spring cloud gateway源码分析，一个请求进来的默认处理流程

1.前言

spring cloud gateway的基本组成和作用就不细赘述，此篇适合对此有一定了解的人阅读。
spring cloud gateway版本: Hoxton.SR1

spring cloud gateway的配置使用yml配置：

server:
  port: 9527y

#根据微服务名称进行动态路由的配置
spring:
  application:
    name: cloud-gateway
  cloud:
    gateway:
      discovery:
        locator:
          enabled: true                     #开启从注册中心动态创建路由的功能，利用微服务名称进行路由
      routes:                
        - id: config-client
          uri: lb://config-client
          predicates:
            - Path=/config/**
          filters:
            - RewritePath=/config/?(?>.*),/config/v1/$\{segment}

2. 流程图

先看一张官网文档给的图，此图大概描述了请求的处理原理，各个组件大致的位置。

3.源码剖析

http底层处理是基于netty，netty是一个高性能异步事件驱动的通讯框架，对于netty的处理流程可以查阅其源码。netty读取完数据经过pipeline管道处理后，最终调用到reactor.netty.http.server.HttpServerHandle#onStateChange方法。然后经过层层方法调用到核心类org.springframework.web.reactive.DispatcherHandler#handle

	public Mono<Void> handle(ServerWebExchange exchange) {
		if (this.handlerMappings == null) {
		    //没有合适的handler返回失败
			return createNotFoundError();
		}
		return Flux.fromIterable(this.handlerMappings)
		        //mapping.getHandler是关键方法，根据handlerMapping找到对应的handler
				.concatMap(mapping -> mapping.getHandler(exchange))
				.next()
				.switchIfEmpty(createNotFoundError())
				//invokeHandler是关键方法，调用处理逻辑
				.flatMap(handler -> invokeHandler(exchange, handler))
				//处理结果，写出
				.flatMap(result -> handleResult(exchange, result));
	}

	private Mono<HandlerResult> invokeHandler(ServerWebExchange exchange, Object handler) {
		if (this.handlerAdapters != null) {
			for (HandlerAdapter handlerAdapter : this.handlerAdapters) {
				if (handlerAdapter.supports(handler)) {
					//查找合适的handlerAdapter处理，默认会调用到SimpleHandlerAdapter#handle
					return handlerAdapter.handle(exchange, handler);
				}
			}
		}
		return Mono.error(new IllegalStateException("No HandlerAdapter: " + handler));
	}

	private Mono<Void> handleResult(ServerWebExchange exchange, HandlerResult result) {
		return getResultHandler(result).handleResult(exchange, result)
				.checkpoint("Handler " + result.getHandler() + " [DispatcherHandler]")
				.onErrorResume(ex ->
						result.applyExceptionHandler(ex).flatMap(exResult -> {
							String text = "Exception handler " + exResult.getHandler() +
									", error=\"" + ex.getMessage() + "\" [DispatcherHandler]";
							return getResultHandler(exResult).handleResult(exchange, exResult).checkpoint(text);
						}));
	}

	private HandlerResultHandler getResultHandler(HandlerResult handlerResult) {
		if (this.resultHandlers != null) {
			for (HandlerResultHandler resultHandler : this.resultHandlers) {
				if (resultHandler.supports(handlerResult)) {
					return resultHandler;
				}
			}
		}
		throw new IllegalStateException("No HandlerResultHandler for " + handlerResult.getReturnValue());
	}

handlerMappings的注入类看下图，最后通过RoutePredicateHandlerMapping找到合适的处理类。handlerMappings中的其他几种Mapping方式，是别的策略或者配置时会用到，可以思考是怎么用的。

先来看看mapping.getHandler的处理逻辑，默认会调用到org.springframework.web.reactive.handler.AbstractHandlerMapping#getHandler

	@Override
	public Mono<Object> getHandler(ServerWebExchange exchange) {
		//getHandlerInternal，根据exchange真正去查找合适的处理handler，根据上面解释，
		//getHandlerInternal调用到RoutePredicateHandlerMapping类中去
		return getHandlerInternal(exchange).map(handler -> {
			if (logger.isDebugEnabled()) {
				logger.debug(exchange.getLogPrefix() + "Mapped to " + handler);
			}
			//跨域处理
			if (hasCorsConfigurationSource(handler)) {
				ServerHttpRequest request = exchange.getRequest();
				CorsConfiguration config = (this.corsConfigurationSource != null ? this.corsConfigurationSource.getCorsConfiguration(exchange) : null);
				CorsConfiguration handlerConfig = getCorsConfiguration(handler, exchange);
				config = (config != null ? config.combine(handlerConfig) : handlerConfig);
				if (!this.corsProcessor.process(config, exchange) || CorsUtils.isPreFlightRequest(request)) {
					return REQUEST_HANDLED_HANDLER;
				}
			}
			return handler;
		});
	}

现在调用到了org.springframework.cloud.gateway.handler.RoutePredicateHandlerMapping#getHandlerInternal方法中，这里一个关键点就来了。Predicates断言，是路由配置的关键，根据predicates的结果，满足的话就会转发请求到对应的Router配置的uri上。

	@Override
	protected Mono<?> getHandlerInternal(ServerWebExchange exchange) {
		// don't handle requests on management port if set and different than server port
		if (this.managementPortType == DIFFERENT && this.managementPort != null
				&& exchange.getRequest().getURI().getPort() == this.managementPort) {
			return Mono.empty();
		}
		exchange.getAttributes().put(GATEWAY_HANDLER_MAPPER_ATTR, getSimpleName());
		
		//lookupRoute(exchange)去查找满足断言条件的路由Router
		return lookupRoute(exchange)
				// 满足的router会被组装到exchange中，然后返回webHandler
				.flatMap((Function<Route, Mono<?>>) r -> {
					exchange.getAttributes().remove(GATEWAY_PREDICATE_ROUTE_ATTR);
					if (logger.isDebugEnabled()) {
						logger.debug(
								"Mapping [" + getExchangeDesc(exchange) + "] to " + r);
					}

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

	protected Mono<Route> lookupRoute(ServerWebExchange exchange) {
		return this.routeLocator.getRoutes()
				// 遍历所有的Router，此行r.getPredicate().apply(exchange)是验证是否满足断言要求，
				// 满足的Router会被返回
				.concatMap(route -> Mono.just(route).filterWhen(r -> {
					// add the current route we are testing
					exchange.getAttributes().put(GATEWAY_PREDICATE_ROUTE_ATTR, r.getId());
					return r.getPredicate().apply(exchange);
				})
						// instead of immediately stopping main flux due to error, log and
						// swallow it
						.doOnError(e -> logger.error(
								"Error applying predicate for route: " + route.getId(),
								e))
						.onErrorResume(e -> Mono.empty()))
				// .defaultIfEmpty() put a static Route not found
				// or .switchIfEmpty()
				// .switchIfEmpty(Mono.empty().log("noroute"))
				.next()
				// TODO: error handling
				.map(route -> {
					if (logger.isDebugEnabled()) {
						logger.debug("Route matched: " + route.getId());
					}
					validateRoute(route, exchange);
					return route;
				});

		/*
		 * TODO: trace logging if (logger.isTraceEnabled()) {
		 * logger.trace("RouteDefinition did not match: " + routeDefinition.getId()); }
		 */
	}

apply调用进入org.springframework.cloud.gateway.handler.AsyncPredicate.DefaultAsyncPredicate#apply方法，查看delegate.test(t)调用的实现类，可以发现所有断言的调用，此处根据我们配置的断言规则调用对应的断言，返回Boolean。

通过断言拿到对应的handler后回到DispatcherHandler#handle方法接下来调用invokeHandler(exchange, handler)

//这个handler 是 FilteringWehHandler
private Mono<HandlerResult> invokeHandler(ServerWebExchange exchange, Object handler) {
		if (this.handlerAdapters != null) {
			for (HandlerAdapter handlerAdapter : this.handlerAdapters) {
				if (handlerAdapter.supports(handler)) {
					//使用SimpleHandlerAdapter来处理
					return handlerAdapter.handle(exchange, handler);
				}
			}
		}
		return Mono.error(new IllegalStateException("No HandlerAdapter: " + handler));
	}

然后调用org.springframework.web.reactive.result.SimpleHandlerAdapter#handle

	@Override
	public Mono<HandlerResult> handle(ServerWebExchange exchange, Object handler) {
		WebHandler webHandler = (WebHandler) handler;
		//handler是FilteringWehHandler，所以调用到FilteringWehHandler.handle
		Mono<Void> mono = webHandler.handle(exchange);
		return mono.then(Mono.empty());
	}

org.springframework.cloud.gateway.handler.FilteringWebHandler#handle

	@Override
	public Mono<Void> handle(ServerWebExchange exchange) {
		Route route = exchange.getRequiredAttribute(GATEWAY_ROUTE_ATTR);
		//取出之前匹配的Router，取出filters，如果配置了的话
		List<GatewayFilter> gatewayFilters = route.getFilters();
		//GatewayFilter和globalFilter合并，并按order排序
		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);
		}
		//进入过滤链调用filters
		return new DefaultGatewayFilterChain(combined).filter(exchange);
	}


		@Override
		public Mono<Void> filter(ServerWebExchange exchange) {
			return Mono.defer(() -> {
				if (this.index < filters.size()) {
					GatewayFilter filter = filters.get(this.index);
					//index 每次+1，设置到chain中，传递到下一次filter，下一次filter时取就是next的filter
					DefaultGatewayFilterChain chain = new DefaultGatewayFilterChain(this,
							this.index + 1);
					//filter链执行
					return filter.filter(exchange, chain);
				}
				else {
					return Mono.empty(); // complete
				}
			});
		}

filter链很重要，是spring cloud gateway的扩展点，可以做扩展逻辑，比如权限校验，登录认证，日志等。默认情况下的filter链如下，需要关注一下LoadBalancerClientFilter和NettyRoutingFilter

org.springframework.cloud.gateway.filter.LoadBalancerClientFilter#filter

	@Override
	@SuppressWarnings("Duplicates")
	public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
		URI url = exchange.getAttribute(GATEWAY_REQUEST_URL_ATTR);
		//使用协议 http还是lb
		String schemePrefix = exchange.getAttribute(GATEWAY_SCHEME_PREFIX_ATTR);
		if (url == null
				|| (!"lb".equals(url.getScheme()) && !"lb".equals(schemePrefix))) {
			return chain.filter(exchange);
		}
		// 保存原始请求url
		addOriginalRequestUrl(exchange, url);

		if (log.isTraceEnabled()) {
			log.trace("LoadBalancerClientFilter url before: " + url);
		}
		//根据注册中心的信息，使用负载均衡算法，找一个可用的服务
		final ServiceInstance instance = choose(exchange);

		if (instance == null) {
			throw NotFoundException.create(properties.isUse404(),
					"Unable to find instance for " + url.getHost());
		}

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

		// if the `lb:` mechanism was used, use `` as the default,
		// if the loadbalancer doesn't provide one.
		String overrideScheme = instance.isSecure() ? "https" : "http";
		if (schemePrefix != null) {
			overrideScheme = url.getScheme();
		}
		//替换成真实服务器的地址，后续调用使用
		URI requestUrl = loadBalancer.reconstructURI(
				new DelegatingServiceInstance(instance, overrideScheme), uri);

		if (log.isTraceEnabled()) {
			log.trace("LoadBalancerClientFilter url chosen: " + requestUrl);
		}

		exchange.getAttributes().put(GATEWAY_REQUEST_URL_ATTR, requestUrl);
		return chain.filter(exchange);
	}

org.springframework.cloud.gateway.filter.NettyRoutingFilter#filter 处理http和https请求的发送

	@Override
	@SuppressWarnings("Duplicates")
	public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
		URI requestUrl = exchange.getRequiredAttribute(GATEWAY_REQUEST_URL_ATTR);

		String scheme = requestUrl.getScheme();
		if (isAlreadyRouted(exchange)
				|| (!"http".equals(scheme) && !"https".equals(scheme))) {
			return chain.filter(exchange);
		}
		setAlreadyRouted(exchange);

		ServerHttpRequest request = exchange.getRequest();

		final HttpMethod method = HttpMethod.valueOf(request.getMethodValue());
		final String url = requestUrl.toASCIIString();

		HttpHeaders filtered = filterRequest(getHeadersFilters(), exchange);

		final DefaultHttpHeaders httpHeaders = new DefaultHttpHeaders();
		filtered.forEach(httpHeaders::set);

		boolean preserveHost = exchange
				.getAttributeOrDefault(PRESERVE_HOST_HEADER_ATTRIBUTE, false);
		Route route = exchange.getAttribute(GATEWAY_ROUTE_ATTR);
		//发送请求
		Flux<HttpClientResponse> responseFlux = httpClientWithTimeoutFrom(route)
				.headers(headers -> {
					headers.add(httpHeaders);
					// Will either be set below, or later by Netty
					headers.remove(HttpHeaders.HOST);
					if (preserveHost) {
						String host = request.getHeaders().getFirst(HttpHeaders.HOST);
						headers.add(HttpHeaders.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(dataBuffer -> ((NettyDataBuffer) dataBuffer)
									.getNativeBuffer()));
				}).responseConnection((res, connection) -> {

					// Defer committing the response until all route filters have run
					// Put client response as ServerWebExchange attribute and write
					// response later NettyWriteResponseFilter
					exchange.getAttributes().put(CLIENT_RESPONSE_ATTR, res);
					exchange.getAttributes().put(CLIENT_RESPONSE_CONN_ATTR, connection);

					ServerHttpResponse response = exchange.getResponse();
					// put headers and status so filters can modify the response
					HttpHeaders headers = new HttpHeaders();

					res.responseHeaders().forEach(
							entry -> headers.add(entry.getKey(), entry.getValue()));

					String contentTypeValue = headers.getFirst(HttpHeaders.CONTENT_TYPE);
					if (StringUtils.hasLength(contentTypeValue)) {
						exchange.getAttributes().put(ORIGINAL_RESPONSE_CONTENT_TYPE_ATTR,
								contentTypeValue);
					}

					setResponseStatus(res, response);

					// make sure headers filters run after setting status so it is
					// available in response
					HttpHeaders filteredResponseHeaders = HttpHeadersFilter.filter(
							getHeadersFilters(), headers, exchange, Type.RESPONSE);

					if (!filteredResponseHeaders
							.containsKey(HttpHeaders.TRANSFER_ENCODING)
							&& filteredResponseHeaders
									.containsKey(HttpHeaders.CONTENT_LENGTH)) {
						// It is not valid to have both the transfer-encoding header and
						// the content-length header.
						// Remove the transfer-encoding header in the response if the
						// content-length header is present.
						response.getHeaders().remove(HttpHeaders.TRANSFER_ENCODING);
					}

					exchange.getAttributes().put(CLIENT_RESPONSE_HEADER_NAMES,
							filteredResponseHeaders.keySet());

					response.getHeaders().putAll(filteredResponseHeaders);

					return Mono.just(res);
				});

		Duration responseTimeout = getResponseTimeout(route);
		if (responseTimeout != null) {
			responseFlux = responseFlux
					.timeout(responseTimeout, Mono.error(new TimeoutException(
							"Response took longer than timeout: " + responseTimeout)))
					.onErrorMap(TimeoutException.class,
							th -> new ResponseStatusException(HttpStatus.GATEWAY_TIMEOUT,
									th.getMessage(), th));
		}

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

以上，就是请求进来的处理过程。