java background, video capture and save a picture frame

FFmpegFrameGrabber ff = FFmpegFrameGrabber.createDefault("视频地址");
ff.start();
int ffLength = ff.getLengthInFrames();
Frame f;
String pngPath = "";
int i = 0;
while (i < ffLength) {
    f = ff.grabImage();
    //截取第6帧
    if( (i>5) && (f.image != null)){
        //截图并放入指定位置
        Java2DFrameConverter converter = new Java2DFrameConverter();
        BufferedImage bi = converter.getBufferedImage(f);
        String pngPath = "图片存放地址";
        File output = new File(pngPath);
        ImageIO.write(bi, "png", output);
        break;
    }
    i++;
}
ff.stop();