servlet显示商品浏览历史记录

假设有一个数据库和书本对象

//数据库
class Db{
	private static Map<String, Book> map = new LinkedHashMap();
	
	static{
		map.put("1", new Book("1","web开发","老张"));
		map.put("2", new Book("2","spring","老毕"));
		map.put("3", new Book("3","struts","老张"));
		map.put("4", new Book("4","jdbc","老黎"));
	}
	
	//拿到书
	public static Map getAll(){
		return map;
	}
}

//书对象
class Book{
	private String id;
	private String name;
	private String author;
	
	public Book() {
		super();
	}

	public Book(String id, String name, String author) {
		super();
		this.id = id;
		this.name = name;
		this.author = author;
	}
	
	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;
	}
	
}

第一个servlet显示商品列表和历史记录

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		
		//设置防止乱码
		response.setCharacterEncoding("utf-8");
		response.setContentType("text/html;charset=utf-8");
		
		//显示商品信息
		PrintWriter out = response.getWriter();
		out.write("有以下商品信息：<br>");
		
		Map<String, Book> map = Db.getAll();
		for(Map.Entry<String, Book> entry : map.entrySet()){
			Book book = entry.getValue();
			out.print("<a href='/day07/CookieDemo3?id="+book.getId()+"'target='_blank'> "+book.getName()+"</a><br>");
			
		}
		
		//显示用户曾经看过什么商品
		out.print("<br>您曾经看过如下商品：<br>");
		Cookie cookies[] = request.getCookies();
		for(int i=0; cookies!=null&&i<cookies.length; i++){
			if(cookies[i].getName().equals("bookHistory")){
				String ids[] = cookies[i].getValue().split("\\,");     //2,2,3
				for(String id : ids){
					Book book = (Book) Db.getAll().get(id);
					out.print(book.getName()+"<br>");
				}
			}
		}
		
	}

第二个servlet显示商品具体信息，存储cookie

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		
		//设置防止乱码
		response.setCharacterEncoding("utf-8");
		response.setContentType("text/html;charset=utf-8");
				
		//显示商品信息
		PrintWriter out = response.getWriter();
		
		//根据用户带来的id显示商品信息
		String id = request.getParameter("id");
		Book book = (Book) Db.getAll().get(id);
		out.print(book.getId()+"<br>");
		out.print(book.getName()+"<br>");
		out.print(book.getAuthor()+"<br>");
		
		//构建Cookie回写给浏览器
		String cookieValue = buildCookie(id,request);
		Cookie cookie = new Cookie("bookHistory", cookieValue);
		cookie.setPath("/day07");
		cookie.setMaxAge(3600*24);
		response.addCookie(cookie);
	}

	
	private String buildCookie(String id, HttpServletRequest request) {
		
		String bookHistory = null;
		Cookie cookies[] = request.getCookies();
		for(int i=0; cookies!=null&&i<cookies.length; i++){
			if(cookies[i].getName().equals("bookHistory")){
				bookHistory = cookies[i].getValue();
			}
		}
		
		/*分情况谈论 
		 * old   id   now
		 * null  1    1
		 * 2,3   1 	  1,2,3
		 * 1      1   1
		 *  ...
		 */
		if(bookHistory==null){
			return id;
		}
		
		//LinkedList便于增删改查
		LinkedList list = new LinkedList(Arrays.asList(bookHistory.split("\\,")));
		if(list.contains(id)){
			list.remove(id);
			list.addFirst(id);
		}else{
			if(list.size()>=3){
				list.removeLast();
				list.addFirst(id);
			}else{
				list.addFirst(id);
			}
		}
		
		StringBuilder sb = new StringBuilder();
		for(Object ob : list){
			sb.append(ob+",");
		}
		
		return sb.deleteCharAt(sb.length()-1).toString();
	}