关于ajax前台验证用户名是否存在

<pre name="code" class="html">                   <div class="login_li">
<div class="login_lt">用户名</div><span  id="userinfo" style="color:red;padding:5px 30px; "></span>
<div class="login_rt">
<input type="text" class="all_in" id="username" name="username" onblur="loadXMLDoc();"/>

    其中需要  onblur 失去焦点事，接下来就是AJAX：

$.ajax({
type: "post",
url:"<%=request.getContextPath()%>/user/check.do",
data: {username: $("input[name='username']").val()},
success: function(msg){
//			alert(msg);
if("success"==msg){
$("#userinfo").html("**用戶已存在");
//				  alert("*********");

                             }else{
$("#userinfo").html("");

 
 }

},
error:  function(){
//  $("#userinfo").html("");
alert("调用异常");
}
});
}

ajax验证是需要后台写代码，写一个验证action的,我用的是springMVC

@RequestMapping("/check.do")
@ResponseBody
public void check(@RequestParam("username")String username,HttpServletResponse response,HttpServletRequest request,Model model){

try {
PrintWriter out = response.getWriter();
User user = new User();
user.setUsername(username);
boolean b=userService.checkUser(user);
if(b){
//				out.print("用户已存在");
out.print("success");
return ;
}
return;
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}

//可额外补充
document.getElementById("sub").disabled=true; //这段代码可以让已经存在的用户不能点击
//按钮颜色变灰色
$("#sub").css("background-color","#C0C0C0");