1 @EnableFeignClients

这个注解标注在springboot的启动类上,作用是开启feign接口扫描
FeignClientsRegistrar.registerFeignClients()扫描被@FeignClient标识的接口生成代理类

    public void registerFeignClients(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {LinkedHashSet<BeanDefinition> candidateComponents = new LinkedHashSet();Map<String, Object> attrs = metadata.getAnnotationAttributes(EnableFeignClients.class.getName());new AnnotationTypeFilter(FeignClient.class);Class<?>[] clients = attrs == null ? null : (Class[])((Class[])attrs.get("clients"));if (clients != null && clients.length != 0) {Class[] var13 = clients;int var15 = clients.length;for(int var17 = 0; var17 < var15; ++var17) {Class<?> clazz = var13[var17];candidateComponents.add(new AnnotatedGenericBeanDefinition(clazz));}} else {ClassPathScanningCandidateComponentProvider scanner = this.getScanner();scanner.setResourceLoader(this.resourceLoader);scanner.addIncludeFilter(new AnnotationTypeFilter(FeignClient.class));Set<String> basePackages = this.getBasePackages(metadata);Iterator var9 = basePackages.iterator();while(var9.hasNext()) {String basePackage = (String)var9.next();candidateComponents.addAll(scanner.findCandidateComponents(basePackage));}}Iterator var14 = candidateComponents.iterator();while(var14.hasNext()) {BeanDefinition candidateComponent = (BeanDefinition)var14.next();if (candidateComponent instanceof AnnotatedBeanDefinition) {AnnotatedBeanDefinition beanDefinition = (AnnotatedBeanDefinition)candidateComponent;AnnotationMetadata annotationMetadata = beanDefinition.getMetadata();Assert.isTrue(annotationMetadata.isInterface(), "@FeignClient can only be specified on an interface");Map<String, Object> attributes = annotationMetadata.getAnnotationAttributes(FeignClient.class.getCanonicalName());String name = this.getClientName(attributes);this.registerClientConfiguration(registry, name, attributes.get("configuration"));this.registerFeignClient(registry, annotationMetadata, attributes);}}}

2 根据接口上的注解创建RequestTemplate

当controller调用feign代理类时，代理类会调用SynchronousMethodHandler.invoke()创建RequestTemplate(url、requestMethod、body)

3 发送请求

接着通过RequestTemplate创建Request，然后client(HttpClient、OkHttp、URLConnection)使用Request发送请求