MediaPlayer getDuration method always return 0 when mp3 loaded from asset folder

I an trying to play the mp3 file from asset folder but its getDuration method always return zero

        try {
             AssetFileDescriptor afd = getAssets().openFd("music.mp3");
            mediaPlayer.setDataSource(afd.getFileDescriptor(), afd.getStartOffset(), afd.getLength());
            afd.close();
            pause.setBackgroundResource(R.drawable.play);
            finalTime = mediaPlayer.getDuration();


        } catch (IOException e) {
            e.printStackTrace();
        }