code: public static String outputVerifyImage(int w, int h, OutputStream os, int verifySize) throws IOException{
          String verifyCode = generateVerifyCode(verifySize);
          outputImage(w, h, os, verifyCode);
          return verifyCode;
      }
docs: 输出随机验证码图片流,并返回验证码值
  @param w
  @param h
  @param os
  @param verifySize
  @return
  @throws IOException