How does dependency injection work in Spring?

Spring's IoC container creates beans, wires their dependencies and manages their lifecycle. Instead of calling new inside a service, the service declares what it needs and the container resolves it. This inverts control over object creation.

@Service
public class OrderService {
    private final OrderRepository repository;

    public OrderService(OrderRepository repository) {
        this.repository = repository;
    }
}

Injection styles and scopes

• Constructor injection for required dependencies and easier unit testing
• Setter injection for optional or reconfigurable dependencies
• Field injection is concise but hides dependencies and complicates tests
• Singleton is the default scope; prototype creates a new bean per lookup
• Use @Qualifier or @Primary when several beans match a type

Constructor injection exposes circular dependencies at startup instead of silently creating proxies. If two beans genuinely depend on each other, the design usually needs to change, often by extracting a third collaborator. Lazy injection can break a cycle but does not fix the underlying coupling.