Spring-cloud Feign 的理解

feign的調用流程

讀取註解信息：EnableFeignClients-->FeignClientsRegistrar-->FeignClientFactoryBean
feigh流程：ReflectiveFeign-->Contract-->SynchronousMethodHandler
相關configuration：FeignClientsConfiguration，FeignAutoConfiguration，DefaultFeignLoadBalancedConfiguration，FeignRibbonClientAutoConfiguration（Ribbon）

在FeignClientsRegistrar中：

@Override
    public void registerBeanDefinitions(AnnotationMetadata metadata,
            BeanDefinitionRegistry registry) {
        //註冊feign配置信息
        registerDefaultConfiguration(metadata, registry);
        //註冊feign client
        registerFeignClients(metadata, registry);
    }

    private void registerFeignClient(BeanDefinitionRegistry registry,
            AnnotationMetadata annotationMetadata, Map<String, Object> attributes) {
        String className = annotationMetadata.getClassName();
        //準備注入FeignClientFactoryBean
        BeanDefinitionBuilder definition = BeanDefinitionBuilder
                .genericBeanDefinition(FeignClientFactoryBean.class);
        ...
    }

查看FeignClientFactoryBean：

@Override
    public Object getObject() throws Exception {
        FeignContext context = applicationContext.getBean(FeignContext.class);
        //構建Feign.Builder
        Feign.Builder builder = feign(context);
        //若是註解沒有指定URL
        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));
        }
        //若是指定了URL
        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) {
                // 由於指定了URL且classpath下有Ribbon，獲取client的delegate（unwrap）
                // 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 targeter.target(this, builder, context, new HardCodedTarget<>(
                this.type, this.name, url));
    }
    
    protected <T> T loadBalance(Feign.Builder builder, FeignContext context,
            HardCodedTarget<T> target) {
        //獲取Feign Client實例
        Client client = getOptional(context, Client.class);
        if (client != null) {
            builder.client(client);
            //DefaultTargeter或者HystrixTargeter
            Targeter targeter = get(context, Targeter.class);
            //調用builder的target，其中就調用了Feign的newInstance
            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?");
    }

在FeignClientsConfiguration配置了Feign.Builder，prototype類型:

@Bean
    @Scope("prototype")
    @ConditionalOnMissingBean
    public Feign.Builder feignBuilder(Retryer retryer) {
        return Feign.builder().retryer(retryer);
    }

Feign的Builder.build返回了一個ReflectiveFeign:

public Feign build() {
      SynchronousMethodHandler.Factory synchronousMethodHandlerFactory =
          new SynchronousMethodHandler.Factory(client, retryer, requestInterceptors, logger,
                                               logLevel, decode404);
      ParseHandlersByName handlersByName =
          new ParseHandlersByName(contract, options, encoder, decoder,
                                  errorDecoder, synchronousMethodHandlerFactory);
      //ReflectiveFeign構造參數
      //ParseHandlersByName做用是經過傳入的target返回代理接口下的方法的各類信息（MethodHandler）
      //Contract：解析接口的方法註解規則，生成MethodMetadata
      //Options：Request超時配置
      //Encoder：請求編碼器
      //Decoder：返回解碼器
      //ErrorDecoder：錯誤解碼器
      //SynchronousMethodHandler.Factory是構建SynchronousMethodHandler的工廠
      //Client：表明真正執行HTTP的組件
      //Retryer：該組決定了在http請求失敗時是否須要重試
      //RequestInterceptor：請求前的攔截器
      //Logger：記錄日誌組件，包含各個階段記錄日誌的方法和留給用戶本身實現的log方法
      //Logger.Level：日誌級別
      //decode404：處理404的策略，返回空仍是報錯
      //synchronousMethodHandlerFactory經過全部的信息去包裝一個synchronousMethodHandler，在調用invoke方法的時候執行HTTP
      return new ReflectiveFeign(handlersByName, invocationHandlerFactory);
    }

在調用Feign.Builder的target的時候，調用了ReflectiveFeign.newInstance:

/**
   * creates an api binding to the {@code target}. As this invokes reflection, care should be taken
   * to cache the result.
   */
  @SuppressWarnings("unchecked")
  @Override
  //接收Target參數（包含feign代理接口的類型class，名稱，http URL）
  public <T> T newInstance(Target<T> target) {
    //首先經過**ParseHandlersByName**解析出接口中包含的方法，包裝RequestTemplate，組裝成<name, MethodHandler>
    Map<String, MethodHandler> nameToHandler = targetToHandlersByName.apply(target);
    Map<Method, MethodHandler> methodToHandler = new LinkedHashMap<Method, MethodHandler>();
    //接口default方法List
    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)));
      }
    }
    //InvocationHandlerFactory.Default()返回了一個ReflectiveFeign.FeignInvocationHandler對象，經過傳入的methodHandler map 調用目標對象的對應方法
    InvocationHandler handler = factory.create(target, methodToHandler);
    //生成JDK代理對象
    T proxy = (T) Proxy.newProxyInstance(target.type().getClassLoader(), new Class<?>[]{target.type()}, handler);
    //綁定接口的默認方法到代理對象
    for(DefaultMethodHandler defaultMethodHandler : defaultMethodHandlers) {
      defaultMethodHandler.bindTo(proxy);
    }
    return proxy;
  }

生成Feign代理對象的基本流程圖：

當調用接口方法時，實際上就是調用代理對象invoke方法：

@Override
  public Object invoke(Object[] argv) throws Throwable {
    //工廠建立請求模版
    RequestTemplate template = buildTemplateFromArgs.create(argv);
    //每次克隆一個新的Retryer
    Retryer retryer = this.retryer.clone();
    while (true) {
      try {
        //這裏調用實際的Feign client execute
        return executeAndDecode(template);
      } catch (RetryableException e) {
        //失敗重試
        retryer.continueOrPropagate(e);
        if (logLevel != Logger.Level.NONE) {
          logger.logRetry(metadata.configKey(), logLevel);
        }
        continue;
      }
    }
  }

在DefaultFeignLoadBalancedConfiguration裏實例化了LoadBalancerFeignClient

@Override
    public Response execute(Request request, Request.Options options) throws IOException {
        try {
            URI asUri = URI.create(request.url());
            String clientName = asUri.getHost();
            URI uriWithoutHost = cleanUrl(request.url(), clientName);
            //delegate這裏是Client.Default實例，底層調用的是java.net原生網絡訪問
            FeignLoadBalancer.RibbonRequest ribbonRequest = new FeignLoadBalancer.RibbonRequest(
                    this.delegate, request, uriWithoutHost);

            IClientConfig requestConfig = getClientConfig(options, clientName);
            //executeWithLoadBalancer會根據ribbon的負載均衡算法構建url，這裏不展開
            return lbClient(clientName).executeWithLoadBalancer(ribbonRequest,
                    requestConfig).toResponse();
        }
        catch (ClientException e) {
            IOException io = findIOException(e);
            if (io != null) {
                throw io;
            }
            throw new RuntimeException(e);
        }
    }