What is dependency injection in Spring and how does the IoC container work?

Expected Interview Answer

Step-by-Step Explanation

1. Step 1

   Define beans

   Annotate classes with @Component/@Service/@Repository or declare them via @Bean methods in a @Configuration class.

2. Step 2

   Declare dependencies

   Express what a class needs through constructor parameters (preferred), setters, or fields marked @Autowired.

3. Step 3

   Container startup

   The ApplicationContext scans configuration, builds a bean definition registry, and instantiates the managed beans.

4. Step 4

   Resolve and inject

   The container resolves each dependency by type (and qualifier if needed) and wires it into the target bean.

5. Step 5

   Manage lifecycle

   The container handles scopes, init/destroy callbacks, and hands ready-to-use beans to your application.

Code Example

@Service
public class OrderService {
    private final PaymentGateway gateway;

    // The container injects PaymentGateway automatically
    public OrderService(PaymentGateway gateway) {
        this.gateway = gateway;
    }

    public void checkout(Order order) {
        gateway.charge(order.total());
    }
}

Follow-up Questions

• What is the difference between BeanFactory and ApplicationContext?
• Why is constructor injection preferred over field injection?
• How does Spring resolve ambiguity when multiple beans match a type?
• What are the different bean scopes in Spring?
• How would you inject a dependency that is only known at runtime?