为什么获取js中的URL的参数的值总是为null(ajax)

我是想验证一个用户名是否存在的，通过前台的input输入，然后写一段js如下实现ajax的功能：


<script type="text/javascript">
var req;
function validate(){
var idField=document.getElementById("userid");
var url="validate.jsp?id = " + escape(idField.value);
//alert(url);          //正常显示
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){
var msg=req.responseXML.getElementsByTagName("msg");
//alert(msg[0].childNodes[0].nodeValue);
setMsg(msg[0].childNodes[0].nodeValue);
}
}
}

function setMsg(msg){
if(msg == "invalid"){
document.getElementById("usermsg").innerHTML = " <font size='-3' color='red'>用户名已存在,请重新输入 </font>";
}else if(msg == "valid"){
document.getElementById("usermsg").innerHTML = " <font color='red'>恭喜您，用户名可用 </font>";
}