code: private void configureHandlerAdapters() {
          Map<String, HandlerAdapter> handlerAdapters = this.factory.getBeansOfType(HandlerAdapter.class);
          if (handlerAdapters.isEmpty()) {
              defineBean(HttpRequestHandlerAdapter.class);
              defineBean(RequestMappingHandlerAdapter.class);
          }
      }
docs: Discovers existing {@link org.springframework.web.servlet.HandlerAdapter handler adapters} in the provided
  context. Provides the default adapters (see original dispatcher servlet) in case no adapters
  exist in the context.