plain old jsp to do this.

There are basically three ways to achieve this:

1. Submit form to a servlet during the onchange event of the 1st dropdown (you can use Javascript for this), let the servlet get the selected item of the 1st dropdown as request parameter, let it obtain the associated values of the 2nd dropdown from the database as a Map<String, String>, let it store them in the request scope. Finally let JSP/JSTL display the values in the 2nd dropdown. You can use JSTL (just drop jstl-1.2.jar in /WEB-INF/lib) c:forEach tag for this. You can prepopulate the 1st list in the doGet() method of the Servlet associated with the JSP page.

    <select name="dd1" onchange="submit()">
        <c:forEach items="${dd1options}" var="option">
            <option value="${option.key}" ${param.dd1 == option.key ? 'selected' : ''}>${option.value}</option>
        </c:forEach>
    </select>
    <select name="dd2" onchange="submit()">
        <c:if test="${empty dd2options}">
            <option>Please select parent</option>
        </c:if>
        <c:forEach items="${dd2options}" var="option">
            <option value="${option.key}" ${param.dd2 == option.key ? 'selected' : ''}>${option.value}</option>
        </c:forEach>
    </select>
    <select name="dd3">
        <c:if test="${empty dd3options}">
            <option>Please select parent</option>
        </c:if>
        <c:forEach items="${dd3options}" var="option">
            <option value="${option.key}" ${param.dd3 == option.key ? 'selected' : ''}>${option.value}</option>
        </c:forEach>
    </select>
   

   Once caveat is however that this will submit the entire form and cause a "flash of content" which may be bad for User Experience. You'll also need to retain the other fields in the same form based on the request parameters. You'll also need to determine in the servlet whether the request is to update a dropdown (child dropdown value is null) or to submit the actual form.

2. Print all possible values of the 2nd and 3rd dropdown out as a Javascript object and make use of a Javascript function to fill the 2nd dropdown based on the selected item of the 1st dropdown during the onchange event of the 1st dropdown. No form submit and no server cycle is needed here.

    <script>
        var dd2options = ${dd2optionsAsJSObject};
        var dd3options = ${dd3optionsAsJSObject};
        function dd1change(dd1) {
            // Fill dd2 options based on selected dd1 value.
            var selected = dd1.options[dd1.selectedIndex].value;
            ...
        }
        function dd2change(dd2) {
            // Fill dd3 options based on selected dd2 value.
            var selected = dd2.options[dd2.selectedIndex].value;
            ...
        }
    </script>
   
    <select name="dd1" onchange="dd1change(this)">
        <c:forEach items="${dd1options}" var="option">
            <option value="${option.key}" ${param.dd1 == option.key ? 'selected' : ''}>${option.value}</option>
        </c:forEach>
    </select>
    <select name="dd2" onchange="dd2change(this)">
        <option>Please select parent</option>
    </select>
    <select name="dd3">
        <option>Please select parent</option>
    </select>
   

   One caveat is however that this may become unnecessarily lengthy and expensive when you have a lot of items. Imagine that you have 3 steps of each 100 possible items, that would mean 100 * 100 * 100 = 1,000,000 items in JS objects. The HTML page would grow over 1MB in length.

3. Make use of XMLHttpRequest in Javascript to fire an asynchronous request to a servlet during the onchange event of the 1st dropdown, let the servlet get the selected item of the 1st dropdown as request parameter, let it obtain the associated values of the 2nd dropdown from the database, return it back as XML or JSON string. Finally let Javascript display the values in the 2nd dropdown through the HTML DOM tree (the Ajax way, as suggested before). The best way for this would be using jQuery.

    <%@ page pageEncoding="UTF-8" %>
    <%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
    <!DOCTYPE html>
    <html lang="en">
        <head>
            <title>SO question 2263996</title>
            <script src="http://code.jquery.com/jquery-latest.min.js"></script>
            <script>
                $(document).ready(function() {
                    $('#dd1').change(function() { fillOptions('dd2', this); });
                    $('#dd2').change(function() { fillOptions('dd3', this); });
                });
                function fillOptions(ddId, callingElement) {
                    var dd = $('#' + ddId);
                    $.getJSON('json/options?dd=' + ddId + '&val=' + $(callingElement).val(), function(opts) {
                        $('>option', dd).remove(); // Clean old options first.
                        if (opts) {
                            $.each(opts, function(key, value) {
                                dd.append($('<option/>').val(key).text(value));
                            });
                        } else {
                            dd.append($('<option/>').text("Please select parent"));
                        }
                    });
                }
            </script>
        </head>
        <body>
            <form>
                <select id="dd1" name="dd1">
                    <c:forEach items="${dd1}" var="option">
                        <option value="${option.key}" ${param.dd1 == option.key ? 'selected' : ''}>${option.value}</option>
                    </c:forEach>
                </select>
                <select id="dd2" name="dd2">
                    <option>Please select parent</option>
                </select>
                <select id="dd3" name="dd3">
                    <option>Please select parent</option>
                </select>
            </form>
        </body>
    </html>
   

   ..where the Servlet behind /json/options can look like this:

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String dd = request.getParameter("dd"); // ID of child DD to fill options for.
        String val = request.getParameter("val"); // Value of parent DD to find associated child DD options for.
        Map<String, String> options = optionDAO.find(dd, val);
        String json = new Gson().toJson(options);
        response.setContentType("application/json");
        response.setCharacterEncoding("UTF-8");
        response.getWriter().write(json);
    }
   

   Here, Gson is Google Gson which eases converting fullworthy Java objects to JSON and vice versa. See also How to use Servlets and Ajax?