session原理解析

cookie机制采用的是在客户端保持状态的方案，而session机制采用的是在服务器端保持状态的方案；

由于采用服务器端保持状态的方案在客户端也需要保存一个标识，所以session机制可能需要借助于cookie机制来达到保存标识的目的。

session实现购物车案例

Book.java 

javabean  

package car;

public class Book {

    private String id;
    private String bookName;
    private String author;
    private int price;
    private String description;
    private int count;
    
    
    
    public Book(String id, String bookName, String author, int price,
            String description) {
        this.id = id;
        this.bookName = bookName;
        this.author = author;
        this.price = price;
        this.description = description;
    }
    public String getId() {
        return id;
    }
    public void setId(String id) {
        this.id = id;
    }
    public String getBookName() {
        return bookName;
    }
    public void setBookName(String bookName) {
        this.bookName = bookName;
    }
    public String getAuthor() {
        return author;
    }
    public void setAuthor(String author) {
        this.author = author;
    }
    public int getPrice() {
        return price;
    }
    public void setPrice(int price) {
        this.price = price;
    }
    public String getDescription() {
        return description;
    }
    public void setDescription(String description) {
        this.description = description;
    }
    public int getCount() {
        return count;
    }
    public void setCount(int count) {
        this.count = count;
    }
    @Override
    public String toString() {
        return "Book [id=" + id + ", bookName=" + bookName + ", author="
                + author + ", price=" + price + ", description=" + description + "]";
    }
    
    
    
}

BookUtils.java

 用来存取数据

package utils;

import java.util.HashMap;
import java.util.Map;

import car.Book;

public class BookUtils {

    private static Map<String,Book> map = new HashMap<String,Book>();
    
    static{
        map.put("1", new Book("1","三年中考五年模拟","李刚",100,"中学生的噩梦"));
        map.put("2", new Book("2","最后一个硬汉","郎咸平",1000,"666的经济学"));
        map.put("3", new Book("3","我的奋斗","李刚",1003,"中学生的噩梦"));
        map.put("4", new Book("4","三年中考五年模拟444","李刚4",400,"中学生的噩梦4"));
        map.put("5", new Book("5","三年中考五年模拟555","5李刚",500,"中学生的噩梦5"));
        map.put("6", new Book("6","666三年中考五年模拟","李6刚",600,"中学生的噩梦6"));
    }
    //获取所有的书
    public static Map<String,Book> getAllBook(){
        return map;
    }
    //根据书的id获取某一本书
    public static Book getBookById(String id){
        return map.get(id);
    }
}

ShowAllBookServlet.java

显示所有的书籍信息，从map中拿取信息

package servlet;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.Map;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import utils.BookUtils;
import car.Book;

public class ShowAllBookServlet extends HttpServlet {

    /**
     * 1.显示所有书的信息
     * 2.提供一个购物车的链接
     * The doGet method of the servlet. <br>
     *
     * This method is called when a form has its tag value method equals to get.
     * 
     * @param request the request send by the client to the server
     * @param response the response send by the server to the client
     * @throws ServletException if an error occurred
     * @throws IOException if an error occurred
     */
    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        request.setCharacterEncoding("utf-8");
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();
        out.write("书库：<br><br>");
        //1.显示所有的书
        Map<String, Book> map = BookUtils.getAllBook();
        //显示获取到的书
        for (Map.Entry<String, Book> entry : map.entrySet()) {
            //拿到每一本的id
            String id = entry.getKey();
            //拿到每一本书
            Book book = entry.getValue();
            //输出书的内容
            out.write(" 《 " + book.getBookName() +" 》 " +"<a href=' "+ request.getContextPath() +"/servlet/ShowBookDetailServlet?id=" + id + " '>显示详细信息<a><br><br>");
        }
        out.write("<br><hr>");
        //提供查看购物车的链接
        out.write("<a href=' " + request.getContextPath() + "/servlet/CarServlet '>查看购物车</a>");
        
    }

    /**
     * The doPost method of the servlet. <br>
     *
     * This method is called when a form has its tag value method equals to post.
     * 
     * @param request the request send by the client to the server
     * @param response the response send by the server to the client
     * @throws ServletException if an error occurred
     * @throws IOException if an error occurred
     */
    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        doGet(request, response);
    }

}

ShowBookDetailServlet.java

显示书籍的详细信息，并可以将书籍加入购物车

package servlet;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import utils.BookUtils;
import car.Book;

public class ShowBookDetailServlet extends HttpServlet {

    /**
     * 1.显示书的详细信息
     * 2.提供购物车的链接
     * 3.返回继续浏览的链接
     * The doGet method of the servlet. <br>
     *
     * This method is called when a form has its tag value method equals to get.
     * 
     * @param request the request send by the client to the server
     * @param response the response send by the server to the client
     * @throws ServletException if an error occurred
     * @throws IOException if an error occurred
     */
    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        request.setCharacterEncoding("utf-8");
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();
        
        //显示书的详细信息
        String id = request.getParameter("id");
        Book book = BookUtils.getBookById(id);
        
        out.write(book + "<a href=' " + request.getContextPath() + "/servlet/ShowAllBookServlet '> 返回主页继续浏览 </a>" + "&nbsp;&nbsp;<a href=' " + request.getContextPath() + "/servlet/BuyServlet?id=" + id + " '>放入购物车</a>");
        
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        doGet(request, response);
    }

}

BuyServlet.java

处理提交的数据，将数据放入session中

package servlet;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.List;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import utils.BookUtils;
import car.Book;

public class BuyServlet extends HttpServlet {

    /**
     * 将购买的书放入购物车 
     * The doGet method of the servlet. <br>
     *
     * This method is called when a form has its tag value method equals to get.
     * 
     * @param request the request send by the client to the server
     * @param response the response send by the server to the client
     * @throws ServletException if an error occurred
     * @throws IOException if an error occurred
     */
    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        request.setCharacterEncoding("UTF-8");
        response.setContentType("text/html;charset=utf-8");
        PrintWriter out = response.getWriter();
        
        //获取页面传递的id
        String id = request.getParameter("id");
        //根据id找到对象
        Book book = BookUtils.getBookById(id);
        //将书存入session中
        HttpSession session = request.getSession();
        //将书放到session的一个集合中
        List<Book> list = (List<Book>) session.getAttribute("carlist");
        
        if(list == null){
            //说明是第一次加入购物车
            list = new ArrayList<Book>();
            //将·书放入
            book.setCount(1);
            list.add(book);
            //将list放入session
            session.setAttribute("carlist", list);
        }else{
            //说明购物车已经有了东西了
            //判断是否购买过书了
            int index = list.indexOf(book);
            if(index == -1){
                //没有
                book.setCount(1);
                list.add(book);
            }else{
                //说明已经买过一次了
                Book b = list.get(index);
                b.setCount(b.getCount() + 1);
            }
            //提醒用户，书已经放入了
            out.write(book.getBookName() + "已经加入购物车，4秒后转向主页 <a href=' " + request.getContextPath() + "/servlet/ShowAllBookServlet '>回到主页</a>");
            response.setHeader("Refresh", "4;url=" + request.getContextPath() + "/servlet/ShowAllBookServlet");
        }
        
    }

    /**
     * The doPost method of the servlet. <br>
     *
     * This method is called when a form has its tag value method equals to post.
     * 
     * @param request the request send by the client to the server
     * @param response the response send by the server to the client
     * @throws ServletException if an error occurred
     * @throws IOException if an error occurred
     */
    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        doGet(request, response);
    }

}

CarServlet.java  存放购物车的信息

package servlet;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.List;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import car.Book;

public class CarServlet extends HttpServlet {

    /**
     * The doGet method of the servlet. <br>
     *
     * This method is called when a form has its tag value method equals to get.
     * 
     * @param request the request send by the client to the server
     * @param response the response send by the server to the client
     * @throws ServletException if an error occurred
     * @throws IOException if an error occurred
     */
    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        request.setCharacterEncoding("utf-8");
        response.setContentType("text/html;charset=utf-8");
        PrintWriter out = response.getWriter();
        
        out.write("欢迎来到购物车<br>");
        
        HttpSession session = request.getSession();
        if(session.isNew()){
            //直接返回主页面
            response.sendRedirect(request.getContextPath() + "/servlet/ShowAllBookServlet");
            return;
        }
        
        //先从session拿到购物车
        List<Book> list = (List<Book>) session.getAttribute("carlist");
        //如果直接过来的，则让他回去
        if(list == null){
            //首次访问
            out.write("您还没有购买任何东西，2秒后返回主页");
            response.setHeader("Refresh", "2;url=" + request.getContextPath() + "/servlet/ShowAllBookServlet");
            return;
        }
        //说明有数
        out.write("您购买的书籍如下：<br>");
        out.write("书名	数量	总价格<br>");
        for (int i = 0; i< list.size(); i++) {
            Book b = list.get(i);
            out.write(b.getBookName() + "	" + b.getCount() + "	" + b.getPrice() * b.getCount() + "<br>");
        }
        out.write("<br><br><a href=' " + request.getContextPath() + "/servlet/ShowAllBookServlet '>返回主页</a>");
        
    }

    /**
     * The doPost method of the servlet. <br>
     *
     * This method is called when a form has its tag value method equals to post.
     * 
     * @param request the request send by the client to the server
     * @param response the response send by the server to the client
     * @throws ServletException if an error occurred
     * @throws IOException if an error occurred
     */
    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        doGet(request, response);
    }

}

可以了。