Spring Cloud Gateway 请求转发源码分析

一、背景

Spring Cloud Gateway 作为一种微服务网关组件，相信大家都不陌生，一个请求经过Spring Cloud Gateway是如何转发出去的，今天我们就来分析一下这部分的源码。

二、正文

下面这张图大家在学习Spring Cloud Gateway的时候肯定见过，在分析源码之前我们再来看下这张图。

其中

Gateway Client： 发送请求到 Spring Cloud Gateway 的客户端
Gateway Handler Mapping： 是处理请求的组件，负责将请求映射到相应的处理器。处理请求将被路由到哪个路由规则，从而选择对应的过滤器链
Gateway Web Handler： 实际处理请求的组件，会依次执行过滤器链，对请求进行处理
Gateway Filter： 过滤器链由多个过滤器组成，每个过滤器执行一些特定代码逻辑
Proxied Service： 被代理的服务，当执行完过滤器链之后会将请求转发到具体的目标服务

官网上有这样一段解释：

📌Clients make requests to Spring Cloud Gateway. If the Gateway Handler Mapping determines that a request matches a route, it is sent to the Gateway Web Handler. This handler runs the request through a filter chain that is specific to the request。

客户端向 Spring Cloud Gateway 发出请求。如果 Gateway Handler Mapping 确定请求与路由匹配，则会将其发送到 Gateway Web Handler。此 handler 通过一个特定于请求的过滤器链来运行请求（这个处理程序会让请求通过一系列只为这个请求设计的过滤器。简单来说，就是请求会经过一组专门为它设置的过滤器。

我们先来看下 gateway 的自动配置类 GatewayAutoConfiguration

Gateway的自动配置

@Configuration(
    proxyBeanMethods = false
)
@ConditionalOnProperty(
    name = {"spring.cloud.gateway.enabled"},
    matchIfMissing = true
)
@EnableConfigurationProperties
//自动配置前置条件：引入了WebFlux 和 HttpHandler 组件
@AutoConfigureBefore({HttpHandlerAutoConfiguration.class, WebFluxAutoConfiguration.class})
//自动配置后置组件：负载均衡组件
@AutoConfigureAfter({GatewayReactiveLoadBalancerClientAutoConfiguration.class, GatewayClassPathWarningAutoConfiguration.class})
@ConditionalOnClass({DispatcherHandler.class})
public class GatewayAutoConfiguration {
    
    @Bean
    @ConditionalOnMissingBean
    public PropertiesRouteDefinitionLocator propertiesRouteDefinitionLocator(GatewayProperties properties) {
        return new PropertiesRouteDefinitionLocator(properties);
    }
    
    @Bean
    @Primary
    public RouteDefinitionLocator routeDefinitionLocator(List<RouteDefinitionLocator> routeDefinitionLocators) {
        return new CompositeRouteDefinitionLocator(Flux.fromIterable(routeDefinitionLocators));
    }
    
    @Bean
    @ConditionalOnMissingBean
    public RoutePredicateHandlerMapping routePredicateHandlerMapping(FilteringWebHandler webHandler, RouteLocator routeLocator, GlobalCorsProperties globalCorsProperties, Environment environment) {
        return new RoutePredicateHandlerMapping(webHandler, routeLocator, globalCorsProperties, environment);
    }
    
    @Bean
    public GatewayProperties gatewayProperties() {
        return new GatewayProperties();
    }
    
    @Bean
    @ConditionalOnEnabledGlobalFilter
    public RouteToRequestUrlFilter routeToRequestUrlFilter() {
        return new RouteToRequestUrlFilter();
    }
    
    /**
     * GlobalFilter自动注入使用的是 @Autowired，不需要显式写出，spring 会自动查找所有类型为 GlobalFilter 的 Bean，并将它们注入到这个List中
     */
    @Bean
    public FilteringWebHandler filteringWebHandler(List<GlobalFilter> globalFilters) {
        return new FilteringWebHandler(globalFilters);
    }
    
    ......
    
}

自动配置类GatewayAutoConfiguration在内部初始化了很多bean，列举几个重要的如下：

PropertiesRouteDefinitionLocator：用于从配置文件（yml/properties）中读取路由配置信息
RouteDefinitionLocator：把 RouteDefinition 转化为 Route
RoutePredicateHandlerMapping：类似于 mvc 的HandlerMapping，用于匹配对应的请求 Route
GatewayProperties：yml配置信息封装在 GatewayProperties 对象中
GlobalFilter实现类：比如RouteToRequestUrlFilter、ForwardRoutingFilter等全局过滤器
FilteringWebHandler：注入了所有实现了 GlobalFilter 接口的过滤器类

FilteringWebHandler 中注入的GlobalFilter如下：

{
  "org.springframework.cloud.gateway.filter.LoadBalancerClientFilter@77856cc5": 10100,
  "org.springframework.cloud.gateway.filter.RouteToRequestUrlFilter@4f6fd101": 10000,
  "org.springframework.cloud.gateway.filter.NettyWriteResponseFilter@32d22650": -1,
  "org.springframework.cloud.gateway.filter.ForwardRoutingFilter@106459d9": 2147483647,
  "org.springframework.cloud.gateway.filter.NettyRoutingFilter@1fbd5e0": 2147483647,
  "org.springframework.cloud.gateway.filter.ForwardPathFilter@33a71d23": 0,
  "org.springframework.cloud.gateway.filter.AdaptCachedBodyGlobalFilter@135064ea": 2147483637,
  "org.springframework.cloud.gateway.filter.WebsocketRoutingFilter@23c05889": 2147483646
}

Gateway的工作流程

所有请求都将由ReactorHttpHandlerAdapter.apply() 方法拦截处理，此时会封装请求对象和响应对象，并传递到HttpWebHandlerAdapter.handle()方法。
HttpWebHandlerAdapter.handle()方法将request和response封装成上下文对象ServerWebExchange，方法通过getDelegate()获取全局异常处理器ExceptionHandlingWebHandler执行全局异常处理

这两步出现的类是在 spring-web 包下
ExceptionHandlingWebHandler执行完成后，调用DispatcherHandler.handle()，循环所有handlerMappings查找处理当前请求的HandlerDispatcherHandler

在 spring-webflux 包下，我们知道Gateway是基于webflux响应式编程
找到Handler后调用DispatcherHandler.invokeHandler()执行找到的Handler，此时会调用FilteringWebHandler.handle()
DefaultGatewayFilterChain.filter()是关键流程，所有过滤器都会在这里执行，比如服务查找、负载均衡、远程调用等，都在这一块。

Gateway源码分析

📌Spring Cloud Gateway版本：spring-cloud-gateway 3.1.4

我们直接从HttpWebHandlerAdapter.handle()看起

HttpWebHandlerAdapter

public Mono<Void> handle(ServerHttpRequest request, ServerHttpResponse response) {
        if (this.forwardedHeaderTransformer != null) {
            try {
                request = this.forwardedHeaderTransformer.apply(request);
            } catch (Throwable var4) {
                Throwable ex = var4;
                if (logger.isDebugEnabled()) {
                    logger.debug("Failed to apply forwarded headers to " + this.formatRequest(request), ex);
                }

                response.setStatusCode(HttpStatus.BAD_REQUEST);
                return response.setComplete();
            }
        }
        // 创建网关上下文对象
        ServerWebExchange exchange = this.createExchange(request, response);
        LogFormatUtils.traceDebug(logger, (traceOn) -> {
            return exchange.getLogPrefix() + this.formatRequest(exchange.getRequest()) + (traceOn ? ", headers=" + this.formatHeaders(exchange.getRequest().getHeaders()) : "");
        });
        // getDelegate()获取当前的Handler
        Mono var10000 = this.getDelegate().handle(exchange).doOnSuccess((aVoid) -> {
            this.logResponse(exchange);
        }).onErrorResume((exx) -> {
            return this.handleUnresolvedError(exchange, exx);
        });
        response.getClass();
        return var10000.then(Mono.defer(response::setComplete));
    }

代码中先创建了网关上下文对象ServerWebExchange，然后getDelegate()获取当前的Handler，我们将断点打到getDelegate() 方法里面：

当前返回的 WebHandler 是ExceptionHandlingWebHandler，而ExceptionHandlingWebHandler的 delegate 是FilteringWebHandler，FilteringWebHandler的 delegate 是DispatcherHandler，所有的 delegate 的handle()方法都会依次执行。

delegate: 委托、委派这里应该是使用了委派模式，不属于23种设计模式，类似代理模式，委派模式主要关注任务的调度和分配，注重结果，可以看作是一种特殊的静态代理，即全权代理。

接下来我们可以把断点放到DispatcherHandler.handler()方法上

DispatcherHandler

  public Mono<Void> handle(ServerWebExchange exchange) {
        if (this.handlerMappings == null) {
            return this.createNotFoundError();
        } else {
            // 1.遍历所有的handlerMapping，获取到当前网关上下文中的多个handlerMapping
            return CorsUtils.isPreFlightRequest(exchange.getRequest()) ? this.handlePreFlight(exchange) : Flux.fromIterable(this.handlerMappings).concatMap((mapping) -> {
                return mapping.getHandler(exchange);
            // 2.获取对应的适配器，调用对应的处理器
            }).next().switchIfEmpty(this.createNotFoundError()).flatMap((handler) -> {
                return this.invokeHandler(exchange, handler);
            // 3. 返回处理结果
            }).flatMap((result) -> {
                return this.handleResult(exchange, result);
            });
        }
    }

可以看到这里所有的handlerMapping ，方法中会调用所有 handlerMappings 的getHandler(exchange)方法，

点进去getHandler(exchange)方法进入一个抽象类AbstractHandlerMapping

AbstractHandlerMapping

public Mono<Object> getHandler(ServerWebExchange exchange) {
        return this.getHandlerInternal(exchange).map((handler) -> {
            if (this.logger.isDebugEnabled()) {
                this.logger.debug(exchange.getLogPrefix() + "Mapped to " + handler);
            }

            ServerHttpRequest request = exchange.getRequest();
            if (this.hasCorsConfigurationSource(handler) || CorsUtils.isPreFlightRequest(request)) {
                CorsConfiguration config = this.corsConfigurationSource != null ? this.corsConfigurationSource.getCorsConfiguration(exchange) : null;
                CorsConfiguration handlerConfig = this.getCorsConfiguration(handler, exchange);
                config = config != null ? config.combine(handlerConfig) : handlerConfig;
                if (config != null) {
                    config.validateAllowCredentials();
                }

                if (!this.corsProcessor.process(config, exchange) || CorsUtils.isPreFlightRequest(request)) {
                    return NO_OP_HANDLER;
                }
            }

            return handler;
        });
    }

类中的getHandler(exchange)方法会调getHandlerInternal(exchange)方法

可以看到getHandlerInternal(exchange)该方法由各个HandlerMapping自行实现，这里有6个重写了getHandlerInternal方法的类，

由于是网关组件，当请求进入时，会先判断路由，所以会进入实现类RoutePredicateHandlerMapping中。

RoutePredicateHandlerMapping

protected Mono<?> getHandlerInternal(ServerWebExchange exchange) {
        if (this.managementPortType == RoutePredicateHandlerMapping.ManagementPortType.DIFFERENT && this.managementPort != null && exchange.getRequest().getURI().getPort() == this.managementPort) {
            return Mono.empty();
        } else {
            exchange.getAttributes().put(ServerWebExchangeUtils.GATEWAY_HANDLER_MAPPER_ATTR, this.getSimpleName());
            return this.lookupRoute(exchange).flatMap((r) -> {
                exchange.getAttributes().remove(ServerWebExchangeUtils.GATEWAY_PREDICATE_ROUTE_ATTR);
                if (this.logger.isDebugEnabled()) {
                    this.logger.debug("Mapping [" + this.getExchangeDesc(exchange) + "] to " + r);
                }

                exchange.getAttributes().put(ServerWebExchangeUtils.GATEWAY_ROUTE_ATTR, r);
                // 此处的webhandler是FilteringWebhandler
                return Mono.just(this.webHandler);
            }).switchIfEmpty(Mono.empty().then(Mono.fromRunnable(() -> {
                exchange.getAttributes().remove(ServerWebExchangeUtils.GATEWAY_PREDICATE_ROUTE_ATTR);
                if (this.logger.isTraceEnabled()) {
                    this.logger.trace("No RouteDefinition found for [" + this.getExchangeDesc(exchange) + "]");
                }

            })));
        }
    }  

 protected Mono<Route> lookupRoute(ServerWebExchange exchange) {
        return this.routeLocator.getRoutes().concatMap((route) -> {
            return Mono.just(route).filterWhen((r) -> {
                exchange.getAttributes().put(ServerWebExchangeUtils.GATEWAY_PREDICATE_ROUTE_ATTR, r.getId());
                return (Publisher)r.getPredicate().apply(exchange);
            }).doOnError((e) -> {
                this.logger.error("Error applying predicate for route: " + route.getId(), e);
            }).onErrorResume((e) -> {
                return Mono.empty();
            });
        }).next().map((route) -> {
            if (this.logger.isDebugEnabled()) {
                this.logger.debug("Route matched: " + route.getId());
            }

            this.validateRoute(route, exchange);
            return route;
        });
    }

lookupRoute中的this.routeLocator.getRoutes()会通过RouteDefinitionRouteLocator拿到yml配置文件中所有的路由断言工厂（Before、After、Path等等），然后把找到的路由转换成Route，再执行apply方法，进行路由匹配，判断是否允许请求通过。

找到对应Route后会返回指定的FilterWebHandler，如下代码：

至此对应着DispatcherHandler中的第一步获取到当前网关上下文中的多个handlerMapping.

接下来我们看DispatcherHandler中的第二步获取对应的适配器，调用对应的处理器.

进入获取对应的适配器方法 invokeHandler(exchange, handler)中

请求对应的适配器是SimpleHandlerAdapter

SimpleHandlerAdapter

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

这里调用了FilteringWebHandler的handle()方法

FilteringWebHandler

    public Mono<Void> handle(ServerWebExchange exchange) {
        Route route = (Route)exchange.getRequiredAttribute(ServerWebExchangeUtils.GATEWAY_ROUTE_ATTR);
        List<GatewayFilter> gatewayFilters = route.getFilters();
        List<GatewayFilter> combined = new ArrayList(this.globalFilters);
        combined.addAll(gatewayFilters);
        AnnotationAwareOrderComparator.sort(combined);
        if (logger.isDebugEnabled()) {
            logger.debug("Sorted gatewayFilterFactories: " + combined);
        }

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

这里面组装了所有使用到的过滤器，使用责任链的设计模式去实现调用，过滤器按照一定顺序排序，order值越小越先执行，

我们看RouteToRequestUrlFilter和ForwardRoutingFilter过滤器.

RouteToRequestUrlFilter：路由转换路由转换，把http://localhost:8888/order/test0 ---> lb://mall-order/order/test0

ForwardRoutingFilter:转发路由网关过滤器。其根据 forward:// 前缀( Scheme )过滤处理，将请求转发到当前网关实例本地接口。

RouteToRequestUrlFilter

public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        // 从exchange中取路由Route对象
        Route route = (Route)exchange.getAttribute(ServerWebExchangeUtils.GATEWAY_ROUTE_ATTR);
        if (route == null) {
            return chain.filter(exchange);
        } else {
            log.trace("RouteToRequestUrlFilter start");
            // 取当前请求uri：http://localhost:8888/order/test0
            URI uri = exchange.getRequest().getURI();
            boolean encoded = ServerWebExchangeUtils.containsEncodedParts(uri);
            // 路由对象中保存的uri，也就是我们在yml文件中配置的值：   lb://mall-order
            URI routeUri = route.getUri();
            if (hasAnotherScheme(routeUri)) {
                exchange.getAttributes().put(ServerWebExchangeUtils.GATEWAY_SCHEME_PREFIX_ATTR, routeUri.getScheme());
                routeUri = URI.create(routeUri.getSchemeSpecificPart());
            }

            if ("lb".equalsIgnoreCase(routeUri.getScheme()) && routeUri.getHost() == null) {
                throw new IllegalStateException("Invalid host: " + routeUri.toString());
            } else {
                // 转换结果为：lb://mall-order/order/test0
                URI mergedUrl = UriComponentsBuilder.fromUri(uri).scheme(routeUri.getScheme()).host(routeUri.getHost()).port(routeUri.getPort()).build(encoded).toUri();
                exchange.getAttributes().put(ServerWebExchangeUtils.GATEWAY_REQUEST_URL_ATTR, mergedUrl);
                return chain.filter(exchange);
            }
        }
    }

再往后就是通过NettyRoutingFilter发起远程调用

NettyRoutingFilter

SpringCloud在实现对后端服务远程调用是基于Netty发送Http请求实现，核心代码在NettyRoutingFilter.filter()中，其中核心代码为send()方法

public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        URI requestUrl = (URI)exchange.getRequiredAttribute(ServerWebExchangeUtils.GATEWAY_REQUEST_URL_ATTR);
        String scheme = requestUrl.getScheme();
        if (!ServerWebExchangeUtils.isAlreadyRouted(exchange) && ("http".equalsIgnoreCase(scheme) || "https".equalsIgnoreCase(scheme))) {
            ServerWebExchangeUtils.setAlreadyRouted(exchange);
            ServerHttpRequest request = exchange.getRequest();
            HttpMethod method = HttpMethod.valueOf(request.getMethodValue());
            String url = requestUrl.toASCIIString();
            HttpHeaders filtered = HttpHeadersFilter.filterRequest(this.getHeadersFilters(), exchange);
            DefaultHttpHeaders httpHeaders = new DefaultHttpHeaders();
            filtered.forEach(httpHeaders::set);
            boolean preserveHost = (Boolean)exchange.getAttributeOrDefault(ServerWebExchangeUtils.PRESERVE_HOST_HEADER_ATTRIBUTE, false);
            Route route = (Route)exchange.getAttribute(ServerWebExchangeUtils.GATEWAY_ROUTE_ATTR);
            Flux<HttpClientResponse> responseFlux = ((HttpClient.RequestSender)this.getHttpClient(route, exchange).headers((headers) -> {
                headers.add(httpHeaders);
                headers.remove("Host");
                if (preserveHost) {
                    String host = request.getHeaders().getFirst("Host");
                    headers.add("Host", host);
                }

            }).request(method).uri(url)).send((req, nettyOutbound) -> {
                if (log.isTraceEnabled()) {
                    nettyOutbound.withConnection((connection) -> {
                        log.trace("outbound route: " + connection.channel().id().asShortText() + ", inbound: " + exchange.getLogPrefix());
                    });
                }

                return nettyOutbound.send(request.getBody().map(this::getByteBuf));
            }).responseConnection((res, connection) -> {
                exchange.getAttributes().put(ServerWebExchangeUtils.CLIENT_RESPONSE_ATTR, res);
                exchange.getAttributes().put(ServerWebExchangeUtils.CLIENT_RESPONSE_CONN_ATTR, connection);
                ServerHttpResponse response = exchange.getResponse();
                HttpHeaders headers = new HttpHeaders();
                res.responseHeaders().forEach((entry) -> {
                    headers.add((String)entry.getKey(), (String)entry.getValue());
                });
                String contentTypeValue = headers.getFirst("Content-Type");
                if (StringUtils.hasLength(contentTypeValue)) {
                    exchange.getAttributes().put("original_response_content_type", contentTypeValue);
                }

                this.setResponseStatus(res, response);
                HttpHeaders filteredResponseHeaders = HttpHeadersFilter.filter(this.getHeadersFilters(), headers, exchange, Type.RESPONSE);
                if (!filteredResponseHeaders.containsKey("Transfer-Encoding") && filteredResponseHeaders.containsKey("Content-Length")) {
                    response.getHeaders().remove("Transfer-Encoding");
                }

                exchange.getAttributes().put(ServerWebExchangeUtils.CLIENT_RESPONSE_HEADER_NAMES, filteredResponseHeaders.keySet());
                response.getHeaders().addAll(filteredResponseHeaders);
                return Mono.just(res);
            });
            Duration responseTimeout = this.getResponseTimeout(route);
            if (responseTimeout != null) {
                responseFlux = responseFlux.timeout(responseTimeout, Mono.error(new TimeoutException("Response took longer than timeout: " + responseTimeout))).onErrorMap(TimeoutException.class, (th) -> {
                    return new ResponseStatusException(HttpStatus.GATEWAY_TIMEOUT, th.getMessage(), th);
                });
            }

            return responseFlux.then(chain.filter(exchange));
        } else {
            return chain.filter(exchange);
        }
    }

上面send方法最终会调用ChannelOperations#send()方法，而该方法其实是基于了Netty实现数据发送。

至此，一次Spring Cloud Gateway请求转发源码分析结束。