Spring Web Flow 2.0 實戰進階（三）：Subflow 實現添加商品到購物車功能

業務場景

購物車頁面展示商品列表，用戶點擊 "加入購物車" 觸發子流程，完成後返回主流程刷新展示。

領域模型實現

CartItem - 購物車條目

public class CartItem implements Serializable {
    private Product product;
    private int quantity;
    
    public CartItem(Product product, int quantity) { ... }
    public int getTotalPrice() { return quantity * product.getPrice(); }
    public void increaseQuantity() { quantity++; }
    // Getter/Setter
}

Cart - 購物車聚合根

public class Cart implements Serializable {
    private Map items = new HashMap<>();
    
    public List getItems() { return new ArrayList<>(items.values()); }
    
    public void addItem(Product product) {
        CartItem item = items.get(product.getId());
        if (item != null) item.increaseQuantity();
        else items.put(product.getId(), new CartItem(product, 1));
    }
    
    public int getTotalPrice() {
        return items.values().stream()
            .mapToInt(i -> i.getProduct().getPrice() * i.getQuantity())
            .sum();
    }
}

關鍵：Cart 非單例，每用戶一個，存放於 conversationScope。

主流程配置 - shopping.xml

<flow>
    <!-- 啟動時創建購物車放入會話範圍 -->
    <var name="mycart" class="samples.webflow.Cart"/>
    <on-start>
        <set name="conversationScope.cart" value="mycart"/>
    </on-start>
    
    <view-state id="viewCart" view="viewCart">
        <on-render>
            <evaluate expression="productService.getProducts()" result="viewScope.products"/>
        </on-render>
        <transition on="submit" to="viewOrder"/>
        <transition on="addToCart" to="addProductToCart"/>
    </view-state>
    
    <!-- 調用子流程 -->
    <subflow-state id="addProductToCart" subflow="addToCart">
        <transition on="productAdded" to="viewCart"/>
    </subflow-state>
    
    <view-state id="viewOrder" .../>
    <view-state id="orderConfirmed" .../>
    <end-state id="returnToIndex" .../>
</flow>

子流程定義 - addToCart.xml

<flow>
    <on-start>
        <!-- 從請求參數獲取 productId -->
        <set name="requestScope.productId" value="requestParameters.productId"/>
    </on-start>
    
    <action-state id="addToCart">
        <evaluate expression="cart.addItem(productService.getProduct(productId))"/>
        <transition to="productAdded"/>
    </action-state>
    
    <end-state id="productAdded"/>
</flow>

註冊子流程 - webflow-config.xml

<webflow:flow-registry id="flowRegistry" flow-builder-services="flowBuilderServices">
    <webflow:flow-location path="/WEB-INF/flows/shopping.xml" id="shopping"/>
    <webflow:flow-location path="/WEB-INF/flows/addToCart.xml" id="addToCart"/>
</webflow:flow-registry>

視圖交互 - viewCart.jsp

<c:choose>
    <c:when test="${empty cart.items}">
        <p>Your cart is empty.</p>
    </c:when>
    <c:otherwise>
        <table>
            <c:forEach var="item" items="${cart.items}">
                <tr>
                    <td>${item.product.description}</td>
                    <td>${item.quantity}</td>
                    <td>${item.product.price}</td>
                    <td>${item.totalPrice}</td>
                </tr>
            </c:forEach>
            <tr><td>TOTAL:</td><td>${cart.totalPrice}</td></tr>
        </table>
    </c:otherwise>
</c:choose>

<a href="${flowExecutionUrl}&_eventId=submit">Submit</a>

<h2>Products for Your Choice</h2>
<table>
    <c:forEach var="product" items="${products}">
        <tr>
            <td>${product.description}</td>
            <td>${product.price}</td>
            <td>
                <a href="${flowExecutionUrl}&_eventId=addToCart&productId=${product.id}">[add to cart]</a>
            </td>
        </tr>
    </c:forEach>
</table>

關鍵點總結

• conversationScope 跨主/子流程共享 Cart
• subflow-state 調用子流程，transition on="productAdded" 接收子流程結束事件
• 子流程 end-state id="productAdded" 與主流程 transition 對應
• 請求參數通過 requestParameters.productId 傳遞
• action-state 純業務邏輯執行，無視圖渲染