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" onblur="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;

	}