@FeignClient(value = "spring-cloud-web-demo")
public interface UserFeign {
    @GetMapping(value = "/user/getUserById", produces = "application/json;charset=utf-8")
    Object getUserById(@RequestParam(value = "id", required = false) Long id);
    //省略
}

@Configuration
@EnableFeignClients("net.teaho.demo.spring.cloud.web.api")
public class ClientAutoConfiguration {
}
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
net.teaho.demo.spring.cloud.web.api.config.ClientAutoConfiguration

@RestController
public class UserController implements UserFeign {
    private static final Logger logger = LoggerFactory.getLogger(UserController.class);
    @Override
    public Object getUserById(Long id) {
        return "{\"id\":1, \"name\": \"test\"}";
    }
    //省略
}

@Component
@Slf4j
public class TestService {
    @Autowired
    private RestTemplate restTemplate;
    public Object getOneUser(){
        return userController.getUserById(1L);
    }
}

@Bean
@LoadBalanced
public RestTemplate restTemplate() {
    return new RestTemplate();
}

// 將配置中標(biāo)注了@LoadBalanced的RestTemplate注入到這里
@LoadBalanced
@Autowired(required = false)
private List<RestTemplate> restTemplates = Collections.emptyList();
//將注冊(cè)的RestTemplateCustomizer（RestTemplate自定義器）集合處理上面的restTemplates集合
@Bean
public SmartInitializingSingleton loadBalancedRestTemplateInitializerDeprecated(
        final ObjectProvider<List<RestTemplateCustomizer>> restTemplateCustomizers) {
    return () -> restTemplateCustomizers.ifAvailable(customizers -> {
        for (RestTemplate restTemplate : LoadBalancerAutoConfiguration.this.restTemplates) {
            for (RestTemplateCustomizer customizer : customizers) {
                customizer.customize(restTemplate);
            }
        }
    });
}

//給接口標(biāo)注成一個(gè)REST調(diào)用方
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface FeignClient {
    //服務(wù)名，可以帶協(xié)議前綴，也可以用${property.name}關(guān)聯(lián)一個(gè)配置值。
    @AliasFor("name")
    String value() default "";
    @Deprecated
    String serviceId() default "";
    //bean name
    String contextId() default "";
    @AliasFor("value")
    String name() default "";
    /**
     * Sets the <code>@Qualifier</code> value for the feign client.
     */
    String qualifier() default "";
    //直接指定一個(gè)地址，比如http://localhost:12345，一般用于調(diào)試
    String url() default "";
    boolean decode404() default false;
    /**
     * A custom <code>@Configuration</code> for the feign client. Can contain override
     * <code>@Bean</code> 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
     */
    //可用于覆蓋FeignClient默認(rèn)設(shè)置
    Class<?>[] configuration() default {};
    //回滾類(lèi)，像我的例子中定義的回滾類(lèi)必須實(shí)現(xiàn)UserFeign接口，看https://cloud.spring.io/spring-cloud-static/Greenwich.SR5/single/spring-cloud.html#spring-cloud-feign-hystrix-fallback
    Class<?> fallback() default void.class;
    //如果需要對(duì)異常做診斷可用此屬性，https://cloud.spring.io/spring-cloud-static/Greenwich.SR5/single/spring-cloud.html#spring-cloud-feign-hystrix-fallback
    Class<?> fallbackFactory() default void.class;
    //路徑前綴
    String path() default "";
    //標(biāo)記bean是否為primary
    boolean primary() default true;
}

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

class FeignClientsRegistrar implements ImportBeanDefinitionRegistrar,
        ResourceLoaderAware, EnvironmentAware {
    public void registerFeignClients(AnnotationMetadata metadata,
            BeanDefinitionRegistry registry) {
        //classPath掃描器
        ClassPathScanningCandidateComponentProvider scanner = getScanner();
        scanner.setResourceLoader(this.resourceLoader);
        //ClassPathScanningCandidateComponentProvider掃描的basePackage集合
        Set<String> basePackages;
        Map<String, Object> attrs = metadata
                .getAnnotationAttributes(EnableFeignClients.class.getName());
        //掃描器用于掃描標(biāo)注了@FeignClient類(lèi)的攔截器
        AnnotationTypeFilter annotationTypeFilter = new AnnotationTypeFilter(
                FeignClient.class);
        final Class<?>[] clients = attrs == null ? null
                : (Class<?>[]) attrs.get("clients");
        //clients屬性為空，以@EnableFeignClients的value、basePackage等為根包掃描
        if (clients == null || clients.length == 0) {
            scanner.addIncludeFilter(annotationTypeFilter);
            basePackages = getBasePackages(metadata);
        }
        //@EnableFeignClients的clients屬性不為空，解析clients的類(lèi)和根包
        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)));
        }
        //1.根據(jù)basePackage找到目標(biāo)@FeignClient接口
        //2.檢查是否為接口
        //3.將找到的接口注冊(cè)為FeignClientFactoryBean
        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);
                }
            }
        }
    }
    private String getClientName(Map<String, Object> client) {
        if (client == null) {
            return null;
        }
        String value = (String) client.get("contextId");
        if (!StringUtils.hasText(value)) {
            value = (String) client.get("value");
        }
        if (!StringUtils.hasText(value)) {
            value = (String) client.get("name");
        }
        if (!StringUtils.hasText(value)) {
            value = (String) client.get("serviceId");
        }
        if (StringUtils.hasText(value)) {
            return value;
        }
        throw new IllegalStateException("Either 'name' or 'value' must be provided in @"
                + FeignClient.class.getSimpleName());
    }
    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();
        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);
    }
}

class FeignClientFactoryBean implements FactoryBean<Object>, InitializingBean,
        ApplicationContextAware {
//省略
    @Override
    public Object getObject() throws Exception {
        return getTarget();
    }
    <T> T getTarget() {
        //1.獲取FeignContext，在FeignAutoConfiguration聲明
        FeignContext context = applicationContext.getBean(FeignContext.class);
        //2.構(gòu)造Feign builder
        Feign.Builder builder = feign(context);
        //3.如果沒(méi)有設(shè)置url參數(shù)
        if (!StringUtils.hasText(this.url)) {
            if (!this.name.startsWith("http")) {
                url = "http://" + this.name;
            }
            else {
                url = this.name;
            }
            //4.設(shè)置path
            url += cleanPath();
            //5.獲取Client（用于執(zhí)行最終HTTP/HTTPS請(qǐng)求，比如LoadBalancerFeignClient），
            //構(gòu)造反射實(shí)例
            return (T) loadBalance(builder, context, new HardCodedTarget<>(this.type,
                    this.name, url));
        }
        //存在url參數(shù)，構(gòu)造非loadBalance的請(qǐng)求實(shí)例（target）
        if (StringUtils.hasText(this.url) && !this.url.startsWith("http")) {
            this.url = "http://" + this.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();
            }
            builder.client(client);
        }
        Targeter targeter = get(context, Targeter.class);
        return (T) targeter.target(this, builder, context, new HardCodedTarget<>(
                this.type, this.name, url));
    }
    //在FeignContext中獲取一些在FeignClientsConfiguration中聲明，F(xiàn)eign需要用到的組件
    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;
    }
    protected <T> T loadBalance(Feign.Builder builder, FeignContext context,
            HardCodedTarget<T> target) {
        //獲取Client
        Client client = getOptional(context, Client.class);
        if (client != null) {
            builder.client(client);
            //從Context獲取Targeter，Targeter用于生成最終target實(shí)例（對(duì)應(yīng)我的例子是被調(diào)用的通過(guò)反射生成的UserFeign實(shí)例）
            Targeter targeter = get(context, Targeter.class);
            return targeter.target(this, builder, context, target);
        }
        throw new IllegalStateException(
                "No Feign Client for loadBalancing defined. Did you forget to include spring-cloud-starter-netflix-ribbon?");
    }
//省略
}

public abstract class Feign {
//省略
  public static class Builder {
    private final List<RequestInterceptor> requestInterceptors =
        new ArrayList<RequestInterceptor>();
    private Logger.Level logLevel = Logger.Level.NONE;
    private Contract contract = new Contract.Default();
    private Client client = new Client.Default(null, null);
    private Retryer retryer = new Retryer.Default();
    private Logger logger = new NoOpLogger();
    private Encoder encoder = new Encoder.Default();
    private Decoder decoder = new Decoder.Default();
    private QueryMapEncoder queryMapEncoder = new QueryMapEncoder.Default();
    private ErrorDecoder errorDecoder = new ErrorDecoder.Default();
    private Options options = new Options();
    private InvocationHandlerFactory invocationHandlerFactory =
        new InvocationHandlerFactory.Default();
    private boolean decode404;
    private boolean closeAfterDecode = true;
    private ExceptionPropagationPolicy propagationPolicy = NONE;
    //省略
    public <T> T target(Class<T> apiType, String url) {
      return target(new HardCodedTarget<T>(apiType, url));
    }
    public <T> T target(Target<T> target) {
      return build().newInstance(target);
    }
    //默認(rèn)實(shí)現(xiàn)就是創(chuàng)建一個(gè)ReflectiveFeign實(shí)例
    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);
    }
  }
//省略
}

public class ReflectiveFeign extends Feign {
  private final ParseHandlersByName targetToHandlersByName;
  private final InvocationHandlerFactory factory;
  private final QueryMapEncoder queryMapEncoder;
  ReflectiveFeign(ParseHandlersByName targetToHandlersByName, InvocationHandlerFactory factory,
      QueryMapEncoder queryMapEncoder) {
    this.targetToHandlersByName = targetToHandlersByName;
    this.factory = factory;
    this.queryMapEncoder = queryMapEncoder;
  }
  ..
  /**
   * creates an api binding to the {@code target}. As this invokes reflection, care should be taken
   * to cache the result.
   */
  @SuppressWarnings("unchecked")
  @Override
  public <T> T newInstance(Target<T> target) {
    //創(chuàng)建方法名和方法處理器的map映射
    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;
      //判斷是否為接口的默認(rèn)方法，DefaultMethodHandler的處理邏輯是直接調(diào)用會(huì)原接口的default方法
      } else if (Util.isDefault(method)) {
        DefaultMethodHandler handler = new DefaultMethodHandler(method);
        defaultMethodHandlers.add(handler);
        methodToHandler.put(method, handler);
      } else {
        //方法處理map
        methodToHandler.put(method, nameToHandler.get(Feign.configKey(target.type(), method)));
      }
    }
    InvocationHandler handler = factory.create(target, methodToHandler);
    //jdk動(dòng)態(tài)代理創(chuàng)建對(duì)象
    T proxy = (T) Proxy.newProxyInstance(target.type().getClassLoader(),
        new Class<?>[] {target.type()}, handler);
    //將默認(rèn)方法處理器也綁定到代理對(duì)象上
    for (DefaultMethodHandler defaultMethodHandler : defaultMethodHandlers) {
      defaultMethodHandler.bindTo(proxy);
    }
    return proxy;
  }
  static class FeignInvocationHandler implements InvocationHandler {
    private final Target target;
    private final Map<Method, MethodHandler> dispatch;
    //省略
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
      //自定義的equals、hashCode和toString的處理
      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();
      }
      //分發(fā)調(diào)用到對(duì)應(yīng)方法的InvocationHandlerFactory.MethodHandler
      return dispatch.get(method).invoke(args);
    }
    //省略
}

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();
}

final class HystrixInvocationHandler implements InvocationHandler {
   //省略
  @Override
  public Object invoke(final Object proxy, final Method method, final Object[] args)
      throws Throwable {
   //省略
    HystrixCommand<Object> hystrixCommand =
        new HystrixCommand<Object>(setterMethodMap.get(method)) {
          //實(shí)際執(zhí)行
          @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 {
              //用配置的fallbackFactory創(chuàng)建fallback實(shí)例
              Object fallback = fallbackFactory.create(getExecutionException());
              Object result = fallbackMethodMap.get(method).invoke(fallback, args);
              //根據(jù)fallback對(duì)象的returntype解析包裝內(nèi)的結(jié)果返回
              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());
            }
          }
        };
    //根據(jù)方法的return去返回結(jié)果
    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();
    }
    return hystrixCommand.execute();
  }
   //省略
}