Dependency Injection

Now that you know what the IoC container does, this page covers how you tell it what to inject. Spring supports three injection styles.

Constructor injection (recommended)

@Service
public class OrderService {
    private final PaymentGateway paymentGateway;

    public OrderService(PaymentGateway paymentGateway) {
        this.paymentGateway = paymentGateway;
    }
}

Dependencies are declared once, in the constructor, and the field can be final. This makes the class impossible to construct in an invalid state — a huge win for testability.

Setter injection

@Service
public class OrderService {
    private PaymentGateway paymentGateway;

    @Autowired
    public void setPaymentGateway(PaymentGateway paymentGateway) {
        this.paymentGateway = paymentGateway;
    }
}

Useful for optional dependencies, but the object can briefly exist without them being set.

Field injection (avoid)

@Service
public class OrderService {
    @Autowired
    private PaymentGateway paymentGateway;
}

Concise, but it hides dependencies from the constructor signature, makes the class harder to unit test without a DI framework, and prevents final fields. Prefer constructor injection by default.

Rule of thumb

• Required dependency → constructor injection.
• Optional dependency → setter injection.
• Field injection → only in quick prototypes or test classes.