BridgeInterceptor

该拦截器是链接客户端代码和网络代码的桥梁

• 将客户端构建的Request对象信息构建成真正的网络请求（设置请求头信息）
• 调用 RealInterceptorChain.proceed()进行网络请求,实际的结果就是会进入到下一个拦截器CacheInterceptor
• 将服务器返回的消息封装成一个Response对象

BridgeInterceptor部分源码

  @Override 
  public Response intercept(Chain chain) throws IOException {
    //获取用户构建的Request对象
    Request userRequest = chain.request();
    Request.Builder requestBuilder = userRequest.newBuilder();

    RequestBody body = userRequest.body();
    //设置Content-Type请求头
    if (body != null) {
      MediaType contentType = body.contentType();
      if (contentType != null) {
        requestBuilder.header("Content-Type", contentType.toString());
      }
      //Content-Length和Transfer-Encoding互斥
      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");
      }
    }
    //设置Host请求头
    if (userRequest.header("Host") == null) {
      requestBuilder.header("Host", hostHeader(userRequest.url(), false));
    }
    //设置Connection请求头
    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());
    //设置Cookie请求头
    if (!cookies.isEmpty()) {
      requestBuilder.header("Cookie", cookieHeader(cookies));
    }
    //设置User-Agent请求头
    if (userRequest.header("User-Agent") == null) {
      requestBuilder.header("User-Agent", Version.userAgent());
    }
    //拦截器链继续往下运行
    Response networkResponse = chain.proceed(requestBuilder.build());
    //响应header， 如果没有自定义配置cookieJar==null，则什么都不做
    HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers());

    Response.Builder responseBuilder = networkResponse.newBuilder()
        .request(userRequest);
    //判断服务器是否支持gzip压缩格式,如果支持则交给kio压缩
    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();
  }

intercept()的代码作用在注释中已经基本完成

1. Request设置User-Agent、Cookie、Accept-Encoding等相关请求头信息
2. 拦截器链继续往下运行,此时剩下拦截器工作就是返回真正的网络响应了（当然此响应可能是缓存的，也可能是服务器返回的最新数据）
3. 将服务器返回的消息封装成一个Response对象