Spring Cloud Gateway 不小心换了个 Web 容器就不能用了

@Configuration(proxyBeanMethods = false)
@ConditionalOnClass({ Undertow.class }) 
public class CustomizedUndertowConfiguration {
    .......
}

public enum WebApplicationType {
  /**
   * 没有 web 服务，不需要 web 容器
   */
  NONE,
  /**
   * 使用基于 servlet 的 web 容器
   */
  SERVLET,
  /**
   * 使用响应式的 web 容器
   */
  REACTIVE;
  private static final String[] SERVLET_INDICATOR_CLASSES = { "javax.servlet.Servlet",
      "org.springframework.web.context.ConfigurableWebApplicationContext" };
  private static final String WEBMVC_INDICATOR_CLASS = "org.springframework.web.servlet.DispatcherServlet";
  private static final String WEBFLUX_INDICATOR_CLASS = "org.springframework.web.reactive.DispatcherHandler";
  private static final String JERSEY_INDICATOR_CLASS = "org.glassfish.jersey.servlet.ServletContainer";
  private static final String SERVLET_APPLICATION_CONTEXT_CLASS = "org.springframework.web.context.WebApplicationContext";
  private static final String REACTIVE_APPLICATION_CONTEXT_CLASS = "org.springframework.boot.web.reactive.context.ReactiveWebApplicationContext";
  static WebApplicationType deduceFromClasspath() {
    if (ClassUtils.isPresent(WEBFLUX_INDICATOR_CLASS, null) && !ClassUtils.isPresent(WEBMVC_INDICATOR_CLASS, null)
        && !ClassUtils.isPresent(JERSEY_INDICATOR_CLASS, null)) {
      return WebApplicationType.REACTIVE;
    }
    for (String className : SERVLET_INDICATOR_CLASSES) {
      if (!ClassUtils.isPresent(className, null)) {
        return WebApplicationType.NONE;
      }
    }
    return WebApplicationType.SERVLET;
  }

@Configuration(proxyBeanMethods = false)
@ConditionalOnMissingBean(ReactiveWebServerFactory.class)
@ConditionalOnClass({ HttpServer.class })
static class EmbeddedNetty {
  @Bean
  @ConditionalOnMissingBean
  ReactorResourceFactory reactorServerResourceFactory() {
    return new ReactorResourceFactory();
  }
  @Bean
  NettyReactiveWebServerFactory nettyReactiveWebServerFactory(ReactorResourceFactory resourceFactory,
      ObjectProvider<NettyRouteProvider> routes, ObjectProvider<NettyServerCustomizer> serverCustomizers) {
    NettyReactiveWebServerFactory serverFactory = new NettyReactiveWebServerFactory();
    serverFactory.setResourceFactory(resourceFactory);
    routes.orderedStream().forEach(serverFactory::addRouteProviders);
    serverFactory.getServerCustomizers().addAll(serverCustomizers.orderedStream().collect(Collectors.toList()));
    return serverFactory;
  }
}
@Configuration(proxyBeanMethods = false)
@ConditionalOnMissingBean(ReactiveWebServerFactory.class)
@ConditionalOnClass({ org.apache.catalina.startup.Tomcat.class })
static class EmbeddedTomcat {
  @Bean
  TomcatReactiveWebServerFactory tomcatReactiveWebServerFactory(
      ObjectProvider<TomcatConnectorCustomizer> connectorCustomizers,
      ObjectProvider<TomcatContextCustomizer> contextCustomizers,
      ObjectProvider<TomcatProtocolHandlerCustomizer<?>> protocolHandlerCustomizers) {
    TomcatReactiveWebServerFactory factory = new TomcatReactiveWebServerFactory();
    factory.getTomcatConnectorCustomizers()
        .addAll(connectorCustomizers.orderedStream().collect(Collectors.toList()));
    factory.getTomcatContextCustomizers()
        .addAll(contextCustomizers.orderedStream().collect(Collectors.toList()));
    factory.getTomcatProtocolHandlerCustomizers()
        .addAll(protocolHandlerCustomizers.orderedStream().collect(Collectors.toList()));
    return factory;
  }
}
@Configuration(proxyBeanMethods = false)
@ConditionalOnMissingBean(ReactiveWebServerFactory.class)
@ConditionalOnClass({ org.eclipse.jetty.server.Server.class, ServletHolder.class })
static class EmbeddedJetty {
  @Bean
  @ConditionalOnMissingBean
  JettyResourceFactory jettyServerResourceFactory() {
    return new JettyResourceFactory();
  }
  @Bean
  JettyReactiveWebServerFactory jettyReactiveWebServerFactory(JettyResourceFactory resourceFactory,
      ObjectProvider<JettyServerCustomizer> serverCustomizers) {
    JettyReactiveWebServerFactory serverFactory = new JettyReactiveWebServerFactory();
    serverFactory.getServerCustomizers().addAll(serverCustomizers.orderedStream().collect(Collectors.toList()));
    serverFactory.setResourceFactory(resourceFactory);
    return serverFactory;
  }
}
@Configuration(proxyBeanMethods = false)
@ConditionalOnMissingBean(ReactiveWebServerFactory.class)
@ConditionalOnClass({ Undertow.class })
static class EmbeddedUndertow {
  @Bean
  UndertowReactiveWebServerFactory undertowReactiveWebServerFactory(
      ObjectProvider<UndertowBuilderCustomizer> builderCustomizers) {
    UndertowReactiveWebServerFactory factory = new UndertowReactiveWebServerFactory();
    factory.getBuilderCustomizers().addAll(builderCustomizers.orderedStream().collect(Collectors.toList()));
    return factory;
  }
}

@Import({ ReactiveWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class,
    ReactiveWebServerFactoryConfiguration.EmbeddedTomcat.class,
    ReactiveWebServerFactoryConfiguration.EmbeddedJetty.class,
    ReactiveWebServerFactoryConfiguration.EmbeddedUndertow.class,
    ReactiveWebServerFactoryConfiguration.EmbeddedNetty.class })

private static <T> Mono<T> cacheRequestBody(ServerWebExchange exchange, boolean cacheDecoratedRequest,
      Function<ServerHttpRequest, Mono<T>> function) {
  ServerHttpResponse response = exchange.getResponse();
  //在这里，强制转换了 bufferFactory 为 NettyDataBufferFactory
  NettyDataBufferFactory factory = (NettyDataBufferFactory) response.bufferFactory();
  // Join all the DataBuffers so we have a single DataBuffer for the body
  return DataBufferUtils.join(exchange.getRequest().getBody())
      .defaultIfEmpty(factory.wrap(new EmptyByteBuf(factory.getByteBufAllocator())))
      .map(dataBuffer -> decorate(exchange, dataBuffer, cacheDecoratedRequest))
      .switchIfEmpty(Mono.just(exchange.getRequest())).flatMap(function);
}