Overview of Spring Core Components, IOC, AOP, and Related Technologies

Spring is a lightweight, non‑intrusive IoC and AOP framework widely used in Java backend development; the article begins with a concise definition and a version‑to‑JDK compatibility table, noting that Spring 5 with Spring Boot 2 on JDK 8 is the current standard.

The core ideas of Spring are introduced, emphasizing IoC/DI support, AOP programming, declarative transactions, testing convenience, integration with other frameworks, and simplification of Java EE APIs.

Spring's modular architecture is described, highlighting the essential Spring Core Container and optional modules such as Context, Web, MVC, DAO, ORM, and AOP, each with a brief purpose.

In-depth discussion of IOC and AOP follows: IOC manages object lifecycles via the container, enabling dependency injection and inversion of control, while AOP separates cross‑cutting concerns using aspects, join points, pointcuts, and advice, with five common advice types.

The article compares JDK dynamic proxies (interface‑based, using InvocationHandler) and CGLIB proxies (subclass‑based, using bytecode manipulation), providing complete Java code examples for both implementations wrapped in ... blocks.

A comparison between Spring AOP (runtime, proxy‑based) and AspectJ (compile‑time or load‑time weaving) is presented in a table, outlining differences in implementation, performance, and capabilities.

BeanFactory and FactoryBean concepts are explained, noting BeanFactory's basic role and limitations, and FactoryBean's ability to customize bean creation.

The Spring bean lifecycle is outlined, covering definition, initialization, usage, and destruction, with configuration options via XML, @Bean(initMethod, destroyMethod), JSR‑250 annotations ( @PostConstruct, @PreDestroy), and lifecycle interfaces.

Common design patterns employed by Spring—Factory, Proxy, Singleton, Template Method, Wrapper, Observer, and Adapter—are listed.

Circular dependency handling is detailed, describing the three‑level cache (singletonObjects, earlySingletonObjects, singletonFactories) and the process of exposing early references to resolve circular references.

The Spring MVC architecture is summarized, naming core components such as DispatcherServlet, HandlerMapping, HandlerAdapter, ModelAndView, and ViewResolver, and outlining the request‑processing workflow.

Spring Boot is introduced as a convention‑over‑configuration tool that simplifies Spring application development, embeds servlet containers, auto‑configures dependencies, and provides starter modules; its startup process and underlying mechanisms are briefly described.

Spring Cloud concepts are touched upon, explaining microservice architecture evolution, service registration, discovery, and common frameworks like Spring Cloud Netflix and Alibaba.

A concise list of frequently used Spring annotations ( @Component, @Autowired, @Bean, @Value, @Configuration, @Qualifier, etc.) is provided for quick reference.

References to additional resources and interview question collections are included at the end.