如图：
代码：
ByteArrayOutputStream jpgout = new ByteArrayOutputStream();
String id = req.getSession().getId();
BufferedImage bimg = new DefaultManageableImageCaptchaService().getImageChallengeForID(id,req.getLocale());
JPEGImageEncoder imgcode = JPEGCodec.createJPEGEncoder(jpgout);
imgcode.encode(bimg);
byte[] captchaChallengeAsJpeg = jpgout.toByteArray();
resp.setHeader("Cache-Control", "no-store");
resp.setHeader("Pragma", "no-cache");
resp.setDateHeader("Expires", 0);
resp.setContentType("image/jpeg");
ServletOutputStream responseOutputStream = resp.getOutputStream();
responseOutputStream.write(captchaChallengeAsJpeg);
responseOutputStream.flush();
responseOutputStream.close();