AJAX用户名异步校验

<tr>
   <th><span class="requiredField">*</span>用户名:</th>
   <td><input type="text" id="username" name="username"
   class="text" maxlength="20" οnblur="checkUsername()">
   <span id="usernameSpan"></span>
   </td>
</tr>

//AJAX用户名异步校验
    function checkUsername(){
      //获得文本框值:
      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("usernameSpan").innerHTML = xhr.responseText;
          }
        }
      
      }
      //3,打开链接
      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{
        xmlHttp = new XMLHttpRequest();
      }catch(e){
        try{
          xmlHttp = new ActiveXobject("Msxm12.XMLHTTP");
        }catch(e){
          try{
            xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
          }catch(e){}
        }
      }
      return xmlHttp;
    }

/**
   * Ajax进行异步校验用户名的执行方法
   * 
   * @throws IOException
   */
  public String findByName() throws IOException {
    // 调用Service进行查询:
    User user = userService.findByUsername(model.getUsername());
    // 获得response对象,向页面输出
    HttpServletResponse response = ServletActionContext.getResponse();
    // response.setCharacterEncoding("text/html;charset=UTF-8");//错误
    response.setContentType("text/html;charset=UTF-8");
    // 判断
    if (user != 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;

UserDao

public User findByUsername(String username) {
    List<User> list = getSession().createQuery(//
        "from User u where u.username = ?")//
        .setParameter(0, username)//
        .list();
    if (list != null && list.size() > 0) {
      return list.get(0);
    }
    return null;

  }