Clean Architecture

// domain/model/Order.java
public class Order {
    private final OrderId id;
    private final List<OrderItem> items;
    private Money total;
    private OrderStatus status;
    private final List<DomainEvent> domainEvents = new ArrayList<>();

    private Order(OrderId id, List<OrderItem> items) {
        this.id = id;
        this.items = new ArrayList<>(items);
        this.status = OrderStatus.PENDING;
        calculateTotal();
    }

    public static Order create(List<OrderItem> items) {
        validateItems(items);
        Order order = new Order(OrderId.generate(), items);
        order.domainEvents.add(new OrderCreatedEvent(order.id, order.total));
        return order;
    }

    public void confirm() {
        if (status != OrderStatus.PENDING) {
            throw new DomainException("Only pending orders can be confirmed");
        }
        this.status = OrderStatus.CONFIRMED;
    }

    public List<DomainEvent> getDomainEvents() {
        return List.copyOf(domainEvents);
    }

    public void clearDomainEvents() {
        domainEvents.clear();
    }
}

// domain/model/Money.java (Value Object)
public record Money(BigDecimal amount, Currency currency) {
    public Money {
        if (amount.compareTo(BigDecimal.ZERO) < 0) {
            throw new DomainException("Amount cannot be negative");
        }
    }

    public static Money zero() {
        return new Money(BigDecimal.ZERO, Currency.getInstance("EUR"));
    }

    public Money add(Money other) {
        if (!this.currency.equals(other.currency)) {
            throw new DomainException("Currency mismatch");
        }
        return new Money(this.amount.add(other.amount), this.currency);
    }
}

// domain/repository/OrderRepository.java (Port)
public interface OrderRepository {
    Order save(Order order);
    Optional<Order> findById(OrderId id);
}

// application/port/in/CreateOrderUseCase.java
public interface CreateOrderUseCase {
    OrderResponse createOrder(CreateOrderRequest request);
}

// application/dto/CreateOrderRequest.java
public record CreateOrderRequest(
    @NotNull UUID customerId,
    @NotEmpty List<OrderItemRequest> items
) {}

// application/service/OrderService.java
@Service
@RequiredArgsConstructor
@Transactional
public class OrderService implements CreateOrderUseCase {
    private final OrderRepository orderRepository;
    private final PaymentGateway paymentGateway;
    private final DomainEventPublisher eventPublisher;

    @Override
    public OrderResponse createOrder(CreateOrderRequest request) {
        List<OrderItem> items = mapItems(request.items());
        Order order = Order.create(items);

        PaymentResult payment = paymentGateway.charge(order.getTotal());
        if (!payment.successful()) {
            throw new PaymentFailedException("Payment failed");
        }

        order.confirm();
        Order saved = orderRepository.save(order);
        publishEvents(order);

        return OrderMapper.toResponse(saved);
    }

    private void publishEvents(Order order) {
        order.getDomainEvents().forEach(eventPublisher::publish);
        order.clearDomainEvents();
    }
}

// infrastructure/persistence/OrderJpaEntity.java
@Entity
@Table(name = "orders")
public class OrderJpaEntity {
    @Id
    private UUID id;
    @Enumerated(EnumType.STRING)
    private OrderStatus status;
    private BigDecimal totalAmount;
    @OneToMany(cascade = CascadeType.ALL, orphanRemoval = true)
    private List<OrderItemJpaEntity> items;
}

// infrastructure/persistence/OrderRepositoryAdapter.java
@Component
@RequiredArgsConstructor
public class OrderRepositoryAdapter implements OrderRepository {
    private final OrderJpaRepository jpaRepository;
    private final OrderJpaMapper mapper;

    @Override
    public Order save(Order order) {
        OrderJpaEntity entity = mapper.toEntity(order);
        return mapper.toDomain(jpaRepository.save(entity));
    }

    @Override
    public Optional<Order> findById(OrderId id) {
        return jpaRepository.findById(id.value()).map(mapper::toDomain);
    }
}

// adapter/rest/OrderController.java
@RestController
@RequestMapping("/api/orders")
@RequiredArgsConstructor
public class OrderController {
    private final CreateOrderUseCase createOrderUseCase;

    @PostMapping
    public ResponseEntity<OrderResponse> createOrder(
            @Valid @RequestBody CreateOrderRequest request) {
        OrderResponse response = createOrderUseCase.createOrder(request);
        URI location = ServletUriComponentsBuilder
            .fromCurrentRequest()
            .path("/{id}")
            .buildAndExpand(response.id())
            .toUri();
        return ResponseEntity.created(location).body(response);
    }
}

class OrderTest {
    @Test
    void shouldCreateOrderWithValidItems() {
        List<OrderItem> items = List.of(
            new OrderItem(new ProductId(UUID.randomUUID()), 2, new Money("10.00", EUR))
        );

        Order order = Order.create(items);

        assertThat(order.getStatus()).isEqualTo(OrderStatus.PENDING);
        assertThat(order.getDomainEvents()).hasSize(1);
    }
}

@ExtendWith(MockitoExtension.class)
class OrderServiceTest {
    @Mock OrderRepository orderRepository;
    @Mock PaymentGateway paymentGateway;
    @Mock DomainEventPublisher eventPublisher;

    @InjectMocks OrderService orderService;

    @Test
    void shouldCreateAndConfirmOrder() {
        when(paymentGateway.charge(any())).thenReturn(new PaymentResult(true, "tx-123"));
        when(orderRepository.save(any())).thenAnswer(i -> i.getArgument(0));

        OrderResponse response = orderService.createOrder(createRequest());

        assertThat(response.status()).isEqualTo(OrderStatus.CONFIRMED);
        verify(eventPublisher).publish(any(OrderCreatedEvent.class));
    }
}