12.1 Assembling Layered Architecture

To keep complex business logic organized, Spring applications generally follow a Layered Architecture. Maintenance and testability improve when each layer has clear responsibilities.

1. Typical 3-Tier Structure

1. Presentation Layer (Controller): The entry point that receives requests and returns responses. It handles data format conversion (JSON/XML) and basic validation.
2. Service Layer (Business Logic): Where the core application logic resides. It sets transaction boundaries and completes business workflows by combining multiple repositories.
3. Data Access Layer (Repository): Responsible for communication with systems like databases or file systems. JPA Repositories belong to this layer.

2. Separation of DTO and Entity

It is best to strictly separate Entities(domain models) from ** DTOs**(data transfer objects).

• Entity: A core class mapped to a database table, containing business rules. It should not be exposed directly to the outside.
• DTO (Data Transfer Object): An object designed to transfer data according to API specifications. Used for movement between layers.

3. Dependency Injection Flow

graph LR
    Controller --> Service
    Service --> Repository
    Repository --> DB

• Controllers depend on Services.
• Services depend on Repositories.
• Using interfaces is encouraged for flexibility.

4. Production Code Example

@Service
@Transactional
@RequiredArgsConstructor
public class OrderService {
    private final OrderRepository orderRepository;
    private final MemberRepository memberRepository;

    public Long order(Long memberId, String itemName, int count) {
        // 1. Retrieve entity
        Member member = memberRepository.findById(memberId).get();
        // 2. Execute business logic and create order
        Order order = Order.createOrder(member, itemName, count);
        // 3. Save order
        orderRepository.save(order);
        return order.getId();
    }
}

🎯 Key Points

• Manage applications by separating them into Controller - Service - Repository layers.
• Handle business logic and manage transactions in the Service Layer.
• Keep Entities for internal database use and DTOs for external communication.