Tokensmith is a Java implementation of an OAuth 2.0 and OIDC Identity server.
Have a look at the documentation on tokensmith.net
Contributing docs are available which hopefully will be sufficient to get started. If you find they are not, then submit an issue in the website repo.
This repo has multiple gradle projects.
Everything related to accepting and responding to HTTP requests
Use cases for supporting OAuth2 and OIDC.
Entities and Repository interfaces
An SDK to interact with Tokensmith (OIDC ID Server).