public interface UserRepository {
    UserEntity save(UserEntity car);
    void update(UserEntity user);
    List<UserEntity> getAll();
    List<UserEntity> getAllByFirstName(String firstName);
    List<UserEntity> getAllByLastName(String lastName);
    List<UserEntity> getAllByGender(String gender);
    UserEntity getById(Long id);
    void remove(Long id);
}

@Repository
@Transactional()
public class UserRepositoryImpl  implements UserRepository {

    @PersistenceContext
    private EntityManager entityManager;

    @Override
    @Transactional
    public UserEntity save(UserEntity user) {
        if(user.getId() != null){
            return entityManager.merge(user);
        } else {
            entityManager.persist(user);
            return user;
        }
    }

    @Override
    public void update(UserEntity user) {

    }

    @Override
    public List<UserEntity> getAll() {
        CriteriaQuery<UserEntity> criteria = entityManager.getCriteriaBuilder().createQuery(UserEntity.class);
        criteria.select(criteria.from(UserEntity.class));
        return entityManager.createQuery(criteria).getResultList();
    }

    @Override
    public List<UserEntity> getAllByFirstName(String firstName) {
         ...
    }

    @Override
    public List<UserEntity> getAllByLastName(String lastName) {
         ...
    }

    @Override
    public List<UserEntity> getAllByGender(String gender) {
         ...
    }

    @Override
    public UserEntity getById(Long id) {
        return entityManager.find(UserEntity.class, id);
    }

    @Override
    public void remove(Long id) {
        UserEntity user = getById(id);
        if(user != null) {
            entityManager.remove(user);
        }
    }
}

public interface UserService {
    UserEntity save(UserEntity userEntityy);
    List<UserEntity> getAll();
    UserEntity getById(long id);
    void remove(long id);
}

@Service
public class UserServiceImpl implements UserService {
    @Autowired
    UserRepository userRepository;

    @Override
    public UserEntity save(UserEntity userEntityy) {
        return userRepository.save(userEntityy);
    }

    @Override
    public List<UserEntity> getAll() {
        return userRepository.getAll();
    }

    @Override
    public UserEntity getById(long id) {
        return userRepository.getById(id);
    }

    @Override
    public void remove(long id) {
        userRepository.remove(id);

    }
}