Spring Cloud全解析：服务调用之Feign的执行流程

Feign的执行流程

• 首先通过@EnableFeignClients注解开启Feign功能，程序启动时开启对@FeignClient注解的扫描

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

• 当接口的方法调用时，通过动态代理SynchronousMethodHandler生成具体的RequestTemplate

  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;
      }
    }
  }

RequestTemplate template = buildTemplateFromArgs.create(argv);

在构建RequestTemplate的时候会进行参数解析，如果参数是@RequestBody的会调用Encoder的encode方法进行编码（BuildEncodedTemplateFromArgs）

encoder.encode(body, metadata.bodyType(), mutable);

• 再根据RequestTemplate生成HTTP的Request对象

• Request对象交给Client处理，可配置HttpURLConnection、HttpClient、OkHttp等

  // 该方法首先会调用RequestInterceptor的apply方法处理请求，之后才会进行请求发送
  // 执行完之后调用Decoder的decode方法来进行解码
  return executeAndDecode(template, options);

• Client被封装到LoadBalanceClient类进行负载均衡

Object executeAndDecode(RequestTemplate template, Options options) throws Throwable {
  // 调用RequestInterceptor的apply方法处理请求
  Request request = targetRequest(template);

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

  Response response;
  long start = System.nanoTime();
  try {
    // 请求发送
    response = client.execute(request, options);
  } 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);
    }
    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 {
        //调用Decoder的decode方法来进行解码
        Object result = decode(response);
        shouldClose = closeAfterDecode;
        return result;
      }
    } else if (decode404 && response.status() == 404 && void.class != metadata.returnType()) {
      Object result = decode(response);
      shouldClose = closeAfterDecode;
      return result;
    } else {
      // 调用ErrorDecoder进行错误解码
      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());
    }
  }
}

参考文献

• Feign的执行流程