如何从MediaHttpDownloader获取内容长度(也就是文件大小)？驱动器file.getSize()始终返回null

import com.google.api.client.googleapis.media.MediaHttpDownloader;
import com.google.api.client.googleapis.media.MediaHttpDownloaderProgressListener;

import java.io.IOException;

public class DownloadExample {
    public static void main(String[] args) {
        String fileId = "your_file_id";
        MediaHttpDownloader downloader = new MediaHttpDownloader.Builder(...)
                .setProgressListener(new CustomProgressListener())
                .build();

        try {
            downloader.download(new File("destination_file"), DriveService.files().get(fileId).executeMedia());
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private static class CustomProgressListener implements MediaHttpDownloaderProgressListener {
        @Override
        public void progressChanged(MediaHttpDownloader downloader) throws IOException {
            long contentLength = downloader.getMediaContentLength();
            System.out.println("Content Length: " + contentLength + " bytes");
        }
    }
}