Hello Andri,
Any deployed App Engine application serves traffic at http://[YOUR_PROJECT_ID].
appspot.com. If you wish to restrict access, you'll need to implement user authentication in your application.
Google offers multiple methods of authenticating users in Cloud Platform applications. Please see the
User Authentication Options available for Java. For example, you can limit access to selected users (or all users) who has a Google account by using the
Users Java API. I recommend going through the different features and their purposes to get yourself more familiar with the options.
I hope that helps. Cheers!