code: public final UdpServer doOnUnbound(Consumer<? super Connection> doOnUnbound) {
  		Objects.requireNonNull(doOnUnbound, "doOnUnbound");
  		return new UdpServerDoOn(this, null, null, doOnUnbound);
  	}
docs: Setup a callback called when {@link io.netty.channel.Channel} is
  unbound.
  
  @param doOnUnbound a consumer observing server stop event
  
  @return a new {@link UdpServer}