一、简介 Feign是一个http请求调用的轻量级框架，可以以Java接口注解的方式调用Http请求，而不用像Java中通过封装HTTP请求报文的方式直接调用。Feign通过处理注解，将请求模板化，当实际
一、简介
Feign是一个http请求调用的轻量级框架，可以以Java接口注解的方式调用Http请求，而不用像Java中通过封装HTTP请求报文的方式直接调用。Feign通过处理注解，将请求模板化，当实际调用的时候，传入参数，根据参数再应用到请求上，进而转化成真正的请求，这种请求相对而言比较直观。
Feign被广泛应用在Spring Cloud 的解决方案中，是学习基于Spring Cloud 微服务架构不可或缺的重要组件。
二、工作原理
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主程序入口添加了@EnableFeignClients注解开启对FeignClient扫描加载处理。根据Feign Client的开发规范，定义接口并加@FeignClient注解。
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当程序启动时，会进行包扫描，扫描所有@FeignClients的注解的类，并且将这些信息注入Spring IOC容器中，当定义的的Feign接口中的方法被调用时，通过JDK动态代理方式，来生成具体的RequestTemplate。当生成代理时，Feign会为每个接口方法创建一个RequestTemplate对象，该对象封装可HTTP请求需要的全部信息，如请求参数名，请求方法等信息都是在这个过程中确定的。
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然后RequestTemplate生成Request，然后把Request交给Client去处理，这里指的Client可以是JDK原生的URLConnection、Apache的HttpClient、也可以是OKhttp，最后Client被封装到LoadBalanceClient类，这个类结合Ribbon负载均衡发起服务之间的调用。
三、源码分析
使用openfegin我们可以不用在yaml文件添加任何关于openfeign的配置，而只需要在一个被@Configuration注释的配置类上或者Application启动类上添加@EnableFeignClients注解。例如：
@EnableFeignClients(basePackages = {"com.yibo.order.center"}) public class OrderCenterApplication { public static void main(String[] args) { SpringApplication.run(OrderCenterApplication.class,args); } }basePackages属性用于指定被@FeignClient注解注释的接口所在的包的包名，或者也可以直接指定clients属性，clients属性可以直接指定一个或多个被@FeignClient注释的类。
basePackages是一个数组，如果被@FeignClient注解注释的接口比较分散，可以指定多个包名，而不使用一个大的包名，这样可以减少包扫描耗费的时间，不拖慢应用的启动速度。
EnableFeignClients
- 该注解引入了FeignClientsRegistrar类
FeignClientsRegistrar实现了ImportBeanDefinitionRegistrar，它是一个动态注入bean的接口，Spring Boot启动的时候，会去调用这个类中的registerBeanDefinitions来实现动态Bean的装载。它的作用类似于ImportSelector。
FeignClientsRegistrar
@EnableFeignClients注解使用@Import导入FeignClientsRegistrar类，这是一个ImportBeanDefinitionRegistrar，因此我们重点关注它的registerBeanDefinitions方法。
class FeignClientsRegistrar implements ImportBeanDefinitionRegistrar, ResourceLoaderAware, EnvironmentAware { @Override public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) { //注册@EnableFeignClients中定义defaultConfiguration属性下的类，包装成FeignClientSpecification，注册到Spring容器。 //在@FeignClient中有一个属性：configuration，这个属性是表示各个FeignClient自定义的配置类， //后面也会通过调用registerClientConfiguration方法来注册成FeignClientSpecification到容器。 //所以，这里可以完全理解在@EnableFeignClients中配置的是做为兜底的配置，在各个@FeignClient配置的就是自定义的情况。 registerDefaultConfiguration(metadata, registry); //该方法负责读取@EnableFeignClients的属性，获取需要扫描的包名， //然后扫描指定的所有包名下的被@FeignClient注解注释的接口， //将扫描出来的接口调用registerFeignClient方法注册到spring容器。 registerFeignClients(metadata, registry); } }registerDefaultConfiguration
- 解析EnableFeignClients属性，注册FeignClientSpecification类型的Bean
registerFeignClients
重点关注registerFeignClients方法，该方法负责读取@EnableFeignClients的属性，获取需要扫描的包名，然后扫描指定的所有包名下的被@FeignClient注解注释的接口，将扫描出来的接口调用registerFeignClient方法注册到spring容器。
class FeignClientsRegistrar implements ImportBeanDefinitionRegistrar, ResourceLoaderAware, EnvironmentAware { 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()); // 扫描带有FeignClient注解的类 AnnotationTypeFilter annotationTypeFilter = new AnnotationTypeFilter( FeignClient.class); //获取@EnableFeignClients 中clients的值 final Class<?>[] clients = attrs == null ? null : (Class<?>[]) attrs.get("clients"); if (clients == null || clients.length == 0) { //如果没有设置，那么设置类型过滤器 scanner.addIncludeFilter(annotationTypeFilter); // 如果没有设置，则扫描的包路径为 @EnableFeignClients 注解所在的包 basePackages = getBasePackages(metadata); } else { final Set<String> clientClasses = new HashSet<>(); basePackages = new HashSet<>(); //设置了则使用注解属性来进行扫描注册 for (Class<?> clazz : clients) { basePackages.add(ClassUtils.getPackageName(clazz)); clientClasses.add(clazz.getCanonicalName()); } AbstractClassTestingTypeFilter filter = new AbstractClassTestingTypeFilter() { @Override protected boolean match(ClassMetadata metadata) { String cleaned = metadata.getClassName().replaceAll("\\$", "."); return clientClasses.contains(cleaned); } }; scanner.addIncludeFilter( new AllTypeFilter(Arrays.asList(filter, annotationTypeFilter))); } //循环扫描注册 for (String basePackage : basePackages) { Set<BeanDefinition> candidateComponents = scanner .findCandidateComponents(basePackage); for (BeanDefinition candidateComponent : candidateComponents) { if (candidateComponent instanceof AnnotatedBeanDefinition) { // verify annotated class is an interface // 验证带注释的类必须是接口 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 = getClientName(attributes); //注册被调用客户端配置 //注册(微服务名).FeignClientSpecification类型的bean //beanname: order-center.FeignClientSpecification registerClientConfiguration(registry, name, attributes.get("configuration")); //注册 FeignClient registerFeignClient(registry, annotationMetadata, attributes); } } } } }registerFeignClient
- 注册 FeignClient，组装BeanDefinition，实质是一个FeignClientFactoryBean，然后注册到Spring IOC容器。
至此，注册完成，注意此时仅仅只是注册到 DefaultListableBeanFactory容器的 beanDefinitionMap中，并没有实例化！
实例化
- Spring容器启动，调用AbstractApplicationContext#refresh方法，
- 在refresh方法内部调用finishBeanFactoryInitialization方法对单例bean进行初始化，
- finishBeanFactoryInitialization方法调用getBean获取name对应的bean实例，如果不存在，则创建一个，即调用doGetBean方法。
- doGetBean调用createBean方法，createBean方法调用doCreateBean方法。
- doCreateBean()方法主要是根据 beanName、mbd、args，使用对应的策略创建 bean 实例，并返回包装类 BeanWrapper。
- doCreateBean方法中调用populateBean对 bean 进行属性填充；其中，可能存在依赖于其他 bean 的属性，则会递归初始化依赖的 bean 实例。
getObjectForBeanInstance
在doGetBean方法中会调用getObjectForBeanInstance方法获取beanName对应的实例对象（主要用于FactoryBean的特殊处理，普通Bean会直接返回sharedInstance本身）
public abstract class AbstractBeanFactory extends FactoryBeanRegistrySupport implements ConfigurableBeanFactory { protected Object getObjectForBeanInstance( Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) { // Don't let calling code try to dereference the factory if the bean isn't a factory. if (BeanFactoryUtils.isFactoryDereference(name)) { if (beanInstance instanceof NullBean) { return beanInstance; } // 1.如果name以“&”为前缀，但是beanInstance不是FactoryBean，则抛异常 if (!(beanInstance instanceof FactoryBean)) { throw new BeanIsNotAFactoryException(beanName, beanInstance.getClass()); } if (mbd != null) { mbd.isFactoryBean = true; } return beanInstance; } // Now we have the bean instance, which may be a normal bean or a FactoryBean. // If it's a FactoryBean, we use it to create a bean instance, unless the // caller actually wants a reference to the factory. // 2.1 如果beanInstance不是FactoryBean（也就是普通bean），则直接返回beanInstance if (!(beanInstance instanceof FactoryBean)) { return beanInstance; } //FactoryBean创建出bean实例返回 // 3.走到这边，代表beanInstance是FactoryBean，但name不带有“&”前缀，表示想要获取的是FactoryBean创建的对象实例 Object object = null; if (mbd != null) { mbd.isFactoryBean = true; } else { // 4.如果mbd为空，则尝试从factoryBeanObjectCache缓存中获取该FactoryBean创建的对象实例 object = getCachedObjectForFactoryBean(beanName); } if (object == null) { // Return bean instance from factory. // 5.只有beanInstance是FactoryBean才能走到这边，因此直接强转 FactoryBean<?> factory = (FactoryBean<?>) beanInstance; // Caches object obtained from FactoryBean if it is a singleton. if (mbd == null && containsBeanDefinition(beanName)) { // 6.mbd为空，但是该bean的BeanDefinition在缓存中存在，则获取该bean的MergedBeanDefinition mbd = getMergedLocalBeanDefinition(beanName); } // 7.mbd是否是合成的（这个字段比较复杂，mbd正常情况都不是合成的，也就是false，有兴趣的可以自己查阅资料看看） boolean synthetic = (mbd != null && mbd.isSynthetic()); // 8.从FactoryBean获取对象实例 object = getObjectFromFactoryBean(factory, beanName, !synthetic); } // 9.返回对象实例 return object; } }getObjectFromFactoryBean
从FactoryBean获取对象实例 getObjectForBeanInstance方法中会调用getObjectFromFactoryBean从FactoryBean获取对象实例，即调用FactoryBean的getObject方法获取对象实例。
public abstract class FactoryBeanRegistrySupport extends DefaultSingletonBeanRegistry { protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) { // 1.如果是单例，并且已经存在于单例对象缓存中 if (factory.isSingleton() && containsSingleton(beanName)) { //又见双重检查锁机制，尝试再从缓存中获取，防止多线程下可能有别的线程已完成该单例Bean的创建 synchronized (getSingletonMutex()) { // 2.从FactoryBean创建的单例对象的缓存中获取该bean实例 Object object = this.factoryBeanObjectCache.get(beanName); if (object == null) { // 3.调用FactoryBean的getObject方法获取对象实例 object = doGetObjectFromFactoryBean(factory, beanName); // Only post-process and store if not put there already during getObject() call above // (e.g. because of circular reference processing triggered by custom getBean calls) Object alreadyThere = this.factoryBeanObjectCache.get(beanName); // 4.如果该beanName已经在缓存中存在，则将object替换成缓存中的 if (alreadyThere != null) { object = alreadyThere; } else { if (shouldPostProcess) { if (isSingletonCurrentlyInCreation(beanName)) { // Temporarily return non-post-processed object, not storing it yet.. return object; } beforeSingletonCreation(beanName); try { // 5.对bean实例进行后置处理，执行所有已注册的BeanPostProcessor的postProcessAfterInitialization方法 //触发BeanPostProcessor，第三方框架可以在此用AOP来包装Bean实例 object = postProcessObjectFromFactoryBean(object, beanName); } catch (Throwable ex) { throw new BeanCreationException(beanName, "Post-processing of FactoryBean's singleton object failed", ex); } finally { //创建完成后，从缓存锁定的名字里清除 afterSingletonCreation(beanName); } } if (containsSingleton(beanName)) { // 6.将beanName和object放到factoryBeanObjectCache缓存中 this.factoryBeanObjectCache.put(beanName, object); } } } // 7.返回object对象实例 return object; } } else { // 8.调用FactoryBean的getObject方法获取对象实例 Object object = doGetObjectFromFactoryBean(factory, beanName); if (shouldPostProcess) { try { // 9.对bean实例进行后置处理，执行所有已注册的BeanPostProcessor的postProcessAfterInitialization方法 object = postProcessObjectFromFactoryBean(object, beanName); } catch (Throwable ex) { throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex); } } // 10.返回object对象实例 return object; } } }主要步骤：
- 调用 FactoryBean 的 getObject 方法获取对象实例。
- 对 bean 实例进行后续处理，执行所有已注册的 BeanPostProcessor 的 postProcessAfterInitialization 方法。
doGetObjectFromFactoryBean
用FactoryBean的getObject方法获取对象实例
public abstract class FactoryBeanRegistrySupport extends DefaultSingletonBeanRegistry { private Object doGetObjectFromFactoryBean(final FactoryBean<?> factory, final String beanName) throws BeanCreationException { Object object; try { // 1.调用FactoryBean的getObject方法获取bean对象实例 if (System.getSecurityManager() != null) { AccessControlContext acc = getAccessControlContext(); try { // 1.1 带有权限验证的 object = AccessController.doPrivileged((PrivilegedExceptionAction<Object>) factory::getObject, acc); } catch (PrivilegedActionException pae) { throw pae.getException(); } } else { // 1.2 不带权限 object = factory.getObject(); } } catch (FactoryBeanNotInitializedException ex) { throw new BeanCurrentlyInCreationException(beanName, ex.toString()); } catch (Throwable ex) { throw new BeanCreationException(beanName, "FactoryBean threw exception on object creation", ex); } // Do not accept a null value for a FactoryBean that's not fully // initialized yet: Many FactoryBeans just return null then. // 2.getObject返回的是空值，并且该FactoryBean正在初始化中，则直接抛异常， // 不接受一个尚未完全初始化的FactoryBean的getObject返回的空值 if (object == null) { if (isSingletonCurrentlyInCreation(beanName)) { throw new BeanCurrentlyInCreationException( beanName, "FactoryBean which is currently in creation returned null from getObject"); } object = new NullBean(); } // 3.返回创建好的bean对象实例 return object; } }很简单的方法，就是直接调用 FactoryBean 的 getObject 方法来获取到对象实例。
细心的同学可以发现，该方法是以 do 开头，以 do 开头的方法是最终进行实际操作的方法，例如本方法就是 FactoryBean 最终实际进行创建 bean 对象实例的方法。
factory.getObject()
该方法会调用到FeignClientFactoryBean的getObject方法
class FeignClientFactoryBean implements FactoryBean<Object>, InitializingBean, ApplicationContextAware { @Override public Object getObject() throws Exception { return getTarget(); } }getObject调用的是getTarget方法，它从applicationContext取出FeignContext，FeignContext继承了NamedContextFactory，它是用来统一维护feign中各个feign客户端相互隔离的上下文。
FeignContext注册到容器是在FeignAutoConfiguration上完成的。
@Configuration(proxyBeanMethods = false) @ConditionalOnClass(Feign.class) @EnableConfigurationProperties({ FeignClientProperties.class, FeignHttpClientProperties.class }) @Import(DefaultGzipDecoderConfiguration.class) public class FeignAutoConfiguration { @Autowired(required = false) private List<FeignClientSpecification> configurations = new ArrayList<>(); @Bean public FeignContext feignContext() { FeignContext context = new FeignContext(); context.setConfigurations(this.configurations); return context; } }在初始化FeignContext时，会把configurations在容器中放入FeignContext中。configurations 的来源就是在前面registerFeignClients方法中将@FeignClient的配置 configuration。
FeignClientFactoryBean#getTarget
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构建feign.builder，在构建时会向FeignContext获取配置的Encoder，Decoder等各种信息。FeignContext在上文中已经提到会为每个Feign客户端分配了一个容器，它们的父容器就是spring容器。
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配置完Feign.Builder之后，再判断是否需要LoadBalance，如果需要，则通过LoadBalance的方法来设置。实际上他们最终调用的是Target.target()方法。
feign(context)
构建feign.builder，在构建时会向FeignContext获取配置的Encoder，Decoder等各种信息。FeignContext在上文中已经提到会为每个Feign客户端分配了一个容器，它们的父容器就是spring容器。
class FeignClientFactoryBean implements FactoryBean<Object>, InitializingBean, ApplicationContextAware { protected Feign.Builder feign(FeignContext context) { FeignLoggerFactory loggerFactory = get(context, FeignLoggerFactory.class); Logger logger = loggerFactory.create(type); // @formatter:off Feign.Builder builder = get(context, Feign.Builder.class) // required values .logger(logger) //日志 .encoder(get(context, Encoder.class)) //编码器 .decoder(get(context, Decoder.class)) //解码器 .contract(get(context, Contract.class));//验证器 // @formatter:on //处理了链接超时、读取超时等配置项 configureFeign(context, builder); return builder; } }FeignClientFactoryBean#loadBalance
- 生成具备负载均衡能力的feign客户端，为feign客户端构建起绑定负载均衡客户端
FeignClientFactoryBean#getOptional(context, Client.class)
Client client = (Client)this.getOptional(context, Client.class); 从上下文中获取一个 Client，默认是LoadBalancerFeignClient。它是在FeignRibbonClientAutoConfiguration这个自动装配类中，通过Import实现的
@Import({ HttpClientFeignLoadBalancedConfiguration.class, OkHttpFeignLoadBalancedConfiguration.class, DefaultFeignLoadBalancedConfiguration.class }) public class FeignRibbonClientAutoConfiguration { //...... }DefaultFeignLoadBalancedConfiguration
这里的通过 DefaultFeignLoadBalancedConfiguration 注入客户端 Client 的实现
@Configuration(proxyBeanMethods = false) class DefaultFeignLoadBalancedConfiguration { @Bean @ConditionalOnMissingBean public Client feignClient(CachingSpringLoadBalancerFactory cachingFactory, SpringClientFactory clientFactory) { return new LoadBalancerFeignClient(new Client.Default(null, null), cachingFactory, clientFactory); } }targeter.target
接下去进入targeter.target(this, builder, context, target) ，携带着构建好的这些对象去创建代理实例 ，这里有两个实现 HystrixTargeter 、DefaultTargeter 很显然，我们没有配置 Hystrix ，这里会走 DefaultTargeter
class DefaultTargeter implements Targeter { @Override public <T> T target(FeignClientFactoryBean factory, Feign.Builder feign, FeignContext context, Target.HardCodedTarget<T> target) { return feign.target(target); } }feign.target
然后会来到 feign.Feign.Builder#target(feign.Target<T>)
public abstract class Feign { public <T> T target(Target<T> target) { return build().newInstance(target); } public Feign build() { Client client = Capability.enrich(this.client, capabilities); Retryer retryer = Capability.enrich(this.retryer, capabilities); List<RequestInterceptor> requestInterceptors = this.requestInterceptors.stream() .map(ri -> Capability.enrich(ri, capabilities)) .collect(Collectors.toList()); Logger logger = Capability.enrich(this.logger, capabilities); Contract contract = Capability.enrich(this.contract, capabilities); Options options = Capability.enrich(this.options, capabilities); Encoder encoder = Capability.enrich(this.encoder, capabilities); Decoder decoder = Capability.enrich(this.decoder, capabilities); InvocationHandlerFactory invocationHandlerFactory = Capability.enrich(this.invocationHandlerFactory, capabilities); QueryMapEncoder queryMapEncoder = Capability.enrich(this.queryMapEncoder, capabilities); SynchronousMethodHandler.Factory synchronousMethodHandlerFactory = new SynchronousMethodHandler.Factory(client, retryer, requestInterceptors, logger, logLevel, decode404, closeAfterDecode, propagationPolicy, forceDecoding); ParseHandlersByName handlersByName = new ParseHandlersByName(contract, options, encoder, decoder, queryMapEncoder, errorDecoder, synchronousMethodHandlerFactory); return new ReflectiveFeign(handlersByName, invocationHandlerFactory, queryMapEncoder); } }ReflectiveFeign.newInstance
最终会调用 ReflectiveFeign.newInstance
这个方法是用来创建一个动态代理的方法，在生成动态代理之前，会根据Contract协议（协议解析规则，解析接口类的注解信息，解析成内部的MethodHandler的处理方式。
从实现的代码中可以看到熟悉的Proxy.newProxyInstance方法产生代理类。而这里需要对每个定义的接口方法进行特定的处理实现，所以这里会出现一个MethodHandler的概念，就是对应方法级别的InvocationHandler。
public class ReflectiveFeign extends Feign { @Override public <T> T newInstance(Target<T> target) { // 解析接口注解信息 //根据接口类和Contract协议解析方式，解析接口类上的方法和注解，转换成内部的MethodHandler处理方式 Map<String, MethodHandler> nameToHandler = targetToHandlersByName.apply(target); Map<Method, MethodHandler> methodToHandler = new LinkedHashMap<Method, MethodHandler>(); List<DefaultMethodHandler> defaultMethodHandlers = new LinkedList<DefaultMethodHandler>(); // 根据方法类型 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))); } } InvocationHandler handler = factory.create(target, methodToHandler); // 基于Proxy.newProxyInstance 为接口类创建动态实现，将所有的请求转换给InvocationHandler 处理。 T proxy = (T) Proxy.newProxyInstance(target.type().getClassLoader(), new Class<?>[] {target.type()}, handler); for (DefaultMethodHandler defaultMethodHandler : defaultMethodHandlers) { defaultMethodHandler.bindTo(proxy); } return proxy; } }targetToHandlersByName.apply
targetToHandlersByName.apply(target) :根据Contract协议规则，解析接口类的注解信息，解析成内部表现：targetToHandlersByName.apply(target);会解析接口方法上的注解，从而解析出方法粒度的特定的配置信息，然后生产一个SynchronousMethodHandler 然后需要维护一个<method，MethodHandler>的map，放入InvocationHandler的实现FeignInvocationHandler中。
public class ReflectiveFeign extends Feign { static final class ParseHandlersByName { public Map<String, MethodHandler> apply(Target target) { List<MethodMetadata> metadata = contract.parseAndValidateMetadata(target.type()); Map<String, MethodHandler> result = new LinkedHashMap<String, MethodHandler>(); for (MethodMetadata md : metadata) { BuildTemplateByResolvingArgs buildTemplate; if (!md.formParams().isEmpty() && md.template().bodyTemplate() == null) { buildTemplate = new BuildFormEncodedTemplateFromArgs(md, encoder, queryMapEncoder, target); } else if (md.bodyIndex() != null) { buildTemplate = new BuildEncodedTemplateFromArgs(md, encoder, queryMapEncoder, target); } else { buildTemplate = new BuildTemplateByResolvingArgs(md, queryMapEncoder, target); } if (md.isIgnored()) { result.put(md.configKey(), args -> { throw new IllegalStateException(md.configKey() + " is not a method handled by feign"); }); } else { result.put(md.configKey(), factory.create(target, md, buildTemplate, options, decoder, errorDecoder)); } } return result; } } }SpringMvcContract：当前Spring Cloud 微服务解决方案中，为了降低学习成本，采用了Spring MVC的部分注解来完成 请求协议解析，也就是说 ，写客户端请求接口和像写服务端代码一样：客户端和服务端可以通过SDK的方式进行约定，客户端只需要引入服务端发布的SDK API，就可以使用面向接口的编码方式对接服务。
OpenFeign调用过程 :
在前面的分析中，我们知道OpenFeign最终返回的是一个 ReflectiveFeign.FeignInvocationHandler 的对象。那么当客户端发起请求时，会进入到 FeignInvocationHandler.invoke 方法中，这个大家都知道，它是一个动态代理的实现。
public class ReflectiveFeign extends Feign { @Override 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(); } // 利用分发器筛选方法，找到对应的handler 进行处理 return dispatch.get(method).invoke(args); } }SynchronousMethodHandler#invoke
而接着，在invoke方法中，会调用 this.dispatch.get(method)).invoke(args) 。this.dispatch.get(method) 会返回一个SynchronousMethodHandler，进行拦截处理。这个方法会根据参数生成完成的RequestTemplate对象，这个对象是Http请求的模版，代码如下。
final class SynchronousMethodHandler implements MethodHandler { @Override public Object invoke(Object[] argv) throws Throwable { RequestTemplate template = buildTemplateFromArgs.create(argv); Options options = findOptions(argv); Retryer retryer = this.retryer.clone(); while (true) { try { return executeAndDecode(template, options); } catch (RetryableException e) { try { 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; } } } }SynchronousMethodHandler#executeAndDecode
经过上述的代码，我们已经将restTemplate拼装完成，上面的代码中有一个 executeAndDecode() 方法，该方法通过RequestTemplate生成Request请求对象，然后利用Http Client获取response，来获取响应信息。
final class SynchronousMethodHandler implements MethodHandler { Object executeAndDecode(RequestTemplate template, Options options) throws Throwable { //转化为Http请求报文 Request request = targetRequest(template); if (logLevel != Logger.Level.NONE) { logger.logRequest(metadata.configKey(), logLevel, request); } Response response; long start = System.nanoTime(); try { //发起远程通信 response = client.execute(request, options); //获取返回结果 response = 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 decoder.decode(response, metadata.returnType()); CompletableFuture<Object> resultFuture = new CompletableFuture<>(); 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; } } }经过上面的分析，这里的 client.execute 的 client 的类型是LoadBalancerFeignClient
LoadBalancerFeignClient#execute
这里就很自然的进入 LoadBalancerFeignClient#execute
public class LoadBalancerFeignClient implements Client { @Override public Response execute(Request request, Request.Options options) throws IOException { try { URI asUri = URI.create(request.url()); String clientName = asUri.getHost(); URI uriWithoutHost = cleanUrl(request.url(), clientName); FeignLoadBalancer.RibbonRequest ribbonRequest = new FeignLoadBalancer.RibbonRequest( this.delegate, request, uriWithoutHost); IClientConfig requestConfig = getClientConfig(options, clientName); return lbClient(clientName) .executeWithLoadBalancer(ribbonRequest, requestConfig).toResponse(); } catch (ClientException e) { IOException io = findIOException(e); if (io != null) { throw io; } throw new RuntimeException(e); } } }其实这个execute里面得流程就是 Ribbon 的那一套。我们可以简单的看一下。首先是构造URI，构造RibbonRequest，选择 LoadBalance，发起调用。
来看一下lbClient 选择负载均衡器的时候做了什么
public class LoadBalancerFeignClient implements Client { private FeignLoadBalancer lbClient(String clientName) { return this.lbClientFactory.create(clientName); } public FeignLoadBalancer create(String clientName) { FeignLoadBalancer client = this.cache.get(clientName); if (client != null) { return client; } IClientConfig config = this.factory.getClientConfig(clientName); ILoadBalancer lb = this.factory.getLoadBalancer(clientName); ServerIntrospector serverIntrospector = this.factory.getInstance(clientName, ServerIntrospector.class); client = this.loadBalancedRetryFactory != null ? new RetryableFeignLoadBalancer(lb, config, serverIntrospector, this.loadBalancedRetryFactory) : new FeignLoadBalancer(lb, config, serverIntrospector); this.cache.put(clientName, client); return client; } }可以得出的结论就是 this.factory.getLoadBalancer(clientName) 跟Ribbon 源码里的获取方式一样，无疑这里获取的就是默认的 ZoneAwareLoadBalancer。然后包装成一个 FeignLoadBalancer 进行返回。
既然负载均衡器选择完了，那么一定还有个地方通过该负载去选择一个服务，接着往下看：
public abstract class AbstractLoadBalancerAwareClient<S extends ClientRequest, T extends IResponse> extends LoadBalancerContext implements IClient<S, T>, IClientConfigAware { public T executeWithLoadBalancer(final S request, final IClientConfig requestConfig) throws ClientException { LoadBalancerCommand<T> command = buildLoadBalancerCommand(request, requestConfig); try { return command.submit( new ServerOperation<T>() { @Override public Observable<T> call(Server server) { URI finalUri = reconstructURIWithServer(server, request.getUri()); S requestForServer = (S) request.replaceUri(finalUri); try { return Observable.just(AbstractLoadBalancerAwareClient.this.execute(requestForServer, requestConfig)); } catch (Exception e) { return Observable.error(e); } } }) .toBlocking() .single(); } catch (Exception e) { Throwable t = e.getCause(); if (t instanceof ClientException) { throw (ClientException) t; } else { throw new ClientException(e); } } } }上面这段代码就是通过获取到的负载进行执行请求，但是这个时候 服务还没有选择，我们跟进去 submit 请求看一看究竟：
public class LoadBalancerCommand<T> { public Observable<T> submit(final ServerOperation<T> operation) { final ExecutionInfoContext context = new ExecutionInfoContext(); if (listenerInvoker != null) { try { listenerInvoker.onExecutionStart(); } catch (AbortExecutionException e) { return Observable.error(e); } } final int maxRetrysSame = retryHandler.getMaxRetriesOnSameServer(); final int maxRetrysNext = retryHandler.getMaxRetriesOnNextServer(); // Use the load balancer Observable<T> o = (server == null ? selectServer() : Observable.just(server)) .concatMap(new Func1<Server, Observable<T>>() { //省略...... }); //省略...... } }可以看到这里有个 selectServer的方法 ，跟进去：
public class LoadBalancerCommand<T> { private final LoadBalancerContext loadBalancerContext; private Observable<Server> selectServer() { return Observable.create(new OnSubscribe<Server>() { @Override public void call(Subscriber<? super Server> next) { try { Server server = loadBalancerContext.getServerFromLoadBalancer(loadBalancerURI, loadBalancerKey); next.onNext(server); next.onCompleted(); } catch (Exception e) { next.onError(e); } } }); } } public class LoadBalancerContext implements IClientConfigAware { public Server getServerFromLoadBalancer(@Nullable URI original, @Nullable Object loadBalancerKey) throws ClientException { String host = null; int port = -1; if (original != null) { host = original.getHost(); } if (original != null) { Pair<String, Integer> schemeAndPort = deriveSchemeAndPortFromPartialUri(original); port = schemeAndPort.second(); } // Various Supported Cases // The loadbalancer to use and the instances it has is based on how it was registered // In each of these cases, the client might come in using Full Url or Partial URL ILoadBalancer lb = getLoadBalancer(); if (host == null) { // Partial URI or no URI Case // well we have to just get the right instances from lb - or we fall back if (lb != null){ Server svc = lb.chooseServer(loadBalancerKey); if (svc == null){ throw new ClientException(ClientException.ErrorType.GENERAL, "Load balancer does not have available server for client: " + clientName); } host = svc.getHost(); if (host == null){ throw new ClientException(ClientException.ErrorType.GENERAL, "Invalid Server for :" + svc); } logger.debug("{} using LB returned Server: {} for request {}", new Object[]{clientName, svc, original}); return svc; } else { // No Full URL - and we dont have a LoadBalancer registered to // obtain a server // if we have a vipAddress that came with the registration, we // can use that else we // bail out if (vipAddresses != null && vipAddresses.contains(",")) { throw new ClientException( ClientException.ErrorType.GENERAL, "Method is invoked for client " + clientName + " with partial URI of (" + original + ") with no load balancer configured." + " Also, there are multiple vipAddresses and hence no vip address can be chosen" + " to complete this partial uri"); } else if (vipAddresses != null) { try { Pair<String,Integer> hostAndPort = deriveHostAndPortFromVipAddress(vipAddresses); host = hostAndPort.first(); port = hostAndPort.second(); } catch (URISyntaxException e) { throw new ClientException( ClientException.ErrorType.GENERAL, "Method is invoked for client " + clientName + " with partial URI of (" + original + ") with no load balancer configured. " + " Also, the configured/registered vipAddress is unparseable (to determine host and port)"); } } else { throw new ClientException( ClientException.ErrorType.GENERAL, this.clientName + " has no LoadBalancer registered and passed in a partial URL request (with no host:port)." + " Also has no vipAddress registered"); } } } else { // Full URL Case // This could either be a vipAddress or a hostAndPort or a real DNS // if vipAddress or hostAndPort, we just have to consult the loadbalancer // but if it does not return a server, we should just proceed anyways // and assume its a DNS // For restClients registered using a vipAddress AND executing a request // by passing in the full URL (including host and port), we should only // consult lb IFF the URL passed is registered as vipAddress in Discovery boolean shouldInterpretAsVip = false; if (lb != null) { shouldInterpretAsVip = isVipRecognized(original.getAuthority()); } if (shouldInterpretAsVip) { Server svc = lb.chooseServer(loadBalancerKey); if (svc != null){ host = svc.getHost(); if (host == null){ throw new ClientException(ClientException.ErrorType.GENERAL, "Invalid Server for :" + svc); } logger.debug("using LB returned Server: {} for request: {}", svc, original); return svc; } else { // just fall back as real DNS logger.debug("{}:{} assumed to be a valid VIP address or exists in the DNS", host, port); } } else { // consult LB to obtain vipAddress backed instance given full URL //Full URL execute request - where url!=vipAddress logger.debug("Using full URL passed in by caller (not using load balancer): {}", original); } } // end of creating final URL if (host == null){ throw new ClientException(ClientException.ErrorType.GENERAL,"Request contains no HOST to talk to"); } // just verify that at this point we have a full URL return new Server(host, port); } }可以看到的是这里获取到了之前构造好的 ZoneAwareLoadBalancer 然后调用 chooseServer 方法获取server ，这个是跟Ribbon 中是一样的流程。
获取到了server 后，会回调先前 executeWithLoadBalancer 方法里构造的 ServerOperation 的 call 方法：
return command.submit( new ServerOperation<T>() { @Override public Observable<T> call(Server server) { URI finalUri = reconstructURIWithServer(server, request.getUri()); S requestForServer = (S) request.replaceUri(finalUri); try { return Observable.just(AbstractLoadBalancerAwareClient.this.execute(requestForServer, requestConfig)); } catch (Exception e) { return Observable.error(e); } } }) .toBlocking() .single();然后会执行 AbstractLoadBalancerAwareClient.this.execute(requestForServer, requestConfig) 进行最后的调用，实际上这里走的是 FeignLoadBalancer#execute
public class FeignLoadBalancer extends AbstractLoadBalancerAwareClient<FeignLoadBalancer.RibbonRequest, FeignLoadBalancer.RibbonResponse> { @Override public RibbonResponse execute(RibbonRequest request, IClientConfig configOverride) throws IOException { Request.Options options; if (configOverride != null) { RibbonProperties override = RibbonProperties.from(configOverride); options = new Request.Options(override.connectTimeout(this.connectTimeout), override.readTimeout(this.readTimeout)); } else { options = new Request.Options(this.connectTimeout, this.readTimeout); } Response response = request.client().execute(request.toRequest(), options); return new RibbonResponse(request.getUri(), response); } }而这里调用的request.client().execute(request.toRequest(), options) 则是 DefaultFeignLoadBalancedConfiguration 注入的 LoadBalancerFeignClient ，在构造 LoadBalancerFeignClient 的时候 ，传递了个 feign.Client.Default ，然后利用 feign.Client.Default 构造了一个 RibbonRequest。
所以这里走 feign.Client.Default#execute ：
public interface Client { class Default implements Client { @Override public Response execute(Request request, Options options) throws IOException { HttpURLConnection connection = convertAndSend(request, options); return convertResponse(connection, request); } } }利用 JDK 提供的 HttpURLConnection 发起远程的 HTTP通讯。至此发起请求的流程就完成了。下面附上一张这个过程的流程图：
对于Ribbon的调用过程请参考：https://www.jianshu.com/p/f3db11f045cc
OpenFeign Configuration ：
针对 feign 的 Configuration，官方给我们提供了很多的个性化配置，具体可以参考 org.springframework.cloud.openfeign.FeignClientProperties.FeignClientConfiguration
public static class FeignClientConfiguration { // 日志 private Logger.Level loggerLevel; // 连接超时 private Integer connectTimeout; private Integer readTimeout; //重试 private Class<Retryer> retryer; //解码 private Class<ErrorDecoder> errorDecoder; private List<Class<RequestInterceptor>> requestInterceptors; // 编码 private Boolean decode404; private Class<Decoder> decoder; private Class<Encoder> encoder; // 解析 private Class<Contract> contract; private ExceptionPropagationPolicy exceptionPropagationPolicy; }这里举个简单的例子，以Logger 为例。我们想为每个不同的 FeignClient 设置日志级别。
1、添加配置类：
@Configuration public class FooConfiguration { @Bean Logger.Level feignLoggerLevel() { return Logger.Level.FULL; } }2、配置日志级别 ,logging.level + FeignClient 包的全路径。
logging.level.com.wuzz.FeignClientService: DEBUG就这样就配置完成了。重启服务就可以看到效果。
参考： https://www.cnblogs.com/lucky-yqy/p/14589434.html
https://www.cnblogs.com/wuzhenzhao/p/13680807.html