怎么样使用AJAX完成用户名是否存在异步校验？

function checkUsername(){ 
  // 获得文件框值: 
  var username = document.getElementById("username").value; 
  // 1.创建异步交互对象 
  var xhr = createXmlHttp(); 
  // 2.设置监听 
  xhr.onreadystatechange = function(){ 
   if(xhr.readyState == 4){ 
    if(xhr.status == 200){ 
     document.getElementById("span1").innerHTML = xhr.responseText; 
    } 
   } 
  } 
  // 3.打开连接 
  xhr.open("GET","${pageContext.request.contextPath}/user_findByName.action?time="+new Date().getTime()+"&username="+username,true); 
  // 4.发送 
  xhr.send(null); 
 } 
 function createXmlHttp(){ 
   var xmlHttp; 
   try{ // Firefox, Opera 8.0+, Safari 
    xmlHttp=new XMLHttpRequest(); 
   } 
   catch (e){ 
    try{// Internet Explorer 
      xmlHttp=new ActiveXObject("Msxml2.XMLHTTP"); 
     } 
    catch (e){ 
     try{ 
      xmlHttp=new ActiveXObject("Microsoft.XMLHTTP"); 
     } 
     catch (e){} 
     } 
   } 
   return xmlHttp; 
   } 

public String findByName() throws IOException { 
  // 调用Service进行查询: 
  User existUser = userService.findByUsername(user.getUsername()); 
  // 获得response对象,项页面输出: 
  HttpServletResponse response = ServletActionContext.getResponse(); 
  response.setContentType("text/html;charset=UTF-8"); 
  // 判断 
  if (existUser != null) { 
   // 查询到该用户:用户名已经存在 
   response.getWriter().println("<font color='red'>用户名已经存在</font>"); 
  } else { 
   // 没查询到该用户:用户名可以使用 
   response.getWriter().println("<font color='green'>用户名可以使用</font>"); 
  } 
  return NONE; 
 } 

private UserDao userDao; 
 public void setUserDao(UserDao userDao) { 
  this.userDao = userDao; 
 } 
 // 按用户名查询用户的方法: 
 public User findByUsername(String username){ 
  return userDao.findByUsername(username); 
 } 

public User findByUsername(String username){ 
  String hql = "from User where username = ?"; 
  List<User> list = this.getHibernateTemplate().find(hql, username); 
  if(list != null && list.size() > 0){ 
   return list.get(0); 
  } 
  return null; 
 } 

<?xml version="1.0" encoding="UTF-8"?> 
<beans xmlns="http://www.springframework.org/schema/beans" 
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
 xmlns:context="http://www.springframework.org/schema/context" 
 xmlns:aop="http://www.springframework.org/schema/aop" 
 xmlns:tx="http://www.springframework.org/schema/tx" 
 xsi:schemaLocation="http://www.springframework.org/schema/beans 
 http://www.springframework.org/schema/beans/spring-beans.xsd 
 http://www.springframework.org/schema/context 
 http://www.springframework.org/schema/context/spring-context.xsd 
 http://www.springframework.org/schema/aop 
 http://www.springframework.org/schema/aop/spring-aop.xsd 
 http://www.springframework.org/schema/tx 
 http://www.springframework.org/schema/tx/spring-tx.xsd"> 
 <!-- 配置连接池: --> 
 <!-- 引入外部属性文件 --> 
 <context:property-placeholder location="classpath:jdbc.properties"/> 
 <!-- 配置C3P0连接池: --> 
 <bean id="dataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource"> 
  <property name="driverClass" value="${jdbc.driver}"/> 
  <property name="jdbcUrl" value="${jdbc.url}"/> 
  <property name="user" value="${jdbc.user}"/> 
  <property name="password" value="${jdbc.password}"/> 
 </bean> 
 <!-- Hibernate的相关信息 --> 
 <bean id="sessionFactory" class="org.springframework.orm.hibernate3.LocalSessionFactoryBean"> 
  <!-- 注入连接池 --> 
  <property name="dataSource" ref="dataSource"/> 
  <!-- 配置Hibernate的其他的属性 --> 
  <property name="hibernateProperties"> 
   <props> 
    <prop key="hibernate.dialect">org.hibernate.dialect.MySQLDialect</prop> 
    <prop key="hibernate.show_sql">true</prop> 
    <prop key="hibernate.format_sql">true</prop> 
    <prop key="hibernate.connection.autocommit">false</prop> 
    <prop key="hibernate.hbm2ddl.auto">update</prop> 
   </props> 
  </property> 
  <!-- 配置Hibernate的映射文件 --> 
  <property name="mappingResources"> 
   <list> 
    <value>cn/itcast/shop/user/vo/User.hbm.xml</value> 
   </list> 
  </property> 
 </bean> 
 <!-- 事务管理: --> 
 <!-- 事务管理器 --> 
 <bean id="transactionManager" class="org.springframework.orm.hibernate3.HibernateTransactionManager"> 
  <property name="sessionFactory" ref="sessionFactory"/> 
 </bean> 
 <!-- 开启注解事务 --> 
 <tx:annotation-driven transaction-manager="transactionManager"/> 
 <!-- Action的配置 ===========================--> 
 <!-- 首页访问的Action --> 
 <bean id="indexAction" class="cn.itcast.shop.index.action.IndexAction" scope="prototype"> 
 </bean> 
 <!-- 配置验证码Action --> 
 <bean id="checkImgAction" class="cn.itcast.shop.user.action.CheckImgAction" scope="prototype"> 
 </bean> 
 <!-- 用户模块的Action --> 
 <bean id="userAction" class="cn.itcast.shop.user.action.UserAction" scope="prototype"> 
  <!-- 注入Service --> 
  <property name="userService" ref="userService"/> 
 </bean> 
 <!-- Service的配置 ===========================--> 
 <bean id="userService" class="cn.itcast.shop.user.service.UserService"> 
  <property name="userDao" ref="userDao"/> 
 </bean> 
 <!-- Dao的配置 ===========================--> 
 <bean id="userDao" class="cn.itcast.shop.user.dao.UserDao"> 
  <property name="sessionFactory" ref="sessionFactory"/> 
 </bean> 
</beans> 
[html] view plain copy 在CODE上查看代码片派生到我的代码片
<?xml version="1.0" encoding="UTF-8" ?> 
<!DOCTYPE struts PUBLIC 
 "-//Apache Software Foundation//DTD Struts Configuration 2.3//EN" 
 "http://struts.apache.org/dtds/struts-2.3.dtd"> 
<struts> 
 <constant name="struts.devMode" value="false" /> 
 <constant name="struts.enable.DynamicMethodInvocation" value="true"/> 
 <package name="shop" extends="struts-default" namespace="/"> 
  <global-results> 
   <result name="msg">/WEB-INF/jsp/msg.jsp</result> 
  </global-results> 
  <!-- 配置首页访问的Action --> 
  <action name="index" class="indexAction"> 
   <result name="index">/WEB-INF/jsp/index.jsp</result> 
  </action> 
  <!-- 配置用户模块的Action --> 
  <action name="user_*" class="userAction" method="{1}"> 
   <result name="registPage">/WEB-INF/jsp/regist.jsp</result> 
   <result name="input">/WEB-INF/jsp/regist.jsp</result> 
   <result name="loginPage">/WEB-INF/jsp/login.jsp</result> 
   <result name="login">/WEB-INF/jsp/login.jsp</result> 
   <result name="loginSuccess" type="redirectAction">index</result> 
   <result name="quit" type="redirectAction">index</result> 
   <result name="checkcodeFail">/WEB-INF/jsp/regist.jsp</result> 
  </action> 
  <!-- 验证码Action --> 
  <action name="checkImg" class="checkImgAction"></action> 
 </package> 
</struts>