gistfile1.java

String returnString = "mystring";
        
Writer writer = new QRCodeWriter();

BitMatrix encoded = null;
try
{
    encoded = writer.encode(returnString, BarcodeFormat.QR_CODE, 300, 300);
} 
catch (WriterException ex)
{
    Logger.getLogger(QRService.class.getName()).log(Level.SEVERE, null, ex);
}

ByteArrayOutputStream outs = new ByteArrayOutputStream();

MatrixToImageWriter.writeToStream(encoded, "PNG", outs);

resp.setContentType("image/png");
resp.getOutputStream().write(outs.toByteArray());