What is Spring Framework?

Spring Framework is a comprehensive, modular framework for developing applications in Java. It provides infrastructural support, freeing developers from routine tasks and allowing them to focus on business logic. The main principles of Spring:

• Inversion of Control (IoC): The container manages the lifecycle of objects and their dependencies.
• Aspect-Oriented Programming (AOP): Allows separation of cross-cutting concerns (logging, transactions) from business logic.
• Abstraction: Spring provides abstractions over various technologies (databases, JMS), simplifying their use.
• Non-invasiveness: The framework does not impose strict requirements on code structure.

Main modules:

• Core Container: IoC container (BeanFactory, ApplicationContext).
• AOP: Support for AOP.
• Data Access/Integration: JDBC, ORM (Hibernate, JPA), JMS, Transaction.
• Web: Spring MVC, Spring WebFlux.
• Test: Support for testing using Spring.

Example of IoC usage: Dependency injection via constructor.

class B {
    // ...
}

class A {
    private final B b;

    // Dependency injection via constructor
    public A(B b) {
        this.b = b;
    }

    // ...
}