spring-cloud-openfeign-core-2.1.1.RELEASE.jar 中 HystrixFeign 的详细构建过程：
@EnableFeignClients -> FeignClientsRegistrar 扫描 @Feign注解的类 -> FeignClientFactoryBean通过Targeter生产FeignClient -> Targeter通过Feign.Builder构建Feign -> Feign.Builder
1. 准备工作（配置）
- FeignAutoConfiguration自动配置类
@Configuration @ConditionalOnClass(name = "feign.hystrix.HystrixFeign") protected static class HystrixFeignTargeterConfiguration { @Bean @ConditionalOnMissingBean public Targeter feignTargeter() { return new HystrixTargeter(); } } @Configuration @ConditionalOnMissingClass("feign.hystrix.HystrixFeign") protected static class DefaultFeignTargeterConfiguration { @Bean @ConditionalOnMissingBean public Targeter feignTargeter() { return new DefaultTargeter(); } }- feign.hystrix.HystrixFeign类存在时，将
HystrixTargeter注册为Targeter类型的 bean - feign.hystrix.HystrixFeign类不存在时，使用
DefaultTargeter。 - 看起来似乎可以使用自定义的Targeter代替Hystrix或默认的，这样就可以自定义各种功能了。实际上不行，因为
Targeter是 package 访问级别的。
- FeignClientsConfiguration
@Configurationpublic class FeignClientsConfiguration { @Bean @ConditionalOnMissingBean public Retryer feignRetryer() { return Retryer.NEVER_RETRY; } @Bean @Scope("prototype") @ConditionalOnMissingBean public 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 以及内部类 Feign.Builder 都是 public 访问级别，可以注入自定义的bean。
2.EnableFeignClients与FeignClientsRegistrar类
将使用@FeignClient注解的类注册成spring bean，并使用注解中的配置
- 在@EnableFeignClients注解中导入FeignClientsRegistrar类
- FeignClientsRegistrar类实现了ImportBeanDefinitionRegistrar类，会由spring框架执行实现方法
registerBeanDefinitions(AnnotationMetaData, BeanDefinitionRegistry) FeignClientsRegistrar中的
registerBeanDefinitions方法调用两个方法registerDefaultConfiguration：注册默认的配置registerFeignClients：注册Feign客户端（重点）
registerFeignClients：获取@EnableFeignClients注解中定义的配置扫描feign客户端registerFeignClients：通过registerFeignClient(BeanDefinitionRegistry, AnnotationMetadata, Map)方法注册每一个feignClient，过程：先获取@FeignClient注解中定义的配置，将配置应用在spring bean 工厂FeignClientFactoryBean， 通过工厂类FeignClientFactoryBean为每一个使用@FeignClient注解的类生产FeignClient，详细过程见下一节
3.FeignClientFactoryBean
FeignClient工厂bean。
class FeignClientFactoryBean implements FactoryBean<Object>, InitializingBean, ApplicationContextAware{ //...}通过实现方法 FactoryBean#getObject()来由spring框架生产FeignClient。
@Overridepublic Object getObject() throws Exception { return getTarget();}/** * 获得目标 * 1. 获得FeignContext * 2. 从FeignContext中获得Feign构建器Feign.Builder * 3. 从FeignContext中获得Client，判断是否进行负载均衡 * 4. 从FeignContext中获得Target，并执行Target的默认方法target(FeignClientFactoryBean, Feign.Builder, FeignContext, Target.HardCodedTarget<T>); * 5.由于一开始注入的Feign.Builder是HystrixFeign.Builder,则此处是调用HystrixFeign.Builder里的对应方法 */<T> T getTarget() { FeignContext context = this.applicationContext.getBean(FeignContext.class); Feign.Builder builder = feign(context); //省略部分代码 // ...... 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 unwrap client = ((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:off Feign.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:on configureFeign(context, builder); return builder; }工厂获得对象（目标）：
1. 获得FeignContext（feign上下文）2. 从FeignContext中获得Feign构建器Feign.Builder（public，可以在此使用自定义构建器）3. 从FeignContext中获得Client，判断是否进行负载均衡4. 从FeignContext中获得Target，并执行Target的默认方法target(FeignClientFactoryBean, Feign.Builder, FeignContext, Target.HardCodedTarget<T>);5. 由于一开始注入的 *Targeter* 是 *HystrixTargeter* ,则此处是调用 HystrixTargeter 里的对应方法（从第一节的配置来看，只要 *feign.hystrix.HystrixFeign* 类存在，就是注入的 *HystrixTargeter *， 否则是 *DefaultTargeter*，对于需要**自定义构建feign的，这里不太重要**）4.Targeter
4.1.HystrixTargeter
class HystrixTargeter implements Targeter { @Override public <T> T target(FeignClientFactoryBean factory, Feign.Builder feign, FeignContext context, Target.HardCodedTarget<T> target) { // 若不是 HystrixFeign，则执行其对应的默认target方法。 // 此处只处理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); } // 调用从Feign.Builder继承的方法。 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只处理 Feign.Builder 类型为 feign.hystrix.HystrixFeign.Builder 的
- 若feign构建器不是 feign.hystrix.HystrixFeign.Builder 类型，则执行注入的 feign 构建器的默认target方法
- 因此，即使注入的 Targeter 是 HystrixTargeter,此处也可以执行自定义 Feign.Builder。
- 理解：Feign.Builder#target(Target) 方法通常不会被 override（后续会讲解为什么不重写此方法）
4.2.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); }}- 执行 Feign.Builder （子）类型对应的 默认 target方法。
- 理解：Feign.Builder#target(Target) 方法通常不会被 override（后续会讲解为什么不重写此方法）
5.FeignBuilder
feign构建器：构建feign对象。
Feign的目的是将 http api 包装成 restful 风格以便开发。在实现中，Feign 是一个为目标http apis 生成 feign对象（
Feign#newInstance）的工厂。
上述步骤目前需要的都是通过对应的 Builder 构建对应的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) 方法里面实际上调用的是 build() 方法来构建对象，因此重写 build() 方法即可，没有必要还重写 target(Target) 方法
- Feign 以及内部类 Feign.Builder 都是 public ，可以重写并注入自定义的bean。
5.1.HystrixFeign
public final class HystrixFeign { public static final class Builder extends Feign.Builder { @Override public Feign build() { return build(null); } // 提供一系列的target方法，支持各种配置：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() { @Override public InvocationHandler create(Target target, Map<Method, MethodHandler> dispatch) { return new HystrixInvocationHandler(target, dispatch, setterFactory, nullableFallbackFactory); } }); super.contract(new HystrixDelegatingContract(contract)); return super.build(); }基本到了这一步，需要设置的东西，都可以配置了。
- 虽然 build 方法中涉及到 InvocationHandler,但基本不需要改什么，而 InvocationHandler 竟然也是 package 访问级别，所以只好复制一个，使用自己的。
- HystrixDelegatingContract 是 public 级别，不需要修改的话，仍然用这个。
5.2示例
以下示例参考 SentinelFeign
@Overridepublic Feign build() { super.invocationHandlerFactory(new InvocationHandlerFactory() { @Override public 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 package Object 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 // 以下逻辑在HystrixTargeter中有，但执行自定义的builder，不会执行到那段逻辑，因此此处加上。 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); } // 此处还是会使用一个默认的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();}需要自定义fallbackFactory，则实现 feign.hystrix.FallbackFactory类，需要自定义fallback，则实现 org.springframework.cglib.proxy.MethodInterceptor即可
6.总结
- 由于Feign构建过程所用到的 Targeter 是 package 访问级别的，不能使用自定义的
- Feign以及Feign.Builder是 publilc，给了我们扩展的空间。
7.参考资料
- feign-hystrix-10.1.0.jar和spring-cloud-openfeign-core-2.1.1.RELEASE.jar
- spring-cloud-alibaba-sentinel-0.9.0.RELEASE.jar中的 sentinelFeign 实现
- Spring Cloud Alibaba Sentinel 整合 Feign 的设计实现