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{// Inter 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://.springframework./schema/beans" 
  xmlns:xsi="http://.w3./2001/XMLSchema-instance" 
  xmlns:context="http://.springframework./schema/context" 
  xmlns:aop="http://.springframework./schema/aop" 
  xmlns:tx="http://.springframework./schema/tx" 
  xsi:schemaLocation="http://.springframework./schema/beans  
  http://.springframework./schema/beans/spring-beans.xsd 
  http://.springframework./schema/context 
  http://.springframework./schema/context/spring-context.xsd 
  http://.springframework./schema/aop 
  http://.springframework./schema/aop/spring-aop.xsd 
  http://.springframework./schema/tx  
  http://.springframework./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=".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=".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">.hibernate.dialect.MySQLDialect</prop> 
        <prop key="hibernate.show_sql">true</prop> 
        <prop key="hibernate.format_sql">true</prop> 
        <prop key="hibernate.connection.automit">false</prop> 
        <prop key="hibernate.hbm2ddl.auto">update</prop> 
      </props> 
    </property> 
    <!-- 配置Hibernate的映射文件 --> 
    <property name="mappingResources"> 
      <list> 
        <value>/itcast/shop/user/vo/User.hbm.xml</value> 
      </list> 
    </property> 
  </bean> 
  <!-- 事务管理: --> 
  <!-- 事务管理器 --> 
  <bean id="transactionManager" class=".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=".itcast.shop.index.action.IndexAction" scope="prototype"> 
  </bean> 
  <!-- 配置验证码Action --> 
  <bean id="checkImgAction" class=".itcast.shop.user.action.CheckImgAction" scope="prototype"> 
  </bean> 
  <!-- 用户模块的Action --> 
  <bean id="userAction" class=".itcast.shop.user.action.UserAction" scope="prototype"> 
    <!-- 注入Service --> 
    <property name="userService" ref="userService"/> 
  </bean> 
  <!-- Service的配置 ===========================--> 
  <bean id="userService" class=".itcast.shop.user.service.UserService"> 
    <property name="userDao" ref="userDao"/> 
  </bean> 
  <!-- Dao的配置 ===========================--> 
  <bean id="userDao" class=".itcast.shop.user.dao.UserDao"> 
    <property name="sessionFactory" ref="sessionFactory"/> 
  </bean> 
</beans> 
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<?xml version="1.0" encoding="UTF-8" ?> 
<!DOCTYPE struts PUBLIC 
  "-//Apache Software Foundation//DTD Struts Configuration 2.3//EN" 
  "http://struts.apache./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="loginSuess" 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>