Google Play Games サービスに対するサーバーサイドのアクセスを有効にする

private static final int RC_SIGN_IN = 9001;
private GoogleSignInClient mGoogleSignInClient;

private void startSignInForAuthCode() {

  // Client ID for your backend server.
  String webClientId = getString(R.string.webclient_id);

  GoogleSignInOptions signInOption = new
      GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_GAMES_SIGN_IN)
      .requestServerAuthCode(webClientId)
      .build();

  GoogleSignInClient signInClient = GoogleSignIn.getClient(this, signInOption);
  Intent intent = signInClient.getSignInIntent();
  startActivityForResult(intent, RC_SIGN_IN);
}

// Auth code to send to backend server.
private String mServerAuthCode;

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
  super.onActivityResult(requestCode, resultCode, data);
  if (requestCode == RC_SIGN_IN) {
    GoogleSignInResult result = Auth.GoogleSignInApi.getSignInResultFromIntent(data);
    if (result.isSuccess()) {
      mServerAuthCode = result.getSignInAccount().getServerAuthCode();
    } else {
      String message = result.getStatus().getStatusMessage();
      if (message == null || message.isEmpty()) {
        message = getString(R.string.signin_other_error);
      }
      new AlertDialog.Builder(this).setMessage(message)
          .setNeutralButton(android.R.string.ok, null).show();
    }
  }
}

/**
 * Exchanges the authcode for an access token credential.  The credential
 * is the associated with the given player.
 *
 * @param authCode - the non-null authcode passed from the client.
 * @param player   - the player object which the given authcode is
 *                 associated with.
 * @return the HTTP response code indicating the outcome of the exchange.
 */
private int exchangeAuthCode(String authCode, Player player) {
try {

    // The client_secret.json file is downloaded from the Google API
    // console.  This is used to identify your web application.  The
    // contents of this file should not be shared.
    //
    File secretFile = new File("client_secret.json");

    // If we don't have the file, we can't access any APIs, so return
    // an error.
    if (!secretFile.exists()) {
        log("Secret file : " + secretFile
                .getAbsolutePath() + "  does not exist!");
        return HttpServletResponse.SC_FORBIDDEN;
    }

    GoogleClientSecrets clientSecrets = GoogleClientSecrets.load(
            JacksonFactory.getDefaultInstance(), new
            FileReader(secretFile));

    // Extract the application id of the game from the client id.
    String applicationId = extractApplicationId(clientSecrets
            .getDetails().getClientId());

    GoogleTokenResponse tokenResponse =
            new GoogleAuthorizationCodeTokenRequest(
            HTTPTransport,
            JacksonFactory.getDefaultInstance(),
            "https://oauth2.googleapis.com/token",
            clientSecrets.getDetails().getClientId(),
            clientSecrets.getDetails().getClientSecret(),
            authCode,
            "")
            .execute();

    log("hasRefresh == " + (tokenResponse.getRefreshToken() != null));
    log("Exchanging authCode: " + authCode + " for token");
    Credential credential = new Credential
            .Builder(BearerToken.authorizationHeaderAccessMethod())
            .setJsonFactory(JacksonFactory.getDefaultInstance())
            .setTransport(HTTPTransport)
            .setTokenServerEncodedUrl("https://www.googleapis.com/oauth2/v4/token")
            .setClientAuthentication(new HttpExecuteInterceptor() {
                @Override
                public void intercept(HttpRequest request)
                        throws IOException {
                        }
            })
            .build()
            .setFromTokenResponse(tokenResponse);

    player.setCredential(credential);

    // Now that we have a credential, we can access the Games API.
    PlayGamesAPI api = new PlayGamesAPI(player, applicationId,
            HTTPTransport, JacksonFactory.getDefaultInstance());

    // Call the verify method, which checks that the access token has
    // access to the Games API, and that the player id used by the
    // client matches the playerId associated with the accessToken.
    boolean ok = api.verifyPlayer();

    // Call a Games API on the server.
    if (ok) {
        ok = api.updatePlayerInfo();
        if (ok) {
            // persist the player.
            savePlayer(api.getPlayer());
        }
    }

    return ok ? HttpServletResponse.SC_OK :
            HttpServletResponse.SC_INTERNAL_SERVER_ERROR;

  } catch (IOException e) {
    e.printStackTrace();
  }
  return HttpServletResponse.SC_INTERNAL_SERVER_ERROR;
}