11.Session

服务端回话跟踪技术，将数据储存在服务端
创建Session对象来实现保存数据

对象方法

1. setAttribute
2. getAttribute
3. remove

创建Session对象：
        Cookie[] cookies = req.getCookies();
        for (Cookie cookie : cookies){
            String name = cookie.getName();
            String value = cookie.getValue();
        }
获取Session对象：
        HttpSession session = req.getSession();
        Object username = session.getAttribute("username");
        System.out.println(username);