How is dependency injection implemented in Bean when they are detected in the context of Spring or similar frameworks?

In Spring, dependency injection (DI) into a Bean occurs automatically when components are detected in the application context. The main methods of injection are:

1. Constructor-based — dependencies are passed through constructor parameters. Spring injects the required objects when creating the Bean.

2. Setter-based (Setter Injection) — dependencies are set through setter methods annotated with @Autowired.

3. Field-based (Field Injection) — class fields are annotated with @Autowired, and Spring injects the required objects directly.

Example of constructor injection:

@Component
public class Service {
    private final Repository repository;

    @Autowired
    public Service(Repository repository) {
        this.repository = repository;
    }
}

When Spring starts, it scans classes with annotations like @Component, @Service, @Repository and creates their instances, automatically resolving dependencies by type from the context. If there are multiple candidates, @Qualifier can be used for clarification.

Thus, dependency injection occurs through the Spring container, which manages the lifecycle of Beans and their relationships.