@Override
    AuthProvider getAuthProvider(String username)
    {
        if (primaryOverrides.contains(username.toLowerCase())) {
            return primaryProvider;
        }
        if (secondaryOverrides.contains(username.toLowerCase())) {
            return secondaryProvider;
        }
        if (tertiaryOverrides.contains(username.toLowerCase())) {
            return tertiaryProvider;
        }
        return null;
    }
Returns an auth provider for the provided username, but only if the username is in the 'override' list for that
provider.

When this method returns null, all providers should be attempted. When this method returns a non-null value, then
only the returned provider is to be used for the provided user.

@param username A user identifier (cannot be null or empty).
@return an auth provider that MUST be used for this user.
xmppserver/src/main/java/org/jivesoftware/openfire/auth/HybridAuthProvider.java