明明已经创建出一个session但是到别的servlet里面竟然获取不到
简单购物车!
写了个小练习  下面这段servlet代码作为首页 首先访问  HttpSession session = request.getSession();  创建了一个session

package com.mth.shopping;



import java.io.IOException;

import java.util.Map;



import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

import javax.servlet.http.HttpSession;



public class ListBook extends HttpServlet {



	public void doGet(HttpServletRequest request, HttpServletResponse response)

			throws ServletException, IOException {

		HttpSession session = request.getSession();

		Map<String, Book> map = DB.getMap();

		session.setAttribute("map", map);

		String url=response.encodeRedirectURL("/Session/BookIndex.jsp");

		System.out.println(url);

		response.sendRedirect(url);

	}



	public void doPost(HttpServletRequest request, HttpServletResponse response)

			throws ServletException, IOException {

		doGet(request, response);

	}



}

但是在另外一个servlet里面 HttpSession session = request.getSession(false);  竟然获取到空的session

package com.mth.shopping;



import java.io.IOException;

import java.util.ArrayList;

import java.util.List;



import javax.servlet.ServletException;

import javax.servlet.http.Cookie;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

import javax.servlet.http.HttpSession;



public class BuyBookServlet extends HttpServlet {

//	private List<Book> list = null;



	@SuppressWarnings("unchecked")

	public void doGet(HttpServletRequest request, HttpServletResponse response)

			throws ServletException, IOException {

		String bookId = request.getParameter("id");

		Book book = DB.getMap().get(bookId);

		HttpSession session = request.getSession(false);

		List<Book> list  = (List<Book>) session.getAttribute("list");

		if (list == null) {

			list = new ArrayList<Book>();

			session.setAttribute("list", list);

		}

		list.add(book);

		

		String url = response.encodeRedirectURL("/Session/ListCart");

		response.sendRedirect(url);

	}



	public void doPost(HttpServletRequest request, HttpServletResponse response)

			throws ServletException, IOException {



		doGet(request, response);

	}



}

请教各位大神 为什么 ?

严重: Servlet.service() for servlet BuyBookServlet threw exceptio