clean-architecture

Clean Architecture, Hexagonal Architecture & DDD for Spring Boot

Overview

This skill provides comprehensive guidance for implementing Clean Architecture, Hexagonal Architecture (Ports & Adapters), and Domain-Driven Design tactical patterns in Java 21+ Spring Boot 3.5+ applications. It ensures clear separation of concerns, framework-independent domain logic, and highly testable codebases through proper layering and dependency management.

When to Use

• Architecting new Spring Boot applications with clear separation of concerns
• Refactoring tightly coupled code into testable, layered architectures
• Implementing domain logic independent of frameworks and infrastructure
• Designing ports and adapters for swappable implementations
• Applying Domain-Driven Design tactical patterns (entities, value objects, aggregates)
• Creating testable business logic without Spring context dependencies

Instructions

1. Understand the Core Concepts

Clean Architecture Layers (Dependency Rule)

Dependencies flow inward. Inner layers know nothing about outer layers.

| Layer | Responsibility | Spring Boot Equivalent | |-------|---------------|----------------------| | Domain | Entities, value objects, domain events, repository interfaces | domain/ - no Spring annotations | | Application | Use cases, application services, DTOs, ports | application/ - @Service, @Transactional | | Infrastructure | Frameworks, database, external APIs | infrastructure/ - @Repository, @Entity | | Adapter | Controllers, presenters, external gateways | adapter/ - @RestController |

Hexagonal Architecture (Ports & Adapters)

• Domain Core: Pure Java business logic, no framework dependencies
• Ports: Interfaces defining contracts (driven and driving)
• Adapters: Concrete implementations (JPA, REST, messaging)

Domain-Driven Design Tactical Patterns

• Entities: Objects with identity and lifecycle (e.g., Order, Customer)
• Value Objects: Immutable, defined by attributes (e.g., Money, Email)
• Aggregates: Consistency boundary with root entity
• Domain Events: Capture significant business occurrences
• Repositories: Persistence abstraction, implemented in infrastructure

2. Organize Package Structure

Follow this feature-based package organization:

com.example.order/
├── domain/
│   ├── model/              # Entities, value objects
│   ├── event/              # Domain events
│   ├── repository/         # Repository interfaces (ports)
│   └── exception/          # Domain exceptions
├── application/
│   ├── port/in/            # Driving ports (use case interfaces)
│   ├── port/out/           # Driven ports (external service interfaces)
│   ├── service/            # Application services
│   └── dto/                # Request/response DTOs
├── infrastructure/
│   ├── persistence/        # JPA entities, repository adapters
│   └── external/           # External service adapters
└── adapter/
    └── rest/               # REST controllers

3. Implement the Domain Layer (Framework-Free)

The domain layer must have zero dependencies on Spring or any framework.

• Use Java records for immutable value objects with built-in validation
• Place business logic in entities, not services (Rich Domain Model)
• Define repository interfaces (ports) in the domain layer
• Use strongly-typed IDs to prevent ID confusion
• Implement domain events for decoupling side effects
• Use factory methods for entity creation to enforce invariants

4. Implement the Application Layer

• Create use case interfaces (driving ports) in application/port/in/
• Create external service interfaces (driven ports) in application/port/out/
• Implement application services with @Service and @Transactional
• Use DTOs for request/response, separate from domain models
• Publish domain events after successful operations

5. Implement the Infrastructure Layer (Adapters)

• Create JPA entities in infrastructure/persistence/
• Implement repository adapters that map between domain and JPA entities
• Use MapStruct or manual mappers for domain-JPA conversion
• Configure conditional beans for swappable implementations
• Keep infrastructure concerns isolated from domain logic

6. Implement the Adapter Layer (REST)

• Create REST controllers in adapter/rest/
• Inject use case interfaces, not implementations
• Use Bean Validation on DTOs
• Return proper HTTP status codes and responses
• Handle exceptions with global exception handlers

7. Apply Best Practices

1. Dependency Rule: Domain has zero dependencies on Spring or other frameworks
2. Immutable Value Objects: Use Java records for value objects with built-in validation
3. Rich Domain Models: Place business logic in entities, not services
4. Repository Pattern: Domain defines interface, infrastructure implements
5. Domain Events: Decouple side effects from primary operations
6. Constructor Injection: Mandatory dependencies via final fields
7. DTO Mapping: Separate domain models from API contracts
8. Transaction Boundaries: Place @Transactional in application services
9. Factory Methods: Use Entity.create() for invariant enforcement during construction
10. Separate JPA Entities: Keep domain entities separate from JPA entities with mappers

8. Validate Architecture Compliance

After implementing each layer, verify the dependency rules are respected:

• Domain Layer Check: Run grep -r "@Service\|@Component\|@Autowired" domain/ to ensure zero Spring imports
• ArchUnit Test: Add dependency tests to verify no infrastructure imports in domain layer:

``java noClasses().that().resideInPackage("..domain..") .should().accessClassesThat().resideInAnyPackage("..spring..", "..infrastructure.."); ``

• Entity Exposure Check: Verify JPA entities are never returned from domain services
• Transaction Check: Confirm @Transactional only on application layer services, never on domain

9. Write Tests

• Domain Tests: Pure unit tests without Spring context, fast execution
• Application Tests: Unit tests with mocked ports using Mockito
• Infrastructure Tests: Integration tests with @DataJpaTest and Testcontainers
• Adapter Tests: Controller tests with @WebMvcTest

Examples

Example 1: Domain Layer - Entity with Domain Events

// 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();
    }
}

Example 2: Domain Layer - Value Object with Validation

// 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) {