How does Spring Boot 4 handle dependency injection and what are the core stereotypes?

// Constructor injection (recommended in Spring Boot 4)
@Service
public class OrderService {

    private final OrderRepository repo;
    private final ProductClient    client;
    private final EventPublisher   events;

    // Spring Boot 4 recommends constructor injection:
    // - Makes dependencies explicit
    // - Enables final fields (immutability)
    // - No @Autowired annotation needed on single-constructor classes
    public OrderService(
            OrderRepository repo,
            ProductClient   client,
            EventPublisher  events) {
        this.repo   = repo;
        this.client = client;
        this.events = events;
    }

    // With Lombok @RequiredArgsConstructor:
    // @Service
    // @RequiredArgsConstructor
    // public class OrderService {
    //     private final OrderRepository repo;
    //     private final ProductClient client;
    //     ...
    // }
}

// @Primary: preferred bean when multiple implementations exist
@Service
@Primary
public class DefaultOrderService implements OrderService { ... }

// @Qualifier: inject specific implementation by name
@Autowired
@Qualifier("premiumOrderService")
private OrderService orderService;