code: public void listenSync(InetAddress address, int port) throws IOException {
          if(auth == null) throw new NullPointerException("The Authenticator is null");
          if(socket != null) throw new IOException("Server already started");
  
          socket = Utils.createServer(port, 50, address, ssl, !explicitSecurity);
  
          while(!socket.isClosed()) {
              update();
          }
      }
docs: Starts the FTP server synchronously, blocking the current thread.
  
  Connections to the server will still create new threads.
  
  @param address The server address or {@code null} for a local address
  @param port The server port or {@code 0} to automatically allocate the port
  @throws IOException When an error occurs while starting the server