So basically I want to protect my APIs with OAuth 2.0 and implement an OAuth Provider to enable acquiry of accessTokens etc.
Can it be done with JOAuth out of the box?
Has anybody already implemented something like this with an Open Source library (Java)?
https://github.com/OpenConextApps/apis is an full-fledged Java Open Source Implementation of an OAuth2 authorization server.
You can try https://github.com/apifest/apifest-oauth20. It's an open-source project written in pure Java and implements the OAuth 2.0 specification - https://tools.ietf.org/html/rfc6749.
You can also use CloudFoundry User Account and Authentication (UAA) Server
It Supports: OAuth2, OpenId Connect, SCIM, SSO
You can clone it from https://github.com/cloudfoundry/uaa.