var req;
function validate()
{
  var idField=document.getElementById("userId");
  //var url="ajaxUserRegister.jsp?userId="+escape(idField.value);//escape对中文有问题
  var url="ajaxUserRegister.jsp?userId="+idField.value;
  url=encodeURI(url); 
    url=encodeURI(url);  //写一个不行。如果写一个就是？？？？号。写2个，则输出 %df%a4这种.
  
  //alert(url);
  if(window.XMLHttpRequest){
    req=new XMLHttpRequest();
  }else if(window.ActiveXObject){
    req=new ActiveXObject("Micrsoft.XMLHTTP");
  }
  req.open("GET",url,true);
  req.onreadystatechange=callback;
  req.send(null);
}
function callback() {
  if(req.readyState == 4) {
    if(req.status == 200) {
      //alert(req.responseText);
      var msg = req.responseXML.getElementsByTagName("msg")[0];
      //alert(msg);
          setMsg(msg.childNodes[0].nodeValue);
    }
  }
}
  
function setMsg(msg){
  //alert(msg);
  var mdiv=document.getElementById("usermsg");
  
  if(msg=="no"){
    mdiv.innerHTML="<font color='red'>username exists</font>"
  }else if(msg=="yes"){
    mdiv.innerHTML="<font color='green'>you can use the name</font>"
  }else if(msg=="isNull"){
    mdiv.innerHTML="<font color='red'>username can't be null</font>"
  }
}


<td bgcolor="#D8F6F6" width="32%">
<html:text property="userId" size="30" maxlength="20" οnblur="validate()"/>
            
<span id="usermsg"></span>
</td>




ajaxUserRegister.jsp： 


<%@ page language="java" import="com.wt.bean.User" %>
<%@ page import="com.wt.action.UserAction;"%>
<%
//response.setCharacterEncoding("UTF-8");
response.setContentType("text/xml; charset=UTF-8");//xml,<msg>yes</msg>
response.setHeader("tCache-Control","no-cache");//HTTP1.1
response.setHeader("Pragma","no-cache");//HTTP1.0
response.setDateHeader("Expires",0);


//System.out.println("request.getParameter()===="+request.getParameter("userId"));
//check database
String userId=request.getParameter("userId");
if(userId==""||userId==null){
  response.getWriter().write("<msg>isNull</msg>");
}else{
  userId=java.net.URLDecoder.decode(userId,"UTF-8");//chinese <-->English
  User user=UserAction.userService.getUser(userId);
  if(user==null){
    response.getWriter().write("<msg>yes</msg>");
  }else{
    response.getWriter().write("<msg>no</msg>");
  }
}
%>