扫描二维码显示问号

public static void main(String[] args) {

int width = 300;

int height = 300;

String format = "png";

//String contents = "http://wx.xingxingbaobei.net/wx/product/index";

String contents = "我是你大爷";

//定义二维码的参数

HashMap hints = new HashMap(); 

hints.put(EncodeHintType.CHARACTER_SET, "utf-8");

hints.put(EncodeHintType.ERROR_CORRECTION, ErrorCorrectionLevel.M);

hints.put(EncodeHintType.MARGIN, 2);

try {

BitMatrix bitMatrix = new MultiFormatWriter().encode(contents, BarcodeFormat.QR_CODE, width, height);

Path file = new File("F:/code/img.png").toPath();

MatrixToImageWriter.writeToPath(bitMatrix, format, file);

} catch (WriterException e) {

e.printStackTrace();

} catch (IOException e) {

e.printStackTrace();

}

}

这样子生成出来的二维码扫描显示的是问号，有多少个中文就有多少问号，这是怎么回事嘞？