AJAX是全稱asynchronous java and xml，其實主要功能是異步數(shù)據(jù)傳輸。下面是一個注冊模塊驗證用戶名是否存在的案例：
Register.jsp
<%@ page language="java" contentType="text/html; charset=utf-8"
    pageEncoding="utf-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Insert title here</title>
<script type="text/javascript">
    var req;
    function validate() {

        var idField = document.getElementById("userid");
        /*if (idField.value.length < 6) {
            alert("fail");
        }

        else {
            alert("success");
        }*/

         var url = "Validate.jsp?id="+ escape(idField.value);
         
         if(window.XMLHttpRequest)
            {
             req = new XMLHttpRequest();
            }
        else if(window.ActiveXObject){
            req = new ActiveXObject("Micorosoft.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];
                setMsg(msg.childNodes[0].nodeValue);
            }
        }
    }

    function setMsg(msg) {
        mdiv = document.getElementById("usermsg");
        if (msg == "valid") {
            mdiv.innerHTML = "<font color='green'>恭喜！您可以使用當前用戶名注冊。</font>";
        } else {
            mdiv.innerHTML = "<font color='red'>對不起！當前用戶名已經(jīng)被注冊，請重新輸入用戶名。</font>";
        }

    }
</script>
</head>
<body>
用戶名：
<input type="text" id="userid" name="username" onBlur="validate();">
<span id="usermsg"></span>
<br>
密碼：
<input type="password" name="password">
</body>
</html>
Validate.jsp
<%
  response.setContentType("text/xml");
  response.setHeader("Cache-Control","no-store");
  response.setHeader("Pragma","no-cache");
  response.setDateHeader("Expires",0);
  //這里自己加驗證邏輯，本例中就只驗證了用戶名長度
  if(length(response.getAttribute("id"))<6)
  {
      response.getWriter().write("<msg>invalid</msg>");
  }
  else
  {
      response.getWriter().write("<msg>valid</msg>");
  }

  
%>