function validate(){
    createXMLHttpRequest();
    var username = document.getElementById("username").value;
    var url = "TestAjax?username="+username;
    xmlHttp.onreadystatechange = callback;
    xmlHttp.open("GET",url,true);
    xmlHttp.send(null);
  }
  
  function callback()
  {
      if(xmlHttp.readyState == 4)
      {
    if(xmlHttp.status == 200)
    {
        var msg = xmlHttp.responseXML.getElementsByTagName("result")[0].firstChild.data;
        setMessage(msg);
    }
      }
  }

public void doGet(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {
  request.setCharacterEncoding("GBK");
  response.setContentType("text/xml");
  response.setHeader("Cache-Control", "no-cache");
  PrintWriter out = response.getWriter();
  String uname = request.getParameter("username").trim();
  System.out.println(uname);
  if(uname.equals("neeke"))
  {
      out.println("<result>welcome to you!</result>");
  }else
  {
      out.println("<result>login false!</result>");
  }
  out.close();
  
    }