如果你想自动化这个过程，你需要创建你自己的com.google.api.client.auth.oauth2.Credential对象和.setRefreshToken。

private static Credential getCredentials() throws IOException {
    InputStream in = GmailService.class.getResourceAsStream(CREDENTIALS_FILE_PATH);
    if (in == null) {
      throw new FileNotFoundException("Resource not found: " + CREDENTIALS_FILE_PATH);
    }
    GoogleClientSecrets clientSecrets = GoogleClientSecrets.load(JSON_FACTORY, new InputStreamReader(in));

    GoogleAuthorizationCodeFlow flow = new GoogleAuthorizationCodeFlow.Builder(
      GoogleNetHttpTransport.newTrustedTransport(), JSON_FACTORY, clientSecrets, SCOPES)
      .setApprovalPrompt("auto")
      .setAccessType("offline")
      .build();

    Credential credential = new Credential.Builder(flow.getMethod())
        .setTransport(flow.getTransport())
        .setJsonFactory(flow.getJsonFactory())
        .setTokenServerEncodedUrl(flow.getTokenServerEncodedUrl())
        .setClientAuthentication(flow.getClientAuthentication())
        .setRequestInitializer(flow.getRequestInitializer())
        .setClock(flow.getClock()).build();

    credential.setRefreshToken("YOUR_REFRESH_TOKEN");

    return credential;
  }

从https://developers.google.com/gmail/api/quickstart/java获取代码