Dependency Injection

Think of Spring Boot Dependency Injection as a marionette master that knows exactly which strings to pull to make your objects dance together.

Pre-Spring

// Without DI
public class CheckoutController {
    private PaymentService paymentService = new PaymentService(
        new FraudCheck(),
        new Database(),
        new EmailService()
        //more things that PaymentService may need...
    );
}

Every new is a commitment. Want to test it? You get the entire dependency tree. Need a mock database? Rewrite all constructors.

Spring's IoC Container

Imagine you're a chef in a kitchen. Instead of running to the farm for eggs, the dairy for milk, and the mill for flour, you shout "I need ingredients!" and they appear on your counter. **That's Spring's IoC (Inversion of Control) Container—**your personal assistant for Java objects.

How It Works

1. Component scan: Spring scans your code for classes marked with annotations such as @Component, @Service, @Controller.
2. Beans creation: Spring instantiates these classes as "beans" (managed objects) and stores them in its container.
3. Dependency injection: When you need a bean, Spring automatically gets it for you from the container.

Ways to Do Dependency Injection

Field Injection

@RestController
public class CheckoutController {

    @Autowired
    private PaymentService paymentService;

    //Controller logic
}

Setter Injection

@RestController
public class CheckoutController {

    private PaymentService paymentService;

    @Autowired
    public void setPaymentService(PaymentService paymentService) {
        this.paymentService = paymentService;
    }

    //Controller logic
}

Constructor Injection

@RestController
public class CheckoutController {

    private final PaymentService paymentService;

    public CheckoutController(PaymentService paymentService) {
        this.paymentService = paymentService;
    }

    //Controller logic
}

Constructor injection is the recommended approach:

• No nulls – Your dependencies arrive fully assembled.
• Immutable – Can’t accidentally swap your database for a toaster mid-request.
• Testing-friendly – No Spring magic needed in unit tests.

Constructor Injection with Lombok

WIth Lombok, Constructor Injection is even simplified (and this is my recommended approach):

@RestController
@RequiredArgConstructor // <-- Lombok annotation
public class CheckoutController {

    private final PaymentService paymentService;

    //Controller logic
}

As a final note on this topic, I really recommend CodeAesthetic’s video “Dependency Injection, The Best Pattern”.