ajax连动下拉菜单

-------------------------jsp中的js代码--------------------

<script type="text/javascript">
  function ajaxFunction(sort){
   var xmlHttp;
   try{
      // Firefox, Opera 8.0+, Safari
       xmlHttp=new XMLHttpRequest();
      } catch (e)
      {
     // Internet Explorer
      try{
          xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");
         }catch (e)
         {
   
         try{
             xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
            }catch (e)
            {
             alert("您的浏览器不支持AJAX！");
             return false;
            }
         }
      }

 

      xmlHttp.onreadystatechange=function(){
        if(xmlHttp.readyState==4){
               document.getElementById("div_product").innerHTML=xmlHttp.responseText;
          }
        }
      xmlHttp.open("POST","problemAction.do?action=showVersions",true);
      xmlHttp.send(null);
   
  }
  
 </script>

----------------------------action中代码-------------------------

 public String showVersions(ActionMapping mapping, ActionForm form,
   HttpServletRequest request, HttpServletResponse response)
 throws Exception {
   String content ="<select id='qbbh' name='qbbh'>";
  content = content + "<option value=''>--请选择--</option>";
   for(int i=0;i<5;i++){
     content = content + "<option value='"+ i +"'>"+ i +"</option>";
    }
  content = content + "</select>";
  response.setContentType( "text/html" );
  response.setCharacterEncoding( "GBK" );
  response.getWriter().write(content);
  return null;
 }