code: public ServletRegistration.Dynamic addServlet(String name, Servlet servlet) {
          final ServletHolder holder = new NonblockingServletHolder(requireNonNull(servlet));
          holder.setName(name);
          handler.getServletHandler().addServlet(holder);
  
          final ServletRegistration.Dynamic registration = holder.getRegistration();
          checkDuplicateRegistration(name, servlets, "servlet");
  
          return registration;
      }
docs: Add a servlet instance.
  
  @param name    the servlet's name
  @param servlet the servlet instance
  @return a {@link javax.servlet.ServletRegistration.Dynamic} instance allowing for further
  configuration