Java Web 實戰：使用 Cookie 記錄並展示用戶最近瀏覽的商品

前言

「最近瀏覽」功能能顯著提升電商轉化率。本文展示基於 Cookie 的輕量級實現：無需數據庫、無需登錄、瀏覽器端持久化，適合匿名用戶行為追蹤。

1. 領域模型

public class Book implements Serializable {
    private String id, name, price, auth, publish, description;
    // 構造器、Getter/Setter 省略
}

2. 數據訪問層（模擬）

public class BookDao {
    private static Map<String, Book> bookMap = new LinkedHashMap<>();
    static {
        bookMap.put("1", new Book("1", "Java核心技術", "99.0", "Cay Horstmann", "機械工業出版社", "經典入門"));
        bookMap.put("2", new Book("2", "Effective Java", "89.0", "Joshua Bloch", "Addison-Wesley", "進階必讀"));
        bookMap.put("3", new Book("3", "Java併發編程實戰", "109.0", "Brian Goetz", "Addison-Wesley", "併發聖經"));
    }
    public static Map<String, Book> getBooks() { return bookMap; }
    public static Book getBook(String id) { return bookMap.get(id); }
}

3. 書籍列表 Servlet（展示所有書 + 最近瀏覽）

@WebServlet("/bookList")
public class BookListServlet extends HttpServlet {
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        resp.setContentType("text/html;charset=UTF-8");
        PrintWriter out = resp.getWriter();

        // 1. 顯示所有書籍鏈接
        for (Map.Entry<String, Book> entry : BookDao.getBooks().entrySet()) {
            Book book = entry.getValue();
            out.write("<a href='" + req.getContextPath() + "/bookDetail?id=" + book.getId() + "'>"
                    + book.getName() + "</a><br>");
        }
        out.write("<hr>");

        // 2. 讀取 Cookie 顯示最近瀏覽
        Cookie[] cookies = req.getCookies();
        Cookie historyCookie = null;
        if (cookies != null) {
            for (Cookie c : cookies) if ("recentBooks".equals(c.getName())) historyCookie = c;
        }

        if (historyCookie == null) {
            out.write("暫無瀏覽記錄！");
        } else {
            out.write("您最近瀏覽過的書籍：<br>");
            String[] ids = historyCookie.getValue().split(",");
            for (String id : ids) {
                Book book = BookDao.getBook(id);
                if (book != null) out.write(book.getName() + "<br>");
            }
        }
    }
}

4. 書籍詳情 Servlet（更新 Cookie）

@WebServlet("/bookDetail")
public class BookDetailServlet extends HttpServlet {
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        resp.setContentType("text/html;charset=UTF-8");
        String id = req.getParameter("id");
        Book book = BookDao.getBook(id);
        if (book == null) {
            resp.getWriter().write("書籍不存在！");
            return;
        }
        // 顯示詳情
        PrintWriter out = resp.getWriter();
        out.write("<h1>" + book.getName() + "</h1>");
        out.write("<p>作者：" + book.getAuth() + "</p>");
        out.write("<p>價格：" + book.getPrice() + "</p>");
        out.write("<p>出版社：" + book.getPublish() + "</p>");
        out.write("<p>描述：" + book.getDescription() + "</p>");

        // 更新最近瀏覽 Cookie（最多保留 3 條，新訪問置頂，去重）
        String newHistory = book.getId();
        Cookie[] cookies = req.getCookies();
        Cookie historyCookie = null;
        if (cookies != null) {
            for (Cookie c : cookies) if ("recentBooks".equals(c.getName())) historyCookie = c;
        }
        if (historyCookie != null) {
            String[] oldIds = historyCookie.getValue().split(",");
            StringBuilder sb = new StringBuilder();
            sb.append(book.getId()).append(",");
            for (String oldId : oldIds) {
                if (!oldId.equals(book.getId()) && sb.toString().split(",").length < 3) {
                    sb.append(oldId).append(",");
                }
            }
            newHistory = sb.substring(0, sb.length() - 1); // 移除尾部逗號
        }

        Cookie newCookie = new Cookie("recentBooks", newHistory);
        newCookie.setMaxAge(3600 * 24 * 30); // 30 天
        newCookie.setPath(req.getContextPath());
        resp.addCookie(newCookie);
    }
}

5. 核心邏輯解析

1. Cookie 命名：recentBooks 存儲書籍 ID 列表，逗號分隔。
2. 去重 + 置頂：新訪問 ID 放最前，舊列表中移除同 ID，保留前 3 個。
3. 有效期：setMaxAge(30*24*3600) 實現 30 天持久化。
4. Path 作用域：setPath(contextPath) 限制在當前應用下發送。

6. 局限性與改進方向

總結

基於 Cookie 的最近瀏覽實現簡單、無狀態、零後端存儲成本，適合中小型項目快速上線。隨著業務增長，建議遷移至服務端 Session/Redis 方案，實現多端同步與個性化推薦。