Session与cookie相比 Session是存储在服务端的数据，使用Session 一定要配合cookie使用

session是不能自己new的，只能 request.getSession 放入参数 setAttribute（“usernmae”，object）

再用session 用 getAttribute（“username”）获取参数

放入session

package com.atguigu.servlet;

import jakarta.servlet.ServletException;
import jakarta.servlet.annotation.WebServlet;
import jakarta.servlet.http.HttpServlet;
import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;
import jakarta.servlet.http.HttpSession;

import java.io.IOException;

@WebServlet("/servlet1")
public class Servlet1 extends HttpServlet {
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //接收请求中的username参数
        String username = req.getParameter("username");
        //获得session对象
        HttpSession session = req.getSession();
        //判断请求中是否有特殊的cookie JSESSIONID 值 **** ****
            //有，
                // 根据JESSIONID去找对应的session对象
                // 1. 找到了
                    //返回之前的session
                // 2. 没找到
                    //创建一个新的session返回 自动resp中存放一个JSESSIONID的cookie
            //没有，则创建一个新的session返回 自动resp中存放一个JSESSIONID的cookie
        System.out.println(session.getId());
        System.out.println(session.isNew());
        //将username存入session
        session.setAttribute("username",username);
        //客户端响应信息
        resp.setContentType("text/html;charset=UTF-8");
        resp.getWriter().write("成功");
    }
}

从session取出

package com.atguigu.servlet;

import jakarta.servlet.ServletException;
import jakarta.servlet.annotation.WebServlet;
import jakarta.servlet.http.HttpServlet;
import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;
import jakarta.servlet.http.HttpSession;

import java.io.IOException;

@WebServlet("/servlet2")
public class Servlet2 extends HttpServlet {
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //获取session对象
        HttpSession session = req.getSession();
        System.out.println(session.getId());
        System.out.println(session.isNew());

        //读取对象
        String username = (String) session.getAttribute("username");
        System.out.println("servlet2 get username: "+username);

    }
}