表单验证AJAX！

有四个text输入框！我现在想做到在提交form表单的时候进行验证！在数据库里查询看看这四个text里输入的是否存在！如果存在给个提示框！能给段代码我参考下最好！

<script type="text/javascript" >
function login(){
var username = document.getElementById("username").value;
var password = document.getElementById("password").value;
createXMLHttpRequest();
var url='Login';
var parameter = "username="+username+"&password="+password;
sendRequest(url,parameter,callBack);
}
function callBack(){
if(xmlHttp.readyState==4){
if(xmlHttp.status==200){
var rootElement = xmlHttp.responseXML.documentElement;
if(rootElement.hasChildNodes()){
var nodes = rootElement.getElementsByTagName("username");
var nickname = document.getElementById("nickname");
nickname.innerHTML = nodes[0].firstChild.nodeValue;
nodes = rootElement.getElementsByTagName("password");
var strpassword = document.getElementById("strpassword");
strpassword.innerHTML = nodes[0].firstChild.nodeValue;
}
}
}
}
</script>



-----------------以下是请求的servlet
package com.briup.servlet;

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;

import com.briup.bean.Logintest;
import com.briup.service.LoginService;
import com.briup.service.impl.LoginServiceImpl;

public class LoginServlet extends HttpServlet {


public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
doPost(request, response);
}


public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

response.setContentType("text/xml");
PrintWriter out = response.getWriter();
String username = request.getParameter("username");
String password = request.getParameter("password");
LoginService service = new LoginServiceImpl();
String xml = "<message>";
try {
Logintest test = service.login(username, password);
xml += "<username>"+test.getUsername()+"</username>";
xml += "<password>"+test.getPassword()+"</password>";
xml += "<email>"+test.getEmail()+"</email>";
xml += "<gender>"+test.getGender()+"</gender>";
xml += "<province>"+test.getPrivince()+"</provice>";
xml += "<telephone>"+test.getTelephone()+"</telephone>";
xml += "<address>"+test.getAddress()+"</address>";
} catch (Exception e) {
e.printStackTrace();
request.setAttribute("error", e.getMessage());
}
xml+="</message>";
out.println(xml);
out.flush();
out.close();
}

}

建议使用jQuery，使用$.post 直接提交数据到服务器端，服务器直接返回数据后js就能直接使用了。