Retrieving session data in JSP which is forwarded from a servlet

I'm trying to retrieve session data which is set in a servlet and pass to a jsp. But its returning NULL . The session IDs are equal.
Scenario:
JSP display the checkbox, user selects it and submits to a servlet. the servlet display the selected items and stores the data in session.
when the user press back button in servlet all i want to do is display those checked box which were selected before as checked in the JSP so that user can edit.
Following is a small test code
JSP
<html>
     <head>
          <title>Item List</title>
     </head>
     <body bgcolor="#fedeab"><br><br><br>
     <p align=center>
          <font size=4>
          Choose the item you want!!!
     </p>
     <form action="/demo/showitem" method="post">
          <%=session.getId()%>
          <div align=center>
          <fieldset style="width:250px">
          <legend style="text-align:center">List of item</legend> <br>
          <%
               if (session.getAttribute("book1")!=null) {
          %>
               <input type="checkbox" name="book1" value="book1" checked > Book 1 <br>
          <% } else {%>
               <input type="checkbox" name="book1" value="book1" > Book 1 <br>
          <% }%>
          <input type="submit" value="Enter">
          </fieldset>
          </div>
     </form>
     </body>
</html>
Servlet
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class ShowItem extends HttpServlet
     public void doPost (HttpServletRequest req, HttpServletResponse res)throws ServletException, IOException
          HttpSession session = req.getSession(true);
          res.setContentType("text/html");
          PrintWriter out = res.getWriter();
          out.println(session.getId());
          out.println("<br>");
          out.println("<form>");
          String book1 = req.getParameter("book1");
          session.setAttribute("book1",book1);
          if (book1 != null)
               out.println ("1." + book1 +"<br>");
               out.println("<br><br><input type=\"submit\" name=\"buy\" value=\"Buy\">");
          else
               out.println("No item to display<br>");
          out.println("&nbsp&nbsp<input type=\"submit\" name=\"back\" value=\"Back\">");
          out.println("</form>");
          RequestDispatcher page = getServletContext().getRequestDispatcher("/jsp/litemlist.jsp");
          if ((req.getParameter("buy"))!= null)
                    page.forward(req, res);
          if ((req.getParameter("back"))!= null)
                    page.forward(req, res);
     public void doGet (HttpServletRequest req, HttpServletResponse res)throws ServletException, IOException
          doPost(req,res);
}

req.getParameter("book1") is not NULL and returning the value of the checkbox "book1". The problem occurs when the back button is press and try to retrieve the session value in the JSP. At that time, session.getAttribute("book1") returns NULL.
But if i convert the servlet to JSP then everything is working fine.
So i wanted to know if there is anything wrong in the way i store the session value in the servlet and forwarding to the JSP.
Thanks

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