Hexagonal Architecture

Package Structure

src/main/java/com/example/
├── domain/                     ← Pure Java. Zero framework dependencies.
│   ├── model/                  ← Entities, value objects, aggregates
│   ├── port/
│   │   ├── in/                 ← Use case interfaces (driving ports)
│   │   └── out/                ← Repository/external interfaces (driven ports)
│   └── service/                ← Domain services (pure business logic)
├── application/                ← Orchestrates use cases. Spring allowed here.
│   └── usecase/                ← @Service implementations of domain ports
└── infrastructure/             ← All framework/DB/HTTP details
    ├── persistence/            ← JPA adapters implementing out ports
    ├── web/                    ← REST controllers (driving adapters)
    └── external/               ← HTTP clients, messaging adapters

Domain Layer — Zero Spring

// domain/model/Order.java — pure Java, no annotations
public class Order {
    private final OrderId id;
    private final CustomerId customerId;
    private OrderStatus status;
    private final List<OrderItem> items;

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

    public static Order create(CustomerId customerId) {
        return new Order(OrderId.generate(), customerId);
    }

    public void addItem(ProductId productId, int quantity, Money price) {
        if (status != OrderStatus.PENDING)
            throw new OrderNotModifiableException(id);
        items.add(new OrderItem(productId, quantity, price));
    }

    // Getters only — no setters
}

// domain/model/OrderId.java — value object
public record OrderId(UUID value) {
    public static OrderId generate() { return new OrderId(UUID.randomUUID()); }
    public static OrderId of(String value) { return new OrderId(UUID.fromString(value)); }
}

Ports — Interfaces Only

// domain/port/in/CreateOrderUseCase.java — driving port
public interface CreateOrderUseCase {
    Order createOrder(CreateOrderCommand command);
}

// domain/port/in/CreateOrderCommand.java
public record CreateOrderCommand(CustomerId customerId, List<OrderItemData> items) {}

// domain/port/out/OrderRepository.java — driven port
public interface OrderRepository {
    Order save(Order order);
    Optional<Order> findById(OrderId id);
    void deleteById(OrderId id);
}

// domain/port/out/InventoryPort.java — driven port
public interface InventoryPort {
    void reserve(List<OrderItem> items);
    void release(List<OrderItem> items);
}

Application Layer — Use Case Implementation

// application/usecase/CreateOrderService.java
@Service  // Spring allowed here
@RequiredArgsConstructor
@Transactional
public class CreateOrderService implements CreateOrderUseCase {

    private final OrderRepository orderRepository;   // domain port (not JPA repo)
    private final InventoryPort inventoryPort;        // domain port

    @Override
    public Order createOrder(CreateOrderCommand command) {
        Order order = Order.create(command.customerId());
        command.items().forEach(item ->
            order.addItem(item.productId(), item.quantity(), item.price()));
        inventoryPort.reserve(order.getItems());
        return orderRepository.save(order);
    }
}

Infrastructure — Adapters

// infrastructure/persistence/JpaOrderRepository.java — implements domain port
@Repository
@RequiredArgsConstructor
public class JpaOrderRepository implements OrderRepository {

    private final SpringDataOrderRepository springDataRepo;
    private final OrderMapper mapper;

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

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

    @Override
    public void deleteById(OrderId id) {
        springDataRepo.deleteById(id.value());
    }
}

// Separate Spring Data interface — infrastructure detail
interface SpringDataOrderRepository extends JpaRepository<OrderJpaEntity, UUID> {}

// infrastructure/web/OrderController.java — driving adapter
@RestController
@RequestMapping("/api/v1/orders")
@RequiredArgsConstructor
public class OrderController {

    private final CreateOrderUseCase createOrderUseCase; // injects use case port

    @PostMapping
    public ResponseEntity<ApiResponse<OrderResponse>> create(@Valid @RequestBody CreateOrderRequest request) {
        Order order = createOrderUseCase.createOrder(request.toCommand());
        return ResponseEntity.status(201).body(ApiResponse.ok(OrderResponse.from(order)));
    }
}

Gotchas

• Agent imports jakarta.persistence in domain classes — domain must be framework-free
• Agent injects JpaRepository directly into use cases — use domain port interfaces
• Agent puts @Transactional on domain services — belongs in application layer
• Agent mixes driving and driven ports — port/in = what app offers, port/out = what app needs
• Agent creates anemic domain with only getters/setters — behavior belongs on domain objects
• Agent stubs driven ports with @MockBean in tests — removed in Boot 4; use @MockitoBean
• Agent puts the adapter's spring-boot-starter-aop dependency for port proxies — renamed spring-boot-starter-aspectj in Boot 4