JavaWeb HttpSession

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/**  * 使用session共享数据  */ public class SessionDemo1 extends HttpServlet {     @Override     protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {         //使用session共享数据         //1.获取session         HttpSession session = req.getSession();         //2.存储数据         session.setAttribute("msg0","HelloSession");     } }   /**  * 获取session共享的数据  */ public class SessionDemo2 extends HttpServlet {     @Override     protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {         //使用session获取数据         //1.获取session         HttpSession session = req.getSession();         //2.获取数据         Object msg = session.getAttribute("msg0");         System.out.println(msg);     } }   /**  * 使用Cookie将Session保存到浏览器，且关闭浏览器再打开后不会消失  */ public class SessionDemo3 extends HttpServlet {     @Override     protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {         //1.获取Session         HttpSession session = req.getSession();         System.out.println(session);           //期望客户端关闭后，session也能相同         Cookie c = new Cookie("JSESSIONID",session.getId());         resp.addCookie(c);     }   }