无法在java代码中拍摄视频截图

我用vlcj编写了一段代码,旨在定期拍摄视频截图。当我尝试输出视频的长度时,它会输出0。问题是什么?

import uk.co.caprica.vlcj.binding.LibVlc;
import uk.co.caprica.vlcj.player.MediaPlayerFactory;
import uk.co.caprica.vlcj.player.embedded.EmbeddedMediaPlayer;
import uk.co.caprica.vlcj.runtime.RuntimeUtil;

import com.sun.jna.Native;
import com.sun.jna.NativeLibrary;

public class ScreenShotCapture {

    public static void main(String[] args){
        NativeLibrary.addSearchPath(RuntimeUtil.getLibVlcLibraryName(), "C:\\Program Files\\VideoLAN\\VLC");
        Native.loadLibrary(RuntimeUtil.getLibVlcLibraryName(), LibVlc.class);
        MediaPlayerFactory mediaPlayerFactory = new MediaPlayerFactory();
        final EmbeddedMediaPlayer mediaPlayer =mediaPlayerFactory.newEmbeddedMediaPlayer();
        mediaPlayer.playMedia("E:\\videos\\Avenged Sevenfold - So Far Away [Music Video].mp4");
        mediaPlayer.setSnapshotDirectory("E:\\vidoes");
        long length = mediaPlayer.getTime();
        long interval = length / 21;
        for(long  i = 1;i <= length;i+= interval){
            mediaPlayer.setTime(i);
            mediaPlayer.saveSnapshot();
        }
    }
}