Dependency Injection is the specific pattern Spring uses to implement IoC: instead of a class creating its own collaborators with `new`, the required dependencies are supplied ('injected') from outside — via constructor parameters, setter methods, or field assignment — by the IoC container, based on bean definitions it manages.

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#Dependency Injection

DI Is a Means, IoC Is the Goal

It's easy to conflate the two, but IoC is the broader principle (control is inverted to an external party), while DI is one concrete technique for achieving it. Spring specifically favors DI over other IoC mechanisms like the Service Locator pattern, because DI makes a class's dependencies explicit and visible in its constructor/setter signature rather than hidden behind an internal lookup call.

Constructor injection · Setter injection · Field injection — three ways the container supplies dependencies

Constructor Injection

java

@Service
class OrderService {
    private final PaymentGateway paymentGateway;

    OrderService(PaymentGateway paymentGateway) {
        this.paymentGateway = paymentGateway;
    }
}