Looking for logging into the Google App Engine User Service site and are not getting the correct working link for it? In such a case, you can use the links given here by searching your desired login page in the search box. If the page is present on our website then you will get all the links related to the Google App Engine User Service site. After that you can login as usual with your login credentials.
https://cloud.google.com/appengine/docs/standard/java/javadoc/com/google/appengine/api/users/UserService?hl=de
https://cloud.google.com/java/getting-started-appengine-standard/authenticate-users
https://stackoverflow.com/questions/6123938/google-app-engine-user-service-to-login-and-logout-java
https://www.youtube.com/watch?v=XQKnl-HW9sw
https://console.cloud.google.com/
https://gist.github.com/merqurio/2f8f55987ccc224e2342
http://www.gwtproject.org/doc/latest/tutorial/appengine.html
https://developers.google.com/identity/protocols/OAuth2ServiceAccount
https://www.vogella.com/tutorials/GoogleAppEngineJava/article.html
https://www.mkyong.com/google-app-engine/gae-java-integrating-google-user-account/
http://webapp2.readthedocs.io/en/latest/tutorials/gettingstarted/usingusers.html
https://developer.chrome.com/webstore/identify_user
https://blog.openbridge.com/the-missing-guide-to-setting-up-google-cloud-service-accounts-for-google-bigquery-6301e509b232