private synchronized void clearCache() {
        knownGroupNamesFromKeys = null;
        knownGroupNamesFromValues = null;
    }
Certain operations imply that our locally cached group list(s) should
be dropped and recreated. For example, when an ad-hoc client command
is used to add a member to a group, the underlying group instance is
actually dropped from the global Group cache, with the effect that our
cache would be referring to an orphaned Group instance.
xmppserver/src/main/java/org/jivesoftware/openfire/group/ConcurrentGroupMap.java