学习笔记八

领域驱动设计核心概念
为什么需要DDD？
java
// 传统CRUD开发的痛点
@Service
public class OrderService {

// 贫血模型：只有数据，没有业务逻辑
@Data
public class Order {
    private Long id;
    private Long userId;
    private BigDecimal amount;
    private String status;
    private List<OrderItem> items;
    // 只有getter/setter，没有业务方法
}

// 业务逻辑散落在Service中
public void submitOrder(Order order) {
    // 1. 校验库存
    checkInventory(order);
    
    // 2. 计算金额
    calculateAmount(order);
    
    // 3. 检查用户余额
    checkUserBalance(order);
    
    // 4. 扣减库存
    reduceInventory(order);
    
    // 5. 创建订单
    saveOrder(order);
    
    // 6. 发送消息
    sendOrderCreatedEvent(order);
}

}

/*
问题：

1. 业务逻辑无法复用

2. 领域知识丢失

3. 需求变更困难

4. 代码难以理解
   */
   DDD核心概念
   java
   // 1. 实体（Entity）：有唯一标识，可变
   @Getter
   public class Order {
   private final OrderId id; // 值对象作为ID
   private OrderStatus status; // 状态枚举
   private Money totalAmount; // 值对象
   private List items;
   private DeliveryAddress deliveryAddress;

   // 实体有业务行为
   public void addItem(Product product, int quantity) {
   // 业务规则：不能添加已取消订单的商品
   if (this.status == OrderStatus.CANCELLED) {
   throw new OrderAlreadyCancelledException();
   }

    OrderItem item = new OrderItem(product, quantity);
    this.items.add(item);
    this.totalAmount = calculateTotalAmount(); // 重新计算总额
   

   }

   public void submit() {
   // 业务规则：订单必须有商品
   if (items.isEmpty()) {
   throw new EmptyOrderException();
   }

    // 业务规则：金额必须大于0
    if (totalAmount.lessThan(Money.ZERO)) {
        throw new InvalidOrderAmountException();
    }
    
    this.status = OrderStatus.SUBMITTED;
    registerEvent(new OrderSubmittedEvent(this.id));
   

   }

   public void cancel() {
   // 业务规则：已支付的订单不能取消
   if (this.status == OrderStatus.PAID) {
   throw new PaidOrderCannotBeCancelledException();
   }

    this.status = OrderStatus.CANCELLED;
    registerEvent(new OrderCancelledEvent(this.id));
   

   }
   }

// 2. 值对象（Value Object）：无标识，不可变
@Value
public class Money {
BigDecimal amount;
Currency currency;

// 业务方法：加法（返回新对象，不修改原对象）
public Money add(Money other) {
    if (!this.currency.equals(other.currency)) {
        throw new CurrencyMismatchException();
    }
    return new Money(this.amount.add(other.amount), this.currency);
}

// 业务规则验证
public boolean isGreaterThan(Money other) {
    return this.amount.compareTo(other.amount) > 0;
}

}

// 3. 聚合（Aggregate）：一致性边界
public class Order extends AggregateRoot {

private List<OrderItem> items;      // 聚合内的实体
private Payment payment;              // 聚合内的值对象
private DeliveryInfo deliveryInfo;    // 聚合内的值对象

// 聚合内的一致性规则
public void pay(CreditCard card) {
    // 规则1：只能支付已提交的订单
    if (this.status != OrderStatus.SUBMITTED) {
        throw new InvalidOrderStatusException();
    }
    
    // 规则2：支付金额必须等于订单总额
    Payment payment = new Payment(this.totalAmount, card);
    this.payment = payment;
    this.status = OrderStatus.PAID;
    
    registerEvent(new OrderPaidEvent(this.id));
}

// 通过ID引用其他聚合
private UserId userId;  // 只引用ID，不直接引用User聚合

}

// 4. 领域服务（Domain Service）：跨多个聚合的业务逻辑
@Service
@DomainService
public class OrderPlacementService {

private final OrderRepository orderRepository;
private final InventoryService inventoryService;
private final PaymentService paymentService;

public Order placeOrder(Order order, PaymentInfo payment) {
    // 跨聚合的业务逻辑
    // 1. 检查库存
    inventoryService.checkAvailability(order.getItems());
    
    // 2. 创建订单（Order聚合的行为）
    order.submit();
    
    // 3. 处理支付（Payment聚合的行为）
    payment.process(order.getTotalAmount());
    
    // 4. 保存订单
    orderRepository.save(order);
    
    return order;
}

}

// 5. 领域事件（Domain Event）：记录重要事件
@Getter
public class OrderSubmittedEvent extends DomainEvent {
private final OrderId orderId;
private final Money totalAmount;
private final LocalDateTime submittedAt;

public OrderSubmittedEvent(OrderId orderId, Money totalAmount) {
    this.orderId = orderId;
    this.totalAmount = totalAmount;
    this.submittedAt = LocalDateTime.now();
}

}

// 事件监听器
@Component
public class OrderEventListener {

@EventListener
public void handleOrderSubmitted(OrderSubmittedEvent event) {
    // 1. 发送通知
    sendNotification(event);
    
    // 2. 更新统计
    updateStatistics(event);
    
    // 3. 触发后续流程
    startWorkflow(event);
}

}