Session案例，显示商品浏览记录

相对于Cookie而言，Session并不经常在服务器中使用，因为session为每一个浏览器用户创建了一个session，这样对服务器的要求无形之中高了很多，而cookie是对每个浏览器发送数据，所以服务器负担小，所以大部分浏览器还是选择使用cookie来实现浏览记录的显示。

当用户的浏览器禁用Cookie的时候，可以使用超链接里面带着指定的id去访问servlet，具体代码为response.encodeURL("/");里面放具体的servlet访问地址,这样的话，就能带着指定的id去访问从而解决了禁用cookie问题,
如果用户没有禁用cookie，则第一次他会将超链接和cookie同时发送回去，但是，当第二次访问的时候，服务器发现他返回了cookie,则就不在重新写超链接了

本案例是使用session来实现模仿当用户在首页点击了商品名字的时候，通过session记录了各个他点击的商品的名字，然后当选择结账的时候，自动将他选择购买的商品显示出来，下面上代码

第一段代码是用来充当首页的servlet,里面假设了如果用户禁用cookie的情况

package com.bird.shopping;

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

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/**
 * @category 模拟网站首页的servlet用来显示所有的商品
 * @author Bird
 *
 */
public class ListBookServlet extends HttpServlet {

	private static final long serialVersionUID = 1L;

	public void doGet(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		response.setCharacterEncoding("UTF-8");
		response.setContentType("text/html;charset=UTF-8");
		PrintWriter out = response.getWriter();
		
		request.getSession();//注意，一定要得到session，否则无法进行自动的id添加
		
		out.write("本网站有如下商品</br>");
		Map<String,Book> map = DB.getAll();
		for(Map.Entry<String, Book> entry : map.entrySet()){
			Book book = entry.getValue();
			String url = response.encodeURL("/Web/servlet/BuyServlet?id="+book.getId()+"");
			out.write(book.getName()+"<a href='"+url+"' target='_blank'>购买</a></br>");
		}
		
	}

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

	}

}

class DB{
	//必须使用链表Map进行有序存储
	private static Map<String,Book> map = new LinkedHashMap<String,Book>();
	static{
		map.put("1", new Book("1","javaweb开发详解","老张","一本好书!!!"));
		map.put("2", new Book("2","jdbc开发详解","老张","一本好书!!!"));
		map.put("3", new Book("3","sping开发详解","老黎","一本好书!!!"));
		map.put("4", new Book("4","struts开发详解","老宇","一本好书!!!"));
		map.put("5", new Book("5","heibrate开发详解","老毕","一本好书!!!"));
	}
	
	public static Map<String,Book> getAll(){
		return map;
	}
}

class Book implements Serializable{
	
	private static final long serialVersionUID = 1L;
	private String id;
	private String name;
	private String author;
	private String description;
	
	public Book(){}
	
	public Book(String id, String name, String author, String description) {
		this.id = id;
		this.name = name;
		this.author = author;
		this.description = description;
	}
	
	public String getId() {
		return id;
	}
	public void setId(String id) {
		this.id = id;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public String getAuthor() {
		return author;
	}
	public void setAuthor(String author) {
		this.author = author;
	}
	public String getDescription() {
		return description;
	}
	public void setDescription(String description) {
		this.description = description;
	}
	
}	

第二段代码，是根据带过来的id将对应的商品名称放入到session中，以备下面的结账，当然，最终的跳转一定要使用浏览器的本地重定向，而不是服务器的请求重定向，否则会反复访问这个servlet干同样的事情，导致多次的购买

package com.bird.shopping;

import java.io.IOException;
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;

/**
 * @category 购买
 * @author Bird
 *
 */
public class BuyServlet extends HttpServlet {

	private static final long serialVersionUID = 1L;

	public void doGet(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		String id = request.getParameter("id");
		Book book = DB.getAll().get(id);
		
		HttpSession session = request.getSession();
		
		//从session获得用户保存的数据（购物车）
		@SuppressWarnings("unchecked")
		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(request.getContextPath()+"/servlet/ListCarServlet");
		//request.getRequestDispatcher("/servlet/ListCarServlet").forward(request, response);
		response.sendRedirect(url);
	}

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

	}

}

第三段代码，用来显示已经购买的商品名称

package com.bird.shopping;

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;

/**
 * @category 显示用户购买的商品
 * @author Bird
 *
 */
public class ListCarServlet extends HttpServlet {

	private static final long serialVersionUID = 1L;

	public void doGet(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		response.setCharacterEncoding("UTF-8");
		response.setContentType("text/html;charset=UTF-8");
		PrintWriter out = response.getWriter();
		
		HttpSession session = request.getSession();
		if(session == null){
			out.write("您没有购买任何商品");
			return;
		}
		
		out.write("您购买了如下商品:</br>");
		@SuppressWarnings("unchecked")
		List<Book> list = (List<Book>) session.getAttribute("list");
		for(Book book: list){
			out.write(book.getName());
			out.write("</br>");
		}
	}

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

	}

}