How are beans initialized in Spring?

Spring beans are initialized by the Spring IoC container (Inversion of Control) at application startup. The main initialization methods are:

• Using annotations: For example, a class is marked with @Component, @Service, @Repository, or @Controller. The container scans packages and creates instances of these classes.
• Using configuration classes: @Configuration is used with methods annotated with @Bean, which return objects to be registered in the context.
• XML configuration: In older projects, beans are described in XML files.

Initialization occurs in several stages:

1. The container scans and creates bean instances (by default, singletons).
2. Dependency injection is performed (via constructors, setters, or fields with @Autowired).
3. Initialization methods are called if defined (@PostConstruct or through the InitializingBean interface).

Example of annotation-based configuration:

@Component
public class MyService {
    @Autowired
    private MyRepository repository;

    @PostConstruct
    public void init() {
        // initialization code
    }
}

@Configuration
public class AppConfig {
    @Bean
    public MyRepository myRepository() {
        return new MyRepository();
    }
}