spring-cloud-openfeign-core-2.1.1.RELEASE.jar 中 HystrixFeign 的詳細構建過程：

@EnableFeignClients -> FeignClientsRegistrar 掃描 @Feign注解的類 -> FeignClientFactoryBean通過Targeter生產FeignClient -> Targeter通過Feign.Builder構建Feign -> Feign.Builder

1. 準備工作（配置）

FeignAutoConfiguration自動配置類

    @Configuration@ConditionalOnClass(name = "feign.hystrix.HystrixFeign")protected static class HystrixFeignTargeterConfiguration {@Bean@ConditionalOnMissingBeanpublic Targeter feignTargeter() {return new HystrixTargeter();}}@Configuration@ConditionalOnMissingClass("feign.hystrix.HystrixFeign")protected static class DefaultFeignTargeterConfiguration {@Bean@ConditionalOnMissingBeanpublic Targeter feignTargeter() {return new DefaultTargeter();}}

feign.hystrix.HystrixFeign類存在時，將 HystrixTargeter 注冊為 Targeter 類型的 bean
feign.hystrix.HystrixFeign類不存在時，使用 DefaultTargeter 。
看起來似乎可以使用自定義的Targeter代替Hystrix或默認的，這樣就可以自定義各種功能了。實際上不行，因為 Targeter 是 package 訪問級別的。
FeignClientsConfiguration

@Configuration
public class FeignClientsConfiguration {@Bean@ConditionalOnMissingBeanpublic Retryer feignRetryer() {return Retryer.NEVER_RETRY;}@Bean@Scope("prototype")@ConditionalOnMissingBeanpublic Feign.Builder feignBuilder(Retryer retryer) {return Feign.builder().retryer(retryer);}@Configuration@ConditionalOnClass({ HystrixCommand.class, HystrixFeign.class })protected static class HystrixFeignConfiguration {@Bean@Scope("prototype")@ConditionalOnMissingBean@ConditionalOnProperty(name = "feign.hystrix.enabled")public Feign.Builder feignHystrixBuilder() {return HystrixFeign.builder();}}
}

重要：Feign 以及內部類 Feign.Builder 都是 public 訪問級別，可以注入自定義的bean。

2.EnableFeignClients與FeignClientsRegistrar類

將使用@FeignClient注解的類注冊成spring bean，并使用注解中的配置

在@EnableFeignClients注解中導入FeignClientsRegistrar類
FeignClientsRegistrar類實現了ImportBeanDefinitionRegistrar類，會由spring框架執行實現方法 registerBeanDefinitions(AnnotationMetaData, BeanDefinitionRegistry)
FeignClientsRegistrar中的 registerBeanDefinitions方法調用兩個方法
1. registerDefaultConfiguration：注冊默認的配置
2. registerFeignClients：注冊Feign客戶端（重點）
registerFeignClients：獲取 @EnableFeignClients注解中定義的配置掃描feign客戶端
registerFeignClients：通過registerFeignClient(BeanDefinitionRegistry, AnnotationMetadata, Map)方法注冊每一個feignClient，過程：先獲取 @FeignClient注解中定義的配置，將配置應用在spring bean 工廠 FeignClientFactoryBean，通過工廠類 FeignClientFactoryBean 為每一個使用@FeignClient注解的類生產 FeignClient，詳細過程見下一節

3.FeignClientFactoryBean

FeignClient工廠bean。

class FeignClientFactoryBeanimplements FactoryBean<Object>, InitializingBean, ApplicationContextAware{//...
}

通過實現方法 FactoryBean#getObject()來由spring框架生產FeignClient。

@Override
public Object getObject() throws Exception {return getTarget();
}/*** 獲得目標* 1. 獲得FeignContext* 2. 從FeignContext中獲得Feign構建器Feign.Builder* 3. 從FeignContext中獲得Client，判斷是否進行負載均衡* 4. 從FeignContext中獲得Target，并執行Target的默認方法target(FeignClientFactoryBean, Feign.Builder,FeignContext, Target.HardCodedTarget<T>);* 5.由于一開始注入的Feign.Builder是HystrixFeign.Builder,則此處是調用HystrixFeign.Builder里的對應方法*/
<T> T getTarget() {FeignContext context = this.applicationContext.getBean(FeignContext.class);Feign.Builder builder = feign(context);//省略部分代碼// ......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 (T) targeter.target(this, builder, context,new HardCodedTarget<>(this.type, this.name, url));
}protected Feign.Builder feign(FeignContext context) {FeignLoggerFactory loggerFactory = get(context, FeignLoggerFactory.class);Logger logger = loggerFactory.create(this.type);// @formatter:offFeign.Builder builder = get(context, Feign.Builder.class)// required values.logger(logger).encoder(get(context, Encoder.class)).decoder(get(context, Decoder.class)).contract(get(context, Contract.class));// @formatter:onconfigureFeign(context, builder);return builder;}

工廠獲得對象（目標）：

1. 獲得FeignContext（feign上下文）
2. 從FeignContext中獲得Feign構建器Feign.Builder（public，可以在此使用自定義構建器）
3. 從FeignContext中獲得Client，判斷是否進行負載均衡
4. 從FeignContext中獲得Target，并執行Target的默認方法target(FeignClientFactoryBean, Feign.Builder,FeignContext, Target.HardCodedTarget<T>);
5. 由于一開始注入的 *Targeter* 是 *HystrixTargeter* ,則此處是調用 HystrixTargeter 里的對應方法（從第一節的配置來看，只要 *feign.hystrix.HystrixFeign* 類存在，就是注入的 *HystrixTargeter *， 否則是 *DefaultTargeter*，對于需要**自定義構建feign的，這里不太重要**）

4.Targeter

4.1.HystrixTargeter

class HystrixTargeter implements Targeter {@Overridepublic <T> T target(FeignClientFactoryBean factory, Feign.Builder feign,FeignContext context, Target.HardCodedTarget<T> target) {// 若不是 HystrixFeign，則執行其對應的默認target方法。// 此處只處理HystrixFeign。if (!(feign instanceof feign.hystrix.HystrixFeign.Builder)) {return feign.target(target);}feign.hystrix.HystrixFeign.Builder builder = (feign.hystrix.HystrixFeign.Builder) feign;SetterFactory setterFactory = getOptional(factory.getName(), context,SetterFactory.class);if (setterFactory != null) {builder.setterFactory(setterFactory);}Class<?> fallback = factory.getFallback();if (fallback != void.class) {return targetWithFallback(factory.getName(), context, target, builder,fallback);}Class<?> fallbackFactory = factory.getFallbackFactory();if (fallbackFactory != void.class) {return targetWithFallbackFactory(factory.getName(), context, target, builder,fallbackFactory);}// 調用從Feign.Builder繼承的方法。return feign.target(target);}private <T> T targetWithFallbackFactory(String feignClientName, FeignContext context,Target.HardCodedTarget<T> target, HystrixFeign.Builder builder,Class<?> fallbackFactoryClass) {FallbackFactory<? extends T> fallbackFactory = (FallbackFactory<? extends T>) getFromContext("fallbackFactory", feignClientName, context, fallbackFactoryClass,FallbackFactory.class);return builder.target(target, fallbackFactory);}private <T> T targetWithFallback(String feignClientName, FeignContext context,Target.HardCodedTarget<T> target, HystrixFeign.Builder builder,Class<?> fallback) {T fallbackInstance = getFromContext("fallback", feignClientName, context,fallback, target.type());return builder.target(target, fallbackInstance);}//...
}

HystrixTarget只處理 Feign.Builder 類型為 feign.hystrix.HystrixFeign.Builder 的
若feign構建器不是 feign.hystrix.HystrixFeign.Builder 類型，則執行注入的 feign 構建器的默認target方法
因此，即使注入的 Targeter 是 HystrixTargeter,此處也可以執行自定義 Feign.Builder。
理解：Feign.Builder#target(Target) 方法通常不會被 override（后續會講解為什么不重寫此方法）

4.2.DefaultTargeter

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

執行 Feign.Builder （子）類型對應的默認 target方法。
理解：Feign.Builder#target(Target) 方法通常不會被 override（后續會講解為什么不重寫此方法）

5.FeignBuilder

feign構建器：構建feign對象。

Feign的目的是將 http api 包裝成 restful 風格以便開發。

在實現中，Feign 是一個為目標http apis 生成 feign對象（Feign#newInstance）的工廠。

上述步驟目前需要的都是通過對應的 Builder 構建對應的Feign。

public abstract class Feign {public static Builder builder() {return new Builder();}public abstract <T> T newInstance(Target<T> target);public static class Builder {public <T> T target(Target<T> target) {return build().newInstance(target);}public Feign build() {SynchronousMethodHandler.Factory synchronousMethodHandlerFactory =new SynchronousMethodHandler.Factory(client, retryer, requestInterceptors, logger, logLevel, decode404, closeAfterDecode, propagationPolicy);ParseHandlersByName handlersByName =new ParseHandlersByName(contract, options, encoder, decoder, queryMapEncoder,errorDecoder, synchronousMethodHandlerFactory);return new ReflectiveFeign(handlersByName, invocationHandlerFactory, queryMapEncoder);}}
}

Feign.Builder#target(Target) 方法里面實際上調用的是 build() 方法來構建對象，因此重寫 build() 方法即可，沒有必要還重寫 target(Target) 方法
Feign 以及內部類 Feign.Builder 都是 public ，可以重寫并注入自定義的bean。

5.1.HystrixFeign

public final class HystrixFeign {public static final class Builder extends Feign.Builder {  @Overridepublic Feign build() {return build(null);}// 提供一系列的target方法，支持各種配置：fallback、FallBackFactory等public <T> T target(Target<T> target, T fallback) {return build(fallback != null ? new FallbackFactory.Default<T>(fallback) : null).newInstance(target);}public <T> T target(Target<T> target, FallbackFactory<? extends T> fallbackFactory) {return build(fallbackFactory).newInstance(target);}public <T> T target(Class<T> apiType, String url, T fallback) {return target(new Target.HardCodedTarget<T>(apiType, url), fallback);}public <T> T target(Class<T> apiType,String url,FallbackFactory<? extends T> fallbackFactory) {return target(new Target.HardCodedTarget<T>(apiType, url), fallbackFactory);}/** Configures components needed for hystrix integration. */Feign build(final FallbackFactory<?> nullableFallbackFactory) {super.invocationHandlerFactory(new InvocationHandlerFactory() {@Overridepublic InvocationHandler create(Target target,Map<Method, MethodHandler> dispatch) {return new HystrixInvocationHandler(target, dispatch, setterFactory,nullableFallbackFactory);}});super.contract(new HystrixDelegatingContract(contract));return super.build();}

基本到了這一步，需要設置的東西，都可以配置了。

雖然 build 方法中涉及到 InvocationHandler,但基本不需要改什么，而 InvocationHandler 竟然也是 package 訪問級別，所以只好復制一個，使用自己的。
HystrixDelegatingContract 是 public 級別，不需要修改的話，仍然用這個。

5.2示例

以下示例參考 SentinelFeign

@Override
public Feign build() {super.invocationHandlerFactory(new InvocationHandlerFactory() {@Overridepublic InvocationHandler create(Target target,Map<Method, MethodHandler> dispatch) {// using reflect get fallback and fallbackFactory properties from// FeignClientFactoryBean because FeignClientFactoryBean is a package// level class, we can not use it in our packageObject feignClientFactoryBean = Builder.this.applicationContext.getBean("&" + target.type().getName());Class fallback = (Class) getFieldValue(feignClientFactoryBean,"fallback");Class fallbackFactory = (Class) getFieldValue(feignClientFactoryBean,"fallbackFactory");String name = (String) getFieldValue(feignClientFactoryBean, "name");Object fallbackInstance;FallbackFactory fallbackFactoryInstance;// check fallback and fallbackFactory properties// 以下邏輯在HystrixTargeter中有，但執行自定義的builder，不會執行到那段邏輯，因此此處加上。if (void.class != fallback) {fallbackInstance = getFromContext(name, "fallback", fallback,target.type());return new PegasusInvocationHandler(target, dispatch, setterFactory,new FallbackFactory.Default(fallbackInstance));}if (void.class != fallbackFactory) {fallbackFactoryInstance = (FallbackFactory) getFromContext(name,"fallbackFactory", fallbackFactory,FallbackFactory.class);return new PegasusInvocationHandler(target, dispatch, setterFactory,fallbackFactoryInstance);}// 此處還是會使用一個默認的FallbackFactory。return new PegasusInvocationHandler(target, dispatch, setterFactory, new PegasusFeignFallbackFactory<>(target));}private Object getFromContext(String name, String type,Class fallbackType, Class targetType) {Object fallbackInstance = feignContext.getInstance(name,fallbackType);if (fallbackInstance == null) {throw new IllegalStateException(String.format("No %s instance of type %s found for feign client %s",type, fallbackType, name));}if (!targetType.isAssignableFrom(fallbackType)) {throw new IllegalStateException(String.format("Incompatible %s instance. Fallback/fallbackFactory of type %s is not assignable to %s for feign client %s",type, fallbackType, targetType, name));}return fallbackInstance;}});super.contract(new HystrixDelegatingContract(contract));return super.build();
}

需要自定義fallbackFactory，則實現 feign.hystrix.FallbackFactory類，需要自定義fallback，則實現 org.springframework.cglib.proxy.MethodInterceptor即可