SpringCloud Gateway默认不支持将请求路由到一个服务的多个端口
本文将结合Gateway的处理流程，提供一些解决思路

需求背景

公司有一个IM项目，对外暴露了两个端口8081和8082，8081是springboot启动使用的端口，对外提供一些http接口，如获取聊天群组成员列表等，相关配置如下

spring: 
  application: 
    name: im
server:
  port: 8081
  servlet:
    context-path: /im

8082是内部启动了一个netty服务器，处理websocket请求，实现即时通讯

......
......
pipeline.addLast(new WebSocketServerProtocolHandler("/ws"));
......
......
bootstrap.bind(8082).sync();

以本地环境为例，直连IM项目的请求地址如下：

http://localhost:8081/im/*** （请求http接口）
ws://localhost:8082/ws（进行websocket协议升级）

项目组希望客户端能统一走网关，实现对这两种接口的调用

问题描述

网关使用springcloud gateway，监听端口8080，注册中心使用nacos
正常情况下，gateway根据服务名转发，配置如下

    - id: im_route
      uri: lb://im
      predicates:
        - Path=/im/**

访问网关http://localhost:8080/im/**，即可将请求转发到IM服务
同理，我们希望访问网关ws://localhost:8080/ws，也能转发到IM的netty服务器

一般情况下，如果要通过gateway转发websocket请求，我们需要做如下配置

    - id: im_ws_route
      uri: lb:ws://im
      predicates:
        - Path=/ws/**

但实际上，lb这个schema告诉gateway要走负载，gateway转发的时候，会到nacos拉取im的服务清单，将其替换为 ip + 端口，而nacos上注册的是springboot项目的端口，也就是8081，所以想转发到IM的8082上，默认是没办法利用gateway的服务发现机制的，只能直接配置服务的ip地址，如下

    - id: im_ws_route
      uri: ws://localhost:8082
      predicates:
        - Path=/ws

不过这样也就失去使用网关的意义了

寻找思路

先观察下gateway转发的流程，找到DispatcherHandler的handle方法

	@Override
	public Mono<Void> handle(ServerWebExchange exchange) {
		if (this.handlerMappings == null) {
			return createNotFoundError();
		}
		if (CorsUtils.isPreFlightRequest(exchange.getRequest())) {
			return handlePreFlight(exchange);
		}
		return Flux.fromIterable(this.handlerMappings)
				.concatMap(mapping -> mapping.getHandler(exchange))
				.next()
				.switchIfEmpty(createNotFoundError())
				.onErrorResume(ex -> handleDispatchError(exchange, ex))
				.flatMap(handler -> handleRequestWith(exchange, handler));
	}

首先遍历内部的HandlerMapping，依次调用其getHandler方法，寻找能处理当前请求的Handler

HandlerMapping共有四个，一般来说我们配置了上述的路由，会在第三个RoutePredicateHandlerMapping返回一个Handler

Handler类型为FilteringWebHandler，其中包含了一组Filter

找到Handler后，在DispatcherHandler # handle方法的最后一行，调了handleRequestWith方法

	private Mono<Void> handleRequestWith(ServerWebExchange exchange, Object handler) {
		if (ObjectUtils.nullSafeEquals(exchange.getResponse().getStatusCode(), HttpStatus.FORBIDDEN)) {
			return Mono.empty();  // CORS rejection
		}
		if (this.handlerAdapters != null) {
			for (HandlerAdapter adapter : this.handlerAdapters) {
				if (adapter.supports(handler)) {
					return adapter.handle(exchange, handler)
							.flatMap(result -> handleResult(exchange, result));
				}
			}
		}
		return Mono.error(new IllegalStateException("No HandlerAdapter: " + handler));
	}

然后遍历了HandlerAdapter，一共有四个

这里起作用的是最后一个SimpleHandlerAdapter，进入其handle方法

	@Override
	public Mono<HandlerResult> handle(ServerWebExchange exchange, Object handler) {
		WebHandler webHandler = (WebHandler) handler;
		Mono<Void> mono = webHandler.handle(exchange);
		return mono.then(Mono.empty());
	}

调用了上面找到Handler的handle方法

	@Override
	public Mono<Void> handle(ServerWebExchange exchange) {
		Route route = exchange.getRequiredAttribute(GATEWAY_ROUTE_ATTR);
		List<GatewayFilter> gatewayFilters = route.getFilters();

		List<GatewayFilter> combined = new ArrayList<>(this.globalFilters);
		combined.addAll(gatewayFilters);
		// TODO: needed or cached?
		AnnotationAwareOrderComparator.sort(combined);

		if (logger.isDebugEnabled()) {
			logger.debug("Sorted gatewayFilterFactories: " + combined);
		}

		return new DefaultGatewayFilterChain(combined).filter(exchange);
	}

最后filter方法依次执行了其中的Filter

		@Override
		public Mono<Void> filter(ServerWebExchange exchange) {
			return Mono.defer(() -> {
				if (this.index < filters.size()) {
					GatewayFilter filter = filters.get(this.index);
					DefaultGatewayFilterChain chain = new DefaultGatewayFilterChain(this, this.index + 1);
					return filter.filter(exchange, chain);
				}
				else {
					return Mono.empty(); // complete
				}
			});
		}
	}

在这些Filter中，有一个ReactiveLoadBalancerClientFilter，会完成从nacos拉取服务清单替换请求地址的任务

看下它的filter方法

    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        URI url = exchange.getAttribute(GATEWAY_REQUEST_URL_ATTR);
        String schemePrefix = exchange.getAttribute(GATEWAY_SCHEME_PREFIX_ATTR);
        if (url == null || (!"lb".equals(url.getScheme()) && !"lb".equals(schemePrefix))) {
            return chain.filter(exchange);
        }
        // preserve the original url
        addOriginalRequestUrl(exchange, url);

        if (log.isTraceEnabled()) {
            log.trace(ReactiveLoadBalancerClientFilter.class.getSimpleName() + " url before: " + url);
        }

        URI requestUri = exchange.getAttribute(GATEWAY_REQUEST_URL_ATTR);
        String serviceId = requestUri.getHost();
        Set<LoadBalancerLifecycle> supportedLifecycleProcessors = LoadBalancerLifecycleValidator
                .getSupportedLifecycleProcessors(clientFactory.getInstances(serviceId, LoadBalancerLifecycle.class),
                        RequestDataContext.class, ResponseData.class, ServiceInstance.class);
        DefaultRequest<RequestDataContext> lbRequest = new DefaultRequest<>(
                new RequestDataContext(new RequestData(exchange.getRequest()), getHint(serviceId)));
        return choose(lbRequest, serviceId, supportedLifecycleProcessors).doOnNext(response -> {

                    if (!response.hasServer()) {
                        supportedLifecycleProcessors.forEach(lifecycle -> lifecycle
                                .onComplete(new CompletionContext<>(CompletionContext.Status.DISCARD, lbRequest, response)));
                        throw NotFoundException.create(properties.isUse404(), "Unable to find instance for " + url.getHost());
                    }

                    ServiceInstance retrievedInstance = response.getServer();

                    URI uri = exchange.getRequest().getURI();

                    // if the `lb:<scheme>` mechanism was used, use `<scheme>` as the default,
                    // if the loadbalancer doesn't provide one.
                    String overrideScheme = retrievedInstance.isSecure() ? "https" : "http";
                    if (schemePrefix != null) {
                        overrideScheme = url.getScheme();
                    }

                    DelegatingServiceInstance serviceInstance = new DelegatingServiceInstance(retrievedInstance,
                            overrideScheme);

                    URI requestUrl = reconstructURI(serviceInstance, uri);

                    if (log.isTraceEnabled()) {
                        log.trace("LoadBalancerClientFilter url chosen: " + requestUrl);
                    }
                    exchange.getAttributes().put(GATEWAY_REQUEST_URL_ATTR, requestUrl);
                    exchange.getAttributes().put(GATEWAY_LOADBALANCER_RESPONSE_ATTR, response);
                    supportedLifecycleProcessors.forEach(lifecycle -> lifecycle.onStartRequest(lbRequest, response));
                }).then(chain.filter(exchange))
                .doOnError(throwable -> supportedLifecycleProcessors.forEach(lifecycle -> lifecycle
                        .onComplete(new CompletionContext<ResponseData, ServiceInstance, RequestDataContext>(
                                CompletionContext.Status.FAILED, throwable, lbRequest,
                                exchange.getAttribute(GATEWAY_LOADBALANCER_RESPONSE_ATTR)))))
                .doOnSuccess(aVoid -> supportedLifecycleProcessors.forEach(lifecycle -> lifecycle
                        .onComplete(new CompletionContext<ResponseData, ServiceInstance, RequestDataContext>(
                                CompletionContext.Status.SUCCESS, lbRequest,
                                exchange.getAttribute(GATEWAY_LOADBALANCER_RESPONSE_ATTR),
                                new ResponseData(exchange.getResponse(), new RequestData(exchange.getRequest()))))));
    }

首先判断uri配置了lb，需要根据服务名做负载转发

在这个filter前，已经根据请求路径/ws和router配置，将请求地址转换成了 服务名:请求路径的形式，也就是将ws://localhost:8080/ws转化成了ws://im/ws

然后根据服务名im到nacos获取到服务的实际地址

 ServiceInstance retrievedInstance = response.getServer();

其中包含了ip和端口

最后替换出实际要转发的地址

所以我们只要在这一步，根据需要将端口8081改成8082，就能实现我们要的效果了

最终解决方案

最初是想自定义一个HandlerMapping完成转发的，但看下来后直接改源码更便捷一些，所以直接在项目中新建一个同路径的类，将源码copy进来，覆盖掉这个Filter

然后在获取到nacos实例后，修改掉端口号，就能改变最终的目标地址

在获取服务实例ServiceInstance retrievedInstance = response.getServer()这行后面加一段代码

if (url.toString().equals("ws://im/ws")) {
    try {
        Field field = retrievedInstance.getClass().getDeclaredField("port");
        field.setAccessible(true);
        field.set(retrievedInstance, 8082);
    } catch (Exception e) {
        e.printStackTrace();
    }
}

im相关的路由配置

    - id: im_route
      uri: lb://im
      predicates:
        - Path=/im/**
      
    - id: im_ws_route
      uri: lb:ws://im
      predicates:
        - Path=/ws/**