實例講解 JSP Model 2 架構（中）：Eshop.jsp、Cart.jsp 與 ShoppingServlet 核心邏輯

Model 2 角色分工

• View (JSP)：Eshop.jsp 主頁面、Cart.jsp 購物車片段 —— 純表現，無業務邏輯
• Controller (Servlet)：ShoppingServlet —— 接收所有請求、協調 Model、決定導向 View
• Model (JavaBean)：CD Bean、Session 中的 Vector<CD> 購物車

Eshop.jsp：主視圖與表單提交

<%@ page session="true" %>
<form name="shoppingForm" action="/examples/servlet/ShoppingServlet" method="POST">
  <select name="CD">
    <option>Yuan | The Guo Brothers | China | $14.95</option>
    <option>Drums of Passion | Babatunde Olatunji | Nigeria | $16.95</option>
    ...
  </select>
  <input type="text" name="qty" size="3" value="1">
  <input type="hidden" name="action" value="ADD">
  <input type="submit" value="Add to Cart">
</form>
<jsp:include page="Cart.jsp" flush="true"/>

• 下拉選單值以 | 分隔：專輯 | 藝人 | 國家 | 價格
• 隱藏欄位 action=ADD 識別操作類型
• <jsp:include> 動態包含購物車片段

Cart.jsp：購物車顯示邏輯

<%@ page session="true" import="java.util.*, shopping.CD" %>
<%
    Vector buylist = (Vector) session.getValue("shopping.shoppingcart");
    if (buylist != null && buylist.size() > 0) {
%>
  <table>
    <tr><th>ALBUM</th><th>ARTIST</th><th>COUNTRY</th><th>PRICE</th><th>QUANTITY</th><th></th></tr>
    <% for (int i=0; i<buylist.size(); i++) {
         CD o = (CD) buylist.elementAt(i); %>
    <tr>
      <td><%= o.getAlbum() %></td> ... 
      <td>
        <form action="/examples/servlet/ShoppingServlet" method="POST">
          <input type="hidden" name="action" value="DELETE">
          <input type="hidden" name="delindex" value="<%= i %>">
          <input type="submit" value="Delete">
        </form>
      </td>
    </tr>
    <% } %>
  </table>
  <form action="/examples/servlet/ShoppingServlet" method="POST">
    <input type="hidden" name="action" value="CHECKOUT">
    <input type="submit" value="Checkout">
  </form>
<% } %>

• 從 Session 取得購物車，為空則不顯示
• 每列商品含獨立刪除表單，攜帶 delindex 識別項目
• 結帳按鈕送出 action=CHECKOUT

ShoppingServlet：中央控制器完整邏輯

public class ShoppingServlet extends HttpServlet {
    public void doPost(HttpServletRequest req, HttpServletResponse res) 
            throws ServletException, IOException {
        HttpSession session = req.getSession(false);
        if (session == null) { 
            res.sendRedirect("https://localhost:8080/error.html"); 
            return;
        }
        Vector buylist = (Vector) session.getValue("shopping.shoppingcart");
        String action = req.getParameter("action");

        if (!"CHECKOUT".equals(action)) {
            if ("DELETE".equals(action)) {
                int idx = Integer.parseInt(req.getParameter("delindex"));
                buylist.removeElementAt(idx);
            } else if ("ADD".equals(action)) {
                CD cd = parseCD(req);
                if (buylist == null) {
                    buylist = new Vector();
                    buylist.addElement(cd);
                } else {
                    boolean merged = false;
                    for (int i=0; i<buylist.size(); i++) {
                        CD existing = (CD) buylist.elementAt(i);
                        if (existing.getAlbum().equals(cd.getAlbum())) {
                            existing.setQuantity(existing.getQuantity() + cd.getQuantity());
                            buylist.setElementAt(existing, i);
                            merged = true; break;
                        }
                    }
                    if (!merged) buylist.addElement(cd);
                }
            }
            session.putValue("shopping.shoppingcart", buylist);
            forward("/jsp/shopping/EShop.jsp", req, res);
        } else {
            float total = 0;
            for (int i=0; i<buylist.size(); i++) {
                CD o = (CD) buylist.elementAt(i);
                total += o.getPrice() * o.getQuantity();
            }
            total += 0.005f; // 四捨五入至分
            String amount = String.valueOf(total).substring(0, String.valueOf(total).indexOf('.')+3);
            req.setAttribute("amount", amount);
            forward("/jsp/shopping/Checkout.jsp", req, res);
        }
    }

    private CD parseCD(HttpServletRequest req) {
        StringTokenizer st = new StringTokenizer(req.getParameter("CD"), "|");
        CD cd = new CD();
        cd.setAlbum(st.nextToken().trim());
        cd.setArtist(st.nextToken().trim());
        cd.setCountry(st.nextToken().trim());
        cd.setPrice(Float.parseFloat(st.nextToken().replace('$',' ').trim()));
        cd.setQuantity(Integer.parseInt(req.getParameter("qty")));
        return cd;
    }

    private void forward(String url, HttpServletRequest req, HttpServletResponse res) 
            throws ServletException, IOException {
        RequestDispatcher rd = getServletContext().getRequestDispatcher(url);
        rd.forward(req, res);
    }
}

關鍵設計亮點

1. 單一入口：所有動作 (ADD/DELETE/CHECKOUT) 由同一 Servlet 根據 action 參數分派
2. Session 為 Model 容器：購物車存在 Session，跨請求保持狀態
3. Forward 不 Redirect：保持請求屬性 (如 amount)，URL 不變，使用者可重新整理
4. 智慧合併同款商品：比對專輯名稱，累加數量而非重複列項
5. 解析邏輯封裝於私有方法：parseCD()、forward() 降低 doPost 複雜度