@Override

public Optional<SignedInUser> getAccessToken(                               RefreshToken refreshToken) {

  return userTokenRepository

      .findByRefreshToken(refreshToken.getRefreshToken())

      .map(ut -> Optional.of(createSignedInUser(             ut.getUser())

          .refreshToken(refreshToken.getRefreshToken())))

      .orElseThrow(() -> new InvalidRefreshTokenException(                                          "Invalid token."));

}