public void close() {
        boolean isPreviouslyClosed = isClosed.getAndSet(true);
        if (isPreviouslyClosed) {
            return;
        }

        AdminClient client;
        while ((client = clientCache.poll()) != null) {
            client.close();
        }
    }
public void close ( ) { boolean isPreviouslyClosed = isClosed . getAndSet ( true ) ; if ( isPreviouslyClosed ) { return ; } AdminClient client ; while ( ( client = clientCache . poll ( ) ) != null ) { client . close ( ) ; } }
close the AdminPool, if no long required.
After closed, all public methods will throw IllegalStateException