Java Web 購物車實作：基於 Session 的商品管理完整範例

前言

購物車 是電商系統的核心功能之一。在不引入資料庫的輕量級場景下，利用 HttpSession 在伺服器端暫存使用者選購的商品，是快速原型開發與中小型專案的常見做法。本文提供完整的 Servlet 架構範例，涵蓋商品列表、加入購物車、查看購物車三大核心流程。

系統架構設計

┌─────────────┐     ┌─────────────┐     ┌─────────────┐
│ ListBook    │────▶│   Buy       │────▶│ SessionCount│
│ Servlet     │     │ Servlet     │     │ Demo        │
│ (商品列表)  │     │ (加入購物車) │     │ (查看購物車) │
└─────────────┘     └─────────────┘     └─────────────┘
                           │
                           ▼
                    ┌─────────────┐
                    │ HttpSession │
                    │ List  │
                    └─────────────┘

核心類別定義

Book 實體類別

package cn.com.shopping;

public class Book {
    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;
    }
    // Getter/Setter 略...
}

模擬資料庫 Db 類別

package cn.com.shopping;

import java.util.LinkedHashMap;
import java.util.Map;

class Db {
    private static Map map = new LinkedHashMap<>();

    static {
        map.put("1", new Book("1", "Java Web 開發實戰", "王曉明", "經典入門好書"));
        map.put("2", new Book("2", "Spring Boot 微服務", "張三", "進階實戰"));
        map.put("3", new Book("3", "MySQL 效能調優", "李四", "DBA 必讀"));
        map.put("4", new Book("4", "Redis 設計與實現", "趙五", "原理深度解析"));
        map.put("5", new Book("5", "Kubernetes 實戰指南", "錢六", "容器編排權威"));
    }

    public static Map getAll() {
        return map;
    }
}

Servlet 實作細節

1. 商品列表 Servlet

package cn.com.shopping;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.Map;
import javax.servlet.ServletException;
import javax.servlet.http.*;

public class ListBookServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();

        out.print("本店商品列表：<br/>");
        Map map = Db.getAll();
        for (Map.Entry entry : map.entrySet()) {
            Book book = entry.getValue();
            // 使用 encodeURL 支援無 Cookie 環境下的 Session 追蹤
            String url = response.encodeURL("/Session/BuyServlet?id=" + book.getId());
            out.print(book.getName() + " <a href='" + url + "' target='_blank'>購買</a><br/>");
        }
    }
}

2. 加入購物車 Servlet

package cn.com.shopping;

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import javax.servlet.ServletException;
import javax.servlet.http.*;

public class BuyServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        String id = request.getParameter("id");
        Book book = Db.getAll().get(id);

        // getSession(false) 不會強制建立新 Session，避免匿名使用者產生垃圾 Session
        HttpSession session = request.getSession(false);
        if (session == null) {
            session = request.getSession(true); // 確認需要時才建立
        }

        @SuppressWarnings("unchecked")
        List cart = (List) session.getAttribute("cart");
        if (cart == null) {
            cart = new ArrayList<>();
            session.setAttribute("cart", cart);
        }
        cart.add(book);

        // 重導向至購物車頁面，使用 encodeRedirectURL 維持 Session ID
        String url = response.encodeRedirectURL("/Session/SessionCountDemo");
        response.sendRedirect(url);
    }
}

3. 查看購物車 Servlet

package cn.com.shopping;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.List;
import javax.servlet.ServletException;
import javax.servlet.http.*;

public class SessionCountDemo extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();

        HttpSession session = request.getSession(false);
        if (session == null) {
            out.write("您尚未購買任何商品！");
            return;
        }

        @SuppressWarnings("unchecked")
        List cart = (List) session.getAttribute("cart");
        if (cart == null || cart.isEmpty()) {
            out.write("購物車是空的！");
            return;
        }

        out.write("您的購物車商品：<br/>");
        for (Book book : cart) {
            out.write("《" + book.getName() + "》 作者：" + book.getAuthor() + "<br/>");
        }
        out.write("<br/><a href='/Session/ListBookServlet'>繼續購物</a>");
    }
}

關鍵技術點總結

實務擴展建議

• 數量管理：將 List 改為 Map 或建立 CartItem 類別包含數量、小計
• 持久化：結合資料表 cart_items(user_id, book_id, quantity) 實現跨裝置同步
• 庫存檢查：加入購物車時即時驗證庫存，或下單時鎖定庫存
• Session 超時處理：配合 HttpSessionListener 記錄棄單行為