Servlet第五篇【介绍会话技术、Cookie的API、详解、应用】（六）

 String id = request.getParameter("id");
        //由于book的id和商品的id是一致的。获取到用户点击的书
        Book book = (Book) DB.getAll().get(id);
        //输出书的详细信息
        printWriter.write("书的编号是：" + book.getId()+"<br/>");
        printWriter.write("书的名称是：" + book.getName()+"<br/>");
        printWriter.write("书的作者是：" + book.getAuthor()+"<br/>");

 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();
            }
        }

if (bookHistory == null) {
            return id;
        }
        //如果Cookie的值不是null的，那么就分解Cookie的得到之前的id。
        String[] strings = bookHistory.split("\\_");
        //为了增删容易并且还要判断id是否存在于该字符串内-----我们使用LinkedList集合装载分解出来的id
        List list = Arrays.asList(strings);
        LinkedList<String> linkedList = new LinkedList<>();
        linkedList.addAll(list);
        if (linkedList.contains(id)) {
            linkedList.remove(id);
            linkedList.addFirst(id);
        }else {
            if (linkedList.size() >= 3) {
                linkedList.removeLast();
                linkedList.addFirst(id);
            } else {
                linkedList.addFirst(id);
            }
        }

 StringBuffer stringBuffer = new StringBuffer();
        //遍历LinkedList集合，添加个下划线“_”
        for (String s : linkedList) {
            stringBuffer.append(s + "_");
        }
        //最后一个元素后面就不需要下划线了
        return stringBuffer.deleteCharAt(stringBuffer.length() - 1).toString();

 String bookHistory = makeHistory(request, id);
        Cookie cookie = new Cookie("bookHistory", bookHistory);
        cookie.setMaxAge(30000);
        response.addCookie(cookie);

   printWriter.write("您曾经浏览过的商品：");
        printWriter.write("<br/>");
        //显示用户浏览过的商品
        Cookie[] cookies = request.getCookies();
        for (int i = 0; cookies != null && i < cookies.length; i++) {
            if (cookies[i].getName().equals("bookHistory")) {
                //获取到的bookHistory是2_3_1之类的
                String bookHistory = cookies[i].getValue();
                //拆解成每一个id值
                String[] ids = bookHistory.split("\\_");
                //得到每一个id值
                for (String id : ids) {
                    //通过id找到每一本书
                    Book book = linkedHashMap.get(id);
                    printWriter.write(book.getName());
                    printWriter.write("<br/>");
                }
                break;
            }
        }