code: public ServerBuilder defaultVirtualHost(VirtualHost defaultVirtualHost) {
          requireNonNull(defaultVirtualHost, "defaultVirtualHost");
          if (updatedDefaultVirtualHostBuilder) {
              throw new IllegalStateException("invoked other default VirtualHost builder methods already");
          }
  
          this.defaultVirtualHost = defaultVirtualHost;
          return this;
      }
docs: Sets the default {@link VirtualHost}, which is used when no other {@link VirtualHost}s match the
  host name of a client request. e.g. the {@code "Host"} header in HTTP or host name in TLS SNI extension
  
  @throws IllegalStateException
  if other default {@link VirtualHost} builder methods have been invoked already, including:
  <ul>
  <li>{@link #tls(SslContext)}</li>
  <li>{@link #service(String, Service)}</li>
  <li>{@link #serviceUnder(String, Service)}</li>
  <li>{@link #service(PathMapping, Service)}</li>
  </ul>
  
  @see #virtualHost(VirtualHost)