Previously UnAuthenticatedServerOAuth2AuthorizedClientRepository used a HashMap for storing OAuth2AuthorizedClients.
UnAuthenticatedServerOAuth2AuthorizedClientRepository and its HashMap are potentially accessed by multiple threads without any synchronization.
Since HashMap is not threadsafe itself, this makes UnAuthenticatedServerOAuth2AuthorizedClientRepository not threadsafe.
Now UnAuthenticatedServerOAuth2AuthorizedClientRepository uses a ConcurrentHashMap for storing OAuth2AuthorizedClients.
Since ConcurrentHashMap is threadsafe, UnAuthenticatedServerOAuth2AuthorizedClientRepository will now be threadsafe as well.
Fixes gh-6717
9520e3a1c0