AbstractEntityClass.java

@MappedSuperclass
@Data
public abstract class AbstractBaseEntity implements Serializable {

	private static final long serialVersionUID = -2700410403541761609L;

	@Id
	@GeneratedValue
	private Long id;
	
	@Column(name="UUID", unique=true, length=36)
	private String uuid;
	
	@PrePersist
	private void generateUuid() {
		if( uuid == null ) {
			uuid = UUID.randomUUID().toString();
		}
	}
}

Role.java

@Data
@Entity
@NoArgsConstructor
@Table(name = "role")
@Transactional
public class Role extends AbstractBaseEntity {

	private static final long serialVersionUID = -4898422772958486777L;

	@NotNull
	@Column(unique=true)
	private String name;
	@NotNull

	@NotNull
	@Column(unique=true)
	private String jpName;
	@NotNull
	
	@ManyToMany(mappedBy="roles")
	private List<User> users;
	
	public Role(String name, String jpName ) {
		this.name = name;
		this.jpName = jpName;
	}
}

RoleRepository.java

public interface RoleRepository extends JpaRepository<Role, Long> {
	Role findByName( String name );
}

User.java

@Data
@NoArgsConstructor
@Entity
@Table(name="user")
@Transactional
public class User extends AbstractBaseEntity {

	private static final long serialVersionUID = -308652147526586990L;

	@NotNull
	private String surname;
	@NotNull
	private String name;
	
	@NotNull
	@Column(unique=true)
	private String email;

	@NotNull
	@Column(unique=true)
	private String login;

	@NotNull
	private String password;
	
	@ManyToMany(cascade=CascadeType.ALL, fetch=FetchType.LAZY)
	@JoinTable(name="user_role")
	private List<Role> roles;
	
	/**
	 * 姓名とメールアドレス指定コンストラクタ
	 *  
	 * @param surname
	 * @param name
	 * @param email
	 * @param login
	 * @param password
	 */
	public User(String surname, String name, String email, String login, String password) {
		this.surname = surname;
		this.name = name;
		this.email = email;
		this.login = login;
		this.password = password;
	}
}

UserRepository.java

public interface UserRepository extends JpaRepository<User, Long>{
	User findByLoginAndPassword(String login, String password);
	User findByEmail(String email);
}

UserService.java

public interface UserService {
	User create(User user);
	User create(String surname, String name, String email, String login, String password );
	User findById(Long id);
}

UserServiceDBImpl.java

@Service
@Transactional
public class UserServiceDBImpl implements UserService {
	private UserRepository userRepo;

	@Autowired
	public UserServiceDBImpl(UserRepository urepo, RoleRepository rrepo) {
		this.userRepo = urepo;
	}

	@Override
	@Transactional
	public User create(User user) {
		User managed_user = userRepo.save(user);

		System.out.println("Role count: " + managed_user.getRoles().size() );	 // NullPointerException
		
		return managed_user;
	}
	
	@Override
	@Transactional
	public User create(String surname, String name, String email, String login, String password ) {
		return this.create(new User(surname, name, email, login, password ));
	}

	@Override
	@Transactional
	public User findById(Long id) {
		return userRepo.findOne(id);
	}

}