can't get parameter from XMLHttpRequest - ajax

Question

I can see parameters from js, but I can't use getParameter to get them. what's the problem?
js code
var email = document.getElementById("email").value;
var password = document.getElementById("password").value;
var req = new XMLHttpRequest();
req.open("POST", "../PersonTest", true);
req.onreadystatechange = function(){
if (req.readyState == 4) {
if (req.status == 200) {
alert(req.responseText);
} else {
alert("HTTP error " + req.status + ": " + req.statusText);
}
}
}
req.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
req.send("email=" + email + "&password=" + password);
servlet code
response.setContentType("text/xml;charset=UTF-8");
PrintWriter out = response.getWriter();
try {
//print request content
ServletInputStream is = request.getInputStream();
...
System.out.println(new String(baos.toByteArray(), "utf-8"));
//get parameter
String email = request.getParameter("email");
String password = request.getParameter("password");
System.out.println(email + " " + password);
}
servlet output:
email=123&password=123
null null