What methods are used to implement dependency injection in the Spring framework?

Dependency Injection (DI) in the Spring Framework is primarily achieved through three methods:

1. Constructor Injection — dependencies are passed through the class constructor parameters. This is the most recommended method as it allows creating immutable objects and simplifies testing.

2. Setter Injection — dependencies are set via setter methods after the object is created.

3. Field Injection — dependencies are directly injected into class fields using the @Autowired annotation. This method is less preferred due to testing complexity and weak encapsulation.

Example of constructor-based dependency injection:

@Component
public class UserService {
    private final UserRepository userRepository;

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

Spring will automatically create and inject the required UserRepository bean when creating UserService.