环境：springboot2.2.13.RELEASE + springcloud Hoxton.SR8

1 开启Feign功能

@SpringCloudApplication
@ServletComponentScan
@EnableFeignClients("com.pack.feign")
public class BaseApplication extends SpringBootServletinitializer {
  @Override
  protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
    return builder.sources(BaseApplication.class);
  }

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

查看EnableFeignClients源码

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

@Import导入了其它类，关于@Import的使用已经介绍过了。直接查看FeignClientsRegistrar

查看核心代码

@Override
public void registerBeanDeFinitions(AnnotationMetadata Metadata, BeanDeFinitionRegistry registry) {
  registerDefaultConfiguration(Metadata, registry);
  registerFeignClients(Metadata, registry);
}

registerFeignClients方法用来扫描指定包下带有@FeignClient注解的类然后注册为Bean。

2 注册FeignClientBean

扫描及注册Bean

public void registerFeignClients(AnnotationMetadata Metadata, BeanDeFinitionRegistry registry) {
    ClasspathScanningCandidateComponentProvider scanner = getScanner();
    scanner.setResourceLoader(this.resourceLoader);

    Set<String> basePackages;

    // 获取添加注解@EnableFeignClients时的相关配置信息
    Map<String, Object> attrs = Metadata
        .getAnnotationAttributes(EnableFeignClients.class.getName());
    // 设置过滤条件，只匹配带有@FeignClient的注解
    AnnotationTypeFilter annotationTypeFilter = new AnnotationTypeFilter(
        FeignClient.class);
    final Class<?>[] clients = attrs == null ? null
        : (Class<?>[]) attrs.get("clients");
    if (clients == null || clients.length == 0) {
        // 过滤条件添加到Scanner中
        scanner.addIncludeFilter(annotationTypeFilter);
        // 获取设置的扫描包信息
        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)));
    }
    // 根据设置的包路径，查找符合条件的组件Bean
    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"));
                // 真正注册Bean
                registerFeignClient(registry, annotationMetadata, attributes);
            }
        }
    }
}

过滤符合条件的Class

private Set<BeanDeFinition> scanCandidateComponents(String basePackage) {
    Set<BeanDeFinition> candidates = new LinkedHashSet<>();
    String packageSearchPath = ResourcePatternResolver.CLAsspATH_ALL_URL_PREFIX +
        resolveBasePackage(basePackage) + '/' + this.resourcePattern;
    Resource[] resources = getResourcePatternResolver().getResources(packageSearchPath);
    for (Resource resource : resources) {
        if (resource.isReadable()) {
            try {
                MetadataReader MetadataReader = getMetadataReaderFactory().getMetadataReader(resource);
                if (isCandidateComponent(MetadataReader)) {
                    ScannedGenericBeanDeFinition sbd = new ScannedGenericBeanDeFinition(MetadataReader);
                    sbd.setSource(resource);
                    if (isCandidateComponent(sbd)) {
                        if (debugEnabled) {
                            logger.debug("Identified candidate component class: " + resource);
                        }
                        candidates.add(sbd);
                    }
                }
            }
        }
    }
    return candidates;
}

查找到所有符合条件的Bean后，接下来就是注册Bean

private void registerFeignClient(BeanDeFinitionRegistry registry,
            AnnotationMetadata annotationMetadata, Map<String, Object> attributes) {
    String className = annotationMetadata.getClassName();
    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);
    String contextId = getContextId(attributes);
    deFinition.addPropertyValue("contextId", contextId);
    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 = contextId + "FeignClient";
    AbstractBeanDeFinition beanDeFinition = deFinition.getBeanDeFinition();
    beanDeFinition.setAttribute(factorybean.OBJECT_TYPE_ATTRIBUTE, className);

    // has a default, won't be null
    boolean primary = (Boolean) attributes.get("primary");

    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);
}

注意下面这行代码

BeanDeFinitionBuilder deFinition = BeanDeFinitionBuilder.genericBeanDeFinition(FeignClientfactorybean.class);

FeignClientfactorybean是一个工厂Bean，实现了factorybean。仅凭一个接口就能调用方法肯定是被代理了。

到这里我们的Feign 相关的Bean就已经注册完了。接下来看看在执行过程中是如何使用的。

3 Feign如何被注入

准备业务代码

@FeignClient(value = "his-insurance")
public interface MedicareSettingFeign {
    @GetMapping("/medicareSetting/queryInsuranceManageInfo")
    public R queryRoles(@RequestParam InsuranceManageQueryDTO queryDTO)  ;
}
@RestController
@RequestMapping("/feign")
public class FeignDemoController {
    @Resource
    private MedicareSettingFeign feign ;
    @GetMapping("/invoke")
    public Object invoke(InsuranceManageQueryDTO dto) {
        return feign.queryRoles(dto) ;
    }
}

当容器在实例化FeignDemoController时，是通过
CommonAnnotationBeanPostProcessor注入MedicareSettingFeign的，接下来查看这个过程

3.1 获取Bean对象

DefaultListablebeanfactory

最后进入到
AbstractAutowireCapablebeanfactory类中的如下方法

protected Object doCreateBean(String beanName, RootBeanDeFinition mbd, @Nullable Object[] args)
            throws BeanCreationException {
    // ...
    try {
        populateBean(beanName, mbd, instanceWrapper);
        exposedobject = initializeBean(beanName, exposedobject, mbd);
    }
    catch (Throwable ex) {
        if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
            throw (BeanCreationException) ex;
        }
        else {
            throw new BeanCreationException(
                mbd.getResourceDescription(), beanName, "Initialization of bean Failed", ex);
        }
    }
    // ...
}

3.1.1 填充Bean属性

populateBean方法填充属性

protected void populateBean(String beanName, RootBeanDeFinition mbd, @Nullable BeanWrapper bw) {
    // 核心代码
    for (BeanPostProcessor bp : getBeanPostProcessors()) {
        if (bp instanceof InstantiationAwareBeanPostProcessor) {
            InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
            PropertyValues pvsToUse = ibp.postProcessproperties(pvs, bw.getWrappedInstance(), beanName);
            if (pvsToUse == null) {
                if (filteredPds == null) {
                    filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
                }
                pvsToUse = ibp.postProcesspropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
                if (pvsToUse == null) {
                    return;
                }
            }
            pvs = pvsToUse;
        }
    }
}

protected void populateBean(String beanName, RootBeanDeFinition mbd, @Nullable BeanWrapper bw) {
    // 核心代码
    for (BeanPostProcessor bp : getBeanPostProcessors()) {
        if (bp instanceof InstantiationAwareBeanPostProcessor) {
            InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
            PropertyValues pvsToUse = ibp.postProcessproperties(pvs, bw.getWrappedInstance(), beanName);
            if (pvsToUse == null) {
                if (filteredPds == null) {
                    filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
                }
                pvsToUse = ibp.postProcesspropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
                if (pvsToUse == null) {
                    return;
                }
            }
            pvs = pvsToUse;
        }
    }
}

这里主要就是执行
CommonAnnotationBeanPostProcessor处理器的postProcessproperties来处理属性注入。

注入方法

public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
    Collection<InjectedElement> checkedElements = this.checkedElements;
    Collection<InjectedElement> elementsToIterate =
        (checkedElements != null ? checkedElements : this.injectedElements);
    if (!elementsToIterate.isEmpty()) {
        for (InjectedElement element : elementsToIterate) {
            element.inject(target, beanName, pvs);
        }
    }
}

通过反射设置Bean的属性

protected void inject(Object target, @Nullable String requestingBeanName, @Nullable PropertyValues pvs)
                throws Throwable {

    if (this.isField) {
        Field field = (Field) this.member;
        ReflectionUtils.makeAccessible(field);
        field.set(target, getResourcetoInject(target, requestingBeanName));
    }
    else {
        // ...
    }
}

target为目标对象（Controller），getResourcetoInject方法获取要注入的资源对象

protected Object getResourcetoInject(Object target, @Nullable String requestingBeanName) {
    return (this.lazyLookup ? buildLazyResourceProxy(this, requestingBeanName) :
            getResource(this, requestingBeanName));
}

进入getResource

protected Object getResource(LookupElement element, @Nullable String requestingBeanName)
            throws NoSuchBeanDeFinitionException {
    return autowireResource(this.resourceFactory, element, requestingBeanName);
}

一路调试最后会进入到Abstractbeanfactory

接着进入到
doGetobjectFromfactorybean 方法调用 factorybean的getobject方法

3.1.2 获取Feign接口的代理对象

private Object doGetobjectFromfactorybean(factorybean<?> factory, String beanName) throws BeanCreationException {
    Object object;
    try {
        if (System.getSecurityManager() != null) {
            AccessControlContext acc = getAccessControlContext();
            try {
                object = AccessController.doPrivileged((PrivilegedExceptionAction<Object>) factory::getobject, acc);
            }
            catch (PrivilegedActionException pae) {
                throw pae.getException();
            }
        }
        else {
            object = factory.getobject();
        }
    }
    // ...
    return object;
}

进入到getobject方法

@Override
public Object getobject() throws Exception {
    return getTarget();
}
<T> T getTarget() {
    FeignContext context = applicationContext.getBean(FeignContext.class);
    Feign.Builder builder = feign(context);

    if (!StringUtils.hasText(url)) {
        if (!name.startsWith("http")) {
            url = "http://" + name;
        }
        else {
            url = name;
        }
        url += cleanPath();
        return (T) loadBalance(builder, context,
                               new HardCodedTarget<>(type, name, url));
    }
    if (StringUtils.hasText(url) && !url.startsWith("http")) {
        url = "http://" + url;
    }
    String url = this.url + cleanPath();
    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();
        }
        if (client instanceof FeignBlockingLoadBalancerClient) {
            // not load balancing because we have a url,
            // but Spring Cloud LoadBalancer is on the classpath, so unwrap
            client = ((FeignBlockingLoadBalancerClient) client).getDelegate();
        }
        builder.client(client);
    }
    Targeter targeter = get(context, Targeter.class);
    return (T) targeter.target(this, builder, context,
                               new HardCodedTarget<>(type, name, url));
}

通过loadBalance方法最后进入到ReflectiveFeign中

3.1.3 创建代理对象

public <T> T newInstance(Target<T> target) {
    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)));
      }
    }
    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;
}

通过JDK创建动态代理对象，其中InvocationHandler使用的是HystrixInvocationHandler对象；通过这个Handler名称也知道Feign调用是具有熔断功能的。

final class HystrixInvocationHandler implements InvocationHandler {
  @Override
  public Object invoke(final Object proxy, final Method method, final 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();
    }

    HystrixCommand<Object> hystrixCommand =
        new HystrixCommand<Object>(setterMethodMap.get(method)) {
          @Override
          protected Object run() throws Exception {
            try {
              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 if (isReturnsCompletableFuture(method)) {
                return ((Future) result).get();
              } else {
                return result;
              }
            } catch (illegalaccessexception e) {
              // shouldn't happen as method is public due to being an interface
              throw new AssertionError(e);
            } catch (InvocationTargetException | ExecutionException e) {
              // Exceptions on fallback are tossed by Hystrix
              throw new AssertionError(e.getCause());
            } catch (InterruptedException e) {
              // Exceptions on fallback are tossed by Hystrix
              Thread.currentThread().interrupt();
              throw new AssertionError(e.getCause());
            }
          }
        };

    if (Util.isDefault(method)) {
      return hystrixCommand.execute();
    } else 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();
    } else if (isReturnsCompletableFuture(method)) {
      return new ObservableCompletableFuture<>(hystrixCommand);
    }
    return hystrixCommand.execute();
  }
}

这里Feign的代理对象就创建完成并且注入到了Controller中。

4 Feign接口调用

所有接口调用的入口都是在HystrixInvocationHandler中的invoke方法中

HystrixInvocationHandler.this.dispatch.get(method).invoke(args);

这里的invoke是调用的SynchronousMethodHandler中的方法

@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;
        }
    }
}

最终业务的执行

Object executeAndDecode(RequestTemplate template, Options options) 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 12
      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;
    }
 }