Inversion of Control (IoC) in Spring Explained

Direct Answer

Inversion of Control means application components declare their dependencies and lifecycle hooks while the Spring container creates and connects the managed object graph. Dependency injection is Spring's main IoC mechanism. Application code may still create ordinary values and short-lived domain objects; the important boundary is whether an object needs container services such as injection, scopes, lifecycle callbacks, transactions, or other proxy advice.

Code Example

@Service
public class OrderService {
    private final PaymentProcessor paymentProcessor;
    private final EmailService emailService;

    // Spring calls this constructor, injects dependencies
    public OrderService(PaymentProcessor paymentProcessor, EmailService emailService) {
        this.paymentProcessor = paymentProcessor;
        this.emailService = emailService;
    }

    public void processOrder(Order order) {
        paymentProcessor.charge(order.getAmount());
        emailService.sendConfirmation(order.getCustomerEmail());
    }
}

// Spring Boot main class — IoC container starts here
@SpringBootApplication
public class ShopApplication {
    public static void main(String[] args) {
        var context = SpringApplication.run(ShopApplication.class, args);
        // Spring already created OrderService with its dependencies
        var orderService = context.getBean(OrderService.class);
    }
}

Common Mistakes & Pitfalls

• Mixing IoC and DI terminology: Freshers often say IoC when they mean Dependency Injection. IoC is the overarching principle (framework controls flow), while DI is one technique to achieve IoC (injecting dependencies). Not all IoC uses DI — event-driven callbacks are also IoC.
• Using new keyword inside beans: Creating dependencies with new PaymentProcessor() defeats IoC entirely. If Spring doesn't create the object, it can't manage lifecycle or apply AOP. Always let Spring instantiate collaborators.
• Forgetting IoC implications for testing: Some freshers mock incorrectly by trying to replace Spring-managed beans with manual instances, breaking the container. Use @MockBean in Spring tests to replace beans within the IoC container itself.