如何使用JPA存储库更新实体中的一个字段

@Entity
@Table(name = "users")
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String username;

    // Getters and setters
}

@Repository
public interface UserRepository extends JpaRepository<User, Long> {
}

@Service
public class UserService {
    @Autowired
    private UserRepository userRepository;

    public void updateUsername(Long userId, String newUsername) {
        Optional<User> optionalUser = userRepository.findById(userId);
        if (optionalUser.isPresent()) {
            User user = optionalUser.get();
            user.setUsername(newUsername);
            userRepository.save(user);
        } else {
            // 处理用户不存在的情况
        }
    }
}