What Java mechanisms are used under the hood of Spring?

Spring uses several key Java mechanisms under the hood:

• Reflection: for dynamically creating and managing objects, calling methods, dependency injection.
• Proxy: for aspect-oriented programming (AOP), for example, through JDK Dynamic Proxy or CGLIB, to add behavior (logging, transactions) without changing the source code.
• Annotations: for component configuration and bean lifecycle management.
• ClassLoader: for loading classes and resources.

Example of using a proxy for AOP:

@Service
public class UserService {
    @Transactional
    public void createUser(User user) {
        // user creation logic
    }
}

Spring will create a proxy that wraps the createUser call to automatically start and end a transaction.