<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-gateway</artifactId>
</dependency>

  <dependencies>
    <dependency>
      <groupId>org.springframework.cloud</groupId>
      <artifactId>spring-cloud-starter</artifactId>
      <version>3.1.1</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework.cloud</groupId>
      <artifactId>spring-cloud-gateway-server</artifactId>
      <version>3.1.1</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-webflux</artifactId>
      <version>2.6.3</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework.cloud</groupId>
      <artifactId>spring-cloud-starter-loadbalancer</artifactId>
      <version>3.1.1</version>
      <scope>compile</scope>
      <optional>true</optional>
    </dependency>
  </dependencies>

@Configuration(proxyBeanMethods = false)
@ConditionalOnProperty(name = "spring.cloud.gateway.enabled", matchIfMissing = true)
@EnableConfigurationProperties
@AutoConfigureBefore({ HttpHandlerAutoConfiguration.class, WebFluxAutoConfiguration.class })
@AutoConfigureAfter({ GatewayReactiveLoadBalancerClientAutoConfiguration.class,
		GatewayClassPathWarningAutoConfiguration.class })
@ConditionalOnClass(DispatcherHandler.class)
public class GatewayAutoConfiguration {			

@Configuration(proxyBeanMethods = false)
@ConditionalOnWebApplication(type = ConditionalOnWebApplication.Type.REACTIVE)
@ConditionalOnClass(WebFluxConfigurer.class)
@ConditionalOnMissingBean({ WebFluxConfigurationSupport.class })
@AutoConfigureAfter({ ReactiveWebServerFactoryAutoConfiguration.class, CodecsAutoConfiguration.class,
		ReactiveMultipartAutoConfiguration.class, ValidationAutoConfiguration.class,
		WebSessionIdResolverAutoConfiguration.class })
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE + 10)
public class WebFluxAutoConfiguration {

@Configuration(proxyBeanMethods = false)
@EnableConfigurationProperties({ WebProperties.class, WebFluxProperties.class })
@Import({ EnableWebFluxConfiguration.class })
@Order(0)
public static class WebFluxConfig implements WebFluxConfigurer {

@Configuration(proxyBeanMethods = false)
@EnableConfigurationProperties({ WebProperties.class, ServerProperties.class })
public static class EnableWebFluxConfiguration extends DelegatingWebFluxConfiguration {

@Configuration(proxyBeanMethods = false)
public class DelegatingWebFluxConfiguration extends WebFluxConfigurationSupport {

@Bean
public DispatcherHandler webHandler() {
    //BeanName為webHandler
	return new DispatcherHandler();
}

public class DispatcherHandler implements WebHandler, PreFlightRequestHandler, ApplicationContextAware {<!--{cke_protected}{C}%3C!%2D%2D%20%2D%2D%3E-->

public interface WebHandler {
	/**
	 * Handle the web server exchange.
	 * @param exchange the current server exchange
	 * @return {@code Mono<Void>} to indicate when request handling is complete
	 */
	Mono<Void> handle(ServerWebExchange exchange);
}

public class HttpWebHandlerAdapter extends WebHandlerDecorator implements HttpHandler {<!--{cke_protected}{C}%3C!%2D%2D%20%2D%2D%3E-->

public class WebHandlerDecorator implements WebHandler {
	private final WebHandler delegate;
	/**
	 * Return the wrapped delegate.
	 */
	public WebHandler getDelegate() {
		return this.delegate;
	}

@Configuration(proxyBeanMethods = false)
@ConditionalOnClass({ DispatcherHandler.class, HttpHandler.class })
@ConditionalOnWebApplication(type = ConditionalOnWebApplication.Type.REACTIVE)
@ConditionalOnMissingBean(HttpHandler.class)
@AutoConfigureAfter({ WebFluxAutoConfiguration.class })
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE + 10)
public class HttpHandlerAutoConfiguration {
	@Configuration(proxyBeanMethods = false)
	public static class AnnotationConfig {
		private final ApplicationContext applicationContext;
		public AnnotationConfig(ApplicationContext applicationContext) {
			this.applicationContext = applicationContext;
		}
		@Bean
		public HttpHandler httpHandler(ObjectProvider<WebFluxProperties> propsProvider) {
			HttpHandler httpHandler = WebHttpHandlerBuilder.applicationContext(this.applicationContext).build();
			WebFluxProperties properties = propsProvider.getIfAvailable();
			if (properties != null && StringUtils.hasText(properties.getBasePath())) {
				Map<String, HttpHandler> handlersMap = Collections.singletonMap(properties.getBasePath(), httpHandler);
				return new ContextPathCompositeHandler(handlersMap);
			}
			return httpHandler;
		}
	}
}

	public static WebHttpHandlerBuilder applicationContext(ApplicationContext context) {
		WebHttpHandlerBuilder builder = new WebHttpHandlerBuilder(
				context.getBean(WEB_HANDLER_BEAN_NAME, WebHandler.class), context);
	    ......
		return builder;		

/** Well-known name for the target WebHandler in the bean factory. */
public static final String WEB_HANDLER_BEAN_NAME = "webHandler";

	private WebHttpHandlerBuilder(WebHandler webHandler, @Nullable ApplicationContext applicationContext) {
		Assert.notNull(webHandler, "WebHandler must not be null");
		this.webHandler = webHandler;
		this.applicationContext = applicationContext;
	}

	public HttpHandler build() {
		WebHandler decorated = new FilteringWebHandler(this.webHandler, this.filters);
		decorated = new ExceptionHandlingWebHandler(decorated,  this.exceptionHandlers);
		HttpWebHandlerAdapter adapted = new HttpWebHandlerAdapter(decorated);
		if (this.sessionManager != null) {
			adapted.setSessionManager(this.sessionManager);
		}
		if (this.codecConfigurer != null) {
			adapted.setCodecConfigurer(this.codecConfigurer);
		}
		if (this.localeContextResolver != null) {
			adapted.setLocaleContextResolver(this.localeContextResolver);
		}
		if (this.forwardedHeaderTransformer != null) {
			adapted.setForwardedHeaderTransformer(this.forwardedHeaderTransformer);
		}
		if (this.applicationContext != null) {
			adapted.setApplicationContext(this.applicationContext);
		}
		adapted.afterPropertiesSet();
		return (this.httpHandlerDecorator != null ? this.httpHandlerDecorator.apply(adapted) : adapted);
	}

//這里的webHandler就是DispatcherHandler對象
WebHandler decorated = new FilteringWebHandler(this.webHandler, this.filters);

	public FilteringWebHandler(WebHandler handler, List<WebFilter> filters) {
		super(handler);
		this.chain = new DefaultWebFilterChain(handler, filters);
	}

private final WebHandler delegate;	
public WebHandlerDecorator(WebHandler delegate) {
		Assert.notNull(delegate, "'delegate' must not be null");
		this.delegate = delegate;
	}

//此時入參中的decorated是FilteringWebHandler對象
decorated = new ExceptionHandlingWebHandler(decorated,  this.exceptionHandlers);

	public ExceptionHandlingWebHandler(WebHandler delegate, List<WebExceptionHandler> handlers) {
		super(delegate);
		List<WebExceptionHandler> handlersToUse = new ArrayList<>();
		handlersToUse.add(new CheckpointInsertingHandler());
		handlersToUse.addAll(handlers);
		this.exceptionHandlers = Collections.unmodifiableList(handlersToUse);
	}

HttpWebHandlerAdapter adapted = new HttpWebHandlerAdapter(decorated);

	public HttpWebHandlerAdapter(WebHandler delegate) {
		super(delegate);
	}