Clean Architecture

Clean Architecture for Spring Boot

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Core Principle: The Dependency Rule

Dependencies point inward only. Inner layers know nothing about outer layers.

┌─────────────────────────────────────────────┐
│              Presentation                    │  ← Controllers, DTOs
│   ┌─────────────────────────────────────┐   │
│   │          Infrastructure              │   │  ← Repositories Impl, External APIs
│   │   ┌─────────────────────────────┐   │   │
│   │   │        Application           │   │   │  ← Use Cases, Ports
│   │   │   ┌─────────────────────┐   │   │   │
│   │   │   │      Domain          │   │   │   │  ← Entities, Value Objects
│   │   │   └─────────────────────┘   │   │   │
│   │   └─────────────────────────────┘   │   │
│   └─────────────────────────────────────┘   │
└─────────────────────────────────────────────┘

Layer Responsibilities

Domain Layer: Key Pattern

// Entity - business identity + behavior
public class Order {
    private OrderId id;
    private OrderStatus status;

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

// Value Object - immutable, equality by value
public record Money(BigDecimal amount, Currency currency) {
    public Money {
        if (amount.compareTo(BigDecimal.ZERO) < 0) {
            throw new IllegalArgumentException("Amount cannot be negative");
        }
    }
    public Money add(Money other) {
        validateSameCurrency(other);
        return new Money(this.amount.add(other.amount), this.currency);
    }
}

Application Layer Pattern

Input Port (UseCase interface) + Output Port (Repository interface) + Service implementation. See references/spring-boot-implementation.md for complete template.

Infrastructure + Presentation

JpaOrderRepository implements OrderRepository — adapters implement ports. OrderController only calls UseCase, no business logic. See references/spring-boot-implementation.md for complete template.

Spring Boot Project Structure

src/main/java/com/example/order/
├── domain/
│   ├── model/          Order.java, OrderId.java, Money.java
│   ├── service/        OrderDomainService.java
│   └── exception/      IllegalOrderStateException.java
├── application/
│   ├── port/in/        CreateOrderUseCase.java
│   ├── port/out/       OrderRepository.java
│   ├── service/        CreateOrderService.java
│   └── dto/            CreateOrderCommand.java
├── infrastructure/
│   ├── persistence/    OrderEntity.java, JpaOrderRepository.java, OrderMapper.java
│   └── config/         PersistenceConfig.java
└── presentation/
    ├── controller/     OrderController.java
    ├── request/        CreateOrderRequest.java
    └── response/       OrderResponse.java

Code Review Checklist

Common Anti-Patterns

1. Framework Coupling in Domain — @Entity in domain layer. Pragmatic resolution: allow JPA annotations in domain if the project is small; strict projects keep JPA entities in infrastructure with a mapper
2. Fat Controllers — business logic in @PostMapping methods; delegate to UseCase instead
3. Anemic Domain Model — order.setStatus(CONFIRMED) from a service; behavior belongs to the entity

When to Apply

• New feature development requiring clear boundaries
• Refactoring legacy code with tangled dependencies
• Designing microservice boundaries

Gotchas

• 不要把 Clean Architecture 套到極致：Spring Boot 小專案用 3 層（Controller / Service / Repository）就夠，ports/adapters 是大型系統的工具
• @Entity 放 domain layer 是務實選擇：嚴格派要求 JPA entity 在 infrastructure，但這需要額外 mapper。若專案規模小，允許 domain 依賴 JPA annotations 是合理的 trade-off
• Domain Event 不要用 Spring ApplicationEvent：這讓 domain 依賴 Spring framework，違反 Dependency Rule
• UseCase 超過 3 個依賴就該審查 SRP：依賴太多通常意味著 use case 做了太多事
• 不要為每個 Entity 都建 Repository interface：只有 Aggregate Root 需要 Repository，其他 Entity 透過 Aggregate Root 存取

Additional Resources

• references/layer-dependencies.md — Dependency rules and violation examples
• references/spring-boot-implementation.md — Complete project templates for all layers
• references/testing-strategy.md — Testing each layer in isolation

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