import com.google.api.client.auth.oauth2.TokenRequest; //导入依赖的package包/类
private String getServiceAccountToken(GoogleCredential credential, String targetAudience)
    throws IOException, GeneralSecurityException {
  final TokenRequest request = new TokenRequest(
      this.httpTransport, JSON_FACTORY,
      new GenericUrl(credential.getTokenServerEncodedUrl()),
      "urn:ietf:params:oauth:grant-type:jwt-bearer");
  final Header header = jwtHeader();
  final Payload payload = jwtPayload(
      targetAudience, credential.getServiceAccountId(), credential.getTokenServerEncodedUrl());
  request.put("assertion", JsonWebSignature.signUsingRsaSha256(
      credential.getServiceAccountPrivateKey(), JSON_FACTORY, header, payload));
  final TokenResponse response = request.execute();
  return (String) response.get("id_token");
}
 

import com.google.api.client.auth.oauth2.TokenRequest; //导入依赖的package包/类
private String getUserToken(GoogleCredential credential) throws IOException {
  final TokenRequest request = new RefreshTokenRequest(
      this.httpTransport, JSON_FACTORY,
      new GenericUrl(credential.getTokenServerEncodedUrl()),
      credential.getRefreshToken())
      .setClientAuthentication(credential.getClientAuthentication())
      .setRequestInitializer(credential);
  final TokenResponse response = request.execute();
  return (String) response.get("id_token");
}
 

import com.google.api.client.auth.oauth2.TokenRequest; //导入依赖的package包/类
@Override
protected TokenResponse executeRefreshToken() throws IOException {
  if (getServiceAccountPrivateKey() == null) {
    return super.executeRefreshToken();
  }
  // service accounts: no refresh token; instead use private key to request new access token
  JsonWebSignature.Header header = new JsonWebSignature.Header();
  header.setAlgorithm("RS256");
  header.setType("JWT");
  header.setKeyId(getServiceAccountPrivateKeyId());
  JsonWebToken.Payload payload = new JsonWebToken.Payload();
  long currentTime = getClock().currentTimeMillis();
  payload.setIssuer(getServiceAccountId());
  payload.setAudience(getTokenServerEncodedUrl());
  payload.setIssuedAtTimeSeconds(currentTime / 1000);
  payload.setExpirationTimeSeconds(currentTime / 1000 + DEFAULT_TOKEN_EXPIRATION_SECONDS);
  payload.setSubject(getServiceAccountUser());
  payload.put("scope", Joiner.on(' ').join(getServiceAccountScopes()));
  try {
    String assertion = JsonWebSignature.signUsingRsaSha256(
        getServiceAccountPrivateKey(), getJsonFactory(), header, payload);
    TokenRequest request = new TokenRequest(
        getTransport(), getJsonFactory(), new GenericUrl(getTokenServerEncodedUrl()),
        "urn:ietf:params:oauth:grant-type:jwt-bearer");
    request.put("assertion", assertion);
    request.setRequestInitializer(getRequestInitializer());
    return request.execute();
  } catch (GeneralSecurityException exception) {
    IOException e = new IOException();
    e.initCause(exception);
    throw e;
  }
}