What methods are used in Spring to implement dependency injection and which of them are considered correct?

In Spring, dependency injection (DI) is implemented in three main ways:

1. Through constructor — dependencies are passed to the class via its constructor. This is considered the most correct and recommended method, as it ensures immutability of dependencies and facilitates testing.

2. Through setters — dependencies are set via setter methods after the object is created. Suitable when dependencies are optional or can change.

3. Through field injection — dependencies are injected directly into the class fields using the @Autowired annotation. This method is less preferred, as it complicates testing and breaks encapsulation.

Example of constructor injection:

@Component
public class UserService {
    private final UserRepository userRepository;

    @Autowired
    public UserService(UserRepository userRepository) {
        this.userRepository = userRepository;
    }
}

Thus, it is recommended to use constructor-based dependency injection to improve reliability and ease of testing.