JSP处理AJAX

<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>

<%
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>");
%>