OkHttp3如何从Http头检索Http内容长度?

我尝试下载一个文件,并检查是否已整体下载:

Request request = new Request.Builder().url("http://example.com/myfile.tar.gz").build();
Response response = client.newCall(request).execute();

// Status code checks goes here
if (downloadedTar == null) {
  throw new SettingsFailedException();
}
ResponseBody downloadedTar = response.body();
double contentLength = Double.parseDouble(response.header("content-length"));

File file = File.createTempFile(System.currentTimeMillis()+"_file", ".tar.gz", getContext().getCacheDir());
FileOutputStream download = new FileOutputStream(file);

download.write(downloadedTar.body().bytes());
download.flush();
download.close();

if(file.exists() && (double)file.length() == contentLength){
  // file has been downloaded
}

但是这句话:

double contentLength = Double.parseDouble(response.header("content-length"));

但是 response.header("content-length") 为Null并且没有整数值,我还尝试了以下变化 response.header("Content-Length") 和 回答标题(“内容长度”) 没有任何成功。

那个么,为什么我无法检索内容长度标题,以及如何确保文件已成功下载?