前台

$.ajax({
       type:"get",//请求方式
       url:"S12",//发送请求地址
       data:'{username:"ssd"}',
       //请求成功后的回调函数有两个参数
       success:function(data,textStatus){
           alert(data);
       }
       });  

后台

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //Cookie [] c =request.getCookies();
         request.setCharacterEncoding("UTF-8");
        PrintWriter out = response.getWriter();
        String s = request.getParameter("username");
        System.out.println(s);
        out.println(s);

后台request没有取到username怎么回事？