Spring Core & Inversion of Control (IoC)

Spring Framework simplifies enterprise Java development through Inversion of Control (IoC) and Dependency Injection (DI).

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1. What is Inversion of Control?

Instead of an application creating its own dependencies with new Class(), control over object instantiation, configuration, and lifecycle is inverted and handed over to the Spring ApplicationContext (IoC Container).

// Traditional Tight Coupling (Without DI)
public class OrderService {
    private PaymentProcessor paymentProcessor = new StripePaymentProcessor(); // Tightly coupled!
}

// Spring Dependency Injection (Loose Coupling)
@Service
public class OrderService {
    private final PaymentProcessor paymentProcessor;

    // Recommended: Constructor Injection
    public OrderService(PaymentProcessor paymentProcessor) {
        this.paymentProcessor = paymentProcessor;
    }

    public OrderResult process(OrderRequest request) {
        return paymentProcessor.charge(request.amount());
    }
}

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2. Spring Bean Scopes

Scope	Description
singleton (default)	Only one shared instance per Spring IoC container.
prototype	A new instance is created every time a bean is requested.
request	One instance per HTTP request (Web-aware).
session	One instance per HTTP session (Web-aware).

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3. Why Constructor Injection is Preferred

1. Immutability: Dependencies can be declared final.
2. Easy Testing: Services can be easily instantiated and tested in unit tests without starting the Spring context.
3. Circular Dependency Detection: Spring throws a clear startup failure if circular dependencies exist.