1.javascript代码:
<script type="text/javascript">
 var req;
 function validate() {
  var idField = document.getElementById("userid");
  var url = "Validate.jsp?id=" + escape(idField.value);
  if(window.XMLHttpRequest) {
   req = new XMLHttpRequest();
  } else if (window.ActiveXObject) {
   req = new ActiveXObject("Microsoft.XMLHTTP");
  }
  req.open("GET", url, true);
  req.onreadystatechange = callback;
  req.send(null);
 }
 
 function callback() {
  if(req.readyState == 4) {
   if(req.status == 200) {
    //alert(req.responseText);
    var msg = req.responseXML.getElementsByTagName("msg")[0];
    //alert(msg);
          setMsg(msg.childNodes[0].nodeValue);
   }
  }
 }
 
 function setMsg(msg) {
  //alert(msg);
  mdiv = document.getElementById("usermsg");
  if(msg == "invalid") {
   mdiv.innerHTML = "<font color='red'>username exists</font>";
  } else {
   mdiv.innerHTML = "<font color='green'>congratulations! you can use this username!</font>";
  }
 }
 

</script>

2.validate.jsp页面代码:

<%
response.setContentType("text/xml");
response.setHeader("Cache-Control", "no-store"); //HTTP1.1
response.setHeader("Pragma", "no-cache"); //HTTP1.0
response.setDateHeader("Expires", 0); //prevents catching at proxy server
System.out.println(request.getParameter("id"));
//check the database
response.getWriter().write("<msg>valid</msg>");
%>