OkHttp 源码学习（四）RetryAndFollowUpIn

在OkHttp 源码学习（二）基本流程这节中曾带大家认识了Interceptor拦截器的概念，不记得的同学可以回顾一下。从本节开始将从源码带角度逐个介绍其中的5个重要拦截器，首先来认识一下RetryAndFollowUpInterceptor

什么是RetryAndFollowUpInterceptor?

用一句话来说，它是okhttp核心拦截器之一，该拦截器主要用于失败重试以及必要的请求重定向。

整个拦截器流程：
step1 初始化StreamAllocation对象
step2 调用RealInterceptorChain.proceed()方法进行网络请求
step3 根据请求响应结果或者异常结果判断是否需要重新请求
step4 对response处理并返回给上一个拦截器

如何初始化

回到RealCall的 getResponseWithInterceptorChain()这个方法里，我们发现这个retryAndFollowUpInterceptor的初始化和其他几个拦截器不太一样。

Response getResponseWithInterceptorChain() throws IOException {
    // Build a full stack of interceptors.
    List<Interceptor> interceptors = new ArrayList<>();
    interceptors.addAll(client.interceptors());
    interceptors.add(retryAndFollowUpInterceptor);
    interceptors.add(new BridgeInterceptor(client.cookieJar()));
    interceptors.add(new CacheInterceptor(client.internalCache()));
    interceptors.add(new ConnectInterceptor(client));
    if (!forWebSocket) {
      interceptors.addAll(client.networkInterceptors());
    }
    interceptors.add(new CallServerInterceptor(forWebSocket));
    ···
    return chain.proceed(originalRequest);
  }

我们找到retryAndFollowUpInterceptor的初始化方法。

private RealCall(OkHttpClient client, Request originalRequest, boolean forWebSocket) {
    this.client = client;
    this.originalRequest = originalRequest;
    this.forWebSocket = forWebSocket;
    this.retryAndFollowUpInterceptor = new RetryAndFollowUpInterceptor(client, forWebSocket);
  }

在RealCall的构造函数中进行了初始化，可以看到RetryAndFollowUpInterceptor在初始化时持有了client的引用。

核心方法intercept()

RetryAndFollowUpInterceptor的核心就是这个方法，我们一起来看下源码。

  @Override public Response intercept(Chain chain) throws IOException {
    Request request = chain.request();
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    Call call = realChain.call();
    EventListener eventListener = realChain.eventListener();

    streamAllocation = new StreamAllocation(client.connectionPool(), createAddress(request.url()),
        call, eventListener, callStackTrace);
    ···
  }

这里都是一些赋值操作，我简单说一下RealInterceptorChain、StreamAllocation这两个类，也是okhttp的核心之一。

• RealInterceptorChain
  okhttp核心之一，拦截器链Interceptor.Chain具体实现类：包含所有应用拦截器application interceptors，所有网络拦截器network interceptors，最终网络的调用者。
• StreamAllocation
  这个类其实是用来建立执行http请求所需要的网络组件。协调Connections、Streams、Calls三者之间的关系。
  Connections:物理套接字连接到远程服务器。这些可能很慢建立，因此有必要能够取消当前正在连接的连接。
  Streams:在连接上分层的逻辑HTTP请求/响应对。每个连接都有自己的分配限制，它定义了连接可以承载多少个并发流。 HTTP / 1.x连接一次可以携带1个流，HTTP / 2通常携带多个流。
  Calls:流的逻辑顺序，通常是初始请求及其后续请求。我们希望将所有单个调用的流保持在相同的连接上以获得更好的行为和局部性。

我们注意到在intercepter()方法中有一段while(true)循环

int followUpCount = 0;
    Response priorResponse = null;
    while (true) {
      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) {
        // The attempt to connect via a route failed. The request will not have been sent.
        if (!recover(e.getLastConnectException(), false, request)) {
          throw e.getLastConnectException();
        }
        releaseConnection = false;
        continue;
      } catch (IOException e) {
        // An attempt to communicate with a server failed. The request may have been sent.
        boolean requestSendStarted = !(e instanceof ConnectionShutdownException);
        if (!recover(e, requestSendStarted, request)) throw e;
        releaseConnection = false;
        continue;
      } finally {
        // We're throwing an unchecked exception. Release any resources.
        if (releaseConnection) {
          streamAllocation.streamFailed(null);
          streamAllocation.release();
        }
      }

      // Attach the prior response if it exists. Such responses never have a body.
      if (priorResponse != null) {
        response = response.newBuilder()
            .priorResponse(priorResponse.newBuilder()
                    .body(null)
                    .build())
            .build();
      }

      Request followUp = followUpRequest(response);

      if (followUp == null) {
        if (!forWebSocket) {
          streamAllocation.release();
        }
        return response;
      }

      closeQuietly(response.body());

      if (++followUpCount > MAX_FOLLOW_UPS) {
        streamAllocation.release();
        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())) {
        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;
    }

代码很多，这一部分主要就是执行前面讲到的流程中step2、step3、step4步骤，注意到这样一句

if (++followUpCount > MAX_FOLLOW_UPS) {
        streamAllocation.release();
        throw new ProtocolException("Too many follow-up requests: " + followUpCount);
      }

MAX_FOLLOW_UPS表示我们最多重试的次数。
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