[Struts 2] How to populate select control after selection of a value in another select control

Hi,

I'm a newbie in Struts 2 and I don't find a good way to link <s:select> on my JSP page.

I have 3 <s:select> populated via 3 linked hierarchic tables : GROUP (id_group, group) > CATEGORY (id_category, id_group, category) > SKILL (id_skill, id_category, skill)

Here is my JSP page, a classic search & list form :

<%@ page contentType="text/html;charset=ISO-8859-1" language="java"%>
<%@ taglib uri="/struts-tags" prefix="s"%>
<html>
	<head>
		<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
		<title>University</title>
		<link href="<%=request.getContextPath()%>/css/style.css" type="text/css" rel="stylesheet"/>
		<link href="<%=request.getContextPath()%>/css/universite.css"	rel="stylesheet" type="text/css">
	</head>
	<body>
		<%@include file="/boot/header.jsp"%>
		<%@include file="/boot/menu.jsp"%>
		<div class="Title">
			<s:property value="message" />
		</div>
		<s:actionerror/>
		<s:form name="TrainerForm" method="POST">
			<table class="tab">
				<tr>
					<td>
						<s:text name="Name"></s:text>
					</td>
					<td>
						<s:textfield id="nameTrainer" name="nameTrainer"/>
					</td>
					<td>
							<s:text name="Group"></s:text>
					</td>
					<td>
						<s:select 	name="group"
									id="group"
									headerKey="0" 
									headerValue="-- Please select --" 
									list="listGroup" 
									listKey="idGroup"  
									listValue="group"
									>
						</s:select>
					</td>
					<td>			
						<s:submit action="listTrainer" value="Search" />
					</td>
				</tr>
				<tr>
					<td>
						<s:text name="Organism"></s:text>
					</td>
					<td>
						<s:textfield id= "nameOrganism" name="nameOrganism" />
					</td>
					<td>
						<s:text name="Category"></s:text>
					</td>
					<td>
						<s:select 	name="category" 
									id="category"
									headerKey="0" 
									headerValue="-- Please select --" 
									list="listCategory"
									listKey="idCategory"
									listValue="category">		
						</s:select>
					</td>
				</tr>
				<tr>
				    <td> </td><td> </td>
					<td>
						<s:text name="Skill"></s:text>
					</td>
					<td>
							<s:select 	name="skill" 
									id="skill"
									headerKey="0" 
									headerValue="-- Please Select --" 
									list="listSkill"
									listKey="idSkill"
									listValue="skill">		
						</s:select>
					</td>
				</tr> 
			</table>
			<table class="tabDisplaySearch">
				<tr>
					<td class="tabTrainerTitle">Fist name</td>
					<td class="tabTrainerTitle">Last name</td>		
					<td class="tabTrainerTitle">Organism</td>	
					<td class="tabTrainerTitle">Evaluation</td>	
					<td class="tabTrainerTitle"> </td>		
				</tr>
				<s:iterator value="listTrainer">
					<tr>
						<td><s:property value="firstname" /></td>
						<td><s:property value="lastname" /></td>
						<td><s:property value="Organism.organism" /></td>
						<td><s:property value="" /></td>
						<td>
							<s:url id="delete" action="deleteTrainer">
		    					<s:param name="idTrainer" value="idTrainer" /> 
							</s:url>
							<s:a href="%{delete}"  onclick="javascript: if (confirm('Continue ?')) return true; else return false;">
								<img border="0" alt="Delete this trainer" src="<%=request.getContextPath()%>/css/images/poubelle.gif">
							</s:a>		
						</td>
					</tr>
				</s:iterator>
			</table>
			<div class="button">
				<a href="<s:url action="addTrainer"/>">
					<img border="0" alt="Create a new trainer" src="<%=request.getContextPath()%>/css/images/button_new.jpg"> 
				</a>
			</div>
		</s:form>
		<%@include file="/boot/footer.jsp" %>
	</body>
</html>

My java class contains methods :

listSkill(), listCategory(), listGroup() and associated Getter and Setter.

When I select a Group, I would like the <select> that display categories only contains categories of the selected Group.
Same thing for Category and Skills.

Thanks a lot !

You need to use javascript.

According to me, a full javascript solution is not a good one. I would like to minimize client role.
Of course, I tried to do that kind of thing :

On my <select> onchange event :

In my listCategory java method, I retrieve the selected idGroup via the associated getter and then, I load the List<Category>.
Finally I set the list with new values.

In Ecclipse debug mode, the list is the right one but on my jsp page, the <select> is not refreshed.

Maybe a cache problem ?

Thanks !