OKHttp拦截器RetryAndFollowUpInterceptor分析

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RetryAndFollowUpInterceptor 拦截器为 OkHttp 拦截器集合中的第一个拦截器，它的作用顾名思义为网络请求失败时发起重试、重定向，下面看一下 intercept 方法的具体代码：

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Request request = chain.request();
 RealInterceptorChain realChain = (RealInterceptorChain) chain;
 Call call = realChain.call();
 // 创建 streamAllocation
 streamAllocation = new StreamAllocation(client.connectionPool(), createAddress(request.url()),
     call, eventListener, callStackTrace);
 int followUpCount = 0;//重定向的次数
 Response priorResponse = null;
 while (true) {
 	//请求被取消
   if (canceled) {
     streamAllocation.release();
     throw new IOException("Canceled");
   }
   Response response;
   boolean releaseConnection = true;
   try {
     response = realChain.proceed(request, streamAllocation, null, null);
     releaseConnection = false;
   } catch (RouteException e) {
   	// 抛出的路由异常
     // The attempt to connect via a route failed. The request will not have been sent.
     // 判断是否能恢复
     if (!recover(e.getLastConnectException(), false, request)) {
       throw e.getLastConnectException();
     }
     releaseConnection = false;
     continue;//继续重试
   } catch (IOException e) {
     // An attempt to communicate with a server failed. The request may have been sent.
     boolean requestSendStarted = !(e instanceof ConnectionShutdownException);
     // 判断是否能恢复
     if (!recover(e, requestSendStarted, request)) throw e;
     releaseConnection = false;
     continue;//继续重试
   } finally {
     streamAllocation.release();
   }
//重定向请求
   Request followUp = followUpRequest(response);
   if (followUp == null) {
   	//无需重定向，直接返回
     return response;
   }
   if (++followUpCount > MAX_FOLLOW_UPS) {
    	//超过了重定向的最大次数
     throw new ProtocolException("Too many follow-up requests: " + followUpCount);
   }
		if (followUp.body() instanceof UnrepeatableRequestBody) {
     streamAllocation.release();
     throw new HttpRetryException("Cannot retry streamed HTTP body", response.code());
   }
   if (!sameConnection(response, followUp.url())) {
   	// 如果连接的地址发生变化则创建一个新的 streamAllocation
     streamAllocation.release();
     streamAllocation = new StreamAllocation(client.connectionPool(),
         createAddress(followUp.url()), call, eventListener, callStackTrace);
   } else if (streamAllocation.codec() != null) {
     throw new IllegalStateException("Closing the body of " + response
         + " didn't close its backing stream. Bad interceptor?");
   }
   request = followUp;
   priorResponse = response;
 }

大致流程为: 在一个循环里发起请求，如果请求失败则判断是否能发起重试，如果允许重试则继续发起请求。请求失败后判断是否重试的逻辑在 recover 方法里:

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private boolean recover(IOException e, boolean requestSendStarted, Request userRequest) {
  streamAllocation.streamFailed(e);
  // The application layer has forbidden retries.
  // 调用者是否配置允许重试，如果不允许的话直接返回 false
  if (!client.retryOnConnectionFailure()) return false;
  // We can't send the request body again.
  // 如果已经发出请求且请求体不可缓存则返回 false
  if (requestSendStarted && userRequest.body() instanceof UnrepeatableRequestBody) return false;
  // This exception is fatal.
  // 根据异常的类型判断是否能重试
  if (!isRecoverable(e, requestSendStarted)) return false;
  // No more routes to attempt.
  // 没有路由地址可用
  if (!streamAllocation.hasMoreRoutes()) return false;
  // For failure recovery, use the same route selector with a new connection.
  return true;
}

如果成功返回了 response 则根据 responseCode 判断是否需要发起重定向请求(当 响应code的值为 3xx 重定向、401、407、408 时)，具体要看一下 followUpRequest 方法:

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private Request followUpRequest(Response userResponse) throws IOException {
  if (userResponse == null) throw new IllegalStateException();
  Connection connection = streamAllocation.connection();
  Route route = connection != null
      ? connection.route()
      : null;
  int responseCode = userResponse.code();
  final String method = userResponse.request().method();
  switch (responseCode) {
  	// HTTP Status-Code 407 缺乏位于浏览器与可以访问所请求资源的服务器之间的代理服务器（proxy server ）要求的身份验证凭证
    case HTTP_PROXY_AUTH:
      Proxy selectedProxy = route != null
          ? route.proxy()
          : client.proxy();
      if (selectedProxy.type() != Proxy.Type.HTTP) {
        throw new ProtocolException("Received HTTP_PROXY_AUTH (407) code while not using proxy");
      }
      return client.proxyAuthenticator().authenticate(route, userResponse);
	// HTTP Status-Code 401: 缺乏目标资源要求的身份验证凭证.
    case HTTP_UNAUTHORIZED:
      return client.authenticator().authenticate(route, userResponse);
	// 临时重定向响应状态码，表示请求的资源暂时地被移动到了响应的 Location 首部所指向的 URL 上
    case HTTP_PERM_REDIRECT:
    // 308 Permanent Redirect（永久重定向）是表示重定向的响应状态码，说明请求的资源已经被永久的移动到了由 Location 首部指定的 URL 上
    case HTTP_TEMP_REDIRECT:
      // "If the 307 or 308 status code is received in response to a request other than GET
      // or HEAD, the user agent MUST NOT automatically redirect the request"
      //对于 307 308，如果请求方法不是 GET 或者 HEAD,则不能发起重定向.可参考文档https://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html
      if (!method.equals("GET") && !method.equals("HEAD")) {
        return null;
      }
      // fall-through
    case HTTP_MULT_CHOICE:
    case HTTP_MOVED_PERM:
    case HTTP_MOVED_TEMP:
    case HTTP_SEE_OTHER:
      // Does the client allow redirects?
      // 客户端是否允许重定向？
      if (!client.followRedirects()) return null;
		// 获取重定向的地址
      String location = userResponse.header("Location");
      if (location == null) return null;
      HttpUrl url = userResponse.request().url().resolve(location);
      // Don't follow redirects to unsupported protocols.
      if (url == null) return null;
      // If configured, don't follow redirects between SSL and non-SSL.
      // 如果禁止 SSL 和非 SSL 间进行重定向，则返回 null
      boolean sameScheme = url.scheme().equals(userResponse.request().url().scheme());
      if (!sameScheme && !client.followSslRedirects()) return null;
      // Most redirects don't include a request body.
      // 大多数请求体不包含请求体
      Request.Builder requestBuilder = userResponse.request().newBuilder();
      if (HttpMethod.permitsRequestBody(method)) {
        final boolean maintainBody = HttpMethod.redirectsWithBody(method);
        if (HttpMethod.redirectsToGet(method)) {
          requestBuilder.method("GET", null);
        } else {
          RequestBody requestBody = maintainBody ? userResponse.request().body() : null;
          requestBuilder.method(method, requestBody);
        }
        //如果不包含请求体则移除相应 header
        if (!maintainBody) {
          requestBuilder.removeHeader("Transfer-Encoding");
          requestBuilder.removeHeader("Content-Length");
          requestBuilder.removeHeader("Content-Type");
        }
      }
      // When redirecting across hosts, drop all authentication headers. This
      // is potentially annoying to the application layer since they have no
      // way to retain them.
      // 如果不是相同的连接，则移除掉 Authorization
      if (!sameConnection(userResponse, url)) {
        requestBuilder.removeHeader("Authorization");
      }
      return requestBuilder.url(url).build();
    case HTTP_CLIENT_TIMEOUT:
      // 408's are rare in practice, but some servers like HAProxy use this response code. The
      // spec says that we may repeat the request without modifications. Modern browsers also
      // repeat the request (even non-idempotent ones.)
      // 响应状态码 408 Request Timeout 表示服务器想要将没有在使用的连接关闭。一些服务器会在空闲连接上发送此信息，即便是在客户端没有发送任何请求的情况下
      if (!client.retryOnConnectionFailure()) {
        // The application layer has directed us not to retry the request.
        return null;
      }
      if (userResponse.request().body() instanceof UnrepeatableRequestBody) {
        return null;
      }
      if (userResponse.priorResponse() != null
          && userResponse.priorResponse().code() == HTTP_CLIENT_TIMEOUT) {
        // We attempted to retry and got another timeout. Give up.
        return null;
      }
		// 重新发起原始请求
      return userResponse.request();
    default:
      return null;
  }
}