Ajax技术的用户注册验证

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>");
%>