js 原始ajax请求实现 struts2

//action代码
public String setYwzslb(ActionMapping mapping, ActionForm form, HttpServletRequest request, HttpServletResponse response) {
List<BtxtjgqxPO> ywzslxList = QfgyBo.getQfgyBo().getywzslxList(request.getParameter("ywlb"),request.getParameter("groupId"));
try {
BtxtjgqxPO btxtjgqxPO = null;

StringBuffer sb = new StringBuffer();
for(int i = 0; i < ywzslxList.size(); i++) {
btxtjgqxPO = ywzslxList.get(i);
sb.append(btxtjgqxPO.getQxdm()).append(":").append(btxtjgqxPO.getQxmc()).append(",");
}
 
response.setContentType("application/json;charset=GBK");   
        response.setCharacterEncoding("GBK");
        PrintWriter pw = response.getWriter();
String str = sb.toString();
pw.write(str);
pw.flush();

} catch (IOException e) {
 this.setMessage("失败！", ErrorLevel.ERROR);
logger.error(e);
}
return null;
}

// 核对对象
var xmlHttp;