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openfeign是通过FeignClientFactoryBean生成动态代理对象的方式实现http客户端无感调用，可以做到像定义接口一样写http客户端调用代码。
配置Feign接口后，我们通常会在SpringBoot项目启动类上标记@EnableFeignClients，这个是生成动态代理对象的入口。

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(FeignClientsRegistrar.class)
public @interface EnableFeignClients {/*** basePackages 配置的别称*/String[] value() default {};/*** 扫描包路径下有@FeignClient注解的接口*/String[] basePackages() default {};/*** 扫描类对应包下有@FeignClient注解的接口*/Class<?>[] basePackageClasses() default {};/*** 默认配置*/Class<?>[] defaultConfiguration() default {};/*** 直接指定标记了@FeignClient的类*/Class<?>[] clients() default {};}

@EnableFeignClients里面的配置主要是为了找到@FeignClient接口，正常情况下可指定basePackages路径，或者直接不加，会直接扫描启动类及其对应包下面的所有类，去找到所有标记了@FiegnClient的接口。
这里最关键的是@Import(FeignClientsRegistrar.class)引入了FeignClientsRegistrar类。
首先看下FeignClientsRegistrar实现的接口ImportBeanDefinitionRegistrar，Spring IOC在创建bean实例的时候
会调ImportBeanDefinitionRegistrar.registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry)方法，这个方法里面是注册默认配置和注册FeignClient的逻辑。

class FeignClientsRegistrar implements ImportBeanDefinitionRegistrar, ResourceLoaderAware, EnvironmentAware {...@Overridepublic void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {// 注册默认配置registerDefaultConfiguration(metadata, registry);// 注册FeignClientregisterFeignClients(metadata, registry);}
}

继续看registerFeignClients(metadata, registry)方法，前面主要是通过@EnableFeignClients注解配置找到所有的Feign接口生成BeanDefinition，最后遍历根据BeanDefinition一个个注册FeignClient

public void registerFeignClients(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {// 通过@EnableFeignClients的配置找到对应的Feign接口并生成对应的BeanDefinitionLinkedHashSet<BeanDefinition> candidateComponents = new LinkedHashSet<>();Map<String, Object> attrs = metadata.getAnnotationAttributes(EnableFeignClients.class.getName());final Class<?>[] clients = attrs == null ? null : (Class<?>[]) attrs.get("clients");if (clients == null || clients.length == 0) {ClassPathScanningCandidateComponentProvider scanner = getScanner();scanner.setResourceLoader(this.resourceLoader);scanner.addIncludeFilter(new AnnotationTypeFilter(FeignClient.class));Set<String> basePackages = getBasePackages(metadata);for (String basePackage : basePackages) {candidateComponents.addAll(scanner.findCandidateComponents(basePackage));}}else {for (Class<?> clazz : clients) {candidateComponents.add(new AnnotatedGenericBeanDefinition(clazz));}}for (BeanDefinition candidateComponent : candidateComponents) {if (candidateComponent instanceof AnnotatedBeanDefinition) {// verify annotated class is an interfaceAnnotatedBeanDefinition 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 = getClientName(attributes);// 注册客户端配置registerClientConfiguration(registry, name, attributes.get("configuration"));// 注册FeignClientregisterFeignClient(registry, annotationMetadata, attributes);}}
}

点进去registerFeignClient(registry, annotationMetadata, attributes)，这个方法才是真正的注册逻辑。
可以看到方法里面主要是创建了一个FeignClientFactoryBean工厂bean，最终通过getObject()生成bean实例。

private void registerFeignClient(BeanDefinitionRegistry registry, AnnotationMetadata annotationMetadata,Map<String, Object> attributes) {String className = annotationMetadata.getClassName();Class clazz = ClassUtils.resolveClassName(className, null);ConfigurableBeanFactory beanFactory = registry instanceof ConfigurableBeanFactory? (ConfigurableBeanFactory) registry : null;String contextId = getContextId(beanFactory, attributes);String name = getName(attributes);FeignClientFactoryBean factoryBean = new FeignClientFactoryBean();factoryBean.setBeanFactory(beanFactory);factoryBean.setName(name);factoryBean.setContextId(contextId);factoryBean.setType(clazz);factoryBean.setRefreshableClient(isClientRefreshEnabled());BeanDefinitionBuilder definition = BeanDefinitionBuilder.genericBeanDefinition(clazz, () -> {factoryBean.setUrl(getUrl(beanFactory, attributes));factoryBean.setPath(getPath(beanFactory, attributes));factoryBean.setDecode404(Boolean.parseBoolean(String.valueOf(attributes.get("decode404"))));Object fallback = attributes.get("fallback");if (fallback != null) {factoryBean.setFallback(fallback instanceof Class ? (Class<?>) fallback: ClassUtils.resolveClassName(fallback.toString(), null));}Object fallbackFactory = attributes.get("fallbackFactory");if (fallbackFactory != null) {factoryBean.setFallbackFactory(fallbackFactory instanceof Class ? (Class<?>) fallbackFactory: ClassUtils.resolveClassName(fallbackFactory.toString(), null));}// 生成bean实例return factoryBean.getObject();});definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);definition.setLazyInit(true);validate(attributes);AbstractBeanDefinition beanDefinition = definition.getBeanDefinition();beanDefinition.setAttribute(FactoryBean.OBJECT_TYPE_ATTRIBUTE, className);beanDefinition.setAttribute("feignClientsRegistrarFactoryBean", factoryBean);// has a default, won't be nullboolean primary = (Boolean) attributes.get("primary");beanDefinition.setPrimary(primary);String[] qualifiers = getQualifiers(attributes);if (ObjectUtils.isEmpty(qualifiers)) {qualifiers = new String[] { contextId + "FeignClient" };}BeanDefinitionHolder holder = new BeanDefinitionHolder(beanDefinition, className, qualifiers);BeanDefinitionReaderUtils.registerBeanDefinition(holder, registry);registerOptionsBeanDefinition(registry, contextId);
}

进入FeignClientFactoryBean的getObject方法，里面又继续调了getTarget方法。这里有两种情况，一种是指定了url，则会通过给的url封装请求，
如果没有则从注册中心获取服务地址，这里继续看loadBalance(builder, context, new HardCodedTarget<>(type, name, url));

public Object getObject() {return getTarget();
}<T> T getTarget() {FeignContext context = beanFactory != null ? beanFactory.getBean(FeignContext.class): applicationContext.getBean(FeignContext.class);Feign.Builder builder = feign(context);// 如果没有指定具体的url，则通过负载均衡从注册中心获取对应的服务地址信息if (!StringUtils.hasText(url)) {if (LOG.isInfoEnabled()) {LOG.info("For '" + name + "' URL not provided. Will try picking an instance via load-balancing.");}if (!name.startsWith("http")) {url = "http://" + name;}else {url = name;}url += cleanPath();return (T) loadBalance(builder, context, new HardCodedTarget<>(type, name, url));}...
}

继续loadBalance方法解析，这里主要是从FeignContext获取到Client实例和Targeter实例，debug的话这两个实例分别对应的类是
Client -> TraceRetryableFeignBlockingLoadBalancerClient
Targeter -> DefaultTargeter

这两个类都很关键

1. TraceRetryableFeignBlockingLoadBalancerClient是后续调用feign接口时通过RetryableFeignBlockingLoadBalancerClient获取对应的负载均衡服务实例
2. DefaultTargeter则是生成动态代理对象

protected <T> T loadBalance(Feign.Builder builder, FeignContext context, HardCodedTarget<T> target) {Client client = getOptional(context, Client.class);if (client != null) {builder.client(client);applyBuildCustomizers(context, builder);Targeter targeter = get(context, Targeter.class);// 点进去看是如何生成代理对象return targeter.target(this, builder, context, target);}throw new IllegalStateException("No Feign Client for loadBalancing defined. Did you forget to include spring-cloud-starter-loadbalancer?");
}

继续看动态代理对象生成的逻辑，DefaultTargeter的target方法里面，主要调用了feign.target(target)

class DefaultTargeter implements Targeter {@Overridepublic <T> T target(FeignClientFactoryBean factory, Feign.Builder feign, FeignContext context,Target.HardCodedTarget<T> target) {return feign.target(target);}}

feign.target先调用了build方法，然后再调newInstance方法。先看builde方法的逻辑，核心是创建了SynchronousMethodHandler.Factory对象，并且返回了ReflectiveFeign对象。
这个工厂对象主要是用来创建SynchronousMethodHandler的，而SynchronousMethodHandler就是feign接口每个方法都会封装一个对应的MethodHandler，里面包含了封装请求，发送请求，解析响应的所有流程。

public <T> T target(Target<T> target) {return build().newInstance(target);
}public Feign build() {super.enrich();SynchronousMethodHandler.Factory synchronousMethodHandlerFactory =new SynchronousMethodHandler.Factory(client, retryer, requestInterceptors,responseInterceptor, logger, logLevel, dismiss404, closeAfterDecode,propagationPolicy, forceDecoding);ParseHandlersByName handlersByName =new ParseHandlersByName(contract, options, encoder, decoder, queryMapEncoder,errorDecoder, synchronousMethodHandlerFactory);return new ReflectiveFeign(handlersByName, invocationHandlerFactory, queryMapEncoder);
}

继续看ReflectiveFeign.newInstance(Target target)方法，通过SpringMvcContract解析了feign接口相关的元信息，
SynchronousMethodHandler.Factory创建了这个类所有方法的MethodHandler，并最终通过InvocationHandlerFactory创建ReflectiveFeign.FeignInvocationHandler，最终返回动态代理对象。

public <T> T newInstance(Target<T> target) {// 通过SpringMvcContract解析了feign接口相关的元信息，创建MethodHandlerMap<String, MethodHandler> nameToHandler = targetToHandlersByName.apply(target);Map<Method, MethodHandler> methodToHandler = new LinkedHashMap<Method, MethodHandler>();List<DefaultMethodHandler> defaultMethodHandlers = new LinkedList<DefaultMethodHandler>();for (Method method : target.type().getMethods()) {if (method.getDeclaringClass() == Object.class) {continue;} else if (Util.isDefault(method)) {DefaultMethodHandler handler = new DefaultMethodHandler(method);defaultMethodHandlers.add(handler);methodToHandler.put(method, handler);} else {methodToHandler.put(method, nameToHandler.get(Feign.configKey(target.type(), method)));}}// 创建ReflectiveFeign.FeignInvocationHandler，生成动态代理对象InvocationHandler handler = factory.create(target, methodToHandler);T proxy = (T) Proxy.newProxyInstance(target.type().getClassLoader(),new Class<?>[] {target.type()}, handler);for (DefaultMethodHandler defaultMethodHandler : defaultMethodHandlers) {defaultMethodHandler.bindTo(proxy);}return proxy;
}

至此所有的feign接口通过FeignInvocationHandler生成了动态代理对象，里面包含了feign接口所有方法的Map<Method, MethodHandler> dispatch。
后续调用feign接口只需要直接注入使用即可。动态代理对象直接对应方法时，最终会调用InvocationHandler的invoke方法。

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {if ("equals".equals(method.getName())) {try {Object otherHandler =args.length > 0 && args[0] != null ? Proxy.getInvocationHandler(args[0]) : null;return equals(otherHandler);} catch (IllegalArgumentException e) {return false;}} else if ("hashCode".equals(method.getName())) {return hashCode();} else if ("toString".equals(method.getName())) {return toString();}// 通过方法获取到对应的SynchronousMethodHandler执行return dispatch.get(method).invoke(args);
}

在调用feign接口方法时，最终会执行SynchronousMethodHandler的invoke方法。

1. 封装请求，执行请求，解码
2. 如果抛RetryableException异常，如果不配置Retryer实例，则默认是不重试

public Object invoke(Object[] argv) throws Throwable {RequestTemplate template = buildTemplateFromArgs.create(argv);Options options = findOptions(argv);Retryer retryer = this.retryer.clone();while (true) {try {// 执行请求并且对响应结果进行解码return executeAndDecode(template, options);} catch (RetryableException e) {try {// 累计重试次数，判断是否继续重试retryer.continueOrPropagate(e);} catch (RetryableException th) {Throwable cause = th.getCause();if (propagationPolicy == UNWRAP && cause != null) {throw cause;} else {throw th;}}if (logLevel != Logger.Level.NONE) {logger.logRetry(metadata.configKey(), logLevel);}continue;}}
}

executeAndDecode方法执行请求并且处理响应结果

Object executeAndDecode(RequestTemplate template, Options options) throws Throwable {Request request = targetRequest(template);if (logLevel != Logger.Level.NONE) {logger.logRequest(metadata.configKey(), logLevel, request);}Response response;long start = System.nanoTime();try {// 发送请求response = client.execute(request, options);// ensure the request is set. TODO: remove in Feign 12response = response.toBuilder().request(request).requestTemplate(template).build();} catch (IOException e) {if (logLevel != Logger.Level.NONE) {logger.logIOException(metadata.configKey(), logLevel, e, elapsedTime(start));}throw errorExecuting(request, e);}long elapsedTime = TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - start);if (decoder != null) {return responseInterceptor.aroundDecode(new InvocationContext(decoder, metadata.returnType(), response));}CompletableFuture<Object> resultFuture = new CompletableFuture<>();// 处理响应结果asyncResponseHandler.handleResponse(resultFuture, metadata.configKey(), response,metadata.returnType(), elapsedTime);try {if (!resultFuture.isDone())throw new IllegalStateException("Response handling not done");return resultFuture.join();} catch (CompletionException e) {Throwable cause = e.getCause();if (cause != null)throw cause;throw e;}
}

RetryableFeignBlockingLoadBalancerClient做了三件事

1. 获取负载均衡服务实例信息
2. 构建完整请求
3. 发送请求并得到响应结果

public Response execute(Request request, Request.Options options) throws IOException {final URI originalUri = URI.create(request.url());String serviceId = originalUri.getHost();Assert.state(serviceId != null, "Request URI does not contain a valid hostname: " + originalUri);final LoadBalancedRetryPolicy retryPolicy = loadBalancedRetryFactory.createRetryPolicy(serviceId,loadBalancerClient);RetryTemplate retryTemplate = buildRetryTemplate(serviceId, request, retryPolicy);return retryTemplate.execute(context -> {Request feignRequest = null;ServiceInstance retrievedServiceInstance = null;Set<LoadBalancerLifecycle> supportedLifecycleProcessors = LoadBalancerLifecycleValidator.getSupportedLifecycleProcessors(loadBalancerClientFactory.getInstances(serviceId, LoadBalancerLifecycle.class),RetryableRequestContext.class, ResponseData.class, ServiceInstance.class);String hint = getHint(serviceId);DefaultRequest<RetryableRequestContext> lbRequest = new DefaultRequest<>(new RetryableRequestContext(null, buildRequestData(request), hint));// On retries the policy will choose the server and set it in the context// and extract the server and update the request being madeif (context instanceof LoadBalancedRetryContext) {LoadBalancedRetryContext lbContext = (LoadBalancedRetryContext) context;ServiceInstance serviceInstance = lbContext.getServiceInstance();if (serviceInstance == null) {if (LOG.isDebugEnabled()) {LOG.debug("Service instance retrieved from LoadBalancedRetryContext: was null. "+ "Reattempting service instance selection");}ServiceInstance previousServiceInstance = lbContext.getPreviousServiceInstance();lbRequest.getContext().setPreviousServiceInstance(previousServiceInstance);supportedLifecycleProcessors.forEach(lifecycle -> lifecycle.onStart(lbRequest));// 负载均衡client获取到服务实例信息retrievedServiceInstance = loadBalancerClient.choose(serviceId, lbRequest);if (LOG.isDebugEnabled()) {LOG.debug(String.format("Selected service instance: %s", retrievedServiceInstance));}lbContext.setServiceInstance(retrievedServiceInstance);}if (retrievedServiceInstance == null) {if (LOG.isWarnEnabled()) {LOG.warn("Service instance was not resolved, executing the original request");}org.springframework.cloud.client.loadbalancer.Response<ServiceInstance> lbResponse = new DefaultResponse(retrievedServiceInstance);supportedLifecycleProcessors.forEach(lifecycle -> lifecycle.onComplete(new CompletionContext<ResponseData, ServiceInstance, RetryableRequestContext>(CompletionContext.Status.DISCARD, lbRequest, lbResponse)));feignRequest = request;}else {if (LOG.isDebugEnabled()) {LOG.debug(String.format("Using service instance from LoadBalancedRetryContext: %s",retrievedServiceInstance));}// 生成带IP和端口的完整服务地址，并构建feign请求String reconstructedUrl = loadBalancerClient.reconstructURI(retrievedServiceInstance, originalUri).toString();feignRequest = buildRequest(request, reconstructedUrl);}}org.springframework.cloud.client.loadbalancer.Response<ServiceInstance> lbResponse = new DefaultResponse(retrievedServiceInstance);LoadBalancerProperties loadBalancerProperties = loadBalancerClientFactory.getProperties(serviceId);// 执行请求得到结果Response response = LoadBalancerUtils.executeWithLoadBalancerLifecycleProcessing(delegate, options,feignRequest, lbRequest, lbResponse, supportedLifecycleProcessors, retrievedServiceInstance != null,loadBalancerProperties.isUseRawStatusCodeInResponseData());int responseStatus = response.status();if (retryPolicy != null && retryPolicy.retryableStatusCode(responseStatus)) {if (LOG.isDebugEnabled()) {LOG.debug(String.format("Retrying on status code: %d", responseStatus));}byte[] byteArray = response.body() == null ? new byte[] {}: StreamUtils.copyToByteArray(response.body().asInputStream());response.close();throw new LoadBalancerResponseStatusCodeException(serviceId, response, byteArray,URI.create(request.url()));}return response;}, new LoadBalancedRecoveryCallback<Response, Response>() {@Overrideprotected Response createResponse(Response response, URI uri) {return response;}});
}

具体的发送请求流程可以详细debug看下，所有细节列出来可能会显得太冗长。过程涉及了很多工厂模式和委托模式，自己debug以下会更加清楚一些细节。比如发送http的客户端，默认是HttpsURLConnection发送请求，LoadBalancerClientConfiguration默认会创建RoundRobinLoadBalancer轮询负载均衡策略，可以看下spring-cloud-openfeign-core下的META-INF\spring.factories自动装备类，进一步结合项目实际情况还可以自定义Encoder，Decoder，ErrorDecoder等。