ajax接收不到servlet返回,ajax请求信息 servlet没接收到

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？