OkHttp源码解析(三)：拦截器

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RetryAndFollowUpInterceptor

RetryAndFollowUpInerceptor 负责了 HTTP 请求的重定向功能。

HTTP 中的重定向

HTTP 协议提供了一种重定向的功能，它通过由服务器返回特定格式的响应从而触发客户端的重定向。其对应的 Response Code 格式为 3XX，并且会在 Response Header 的 Location 字段中放入新的 URL，这样我们客户端就可以根据该 Location 字段所指定的 URL 重新请求从而得到需要的数据。

重定向与服务器转发的区别

  • 重定向是客户端行为，而服务器转发则是服务端行为
  • 重定向我们的客户端发出了多次请求，而转发我们的客户端只发出了一次请求。
  • 重定向的控制权在客户端，转发的控制权在服务端。
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@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 {
        // 调用chain的proceed方法获取下层得到的结果
      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.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, 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();
    }
   //根据response code获取重定向后的request
    Request followUp = followUpRequest(response, streamAllocation.route());

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

    closeQuietly(response.body());
    //最大重试次数20次，超过就抛异常
    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?");
    }

    // 修改下次重定向的request
    request = followUp;
    // 记录上一次的response
    priorResponse = response;
  }
}
  • 调用 chain.proceed 方法进行请求获取 Response
  • 若出现其他未知的异常，则通过抛出异常释放资源
  • 在本次 Response 中设置上一次的 Response priorResponse，且body为空
  • 根据 Response 中的 response code 进行重定向，调用 followUpRequest 方法获取重定向后的 request followUp
  • 若重定向后的 followUp 为 null，说明不再需要重定向，停止 timeout 计时并返回 Response
  • 若重定向超过指定次数（默认 20 次），则抛出异常。
  • 若仍未返回，则需要下一次重定向，对下一次的 request 等变量进行赋值。
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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:// 407代理身份认证
      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:// 401身份认证
      return client.authenticator().authenticate(route, userResponse);

    case HTTP_PERM_REDIRECT:// 308 (307、308 两种状态码不对 GET、HEAD 以外的请求重定向)
    case HTTP_TEMP_REDIRECT:// 307
      // "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:// 300
    case HTTP_MOVED_PERM:// 301
    case HTTP_MOVED_TEMP:// 302
    case HTTP_SEE_OTHER:// 303
       // 若客户端关闭了重定向，则直接返回 null
      if (!client.followRedirects()) return null;

      String location = userResponse.header("Location");
      if (location == null) return null;
      HttpUrl url = userResponse.request().url().resolve(location);
      if (url == null) return null;
      boolean sameScheme = url.scheme().equals(userResponse.request().url().scheme());
      if (!sameScheme && !client.followSslRedirects()) return null;
      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");
        }
      }

      if (!sameConnection(userResponse, url)) {
        requestBuilder.removeHeader("Authorization");
      }
      // 重新构建request
      return requestBuilder.url(url).build();

    case HTTP_CLIENT_TIMEOUT: // 408 说明需要重新发送一次相同的请求
      if (!client.retryOnConnectionFailure()) {
        return null;
      }
      if (userResponse.request().body() instanceof UnrepeatableRequestBody) {
        return null;
      }
      if (userResponse.priorResponse() != null
          && userResponse.priorResponse().code() == HTTP_CLIENT_TIMEOUT) {
        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;
  }
}

BridgeInterceptor

BridgeInterceptor 的名字取的非常形象，它就像一座桥梁，连接了用户与服务器。在用户向服务器发送请求时，它会把用户所构建的请求转换为向服务器请求的真正的 Request，而在服务器返回了响应后，它又会将服务器所返回的响应转换为用户所能够使用的 Response。

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@Override public Response intercept(Chain chain) throws IOException {
  Request userRequest = chain.request();
  Request.Builder requestBuilder = userRequest.newBuilder();

  RequestBody body = userRequest.body();
  if (body != null) {
    MediaType contentType = body.contentType();
    if (contentType != null) {
      requestBuilder.header("Content-Type", contentType.toString());
    }

    long contentLength = body.contentLength();
    if (contentLength != -1) {
      requestBuilder.header("Content-Length", Long.toString(contentLength));
      requestBuilder.removeHeader("Transfer-Encoding");
    } else {
      requestBuilder.header("Transfer-Encoding", "chunked");
      requestBuilder.removeHeader("Content-Length");
    }
  }

  if (userRequest.header("Host") == null) {
    requestBuilder.header("Host", hostHeader(userRequest.url(), false));
  }

  if (userRequest.header("Connection") == null) {
    requestBuilder.header("Connection", "Keep-Alive");
  }

  // If we add an "Accept-Encoding: gzip" header field we're responsible for also decompressing
  // the transfer stream.
  boolean transparentGzip = false;
  if (userRequest.header("Accept-Encoding") == null && userRequest.header("Range") == null) {
    transparentGzip = true;
    requestBuilder.header("Accept-Encoding", "gzip");
  }

  List<Cookie> cookies = cookieJar.loadForRequest(userRequest.url());
  if (!cookies.isEmpty()) {
    requestBuilder.header("Cookie", cookieHeader(cookies));
  }

  if (userRequest.header("User-Agent") == null) {
    requestBuilder.header("User-Agent", Version.userAgent());
  }

  Response networkResponse = chain.proceed(requestBuilder.build());

  HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers());

  Response.Builder responseBuilder = networkResponse.newBuilder()
      .request(userRequest);

  if (transparentGzip
      && "gzip".equalsIgnoreCase(networkResponse.header("Content-Encoding"))
      && HttpHeaders.hasBody(networkResponse)) {
    GzipSource responseBody = new GzipSource(networkResponse.body().source());
    Headers strippedHeaders = networkResponse.headers().newBuilder()
        .removeAll("Content-Encoding")
        .removeAll("Content-Length")
        .build();
    responseBuilder.headers(strippedHeaders);
    String contentType = networkResponse.header("Content-Type");
    responseBuilder.body(new RealResponseBody(contentType, -1L, Okio.buffer(responseBody)));
  }

  return responseBuilder.build();
}

可以看到，这里主要对 Header 进行处理，将一些原来 request 中的 Header 进行处理后设置进了新 request，并用其进行请求。其中若调用者未设置 Accept-Encoding，则它会默认设置 gzip。
而在对 response 处理时，若之前设置了 gzip，则进行 gzip 解压。这种自动解压会自动将 Content-Length、Content-Encoding 字段从 Header 中移除。

CacheInterceptor

CacheInterceptor 主要负责了对缓存的读取以及更新。

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@Override public Response intercept(Chain chain) throws IOException {
  Response cacheCandidate = cache != null
      ? cache.get(chain.request())
      : null;

  long now = System.currentTimeMillis();

  CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
  Request networkRequest = strategy.networkRequest;
  Response cacheResponse = strategy.cacheResponse;

  if (cache != null) {
    cache.trackResponse(strategy);
  }

  if (cacheCandidate != null && cacheResponse == null) {
    closeQuietly(cacheCandidate.body()); // The cache candidate wasn't applicable. Close it.
  }

  // If we're forbidden from using the network and the cache is insufficient, fail.
  if (networkRequest == null && cacheResponse == null) {
    return new Response.Builder()
        .request(chain.request())
        .protocol(Protocol.HTTP_1_1)
        .code(504)
        .message("Unsatisfiable Request (only-if-cached)")
        .body(Util.EMPTY_RESPONSE)
        .sentRequestAtMillis(-1L)
        .receivedResponseAtMillis(System.currentTimeMillis())
        .build();
  }

  // If we don't need the network, we're done.
  if (networkRequest == null) {
    return cacheResponse.newBuilder()
        .cacheResponse(stripBody(cacheResponse))
        .build();
  }

  Response networkResponse = null;
  try {
    networkResponse = chain.proceed(networkRequest);
  } finally {
    // If we're crashing on I/O or otherwise, don't leak the cache body.
    if (networkResponse == null && cacheCandidate != null) {
      closeQuietly(cacheCandidate.body());
    }
  }

  // If we have a cache response too, then we're doing a conditional get.
  if (cacheResponse != null) {
    if (networkResponse.code() == HTTP_NOT_MODIFIED) {
      Response response = cacheResponse.newBuilder()
          .headers(combine(cacheResponse.headers(), networkResponse.headers()))
          .sentRequestAtMillis(networkResponse.sentRequestAtMillis())
          .receivedResponseAtMillis(networkResponse.receivedResponseAtMillis())
          .cacheResponse(stripBody(cacheResponse))
          .networkResponse(stripBody(networkResponse))
          .build();
      networkResponse.body().close();

      // Update the cache after combining headers but before stripping the
      // Content-Encoding header (as performed by initContentStream()).
      cache.trackConditionalCacheHit();
      cache.update(cacheResponse, response);
      return response;
    } else {
      closeQuietly(cacheResponse.body());
    }
  }

  Response response = networkResponse.newBuilder()
      .cacheResponse(stripBody(cacheResponse))
      .networkResponse(stripBody(networkResponse))
      .build();

  if (cache != null) {
    if (HttpHeaders.hasBody(response) && CacheStrategy.isCacheable(response, networkRequest)) {
      // Offer this request to the cache.
      CacheRequest cacheRequest = cache.put(response);
      return cacheWritingResponse(cacheRequest, response);
    }

    if (HttpMethod.invalidatesCache(networkRequest.method())) {
      try {
        cache.remove(networkRequest);
      } catch (IOException ignored) {
        // The cache cannot be written.
      }
    }
  }

  return response;
}

  • 尝试从缓存中获取了缓存的 response
  • 根据 当前时间、request、缓存的response 构建缓存策略。
  • 若缓存策略不能使用网络（networkRequest == null)，且无缓存（cacheResponse == null），则直接请求失败。
  • 若缓存策略不能使用网络，由于前面有判断所以可以确定有缓存，直接构建缓存的 response 并返回。
  • 调用 chain.proceed 网络请求获取 response
  • 对 code 304 作出处理，结合本地及网络返回数据构建 response 并返回
  • 构建网络请求的所获得的 response ，并且由于该网络请求并未进行过缓存，进行缓存并返回结果

ConnectInterceptor

主要用来建立连接。

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public final class ConnectInterceptor implements Interceptor {
  public final OkHttpClient client;

  public ConnectInterceptor(OkHttpClient client) {
    this.client = client;
  }

  @Override public Response intercept(Chain chain) throws IOException {
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    Request request = realChain.request();
    StreamAllocation streamAllocation = realChain.streamAllocation();

    // We need the network to satisfy this request. Possibly for validating a conditional GET.
    boolean doExtensiveHealthChecks = !request.method().equals("GET");
    HttpCodec httpCodec = streamAllocation.newStream(client, chain, doExtensiveHealthChecks);
    RealConnection connection = streamAllocation.connection();

    return realChain.proceed(request, streamAllocation, httpCodec, connection);
  }
}

CallServerInterceptor

负责请求网络。

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@Override public Response intercept(Chain chain) throws IOException {
  RealInterceptorChain realChain = (RealInterceptorChain) chain;
  HttpCodec httpCodec = realChain.httpStream();
  StreamAllocation streamAllocation = realChain.streamAllocation();
  RealConnection connection = (RealConnection) realChain.connection();
  Request request = realChain.request();

  long sentRequestMillis = System.currentTimeMillis();

  realChain.eventListener().requestHeadersStart(realChain.call());
  httpCodec.writeRequestHeaders(request);
  realChain.eventListener().requestHeadersEnd(realChain.call(), request);

  Response.Builder responseBuilder = null;
  if (HttpMethod.permitsRequestBody(request.method()) && request.body() != null) {
    // If there's a "Expect: 100-continue" header on the request, wait for a "HTTP/1.1 100
    // Continue" response before transmitting the request body. If we don't get that, return
    // what we did get (such as a 4xx response) without ever transmitting the request body.
    if ("100-continue".equalsIgnoreCase(request.header("Expect"))) {
      httpCodec.flushRequest();
      realChain.eventListener().responseHeadersStart(realChain.call());
      responseBuilder = httpCodec.readResponseHeaders(true);
    }

    if (responseBuilder == null) {
      // Write the request body if the "Expect: 100-continue" expectation was met.
      realChain.eventListener().requestBodyStart(realChain.call());
      long contentLength = request.body().contentLength();
      CountingSink requestBodyOut =
          new CountingSink(httpCodec.createRequestBody(request, contentLength));
      BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut);

      request.body().writeTo(bufferedRequestBody);
      bufferedRequestBody.close();
      realChain.eventListener()
          .requestBodyEnd(realChain.call(), requestBodyOut.successfulCount);
    } else if (!connection.isMultiplexed()) {
      // If the "Expect: 100-continue" expectation wasn't met, prevent the HTTP/1 connection
      // from being reused. Otherwise we're still obligated to transmit the request body to
      // leave the connection in a consistent state.
      streamAllocation.noNewStreams();
    }
  }

  httpCodec.finishRequest();

  if (responseBuilder == null) {
    realChain.eventListener().responseHeadersStart(realChain.call());
    responseBuilder = httpCodec.readResponseHeaders(false);
  }

  Response response = responseBuilder
      .request(request)
      .handshake(streamAllocation.connection().handshake())
      .sentRequestAtMillis(sentRequestMillis)
      .receivedResponseAtMillis(System.currentTimeMillis())
      .build();

  int code = response.code();
  if (code == 100) {
    // server sent a 100-continue even though we did not request one.
    // try again to read the actual response
    responseBuilder = httpCodec.readResponseHeaders(false);

    response = responseBuilder
            .request(request)
            .handshake(streamAllocation.connection().handshake())
            .sentRequestAtMillis(sentRequestMillis)
            .receivedResponseAtMillis(System.currentTimeMillis())
            .build();

    code = response.code();
  }

  realChain.eventListener()
          .responseHeadersEnd(realChain.call(), response);

  if (forWebSocket && code == 101) {
    // Connection is upgrading, but we need to ensure interceptors see a non-null response body.
    response = response.newBuilder()
        .body(Util.EMPTY_RESPONSE)
        .build();
  } else {
    response = response.newBuilder()
        .body(httpCodec.openResponseBody(response))
        .build();
  }

  if ("close".equalsIgnoreCase(response.request().header("Connection"))
      || "close".equalsIgnoreCase(response.header("Connection"))) {
    streamAllocation.noNewStreams();
  }

  if ((code == 204 || code == 205) && response.body().contentLength() > 0) {
    throw new ProtocolException(
        "HTTP " + code + " had non-zero Content-Length: " + response.body().contentLength());
  }

  return response;
}

  • 写入Request Header
  • 写入Request Body
  • 读取Response Header
  • 读取Response Body
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