https://www.cnblogs.com/xiaoxing/p/5836835.html
The most important thing about spring is IOC (Inversion of Control) and AOP (Aspect Oriented Programming), DI ( Dependency injection )
IOC inversion is that we can automatically generate the implementation class of DAO for us through the container (such as spring). In this way, the reverse control can be truly realized. As a result, we can perform data migration without modifying any code in the case of changing the database at any time .