1.启动加载流程

spring-cloud-gateway-sample项目启动，
需要添加注释@EnableAutoConfiguration默认加载spring-could-gateway-core里的spring-factories里的配置默认启动类
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.cloud.gateway.config.GatewayClassPathWarningAutoConfiguration,\
org.springframework.cloud.gateway.config.GatewayAutoConfiguration,\
org.springframework.cloud.gateway.config.GatewayLoadBalancerClientAutoConfiguration,\
org.springframework.cloud.gateway.config.GatewayRedisAutoConfiguration,\
org.springframework.cloud.gateway.discovery.GatewayDiscoveryClientAutoConfiguration
在以上自动加载配置的类中，GatewayClassPathWarningAutoConfiguration，GatewayRedisAutoConfiguration, GatewayDiscovRibbonAutoConfiguratioeryClientAutoConfiguration
都是在GatewayAutoConfiguration加载之后再加载，GatewayLoadBalancerClientAutoConfiguration加载之后记载RibbonAutoConfiguration类。
下面看下GatewayAutoConfiguration类的加载逻辑
  注解如下：
  @ConditionalOnProperty(name = "spring.cloud.gateway.enabled", matchIfMissing = true) 
  应用环境中需要有spring.cloud.gateway.enabled配置，但不是必须的
  
  @EnableConfigurationProperties
  应用环境中有带有@ConfigurationProperties的bean提供有效的支持
  
  @AutoConfigureBefore(HttpHandlerAutoConfiguration.class)
  加载GatewayAutoConfiguration类前先加载HttpHandlerAutoConfiguration的bean这里用到webflux
  
  @AutoConfigureAfter({GatewayLoadBalancerClientAutoConfiguration.class, GatewayClassPathWarningAutoConfiguration.class})
  加载完GatewayAutoConfiguration类之后初始化GatewayLoadBalancerClientAutoConfiguration，GatewayClassPathWarningAutoConfiguration类
  
  @ConditionalOnClass(DispatcherHandler.class)
  类加载中存在DispatcherHandler.class,才初始化这个bean
  
  代码如下：
  1.初始化NettyConfiguration这个bean，同时检查类路径中存在HttpClient.class,
    并在内部初始化HttpClient,Consumer,HttpClientProperties，NettyRoutingFilter，NettyWriteResponseFilter，ReactorNettyWebSocketClient
    
  2.初始化RouteLocatorBuilder，PropertiesRouteDefinitionLocator(没有则创建)，
        InMemoryRouteDefinitionRepository(RouteDefinitionRepository(没有则创建))，RouteDefinitionLocator,
        RouteLocator(实际初始化bean对象为RouteDefinitionRouteLocator,CachingRouteLocator(首选bean)),RouteRefreshListener,
        FilteringWebHandler,RoutePredicateHandlerMapping
        //ConfigurationProperty beans
        GatewayProperties,SecureHeadersProperties
        // HttpHeaderFilter beans
        ForwardedHeadersFilter,RemoveHopByHopHeadersFilter,XForwardedHeadersFilter
        // GlobalFilter beans
        AdaptCachedBodyGlobalFilter,RouteToRequestUrlFilter,ForwardRoutingFilter,ForwardPathFilter,WebsocketRoutingFilter,WeightCalculatorWebFilter
        // Predicate Factory beans
        // 1.路由Filter Facatory
        AfterRoutePredicateFactory,BeforeRoutePredicateFactory,BetweenRoutePredicateFactory,CookieRoutePredicateFactory,HeaderRoutePredicateFactory,
        HostRoutePredicateFactory,MethodRoutePredicateFactory,PathRoutePredicateFactory,QueryRoutePredicateFactory,ReadBodyPredicateFactory,
        RemoteAddrRoutePredicateFactory,WeightRoutePredicateFactory,
        // 2.WebFilter Facatory
        AddRequestHeaderGatewayFilterFactory,AddRequestParameterGatewayFilterFactory,AddResponseHeaderGatewayFilterFactory
        ModifyRequestBodyGatewayFilterFactory,ModifyResponseBodyGatewayFilterFactory,PrefixPathGatewayFilterFactory,
        PreserveHostHeaderGatewayFilterFactory,RedirectToGatewayFilterFactory,RemoveRequestHeaderGatewayFilterFactory,
        RemoveResponseHeaderGatewayFilterFactory,SetPathGatewayFilterFactory,SecureHeadersGatewayFilterFactory,
        SetRequestHeaderGatewayFilterFactory,SetResponseHeaderGatewayFilterFactory,SetStatusGatewayFilterFactory,
        SaveSessionGatewayFilterFactory,StripPrefixGatewayFilterFactory,RequestHeaderToRequestUriGatewayFilterFactory
        // 3.熔断，限流，重写，重试等Filter Facatory
        // 初始化熔断器配置，熔断工厂filter
        @Configuration
        @ConditionalOnClass({HystrixObservableCommand.class, RxReactiveStreams.class})
        protected static class HystrixConfiguration {
          @Bean
          public HystrixGatewayFilterFactory hystrixGatewayFilterFactory(DispatcherHandler dispatcherHandler) {
            return new HystrixGatewayFilterFactory(dispatcherHandler);
          }
        }
        
        // 初始化限流工厂filter
        @Bean(name = PrincipalNameKeyResolver.BEAN_NAME)
        @ConditionalOnBean(RateLimiter.class)
        public PrincipalNameKeyResolver principalNameKeyResolver() {
          return new PrincipalNameKeyResolver();
        }

        @Bean
        @ConditionalOnBean({RateLimiter.class, KeyResolver.class})
        public RequestRateLimiterGatewayFilterFactory requestRateLimiterGatewayFilterFactory(RateLimiter rateLimiter, PrincipalNameKeyResolver resolver) {
          return new RequestRateLimiterGatewayFilterFactory(rateLimiter, resolver);
        }
        RewritePathGatewayFilterFactory,RetryGatewayFilterFactory,
        
        //初始化spring-boot执行器 用于心跳检测
        @Configuration
        @ConditionalOnClass(Health.class)
        protected static class GatewayActuatorConfiguration {

          @Bean
          @ConditionalOnEnabledEndpoint
          public GatewayControllerEndpoint gatewayControllerEndpoint(RouteDefinitionLocator routeDefinitionLocator, List<GlobalFilter> globalFilters,
                                      List<GatewayFilterFactory> GatewayFilters, RouteDefinitionWriter routeDefinitionWriter,
                                      RouteLocator routeLocator) {
            return new GatewayControllerEndpoint(routeDefinitionLocator, globalFilters, GatewayFilters, routeDefinitionWriter, routeLocator);
          }
        }