1．Java代码如下:
 package com.eduask;
 import java.io.IOException;
 import javax.servlet.ServletException;
 import javax.servlet.http.HttpServlet;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
 //测试用户名在服务器中的测试;
 public class Demo2 extends HttpServlet{
 @Override
 protected void service(HttpServletRequest req, HttpServletResponse resp)
 throws ServletException, IOException {
 resp.setContentType("text/html;charset=utf-8");
 String name=req.getParameter("name1");
 name=new String(name.getBytes("ISO8859-1"),"utf-8");
 if("tom".equals(name)){
 resp.getWriter().write("<font color=red>该用户已存在！</font>");
 }else{
 resp.getWriter().write("<font color=blue>该用户可以注册！</font>");
 }
 }
 }
 2.demo2.jsp代码如下:
 <body>
 请输入用户名: <input id="input1" type="text" name="name1">
 <span id="span1"></span>
 <script type="text/javascript" language="javascript">
 var input1=document.getElementById("input1");
 input1.οnblur=function(){
 var xhr=getXhr();
 xhr.onreadystatechange=function(){
 if(xhr.readyState==4){
 if(xhr.status==200){
 var msg=xhr.responseText;
 var span1=document.getElementById("span1");
 span1.innerHTML=msg;
 }
 }
 }
 xhr.open("get","/ajax/Demo2?name1="+input1.value,true);
 xhr.send(null);
 }
 function getXhr(){
 var xml;
 if(window.XMLHttpRequest){
 xml=new XMLHttpRequest();
 } else{
 xml=new ActiveXObject("microsoft,xmlhttp");
 }
 return xml;
 }
 </script>
 3.web.xml配置文件如下:
 <!-- demo2 -->
 <servlet>
 <servlet-name>Demo2</servlet-name>
 <servlet-class>com.eduask.Demo2</servlet-class>
 </servlet>
 <servlet-mapping>
 <servlet-name>Demo2</servlet-name>
 <url-pattern>/Demo2</url-pattern>
 </servlet-mapping>