openfeign实现原理

1.openfeign简介

OpenFeign 提供了一种声明式的远程调用接口，它可以大幅简化远程调用的编程体验。调用其他服务接口像调用本地服务service方法一样丝滑顺畅。使用示例如下：

引入依赖

<dependency>
  <groupId>org.springframework.cloudgroupId>
  <artifactId>spring-cloud-starter-openfeignartifactId>
  <version>2.2.3.RELEASEversion>
dependency>

order服务调product服务接口

@FeignClient(name = "${micro-server.mall-product}", path = "/api/mall/product")
public interface ProductService {

    @GetMapping("/sku/{skuId}")
    ResponseVO<ProductSku> getSku(@PathVariable("skuId") Long skuId);

    @PostMapping("/sku/price")
    ResponseVO<List<ProductSku>> getSkuPrice(@RequestBody SkuQuery query);
}

扫描feignClient

@SpringBootApplication
@EnableDiscoveryClient
@MapperScan(basePackages = "com.shepherd.mallorder.dao")
@ComponentScan(basePackages = {"com.shepherd"})
@EnableFeignClients(basePackages = "com.shepherd.mallorder")
@EnableRabbit    //要想监听队列接收消息必须开启此注解
public class MallOrderServiceApplication {

    public static void main(String[] args) {
        SpringApplication.run(MallOrderServiceApplication.class, args);
    }

}

基于以上操作就可以完美实现服务之间接口的调用了。

2.openfeign实现原理

OpenFeign 使用了动态代理技术来封装远程服务调用的过程。通过上面示例我们知道远程服务调用的信息被写在了 FeignClient 接口中。在上面的代码里，你可以看到，服务的名称、接口类型、访问路径已经通过注解做了声明。OpenFeign 通过解析这些注解标签生成一个“动态代理类”，这个代理类会将接口调用转化为一个远程服务调用的 Request，并发送给目标服务。

当我们项目启动时，会通过@EnableFeignClients(basePackages = "com.shepherd.mallorder")扫描相应包路径下的feignClient，然后使用动态代理技术生成代理类注入到spring容器之中，这样当我们使用feign接口方法发起远程调用，其实就是使用生成的动态代理类去实现请求。

上图中的步骤 1 到步骤 3 是在项目启动阶段加载完成的，只有第 4 步“调用远程服务”是发生在项目的运行阶段。

下面我来解释一下上图中的几个关键步骤。

首先，在项目启动阶段，OpenFeign 框架会发起一个主动的扫包流程，从指定的目录下扫描并加载所有被 @FeignClient 注解修饰的接口。然后，OpenFeign 会针对每一个 FeignClient 接口生成一个动态代理对象，即图中的FeignProxyService，这个代理对象在继承关系上属于 FeignClient 注解所修饰的接口的实例。

接下来，这个动态代理对象会被添加到 Spring 上下文中，并注入到对应的服务里，也就是图中的 LocalService 服务最后，LocalService 会发起底层方法调用。实际上这个方法调用会被 OpenFeign 生成的代理对象接管，由代理对象发起一个远程服务调用，并将调用的结果返回给LocalService。

那么， OpenFeign 是如何通过动态代理技术创建代理对象，并放到spring容器上下文中呢？

这时候我们想起了FactoryBean就是为了创建复杂的bean，我们这里生成代理类并放到spring容器中正好可以由它实现。

1.项目加载：在项目的启动阶段，EnableFeignClients 注解扮演了“启动开关”的角色，它使用 Spring 框架的 Import 注解导入了 FeignClientsRegistrar 类，开始了OpenFeign 组件的加载过程。

2.扫包：FeignClientsRegistrar 负责 FeignClient 接口的加载，它会在指定的包路径下扫描所有的 FeignClients 类，并构造 FeignClientFactoryBean 对象来解析FeignClient 接口。

3.解析 FeignClient 注解：FeignClientFactoryBean 有两个重要的功能，一个是解析FeignClient 接口中的请求路径和降级函数的配置信息；另一个是触发动态代理的构造过程。其中，动态代理构造是由更下一层的 ReflectiveFeign 完成的。

4.构建动态代理对象：ReflectiveFeign 包含了 OpenFeign 动态代理的核心逻辑，它主要负责创建出 FeignClient 接口的动态代理对象。ReflectiveFeign 在这个过程中有两个重要任务，一个是解析 FeignClient 接口上各个方法级别的注解，将其中的远程接口URL、接口类型（GET、POST 等）、各个请求参数等封装成元数据，并为每一个方法生成一个对应的 MethodHandler 类作为方法级别的代理；另一个重要任务是将这些MethodHandler 方法代理做进一步封装，通过 Java 标准的动态代理协议，构建一个实现了 InvocationHandler 接口的动态代理对象，并将这个动态代理对象绑定到FeignClient 接口上。这样一来，所有发生在 FeignClient 接口上的调用，最终都会由它背后的动态代理对象来承接。

MethodHandler 的构建过程涉及到了复杂的元数据解析，OpenFeign 组件将FeignClient 接口上的各种注解封装成元数据，并利用这些元数据把一个方法调用“翻译”成一个远程调用的 Request 请求。

那么上面说到的“元数据的解析”是如何完成的呢？它依赖于 OpenFeign 组件中的Contract 协议解析功能。Contract 是 OpenFeign 组件中定义的顶层抽象接口，它有一系列的具体实现，其中和我们实战项目有关的是 SpringMvcContract 这个类，从这个类的名字中我们就能看出来，它是专门用来解析 Spring MVC 标签的。

3.openfeign核心源码

接下来对openfeign的核心源码实现类和方法进行浅浅的分析。从EnableFeignClients注解类开始：

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(FeignClientsRegistrar.class)
public @interface EnableFeignClients {

	/**
	 * Alias for the {@link #basePackages()} attribute. Allows for more concise annotation
	 * declarations e.g.: {@code @ComponentScan("org.my.pkg")} instead of
	 * {@code @ComponentScan(basePackages="org.my.pkg")}.
	 * @return the array of 'basePackages'.
	 */
	String[] value() default {};

	/**
	 * Base packages to scan for annotated components.
	 * 

	 * {@link #value()} is an alias for (and mutually exclusive with) this attribute.
	 * 

	 * Use {@link #basePackageClasses()} for a type-safe alternative to String-based
	 * package names.
	 * @return the array of 'basePackages'.
	 */
	String[] basePackages() default {};

	/**
	 * Type-safe alternative to {@link #basePackages()} for specifying the packages to
	 * scan for annotated components. The package of each class specified will be scanned.
	 * 

	 * Consider creating a special no-op marker class or interface in each package that
	 * serves no purpose other than being referenced by this attribute.
	 * @return the array of 'basePackageClasses'.
	 */
	Class<?>[] basePackageClasses() default {};

	/**
	 * A custom @Configuration for all feign clients. Can contain override
	 * @Bean definition for the pieces that make up the client, for instance
	 * {@link feign.codec.Decoder}, {@link feign.codec.Encoder}, {@link feign.Contract}.
	 *
	 * @see FeignClientsConfiguration for the defaults
	 * @return list of default configurations
	 */
	Class<?>[] defaultConfiguration() default {};

	/**
	 * List of classes annotated with @FeignClient. If not empty, disables classpath
	 * scanning.
	 * @return list of FeignClient classes
	 */
	Class<?>[] clients() default {};

}

这个类的作用就是引入FeignClientsRegistrar类：

class FeignClientsRegistrar implements ImportBeanDefinitionRegistrar, ResourceLoaderAware, EnvironmentAware {
	// 注册动态代理类feignProxyService入口
	@Override
	public void registerBeanDefinitions(AnnotationMetadata metadata,
			BeanDefinitionRegistry registry) {
		registerDefaultConfiguration(metadata, registry);
		registerFeignClients(metadata, registry);
	}
}

registerFeignClients(metadata, registry)方法就是实现注册动态代理类放到spring容器中的：

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());
		//指定扫描类注解类型为@FeignClient的类
		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() {
				@Override
				protected boolean match(ClassMetadata metadata) {
					String cleaned = metadata.getClassName().replaceAll("\\$", ".");
					return clientClasses.contains(cleaned);
				}
			};
			scanner.addIncludeFilter(
					new AllTypeFilter(Arrays.asList(filter, annotationTypeFilter)));
		}
		//扫描定义的Feign组件包路径
		for (String basePackage : basePackages) {
			Set<BeanDefinition> candidateComponents = scanner
					.findCandidateComponents(basePackage);
			for (BeanDefinition candidateComponent : candidateComponents) {
				if (candidateComponent instanceof AnnotatedBeanDefinition) {
					// verify annotated class is an interface
					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);
					registerClientConfiguration(registry, name,
							attributes.get("configuration"));

					registerFeignClient(registry, annotationMetadata, attributes);
				}
			}
		}
	}

registerFeignClient(registry, annotationMetadata, attributes):

private void registerFeignClient(BeanDefinitionRegistry registry,
			AnnotationMetadata annotationMetadata, Map<String, Object> attributes) {
		String className = annotationMetadata.getClassName();
		//定义Feign组件的创建工厂FeignClientFactoryBean
		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 null

		beanDefinition.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);
	}

这里可以看到使用了factoryBean创建了feignclient组件bean

//定义Feign组件的创建工厂
BeanDefinitionBuilder definition = BeanDefinitionBuilder.genericBeanDefinition(FeignClientFactoryBean.class);

接下来看看Feign代理Bean实例是如何创建的，FeignClientFactoryBean：

class FeignClientFactoryBean implements FactoryBean<Object>, InitializingBean, ApplicationContextAware {
      	@Override
        public Object getObject() throws Exception {
        return getTarget();
        }
        
        <T> T getTarget() {
        	FeignContext context = applicationContext.getBean(FeignContext.class);
          //从Spring Context中获取到Feign的Builder
          Feign.Builder builder = feign(context);
          //@FeignClient注解没有配置URL属性
          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));
          }
          //处理@FeignClient URL属性(主机名)存在的情况
          if (StringUtils.hasText(this.url) && !this.url.startsWith("http")) {
            this.url = "http://" + this.url;
          }
          String url = this.url + cleanPath();
          //获取到调用客户端：Spring封装了基于Ribbon的客户端（LoadBalancerFeignClient）
          //1、Feign自己封装的Request（基于java.net原生），2、OkHttpClient（新一代/HTTP2），3、ApacheHttpClient（常规）
          Client client = getOptional(context, Client.class);
          if (client != null) {
            if (client instanceof LoadBalancerFeignClient) {
              // not lod balancing because we have a url,
              // but ribbon is on the classpath, so unwrap
              client = ((LoadBalancerFeignClient)client).getDelegate();
            }
            //设置调用客户端
            builder.client(client);
          }
          //DefaultTargeter或者HystrixTargeter，其中HystrixTargeter带熔断和降级功能
          //主要用户在Builder中配置调用失败回调方法
          Targeter targeter = get(context, Targeter.class);
          //Bean创建实际目标封装，最终生成InvocationHandler
          return targeter.target(this, builder, context, new HardCodedTarget<>(
              this.type, this.name, url));

        }
		}
		
		

执行target方法时，最终会来到ReflectiveFeign的newInstance方法：

public <T> T newInstance(Target<T> target) {
//核心方法，解析定义的@FeignClient组件中的方法和请求路径
   Map<String, MethodHandler> nameToHandler = targetToHandlersByName.apply(target);
   Map<Method, MethodHandler> methodToHandler = new LinkedHashMap<Method, MethodHandler>();
   List<DefaultMethodHandler> defaultMethodHandlers = new LinkedList<DefaultMethodHandler>();

   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)));
     }
   }
//调用工厂Bean，创建执行Handler
   InvocationHandler handler = factory.create(target, methodToHandler);
   T proxy = (T) Proxy.newProxyInstance(target.type().getClassLoader(), new Class<?>[]{target.type()}, handler);

   for(DefaultMethodHandler defaultMethodHandler : defaultMethodHandlers) {
     defaultMethodHandler.bindTo(proxy);
   }
   return proxy;
 }

请求方法和路径解析器ParseHandlersByName:

static final class ParseHandlersByName {

   private final Contract contract;
   private final Options options;
   private final Encoder encoder;
   private final Decoder decoder;
   private final ErrorDecoder errorDecoder;
   private final SynchronousMethodHandler.Factory factory;

   ParseHandlersByName(Contract contract, Options options, Encoder encoder, Decoder decoder,
                       ErrorDecoder errorDecoder, SynchronousMethodHandler.Factory factory) {
     this.contract = contract;
     this.options = options;
     this.factory = factory;
     this.errorDecoder = errorDecoder;
     this.encoder = checkNotNull(encoder, "encoder");
     this.decoder = checkNotNull(decoder, "decoder");
   }

   public Map<String, MethodHandler> apply(Target key) {
 //核心方法：解析@FeignClient组件，Feign自带的则解析自己的注解格式，Spring提供了解析MVC注解的SpringMvcContract
     List<MethodMetadata> metadata = contract.parseAndValidatateMetadata(key.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);
       } else if (md.bodyIndex() != null) {
         buildTemplate = new BuildEncodedTemplateFromArgs(md, encoder);
       } else {
         buildTemplate = new BuildTemplateByResolvingArgs(md);
       }
       result.put(md.configKey(),
                  factory.create(key, md, buildTemplate, options, decoder, errorDecoder));
     }
     return result;
   }
 }

到这里，feignClient和接口方法的解析、封装已完成，并放入到spring容器之中。

接下来就是使用feignClient调用远程服务了，根据创建流程中Targeter对象，有2个实现，一个为DefaultTargeter，源码如下:

class DefaultTargeter implements Targeter {

	@Override
	public <T> T target(FeignClientFactoryBean factory, Feign.Builder feign, FeignContext context,
						Target.HardCodedTarget<T> target) {
		//简单暴力，直接调用Feign Builer字段的设置目标对象方法
		return feign.target(target);
	}
}

上面的方法最终会调用Feign原生的InvocationHandlerFactory.Default工厂来创建FeignInvocationHandler:

//第一部分：封装处理请求的Hander
static class FeignInvocationHandler implements InvocationHandler {

    private final Target target;
    private final Map<Method, MethodHandler> dispatch;

    FeignInvocationHandler(Target target, Map<Method, MethodHandler> dispatch) {
      this.target = checkNotNull(target, "target");
      this.dispatch = checkNotNull(dispatch, "dispatch for %s", target);
    }

    @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();
      }
      return dispatch.get(method).invoke(args);
    }

    @Override
    public boolean equals(Object obj) {
      if (obj instanceof FeignInvocationHandler) {
        FeignInvocationHandler other = (FeignInvocationHandler) obj;
        return target.equals(other.target);
      }
      return false;
    }

    @Override
    public int hashCode() {
      return target.hashCode();
    }

    @Override
    public String toString() {
      return target.toString();
    }
  }
  //第二部分：解析出来的实际处理请求的方法
  final class SynchronousMethodHandler implements MethodHandler {

  private static final long MAX_RESPONSE_BUFFER_SIZE = 8192L;

  private final MethodMetadata metadata;
  private final Target<?> target;
  private final Client client;
  private final Retryer retryer;
  private final List<RequestInterceptor> requestInterceptors;
  private final Logger logger;
  private final Logger.Level logLevel;
  private final RequestTemplate.Factory buildTemplateFromArgs;
  private final Options options;
  private final Decoder decoder;
  private final ErrorDecoder errorDecoder;
  private final boolean decode404;

  private SynchronousMethodHandler(Target<?> target, Client client, Retryer retryer,
                                   List<RequestInterceptor> requestInterceptors, Logger logger,
                                   Logger.Level logLevel, MethodMetadata metadata,
                                   RequestTemplate.Factory buildTemplateFromArgs, Options options,
                                   Decoder decoder, ErrorDecoder errorDecoder, boolean decode404) {
    this.target = checkNotNull(target, "target");
    this.client = checkNotNull(client, "client for %s", target);
    this.retryer = checkNotNull(retryer, "retryer for %s", target);
    this.requestInterceptors =
        checkNotNull(requestInterceptors, "requestInterceptors for %s", target);
    this.logger = checkNotNull(logger, "logger for %s", target);
    this.logLevel = checkNotNull(logLevel, "logLevel for %s", target);
    this.metadata = checkNotNull(metadata, "metadata for %s", target);
    this.buildTemplateFromArgs = checkNotNull(buildTemplateFromArgs, "metadata for %s", target);
    this.options = checkNotNull(options, "options for %s", target);
    this.errorDecoder = checkNotNull(errorDecoder, "errorDecoder for %s", target);
    this.decoder = checkNotNull(decoder, "decoder for %s", target);
    this.decode404 = decode404;
  }

  @Override
  public Object invoke(Object[] argv) throws Throwable {
    RequestTemplate template = buildTemplateFromArgs.create(argv);
    Retryer retryer = this.retryer.clone();
    while (true) {
      try {
		//执行请求并解析
        return executeAndDecode(template);
      } catch (RetryableException e) {
        retryer.continueOrPropagate(e);
        if (logLevel != Logger.Level.NONE) {
          logger.logRetry(metadata.configKey(), logLevel);
        }
        continue;
      }
    }
  }

  Object executeAndDecode(RequestTemplate template) throws Throwable {
    Request request = targetRequest(template);

    if (logLevel != Logger.Level.NONE) {
      logger.logRequest(metadata.configKey(), logLevel, request);
    }

    Response response;
    long start = System.nanoTime();
    try {
      response = client.execute(request, options);
      // ensure the request is set. TODO: remove in Feign 10
      response.toBuilder().request(request).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);

    boolean shouldClose = true;
    try {
      if (logLevel != Logger.Level.NONE) {
        response =
            logger.logAndRebufferResponse(metadata.configKey(), logLevel, response, elapsedTime);
        // ensure the request is set. TODO: remove in Feign 10
        response.toBuilder().request(request).build();
      }
      if (Response.class == metadata.returnType()) {
        if (response.body() == null) {
          return response;
        }
        if (response.body().length() == null ||
                response.body().length() > MAX_RESPONSE_BUFFER_SIZE) {
          shouldClose = false;
          return response;
        }
        // Ensure the response body is disconnected
        byte[] bodyData = Util.toByteArray(response.body().asInputStream());
        return response.toBuilder().body(bodyData).build();
      }
      if (response.status() >= 200 && response.status() < 300) {
        if (void.class == metadata.returnType()) {
          return null;
        } else {
          return decode(response);
        }
      } else if (decode404 && response.status() == 404) {
        return decode(response);
      } else {
        throw errorDecoder.decode(metadata.configKey(), response);
      }
    } catch (IOException e) {
      if (logLevel != Logger.Level.NONE) {
        logger.logIOException(metadata.configKey(), logLevel, e, elapsedTime);
      }
      throw errorReading(request, response, e);
    } finally {
      if (shouldClose) {
        ensureClosed(response.body());
      }
    }
  }

  long elapsedTime(long start) {
    return TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - start);
  }

  Request targetRequest(RequestTemplate template) {
    for (RequestInterceptor interceptor : requestInterceptors) {
      interceptor.apply(template);
    }
    return target.apply(new RequestTemplate(template));
  }

  Object decode(Response response) throws Throwable {
    try {
      return decoder.decode(response, metadata.returnType());
    } catch (FeignException e) {
      throw e;
    } catch (RuntimeException e) {
      throw new DecodeException(e.getMessage(), e);
    }
  }

再看一下带熔断功能的HystrixTargeter：

class HystrixTargeter implements Targeter {

	@Override
	public <T> T target(FeignClientFactoryBean factory, Feign.Builder feign, FeignContext context,
						Target.HardCodedTarget<T> target) {
		//Feign实例不是HystrixFeign.Builder的则忽略，调用原生的构建方法
		if (!(feign instanceof feign.hystrix.HystrixFeign.Builder)) {
			return feign.target(target);
		}
		//以下方法则使用HystrixFeign Builder （自定义了构建实际代理对象HystrixInvocationHandler）构建一个执行处理器
		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);
		}

		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);
		/* We take a sample fallback from the fallback factory to check if it returns a fallback
		that is compatible with the annotated feign interface. */
		Object exampleFallback = fallbackFactory.create(new RuntimeException());
		Assert.notNull(exampleFallback,
			String.format(
			"Incompatible fallbackFactory instance for feign client %s. Factory may not produce null!",
				feignClientName));
		if (!target.type().isAssignableFrom(exampleFallback.getClass())) {
			throw new IllegalStateException(
				String.format(
					"Incompatible fallbackFactory instance for feign client %s. Factory produces instances of '%s', but should produce instances of '%s'",
					feignClientName, exampleFallback.getClass(), target.type()));
		}
		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);
	}

	private <T> T getFromContext(String fallbackMechanism, String feignClientName, FeignContext context,
								 Class<?> beanType, Class<T> targetType) {
		Object fallbackInstance = context.getInstance(feignClientName, beanType);
		if (fallbackInstance == null) {
			throw new IllegalStateException(String.format(
				"No " + fallbackMechanism + " instance of type %s found for feign client %s",
				beanType, feignClientName));
		}

		if (!targetType.isAssignableFrom(beanType)) {
			throw new IllegalStateException(
					String.format(
						"Incompatible " + fallbackMechanism + " instance. Fallback/fallbackFactory of type %s is not assignable to %s for feign client %s",
						beanType, targetType, feignClientName));
		}
		return (T) fallbackInstance;
	}

	private <T> T getOptional(String feignClientName, FeignContext context,
		Class<T> beanType) {
		return context.getInstance(feignClientName, beanType);
	}

最终HystrixTargeter创建出HystrixInvocationHandler，其源码为：

final class HystrixInvocationHandler implements InvocationHandler {

  private final Target<?> target;
  private final Map<Method, MethodHandler> dispatch;
  private final FallbackFactory<?> fallbackFactory; // Nullable
  private final Map<Method, Method> fallbackMethodMap;
  private final Map<Method, Setter> setterMethodMap;

  HystrixInvocationHandler(Target<?> target, Map<Method, MethodHandler> dispatch,
                           SetterFactory setterFactory, FallbackFactory<?> fallbackFactory) {
    this.target = checkNotNull(target, "target");
    this.dispatch = checkNotNull(dispatch, "dispatch");
    this.fallbackFactory = fallbackFactory;
    this.fallbackMethodMap = toFallbackMethod(dispatch);
    this.setterMethodMap = toSetters(setterFactory, target, dispatch.keySet());
  }

  /**
   * If the method param of InvocationHandler.invoke is not accessible, i.e in a package-private
   * interface, the fallback call in hystrix command will fail cause of access restrictions. But
   * methods in dispatch are copied methods. So setting access to dispatch method doesn't take
   * effect to the method in InvocationHandler.invoke. Use map to store a copy of method to invoke
   * the fallback to bypass this and reducing the count of reflection calls.
   *
   * @return cached methods map for fallback invoking
   */
  static Map<Method, Method> toFallbackMethod(Map<Method, MethodHandler> dispatch) {
    Map<Method, Method> result = new LinkedHashMap<Method, Method>();
    for (Method method : dispatch.keySet()) {
      method.setAccessible(true);
      result.put(method, method);
    }
    return result;
  }

  /**
   * Process all methods in the target so that appropriate setters are created.
   */
  static Map<Method, Setter> toSetters(SetterFactory setterFactory, Target<?> target,
                                       Set<Method> methods) {
    Map<Method, Setter> result = new LinkedHashMap<Method, Setter>();
    for (Method method : methods) {
      method.setAccessible(true);
      result.put(method, setterFactory.create(target, method));
    }
    return result;
  }

  @Override
  public Object invoke(final Object proxy, final Method method, final Object[] args)
      throws Throwable {
    // early exit if the invoked method is from java.lang.Object
    // code is the same as ReflectiveFeign.FeignInvocationHandler
    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();
    }
	//核心方法位置，在此，则实现把客户端请求封装为Hystrix命令的方式进行调用，实现了调用的熔断与降级
    HystrixCommand<Object> hystrixCommand = new HystrixCommand<Object>(setterMethodMap.get(method)) {
      @Override
      protected Object run() throws Exception {
        try {
		//实际调用的请求方法已在上一个默认Handler的第二部分指出
          return HystrixInvocationHandler.this.dispatch.get(method).invoke(args);
        } catch (Exception e) {
          throw e;
        } catch (Throwable t) {
          throw (Error) t;
        }
      }

      @Override
      protected Object getFallback() {
        if (fallbackFactory == null) {
          return super.getFallback();
        }
        try {
          Object fallback = fallbackFactory.create(getExecutionException());
          Object result = fallbackMethodMap.get(method).invoke(fallback, args);
          if (isReturnsHystrixCommand(method)) {
            return ((HystrixCommand) result).execute();
          } else if (isReturnsObservable(method)) {
            // Create a cold Observable
            return ((Observable) result).toBlocking().first();
          } else if (isReturnsSingle(method)) {
            // Create a cold Observable as a Single
            return ((Single) result).toObservable().toBlocking().first();
          } else if (isReturnsCompletable(method)) {
            ((Completable) result).await();
            return null;
          } else {
            return result;
          }
        } catch (IllegalAccessException e) {
          // shouldn't happen as method is public due to being an interface
          throw new AssertionError(e);
        } catch (InvocationTargetException e) {
          // Exceptions on fallback are tossed by Hystrix
          throw new AssertionError(e.getCause());
        }
      }
    };

    if (isReturnsHystrixCommand(method)) {
      return hystrixCommand;
    } else if (isReturnsObservable(method)) {
      // Create a cold Observable
      return hystrixCommand.toObservable();
    } else if (isReturnsSingle(method)) {
      // Create a cold Observable as a Single
      return hystrixCommand.toObservable().toSingle();
    } else if (isReturnsCompletable(method)) {
      return hystrixCommand.toObservable().toCompletable();
    }
    return hystrixCommand.execute();
  }

至此，一个Feign代理Bean的创建过程和使用该bean发起远程调用逻辑已经讲述完毕。