public Spec spec() {
        if ( ! serverChannel.isOpen()) {
            return null;
        }
        return new Spec(serverChannel.socket().getInetAddress().getHostName(),
                        serverChannel.socket().getLocalPort());
    }
Obtain the Spec for the local port and host interface this Acceptor
is listening to.  If this Acceptor is no longer listening (it has
been shut down), null will be returned.

@return listening spec, or null if not listening
jrt/src/com/yahoo/jrt/Acceptor.java