What is Spring?

Spring is an open-source framework for building enterprise Java applications. It provides a comprehensive infrastructure for development, focusing on:

• dependency injection (DI): Managing dependencies between components.
• aspect-oriented programming (AOP): Modularization of cross-cutting concerns (logging, security).
• backward compatibility with previous versions.

It consists of many modules, such as:

• Core Container: DI, AOP.
• Data Access/Integration: JDBC, ORM, JMS.
• Web: MVC, WebSocket.
• Testing: Support for unit and integration tests.

The main principle of Spring is the use of POJO (Plain Old Java Objects) and providing infrastructure for their management and interaction. Thus, applications become more flexible, testable, and easy to maintain.