一.OpenFeign介绍

OpenFeign是实现微服务间调用的工具，功能包括编解码、构造http请求等。同时OpenFeign又集成了ribbon功能实现客户端负载均衡能力，Bibbon默认的客户端负载均衡能力是与Eurake集成，Nacos通过重写ribbon的ServerList功能实现ribbon与Nacos集成。

二.OpenFeign初始化原理

OpenFeign入口在@EnableFeignClients注解，同时它又导入了FeignClientsRegistrar类

FeignClientsRegistrar类实现了ImportBeanDefinitionRegistrar类，这个类是Spring的一个扩展点，可以注册BeanDefinition，对外暴露了一个接口

@Override
public void registerBeanDefinitions(AnnotationMetadata metadata,BeanDefinitionRegistry registry) {registerDefaultConfiguration(metadata, registry);registerFeignClients(metadata, registry);
}

再进入registerFeignClients(metadata, registry)方法

public void registerFeignClients(AnnotationMetadata metadata,BeanDefinitionRegistry registry) {ClassPathScanningCandidateComponentProvider scanner = getScanner();scanner.setResourceLoader(this.resourceLoader);Set<String> basePackages;Map<String, Object> attrs = metadata.getAnnotationAttributes(EnableFeignClients.class.getName());AnnotationTypeFilter annotationTypeFilter = new AnnotationTypeFilter(FeignClient.class);final Class<?>[] clients = attrs == null ? null: (Class<?>[]) attrs.get("clients");if (clients == null || clients.length == 0) {scanner.addIncludeFilter(annotationTypeFilter);basePackages = getBasePackages(metadata);}else {final Set<String> clientClasses = new HashSet<>();basePackages = new HashSet<>();for (Class<?> clazz : clients) {basePackages.add(ClassUtils.getPackageName(clazz));clientClasses.add(clazz.getCanonicalName());}AbstractClassTestingTypeFilter filter = new AbstractClassTestingTypeFilter() {@Overrideprotected boolean match(ClassMetadata metadata) {String cleaned = metadata.getClassName().replaceAll("\\$", ".");return clientClasses.contains(cleaned);}};scanner.addIncludeFilter(new AllTypeFilter(Arrays.asList(filter, annotationTypeFilter)));}for (String basePackage : basePackages) {Set<BeanDefinition> candidateComponents = scanner.findCandidateComponents(basePackage);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);registerClientConfiguration(registry, name,attributes.get("configuration"));registerFeignClient(registry, annotationMetadata, attributes);}}}
}

又进入registerFeignClient(registry, annotationMetadata, attributes)方法

private void registerFeignClient(BeanDefinitionRegistry registry,AnnotationMetadata annotationMetadata, Map<String, Object> attributes) {String className = annotationMetadata.getClassName();BeanDefinitionBuilder definition = BeanDefinitionBuilder.genericBeanDefinition(FeignClientFactoryBean.class);validate(attributes);definition.addPropertyValue("url", getUrl(attributes));definition.addPropertyValue("path", getPath(attributes));String name = getName(attributes);definition.addPropertyValue("name", name);definition.addPropertyValue("type", className);definition.addPropertyValue("decode404", attributes.get("decode404"));definition.addPropertyValue("fallback", attributes.get("fallback"));definition.addPropertyValue("fallbackFactory", attributes.get("fallbackFactory"));definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);String alias = name + "FeignClient";AbstractBeanDefinition beanDefinition = definition.getBeanDefinition();boolean primary = (Boolean)attributes.get("primary"); // has a default, won't be nullbeanDefinition.setPrimary(primary);String qualifier = getQualifier(attributes);if (StringUtils.hasText(qualifier)) {alias = qualifier;}BeanDefinitionHolder holder = new BeanDefinitionHolder(beanDefinition, className,new String[] { alias });BeanDefinitionReaderUtils.registerBeanDefinition(holder, registry);
}

这个方法是整个OpenFeign初始化客户端的核心，它注册了一个FeignClientFactoryBean类，这个类实现了FactoryBean接口，它是Spring中一个特殊的类，它可以返回“一类”对象，主要方法是getObject方法

@Override
public Object getObject() throws Exception {FeignContext context = applicationContext.getBean(FeignContext.class);Feign.Builder builder = feign(context);if (!StringUtils.hasText(this.url)) {String url;if (!this.name.startsWith("http")) {url = "http://" + this.name;}else {url = this.name;}url += cleanPath();return loadBalance(builder, context, new HardCodedTarget<>(this.type,this.name, url));}if (StringUtils.hasText(this.url) && !this.url.startsWith("http")) {this.url = "http://" + this.url;}String url = this.url + cleanPath();Client client = getOptional(context, Client.class);if (client != null) {if (client instanceof LoadBalancerFeignClient) {// not load balancing because we have a url,// but ribbon is on the classpath, so unwrapclient = ((LoadBalancerFeignClient)client).getDelegate();}builder.client(client);}Targeter targeter = get(context, Targeter.class);return targeter.target(this, builder, context, new HardCodedTarget<>(this.type, this.name, url));
}

三.OpenFeign调用原理

每次当我们注入Feign的接口时，Spring都会为我们创建一个接口的代理，而这个代理的生成都会走getObject方法。当我们没有为接口配置具体url时，使用服务名查找，而服务名查找就会走负载均衡方法，也就是loadBalance方法

这个方法中的client实现为LoadBalanceFeignClient类

进入target方法

最终生成代理

最终执行invoke方法

使用LoadBalancerFeignClient的实现

最终调用AbstractLoadBalancerAwareClient中的executeWithLoadBalancer方法，此时已经ribbon的方法了。

进入executeWithLoadBalancer方法

进入submit方法，注意看selectServer()方法

private Observable<Server> selectServer() {return Observable.create(new OnSubscribe<Server>() {@Overridepublic void call(Subscriber<? super Server> next) {try {Server server = loadBalancerContext.getServerFromLoadBalancer(loadBalancerURI, loadBalancerKey);   next.onNext(server);next.onCompleted();} catch (Exception e) {next.onError(e);}}});
}