[Design Patterns] 도메인 주도 설계와 디자인 패턴

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// Entity의 핵심 - Identity와 생명주기 관리
public abstract class Entity<ID> {
    protected ID id;
    
    protected Entity(ID id) {
        this.id = Objects.requireNonNull(id, "Entity ID cannot be null");
    }
    
    public ID getId() {
        return id;
    }
    
    // Identity-based equality
    @Override
    public boolean equals(Object obj) {
        if (this == obj) return true;
        if (obj == null || getClass() != obj.getClass()) return false;
        
        Entity<?> entity = (Entity<?>) obj;
        return Objects.equals(id, entity.id);
    }
    
    @Override
    public int hashCode() {
        return Objects.hash(id);
    }
    
    // Template Method 패턴으로 비즈니스 규칙 검사
    protected final void checkBusinessRules(BusinessRule... rules) {
        for (BusinessRule rule : rules) {
            if (!rule.isSatisfied()) {
                throw new BusinessRuleViolationException(rule.getMessage());
            }
        }
    }
}

// Value Object 패턴 구현
public class Money {
    private final BigDecimal amount;
    private final Currency currency;
    
    public Money(BigDecimal amount, Currency currency) {
        this.amount = Objects.requireNonNull(amount);
        this.currency = Objects.requireNonNull(currency);
    }
    
    public Money add(Money other) {
        ensureSameCurrency(other);
        return new Money(this.amount.add(other.amount), this.currency);
    }
    
    private void ensureSameCurrency(Money other) {
        if (!this.currency.equals(other.currency)) {
            throw new IllegalArgumentException("Cannot operate on different currencies");
        }
    }
    
    @Override
    public boolean equals(Object obj) {
        if (this == obj) return true;
        if (obj == null || getClass() != obj.getClass()) return false;
        
        Money money = (Money) obj;
        return Objects.equals(amount, money.amount) && 
               Objects.equals(currency, money.currency);
    }
    
    @Override
    public int hashCode() {
        return Objects.hash(amount, currency);
    }
}

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// Aggregate Root - 복합체 패턴과 도메인 이벤트 결합
public abstract class AggregateRoot<ID> extends Entity<ID> {
    private final List<DomainEvent> domainEvents = new ArrayList<>();
    
    protected AggregateRoot(ID id) {
        super(id);
    }
    
    protected void addDomainEvent(DomainEvent event) {
        domainEvents.add(event);
    }
    
    public List<DomainEvent> getDomainEvents() {
        return Collections.unmodifiableList(domainEvents);
    }
    
    public void clearDomainEvents() {
        domainEvents.clear();
    }
}

// Order Aggregate 예시
public class Order extends AggregateRoot<OrderId> {
    private CustomerId customerId;
    private List<OrderLine> orderLines;
    private OrderStatus status;
    private Money totalAmount;
    
    // Factory Method 패턴
    public static Order create(CustomerId customerId, ShippingAddress address) {
        OrderId orderId = OrderId.generate();
        Order order = new Order(orderId, customerId);
        
        order.addDomainEvent(new OrderCreatedEvent(orderId, customerId));
        return order;
    }
    
    private Order(OrderId id, CustomerId customerId) {
        super(id);
        this.customerId = Objects.requireNonNull(customerId);
        this.orderLines = new ArrayList<>();
        this.status = OrderStatus.DRAFT;
    }
    
    public void addOrderLine(ProductId productId, int quantity, Money unitPrice) {
        checkBusinessRules(
            new OrderCanBeModifiedRule(this.status),
            new QuantityMustBePositiveRule(quantity)
        );
        
        OrderLine orderLine = new OrderLine(productId, quantity, unitPrice);
        orderLines.add(orderLine);
        
        addDomainEvent(new OrderLineAddedEvent(this.getId(), productId, quantity));
    }
    
    public void confirm() {
        checkBusinessRules(
            new OrderMustHaveItemsRule(this.orderLines),
            new OrderCanBeConfirmedRule(this.status)
        );
        
        this.status = OrderStatus.CONFIRMED;
        addDomainEvent(new OrderConfirmedEvent(this.getId(), this.totalAmount));
    }
}

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// Repository의 도메인 중심 설계
public interface Repository<T extends AggregateRoot<ID>, ID> {
    void save(T aggregate);
    void delete(T aggregate);
    Optional<T> findById(ID id);
    boolean exists(ID id);
}

// 구체적인 Repository 인터페이스
public interface OrderRepository extends Repository<Order, OrderId> {
    List<Order> findByCustomerId(CustomerId customerId);
    List<Order> findByStatus(OrderStatus status);
    List<Order> findBySpecification(OrderSpecification specification);
}

// Repository 구현체 - Adapter 패턴
@Repository
public class JpaOrderRepository implements OrderRepository {
    private final JpaOrderDataRepository jpaRepository;
    private final OrderMapper orderMapper;
    
    @Override
    public void save(Order order) {
        OrderEntity entity = orderMapper.toEntity(order);
        jpaRepository.save(entity);
        
        // 도메인 이벤트 발행
        publishDomainEvents(order);
    }
    
    @Override
    public Optional<Order> findById(OrderId id) {
        return jpaRepository.findById(id.getValue())
                           .map(orderMapper::toDomain);
    }
    
    private void publishDomainEvents(Order order) {
        order.getDomainEvents().forEach(event -> {
            DomainEventPublisher.instance().publish(event);
        });
        order.clearDomainEvents();
    }
}

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// Command 측면
public interface OrderCommandService {
    OrderId placeOrder(PlaceOrderCommand command);
    void cancelOrder(CancelOrderCommand command);
}

// Query 측면  
public interface OrderQueryService {
    OrderSummary getOrderSummary(OrderId orderId);
    List<OrderListItem> getOrdersByCustomer(CustomerId customerId);
}

// Domain Service - 여러 Aggregate를 조정
@Service
public class OrderProcessingService {
    private final OrderRepository orderRepository;
    private final CustomerRepository customerRepository;
    
    @Transactional
    public OrderId processOrder(PlaceOrderCommand command) {
        // 1. 고객 조회 및 검증
        Customer customer = customerRepository.findById(command.getCustomerId())
            .orElseThrow(() -> new CustomerNotFoundException(command.getCustomerId()));
        
        // 2. 주문 생성
        Order order = Order.create(command.getCustomerId(), command.getShippingAddress());
        
        // 3. 주문 항목 추가
        for (OrderItemRequest item : command.getItems()) {
            order.addOrderLine(item.getProductId(), item.getQuantity(), item.getUnitPrice());
        }
        
        // 4. 주문 확정
        order.confirm();
        
        // 5. 저장
        orderRepository.save(order);
        
        return order.getId();
    }
}

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// Event Store 패턴
public interface EventStore {
    void saveEvents(String aggregateId, List<DomainEvent> events, int expectedVersion);
    List<DomainEvent> getEvents(String aggregateId);
}

// Event Sourcing을 지원하는 Aggregate Root
public abstract class EventSourcedAggregateRoot<ID> {
    private ID id;
    private int version = 0;
    private final List<DomainEvent> uncommittedEvents = new ArrayList<>();
    
    // Event를 적용하여 상태 복원
    public void loadFromHistory(List<DomainEvent> events) {
        for (DomainEvent event : events) {
            applyEvent(event, false);
            version++;
        }
    }
    
    // 새로운 Event 적용
    protected void applyEvent(DomainEvent event) {
        applyEvent(event, true);
    }
    
    private void applyEvent(DomainEvent event, boolean isNew) {
        // Event Handler 메서드 호출 로직
        handleEvent(event);
        
        if (isNew) {
            uncommittedEvents.add(event);
        }
    }
    
    protected abstract void handleEvent(DomainEvent event);
    
    public List<DomainEvent> getUncommittedEvents() {
        return Collections.unmodifiableList(uncommittedEvents);
    }
    
    public void markEventsAsCommitted() {
        uncommittedEvents.clear();
    }
}