Just let the servlet return the desired dropdown options in the desired format based on the request parameters in the doGet() method. Here's an example which returns it in JSON format with help of Google Gson:
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String type = request.getParameter("type"); // Returns "country" or "state".
String value = request.getParameter("value"); // Value of selected country or state.
Map<String, String> options = optionDAO.find(type, id); // Do your thing to obtain them from DB. Map key is option value and map value is option label.
String json = new Gson().toJson(options); // Convert Java object to JSON string.
response.setContentType("application/json"); // Inform client that you're returning JSON.
response.setCharacterEncoding("UTF-8"); // Important if you want world domination.
response.getWriter().write(json); // Write JSON string to response.
}
Assuming that the above servlet is mapped on an url-pattern of /options, you could use it as in the following JSP example with help of jQuery. I am recommending jQuery because you would otherwise end up with 10 times as much JavaScript code needed for this "simple" task.
<%@ page pageEncoding="UTF-8" %>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<!DOCTYPE html>
<html lang="en">
<head>
<title>SO question 3983929</title>
<script src="http://code.jquery.com/jquery-latest.min.js"></script>
<script>
$(document).ready(function() {
$('#country').change(function() { fillOptions('state'); });
$('#state').change(function() { fillOptions('city'); });
});
function fillOptions(dropdownId) {
var dropdown = $('#' + dropdownId);
$.getJSON('options?type=' + dropdownId + '&value=' + $(this).val(), function(opts) {
$('>option', dropdown).remove(); // Clean old options first.
if (opts) {
$.each(opts, function(key, value) {
dropdown.append($('<option/>').val(key).text(value));
});
} else {
dropdown.append($('<option/>').text('Please select ' + dropdownId));
}
});
}
</script>
</head>
<body>
<form>
<select id="country" name="country">
<c:forEach items="${countries}" var="country">
<option value="${country.key}" ${param.country == option.key ? 'selected' : ''}>${country.value}</option>
</c:forEach>
</select>
<select id="state" name="state">
<option>Please select country</option>
</select>
<select id="city" name="city">
<option>Please select state</option>
</select>
</form>
</body>
</html>
Here I am assuming that you've already prepopulated the ${countries} as being a Map<String, String> in a servlet which has preprocessed the request to this JSP for the initial display.