<pre name="code" class="html">                   <div class="login_li">
                    <div class="login_lt">使用者名稱</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 失去焦點事，接下來就是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("**使用者已存在");
//				  alert("*********");
                   
                             }else{
				  $("#userinfo").html("");

                    
 }
		
	   },
		error:  function(){
			//  $("#userinfo").html("");
				alert("呼叫異常");
		}   
   });
        }
 

        @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("使用者已存在");
					out.print("success");
					return ;
				}
				return;
			} catch (Exception e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
		}
 

//可額外補充

 document.getElementById("sub").disabled=true; //這段程式碼可以讓已經存在的使用者不能點選
//按鈕顏色變灰色
$("#sub").css("background-color","#C0C0C0");