将servlet值传递到jsp页面

Question

I am trying to get values to a drop down list based on the previous value selected from another drop down list. I am able to call to java class from the servlet and the values are returned to servlet but it is not getting passed down to jsp page

jsp page part(index.jsp)

<select id="region" name="region" class="form-control" onchange="getgroups()" required="required" >
<option value="" default selected>Select region</option>
<option value="region1">region1</option>
<option value="region2">region2</option>
<option value="region3">region3</option>
<option value="region4">region4</option></select>

The onchange value in region calls this function

function getgroups(){

var j = document.getElementById("region");
var s = document.getElementById("secret_key");
var a = document.getElementById("access_key");

   var output = $.ajax({
     type: 'POST',
     url: 'http://localhost:8066/VMMigratorNew/ec2util',
     data: {"region":j.value,
            "secret_key":s.value,
            "access_key":a.value,
            },
     success: function(response) {
         return output.responseText;
     }

});

}

and in servlet

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        String region = request.getParameter("region");
        String secretKey = request.getParameter("secret_key");
        String accessKey = request.getParameter("access_key");




            List<String> vpcs = RunEc2Command.getVPCForUSer(AWS_ACCESS_KEY, AWS_SECRET_KEY, region);

            if(vpcs != null && vpcs.size() > 0) {
                                request.setAttribute("vpclist", vpcs);
            }

        request.getRequestDispatcher("/index.jsp").forward(request, response);
    }
}

and in the same index.jsp page , I am trying to show the list values in a drop down box which is in index.jsp itself.

          <select id="vpc" name="vpc" class="form-control" placeholder="Choose VPC"  >
                                        <option value="" default selected>Select VPC</option>

                                            <c:forEach items="${vpclist}" var="vpcs">    
                                                 <option>
                                                   ${vpcs}
                                                 </option>
                                            </c:forEach> 
                                    </select> 

I need to get the value of VPC in drop dron box after selecting the value from region drop down box. Thanks for the help in advance.

Answer 1

Currently, the workflow in your servlet is suitable for a synchronous request, which requires the servlet to return the whole HTML in the HTTP response for the browser to render. However, it seems to me that your intent is instead of querying the servlet in order to obtain a VPC list, and you are trying to pose the query asynchronously using AJAX.

Hence, my suggestion would be to let the servlet return the VPC list in some format (eg, JSON). In the success callback, you should modify the DOM in order to insert the list you received.