探究清楚 feign 的原理，自定义 feign 功能

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

@Configuration
public 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
1. registerDefaultConfiguration：注册默认的配置
2. 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。

@Override
public 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
其中的 YiFeiXiInvocationHandler 和 YiFeiXiFeignFallbackFactory是自定义的。

@Override
public 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 YiFeiXiInvocationHandler(target, dispatch, setterFactory,
                                                    new FallbackFactory.Default(fallbackInstance));
            }
            if (void.class != fallbackFactory) {
                fallbackFactoryInstance = (FallbackFactory) getFromContext(name,
                                                                           "fallbackFactory", fallbackFactory,
                                                                           FallbackFactory.class);
                return new YiFeiXiInvocationHandler(target, dispatch, setterFactory,
                                                    fallbackFactoryInstance);
            }
            // 若注解中没有使用fallback或fallbackFactory，则使用一个默认的FallbackFactory。
            return new YiFeiXiInvocationHandler(target, dispatch, setterFactory, new YiFeiXiFeignFallbackFactory<>(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

Feign详细构建过程及自定义扩展