Ajax问题

代码如下
JScript code:
<script type="text/javascript">
var xmlHttp;
function createXMLHttpRequest(){
if (window.ActiveXObject) {
xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
}
else if (window.XMLHttpRequest) {
xmlHttp = new XMLHttpRequest();
}
}
function createString(){
createXMLHttpRequest();
var username = document.getElementById("username").value;
var cString = "Aservlet?username=" + username;
xmlHttp.open("GET", cString, true);
xmlHttp.onreadystatechange = callback;
xmlHttp.send(null);
}
function callback(){
if(xmlHttp.readyState == 4) {
if(xmlHttp.status == 200) {
fun();
}
}
}
function fun() {
var username = xmlHttp.responseText;
document.getElementById(email).value=username;
}
</script>


<input type="text" name="username" id="username">
<input type="button" id="b1" onclick="createString()" value="注册">
<input ty