回归到最原始的版本，保存作用域的方法有四种page（页面级，目前开发中几乎不使用）、request、session、application

1、Request：一次请求响应范围

//演示request保存作用域（demo01和demo02）
@WebServlet("/demo01")
public class Demo01Servlet extends HttpServlet {
    @Override
    protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1.向request保存作用域保存数据
        request.setAttribute("uname","lili");
        //2.客户端重定向
        //response.sendRedirect("demo02"); //不能获取demo01的key
        //3.服务器端转发   request.getRequestDispatcher("demo02").forward(request,response); //可以获取
    }
}

@WebServlet("/demo02")
public class Demo02Servlet extends HttpServlet {
    @Override
    protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1.获取request保存作用域保存的数据，key为uname
        Object unameObj = request.getAttribute("uname");
        System.out.println("unameObj = " + unameObj);
    }
}

2、Session：一次绘画范围有效（关闭浏览器session失效）

//演示session保存作用域（demo03和demo04）
@WebServlet("/demo03")
public class Demo03Servlet extends HttpServlet {
    @Override
    protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1.向session保存作用域保存数据
        request.getSession().setAttribute("uname","lili");
        //2.客户端重定向
        response.sendRedirect("demo04");
        //3.服务器端转发
        //request.getRequestDispatcher("demo04").forward(request,response);
    }
}

@WebServlet("/demo04")
public class Demo04Servlet extends HttpServlet {
    @Override
    protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1.获取session保存作用域保存的数据，key为uname
        Object unameObj = request.getSession().getAttribute("uname");
        System.out.println("unameObj = " + unameObj);
    }
}

3、Application：一次应用程序范围内有效（只要tomcat不关闭，一直都可以访问）

//演示application保存作用域（demo05和demo06）
@WebServlet("/demo05")
public class Demo05Servlet extends HttpServlet {
    @Override
    protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1.向application保存作用域保存数据
        //ServletContext : Servlet上下文
        ServletContext application = request.getServletContext();
        application.setAttribute("uname","lili");
        //2.客户端重定向
        response.sendRedirect("demo06");
        //3.服务器端转发
        //request.getRequestDispatcher("demo04").forward(request,response);
    }
}

@WebServlet("/demo06")
public class Demo06Servlet extends HttpServlet {
    @Override
    protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1.获取application保存作用域保存的数据，key为uname
        ServletContext application = request.getServletContext() ;
        Object unameObj = application.getAttribute("uname");
        System.out.println("unameObj = " + unameObj);
    }
}