How can I implement a java backend probably on top of Spring, we will use only REST api's to access backend. user can login through standard login/password, and social authentication (facebook, google etc.). Is there a standard oauth2 implementation which we can use for this requirement. Any help will be appreciated. Thanks!
You could use Apache Oltu Client library to implement login through Facebook, Google, ... You have to implement your own user management for storing the API token, and to provide your login/password authentication. https://cwiki.apache.org/confluence/display/OLTU/OAuth+2.0+Client+Quickstart