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