@Override
    public UserProvider getUserProvider( String username )
    {
        // TODO add optional caching, to prevent retrieving the administrative users upon every invocation.
        final JID jid = XMPPServer.getInstance().createJID( username, null );
        final boolean isAdmin = AdminManager.getAdminProvider().getAdmins().contains( jid );

        if ( isAdmin )
        {
            return adminProvider;
        } else
        {
            return userProvider;
        }
    }
Serves the regular, non-administrative users.
xmppserver/src/main/java/org/jivesoftware/openfire/user/AuthorizationBasedUserProviderMapper.java