...
String QRcode = "...";
new generateQrcode(qrcodeImageview).execute(QRcode);
...
private class generateQrcode extends AsyncTask<String, Void, Bitmap> {
        public final static int WIDTH = 400;
        ImageView bmImage;

        public generateQrcode(ImageView bmImage) {
            this.bmImage = bmImage;
        }

        protected Bitmap doInBackground(String... urls) {
            String Value = urls[0];
            com.google.zxing.Writer writer = new QRCodeWriter();
            Bitmap bitmap = null;
            BitMatrix bitMatrix = null;
            try {
                bitMatrix = writer.encode(Value, com.google.zxing.BarcodeFormat.QR_CODE, WIDTH, WIDTH,
                        ImmutableMap.of(EncodeHintType.MARGIN, 1));
                bitmap = Bitmap.createBitmap(400, 400, Bitmap.Config.ARGB_8888);
                for (int i = 0; i < 400; i++) {
                    for (int j = 0; j < 400; j++) {
                        bitmap.setPixel(i, j, bitMatrix.get(i, j) ? Color.BLACK
                                : Color.WHITE);
                    }
                }
            } catch (WriterException e) {
                e.printStackTrace();
            }
            return bitmap;
        }

        protected void onPostExecute(Bitmap result) {
            bmImage.setImageBitmap(result);
        }
    }