Dependency Injection in Spring Boot

Instead of a class creating its own dependencies, Spring 'injects' them — making code loosely coupled and easy to test.

@Service
public class OrderService {
    private final PaymentService paymentService;

    // Constructor injection — the recommended approach
    public OrderService(PaymentService paymentService) {
        this.paymentService = paymentService;
    }
}

Constructor injection — preferred; makes dependencies explicit and final
Field injection (@Autowired on a field) — concise but harder to unit test
Setter injection — used for optional dependencies

Tip

Always recommend constructor injection when asked — it's the current best practice and enables immutability.