Okhttp3源码解析(5)-拦截器RetryAndFollowUpInterceptor

前言

回顾：
Okhttp的基本用法
Okhttp3源码解析(1)-OkHttpClient分析
Okhttp3源码解析(2)-Request分析
Okhttp3源码解析(3)-Call分析(整体流程)
Okhttp3源码解析(4)-拦截器与设计模式

上节讲了拦截器与设计模式，今天讲RetryAndFollowUpInterceptor，如果我们没有去自定义拦截器， 那RetryAndFollowUpInterceptor是第一个拦截器。

初始化

首先先看RetryAndFollowUpInterceptor被添加的位置：

初始化位置：
call实例化方法中：

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

找到了初始化的位置， 下面去RetryAndFollowUpInterceptor种分析！

RetryAndFollowUpInterceptor解析

从上节我们就知道拦截器中的intercept()是核心！ 这里贴出代码：

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

    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(), streamAllocation, false, request)) {
          throw e.getFirstConnectException();
        }
        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, streamAllocation, 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;
      try {
        followUp = followUpRequest(response, streamAllocation.route());
      } catch (IOException e) {
        streamAllocation.release();
        throw e;
      }

      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);
        this.streamAllocation = streamAllocation;
      } 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;
    }
  }

我先贴出一个while循环的流程图：

根据流程图和源码可以分析RetryAndFollowUpInterceptor主要做了以下内容，后两点都是发生在while循环中：

• 初始化StreamAllocation 对象
• 网络请求-chain.proceed() ，对在请求时发生的异常进行捕获以及对应的重连机制
• followUpRequest 对响应码进行处理

下面可以逐块代码分析：

1.初始化StreamAllocation 对象

StreamAllocation类是协调三个实体之间的关系 三个实体是：Connections，Streams，Calls
我们请求网络时需要传递它

StreamAllocation在这大家简单了解一下就可以了.

2.网络请求时异常捕获-以及重连机制

网络请求如下：

response = realChain.proceed(request, streamAllocation, null, null);

如果请求发现异常，我们通过try/catch捕获

catch (RouteException e) {
        // The attempt to connect via a route failed. The request will not have been sent.
        if (!recover(e.getLastConnectException(), streamAllocation, false, request)) {
          throw e.getFirstConnectException();
        }
        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, streamAllocation, requestSendStarted, request)) throw e;
        releaseConnection = false;
        continue;
      }

-RouteException 路由异常

• IOException IO异常

捕获后都做了recover()重连判断，具体代码如下，就不细说了：

private boolean recover(IOException e, StreamAllocation streamAllocation,
      boolean requestSendStarted, Request userRequest) {
    streamAllocation.streamFailed(e);

    // The application layer has forbidden retries.
    if (!client.retryOnConnectionFailure()) return false;

    // We can't send the request body again.
    if (requestSendStarted && userRequest.body() instanceof UnrepeatableRequestBody) return false;

    // This exception is fatal.
    if (!isRecoverable(e, requestSendStarted)) return false;

    // No more routes to attempt.
    if (!streamAllocation.hasMoreRoutes()) return false;

    // For failure recovery, use the same route selector with a new connection.
    return true;
  }

这里需要注意的是如果可以重连，执行 continue;
continue含义： 继续循环，（不执行 循环体内continue 后面的语句，直接进行下一循环）

3. followUpRequest 对响应码进行处理

先看看具体followUpRequest 方法：

private Request followUpRequest(Response userResponse, Route route) throws IOException {
    if (userResponse == null) throw new IllegalStateException();
    int responseCode = userResponse.code();

    final String method = userResponse.request().method();
    switch (responseCode) {
      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);

      case HTTP_UNAUTHORIZED:
        return client.authenticator().authenticate(route, userResponse);

      case HTTP_PERM_REDIRECT:
      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"
        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?
        if (!client.followRedirects()) return null;

        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.
        boolean sameScheme = url.scheme().equals(userResponse.request().url().scheme());
        if (!sameScheme && !client.followSslRedirects()) return null;

        // Most redirects don't include a request body.
        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);
          }
          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.
        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.)
        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;
        }

        if (retryAfter(userResponse, 0) > 0) {
          return null;
        }

        return userResponse.request();

      case HTTP_UNAVAILABLE:
        if (userResponse.priorResponse() != null
            && userResponse.priorResponse().code() == HTTP_UNAVAILABLE) {
          // We attempted to retry and got another timeout. Give up.
          return null;
        }

        if (retryAfter(userResponse, Integer.MAX_VALUE) == 0) {
          // specifically received an instruction to retry without delay
          return userResponse.request();
        }

        return null;

      default:
        return null;
    }
  }

不难看出， 是根据响应码进行判断的。

• HTTP_PROXY_AUTH 407 代理身份验证
• HTTP_UNAUTHORIZED 401 未授权
• HTTP_PERM_REDIRECT 308 重定向
• HTTP_TEMP_REDIRECT 307 重定向
• HTTP_MULT_CHOICE 300 Multiple Choices
• HTTP_MOVED_PERM 301 Moved Permanently
• HTTP_MOVED_TEMP 302 Temporary Redirect
• HTTP_SEE_OTHER 303 See Other
• HTTP_CLIENT_TIMEOUT 408 Request Time-Out
• HTTP_UNAVAILABLE 503 Service Unavailable

对于这些响应码都做了处理：
1.返回null

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

2.其他异常情况直接抛异常了

强调：
MAX_FOLLOW_UPS字段， 表示最大的重定向次数

/**
   * How many redirects and auth challenges should we attempt? Chrome follows 21 redirects; Firefox,
   * curl, and wget follow 20; Safari follows 16; and HTTP/1.0 recommends 5.
   */
  private static final int MAX_FOLLOW_UPS = 20;

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

这节就说到这，希望对大家有所帮助.....

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