Dependency Injection and Beans

Spring's IoC container creates objects called beans and injects their dependencies. Mark classes with stereotypes (@Component, @Service, @Repository, @RestController) or declare beans with @Bean methods in @Configuration classes (useful for third-party types). Prefer constructor injection with final fields — dependencies are explicit, immutable and easy to test. With a single constructor, no @Autowired is needed.

public interface PaymentGateway {
    PaymentResult charge(Money amount, String token);
}

@Service
public class CheckoutService {
    private final OrderRepository orders;
    private final PaymentGateway payments;

    public CheckoutService(OrderRepository orders, PaymentGateway payments) {   // constructor injection
        this.orders = orders;
        this.payments = payments;
    }

    public Order checkout(Cart cart, String token) {
        var result = payments.charge(cart.total(), token);
        return orders.save(Order.paid(cart, result.reference()));
    }
}

@Configuration
class PaymentConfig {
    @Bean
    PaymentGateway paymentGateway(StripeProperties props) {
        return new StripeGateway(props.apiKey());
    }
}

Common pitfalls

• Field injection (@Autowired private Foo foo;) — hides dependencies and complicates tests.
• Circular dependencies between services (a design smell; Boot fails fast by default).
• Multiple beans of the same type without @Primary or @Qualifier.

Key points

• Beans are container-managed objects
• Constructor injection with final fields
• @Bean for third-party classes

Further reading: Spring: Dependency injection