教一下后台如何去接收前端通过ajax发送的数据呢

Question

$.ajax({     type:"post",     url:"/servlet/js.do",     dataType:"json",     data:"{username:${'#username'}.val(),password:${'#password'}.val()}",     contentType:"application/x-www-form-urlencoded;charset=utf-8",     success:function (data) {         console.log(data);     },     error:function (e) {         console.log(e);     } });是了很久，请教一下后台如何去接收前端通过ajax发送的数据呢