Dependency Injection and Layers

Dependency injection supplies collaborators from outside an object instead of letting it construct infrastructure internally. Constructor injection makes required dependencies explicit and supports immutable fields.

@Service
final class BookService {
    private final BookRepository repository;
    private final Clock clock;

    BookService(BookRepository repository, Clock clock) {
        this.repository = repository;
        this.clock = clock;
    }
}

Injecting Clock makes time an explicit dependency and permits deterministic tests. Avoid using the container as a global service locator.

Responsibilities

Layer	Typical responsibility
Transport	HTTP parsing, representation, status, headers
Application	Use-case orchestration and transaction intent
Domain	Business invariants and domain decisions
Infrastructure	Persistence, messaging, remote-system adapters

This is a dependency guideline, not a requirement for four packages in every small application. Complexity should justify structure.

Ports and adapters

The application can depend on a domain-oriented repository interface while an adapter implements it with JPA or another mechanism. The benefit is not “zero framework imports” as an end in itself; it is an inward dependency direction that keeps core policy testable and changeable.

Bean lifecycle cautions

Singleton scope means one bean instance per application context, not global immutability. Avoid mutable request-specific fields in singleton beans. Know when proxies provide behavior such as transactions, method security, caching, or async execution; calls that bypass a proxy can bypass its advice.

Configuration

Use @ConfigurationProperties for cohesive, type-safe external configuration. Validate required properties at startup. Treat profiles as coarse environment or feature groupings, not as a substitute for well-defined configuration.