** * 生成验证码 *  * @param request* @param response* @throws Exception */@RequestMapping(value = "captcha.htm", method = RequestMethod.GET)public void captcha(HttpServletRequest request, HttpServletResponse response)      throws Exception {   response.setDateHeader("Expires", 0);   response.setHeader("Cache-Control",         "no-store, no-cache, must-revalidate");   response.addHeader("Cache-Control", "post-check=0, pre-check=0");   response.setHeader("Pragma", "no-cache");   response.setContentType("image/jpeg");   String capText = captchaProducer.createText();   request.getSession().setAttribute(         com.google.code.kaptcha.Constants.KAPTCHA_SESSION_KEY, capText);   BufferedImage bi = captchaProducer.createImage(capText);   ServletOutputStream out = response.getOutputStream();   ImageIO.write(bi, "jpg", out);   try {      out.flush();   } finally {      out.close();   }}