@Override public Response intercept(Chain chain) throws IOException {
    Request userRequest = chain.request();
    // 构建一个新的Builder 
    Request.Builder requestBuilder = userRequest.newBuilder();
    
    RequestBody body = userRequest.body();
    if (body != null) {
      MediaType contentType = body.contentType();
      if (contentType != null) {
        // 把ContentType这个枚举转换为String  
        requestBuilder.header("Content-Type", contentType.toString());
      }
       
      long contentLength = body.contentLength();
      if (contentLength != -1) {
        // ContentLength转换为String  
        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");
    }

    // 如果还没设置Accept-Encoding，自动设置gzip
    boolean transparentGzip = false;
    if (userRequest.header("Accept-Encoding") == null && userRequest.header("Range") == null) {
      transparentGzip = true;
      requestBuilder.header("Accept-Encoding", "gzip");
    }
    // 把原请求中的cookie设置到新的builder中
    List<Cookie> cookies = cookieJar.loadForRequest(userRequest.url());
    if (!cookies.isEmpty()) {
      // 把Cookie转换成String  
      requestBuilder.header("Cookie", cookieHeader(cookies));
    }

    if (userRequest.header("User-Agent") == null) {
      requestBuilder.header("User-Agent", Version.userAgent());
    }
    // 从下层拿到Response
    Response networkResponse = chain.proceed(requestBuilder.build());
    
    HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers());
    // 构建一个新的Response用做转换，理所当然的把request设置为转换前的Request
    Response.Builder responseBuilder = networkResponse.newBuilder()
        .request(userRequest);
    // 如果设置了Gzip压缩，则要进行Gzip解压
    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();
  }