利用Java如何实现一个随机生成动态验证码功能

  @RequestMapping(value="getGifCode",method=RequestMethod.GET) public void getGifCode(HttpServletResponse response,HttpServletRequest request){  try {  response.setHeader("Pragma", "No-cache");    response.setHeader("Cache-Control", "no-cache");    response.setDateHeader("Expires", 0);    response.setContentType("image/gif");       Captcha captcha = new GifCaptcha(146,33,4);   //输出   captcha.out(response.getOutputStream());   HttpSession session = request.getSession(true);    //存入Session   session.setAttribute("_code",captcha.text().toLowerCase());   } catch (Exception e) {  LoggerUtils.fmtError(getClass(),e, "获取验证码异常：%s",e.getMessage());  } }