Spring and dependency injection

The problem: new everywhere

public class OrderService {
    private final PaymentGateway gateway = new RazorpayGateway("live-key");   // hard-wired
    private final SmsSender sms = new TwilioSmsSender();                     // hard-wired
}

OrderService decides which payment gateway and SMS provider to use, creates them itself, and even holds a key. To test it without charging real cards, or to switch providers, you must edit OrderService. The classes are tightly coupled.

Dependency injection

Dependency injection (DI) means a class RECEIVES the objects it depends on, usually through its constructor, instead of creating them. Something outside (in Spring's case, the container) decides which implementation to pass. You already did this by hand with OtpService(SmsSender) in module 5.

@Service
public class OrderService {
    private final PaymentGateway gateway;
    private final SmsSender sms;

    public OrderService(PaymentGateway gateway, SmsSender sms) {   // constructor injection
        this.gateway = gateway;
        this.sms = sms;
    }
}

@Component
public class RazorpayGateway implements PaymentGateway { … }      // Spring creates this bean
@Component
public class SmsGatewaySender implements SmsSender { … }

The container and beans

At start-up, Spring scans your packages for classes annotated as components, creates one object (a bean) of each by default (singleton scope), works out which beans need which others, and wires them together. This is Inversion of Control: the framework controls object creation, not your code.

@Configuration
public class ClockConfig {
    @Bean
    public java.time.Clock clock() {
        return java.time.Clock.system(java.time.ZoneId.of("Asia/Kolkata"));   // a bean you can inject anywhere
    }
}

Constructor versus field injection

You will see @Autowired on fields in older code (@Autowired private SmsSender sms;). Prefer constructor injection: the fields can be final, the dependencies are obvious, the object can never exist half-initialised, and tests can simply call new OrderService(fakeGateway, fakeSms) without Spring.