Ajax status 0 when calling the servlet - ajax

Question

Hi I'm trying to just get a simple string returned from the servlet using Ajax, but nothing was ever returned 'cause the status is always 0 while readystate is 4.
Here's the .js code
function validate(choice) {
//var url = "http://localhost:8080/examples/validate.do?id=" + escape(choice);
var url = "../../validate.do"
if(window.XMLHttpRequest) {
req = new XMLHttpRequest();
}else if(window.ActiveXObject) {
req = new ActiveXObject("MSXML2.XMLHTTP.3.0");
}
alert("IM IN VALIDATE() with " + choice);
req.open("GET", url, true);
req.onreadystatechange = callback;
req.send(null);
return false;
}
function callback() {
if(req.readyState == 4 ) {
if(req.status == 200){
var check = req.responseText;
alert(check);
}
else
alert(req.status);
}
}
and Java code
package model;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class DoAjaxServlet extends HttpServlet {
protected void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html;charset=UTF-8");
PrintWriter out = response.getWriter();
try {
response.setContentType("text/html");
response.setHeader("Access-Control-Allow-Origin", "*");
response.setHeader("Cache-Control", "no-store");
response.setHeader("Pragma", "no-cache");
response.setDateHeader("Expires", 0);
String resultStr = "JUST RETURNING THIS STRING";
out.write(resultStr);
} finally {
out.close();
}
}
#Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
#Override
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
#Override
public String getServletInfo() {
return "Short description";
}
}
I'm running this on Tomcat 7 using Chrome, and accessed the html file from localhost:8080 not instead of running local, so a lot of solutions floating around won't work.
Going to
http://localhost:8080/examples/validate.do
in Chrome it prints the string just fine, so I think I didn't write the url wrong. The .js file are at somewhere like
http://localhost:8080/examples/jsp/HTE/my.js
I also tried using "http://localhost:8080/examples/validate.do" directly as url in .js and adding the setHeader("Access-Control-Allow-Origin", "*") to Java file but nothing changes.
After searching around in the posts I'm running of ideas on this one... Would you kindly tell me where this might go wrong?