客户端跳转：//不保存request属性到下个页面，通过
     传参response.sendRedirect("index2.jsp");response.setHeader("Refresh","3;URL=index2.jsp");//定时跳转
       //定时跳转 服务器端跳转：//保存request属性到下个页面，通过对url地址的重写传参
     request.getRequestDispatcher("index2.jsp").forward(request,response);