What is the difference between Constructor Injection, Setter Injection, and Field Injection? कंस्ट्रक्टर इंजेक्शन, सेटर इंजेक्शन और फील्ड इंजेक्शन में क्या अंतर है?

Constructor Injection supplies dependencies through a class constructor, making them final and guaranteeing the object is never in a partially-constructed, invalid state. Setter Injection supplies dependencies through public setter methods after construction, useful for optional dependencies. Field Injection uses @Autowired directly on a field, which is the most concise but hardest to test and hides dependencies from the constructor signature.

Constructor Injection is the officially recommended approach by the Spring team: it enables immutability (final fields), makes required dependencies explicit and impossible to forget, works well with unit testing without a Spring context, and surfaces circular dependencies at startup instead of silently working around them.

// Recommended: Constructor Injection
@Service
public class OrderService {
    private final PaymentGateway paymentGateway;
    public OrderService(PaymentGateway paymentGateway) {
        this.paymentGateway = paymentGateway;
    }
}

// Discouraged: Field Injection
@Service
public class OrderService {
    @Autowired
    private PaymentGateway paymentGateway;
}