okhttp源码2

前言

本章分析访问网络的逻辑,即RealCall.getResponseWithInterceptorChain()这个方法.这里okHttp框架使用了拦截链来对网络访问进行干预处理.每一条拦截器代表了一部分的业务逻辑.
上代码:

Response getResponseWithInterceptorChain() throws IOException {
    // Build a full stack of interceptors.
    //先构建一个拦截链条
    List<Interceptor> interceptors = new ArrayList<>();
    //将我们自定义的拦截器加入
    interceptors.addAll(client.interceptors());
    //重试和重定向拦截器
    interceptors.add(retryAndFollowUpInterceptor);
    //桥接拦截器,将request转化为网络可用的请求,将网络返回的response转化为对用户的response
    interceptors.add(new BridgeInterceptor(client.cookieJar()));
    //缓存拦截器
    interceptors.add(new CacheInterceptor(client.internalCache()));
    //连接拦截器
    interceptors.add(new ConnectInterceptor(client));
    //如果不是socket,添加自定义的网络拦截器
    if (!forWebSocket) {
      interceptors.addAll(client.networkInterceptors());
    }
    //连接服务器拦截器,真正请求数据的业务
    interceptors.add(new CallServerInterceptor(forWebSocket));

    //下面开始,会按照添加到连接器链条的顺序,依次执行拦截器
    Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0,
        originalRequest, this, eventListener, client.connectTimeoutMillis(),
        client.readTimeoutMillis(), client.writeTimeoutMillis());

    return chain.proceed(originalRequest);
  }

从代码可以看到,在将所有拦截器添加到链条之后,new了一个RealInterceptorChain实例,并执行proceed方法.下面分析这个方法.

public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,
      RealConnection connection) throws IOException {
    if (index >= interceptors.size()) throw new AssertionError();
    // Call the next interceptor in the chain.
    //新建一个chain,调用下一个拦截器的逻辑
    RealInterceptorChain next = new RealInterceptorChain(interceptors, streamAllocation, httpCodec,
        connection, index + 1, request, call, eventListener, connectTimeout, readTimeout,
        writeTimeout);
    Interceptor interceptor = interceptors.get(index);
    Response response = interceptor.intercept(next);
    return response;
  }

依靠每个拦截其中调用的chain.processd方法,即实现拦截器的依次执行.

RetryAndFollowUpInterceptor

除了我们自定义的拦截器,这是框架执行的第一个拦截器.再这个拦截器中,我们拿到网络请求的结果后,会进行失败重试或重定向.

@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;
    //在一个循环中不断重试,直到达成指定条件
    while (true) {
      //取消
      if (canceled) {
        streamAllocation.release();
        throw new IOException("Canceled");
      }

      Response response;
      boolean releaseConnection = true;
      try {
      //调用获取网络返回结果
        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.
        //连接路由失败,请求尚未发送,recover放回true表示可恢复的请求,false表不可,如果false则throw异常,终端重试
        if (!recover(e.getLastConnectException(), streamAllocation, false, request)) {
          throw e.getFirstConnectException();
        }
        //可恢复请求,继续
        releaseConnection = false;
        continue;
      } catch (IOException e) {
        // An attempt to communicate with a server failed. The request may have been sent.
      //如果出现io异常,判断是否请求可恢复
        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.
      //priorResponse 是上次循环中的response
      if (priorResponse != null) {
        response = response.newBuilder()
            .priorResponse(priorResponse.newBuilder()
                    .body(null)
                    .build())
            .build();
      }

      //重定向request
      Request followUp;
      try {
        //这里会根据response的返回码,处理不同的情况.比如返回码是重定向,即重新封装一个到重定向地址的request返回
        followUp = followUpRequest(response, streamAllocation.route());
      } catch (IOException e) {
        streamAllocation.release();
        throw e;
      }

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

      closeQuietly(response.body());
      //重定向次数超过上限,则退出
      if (++followUpCount > MAX_FOLLOW_UPS) {
        streamAllocation.release();
        throw new ProtocolException("Too many follow-up requests: " + followUpCount);
      }
      //不可重复的请求体
      if (followUp.body() instanceof UnrepeatableRequestBody) {
        streamAllocation.release();
        throw new HttpRetryException("Cannot retry streamed HTTP body", response.code());
      }
      //比较重定向url和当前url是否相同,如不同,则新建streamAllocation
      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;
    }
  }

可以看到,重试和重定向拦截器是在一个循环中不断重试,知道不在满足重试的条件.

BridgeInterceptor

这是一个将应用层request转化为网络岑request,比如添加一些请求头,如Content-Type,User-Agent,Accept-Encoding;并将网络层response转化为应用层response的拦截器.代码如下,不再赘述.

@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();
  }

CacheInterceptor

okHttp的缓存策略和http的缓存策略是一致的.http缓存机制请参考连接.下面分析代码:

@Override public Response intercept(Chain chain) throws IOException {
     //取出缓存的response,cache是一个本地缓存方案,以文件方式缓存到本地,
    //可以根据request取出相应的response,类似map
    Response cacheCandidate = cache != null
        ? cache.get(chain.request())
        : null;

    long now = System.currentTimeMillis();
    //缓存策略,这个类解析了request的缓存设置`get`方法根据不同的策略设置缓存
    CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
    Request networkRequest = strategy.networkRequest;
    Response cacheResponse = strategy.cacheResponse;
    //统计网络请求,和缓存response计数,有则加一
    if (cache != null) {
      cache.trackResponse(strategy);
    }
    //本地缓存不为空,但是根据缓存策略为空,所以本地缓存不可用,关闭
    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.
    //当不能进行网络请求,并且缓存response已经过期时,返回504.
    //从CacheStrategy.Factory.get方法可知,当request设置为OnlyIfCached时达成此条件
    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.
    //缓存有效时,直接返回缓存
    if (networkRequest == null) {
      return cacheResponse.newBuilder()
          .cacheResponse(stripBody(cacheResponse))
          .build();
    }

    //网络获取数据
    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.
    //当网络数据和缓存数据都存在时,
    if (cacheResponse != null) {
      //当网络返回码是内容未改变,组装缓存数据返回
      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());
      }
    }
    //网络返回数据
    Response response = networkResponse.newBuilder()
        .cacheResponse(stripBody(cacheResponse))
        .networkResponse(stripBody(networkResponse))
        .build();

    if (cache != null) {
      //网络数据有body并且可缓存时,缓存数据
      if (HttpHeaders.hasBody(response) && CacheStrategy.isCacheable(response, networkRequest)) {
        // Offer this request to the cache.
        CacheRequest cacheRequest = cache.put(response);
        return cacheWritingResponse(cacheRequest, response);
      }
      //此请求不可缓存时删除
      if (HttpMethod.invalidatesCache(networkRequest.method())) {
        try {
          cache.remove(networkRequest);
        } catch (IOException ignored) {
          // The cache cannot be written.
        }
      }
    }
    return response;
  }