一个小小的Ajax例子
HTML:

<input type="text" name="username" size="30" maxlength="20" onblur="validate()">
                <div id=usermsg></div>

javascript:

<script type="text/javascript">
var req;
function validate(){
  var idField = document.getElementById("userid");
  var url = "validate.jsp?id=" + escape(idField);
  if(window.XMLHttpRequest){
   req = new XMLHttpRequest();
   alert("ddd");
  }else if(window.ActiveXObject){
   req = new ActiveXObject("Microsoft.XMLHTTP");
  }
  req.open("GET", url , true);
  req.onreadystatechange = callback;
  req.send(null);
}

function callback(){
   if(req.readyState == 4){
    if(req.status ==200){
     var msg = req.responseXML.getElementsByTagName("msg")[0];
     //alert(msg.childNodes[0].nodeValue);
     setMsg(msg.childNodes[0].nodeValue);
    }
   }
}

function setMsg(msg){
  if(msg == "invalid"){
   document.getElementById("usermsg").innerHTML = "<font color='red'>用户名已存在</font>";
  }else{
   document.getElementById("usermsg").innerHTML = "<font color='green'>可以注册</font>";
  }
}

</script>

validate.jsp

<%
response.setContentType("text/xml");
response.setHeader("Cache-Contorl", "no-store");//HTTP1.1
//response.setHeader("Pragma", "no-cache");//HTTP1.0
response.setDateHeader("Expires", 0);//prevents caching at the proxy server
response.getWriter().write("<msg>invalid</msg>");
%>