code: public static String outputVerifyImage(int w, int h, File outputFile, int verifySize) throws IOException{
          String verifyCode = generateVerifyCode(verifySize);
          outputImage(w, h, outputFile, verifyCode);
          return verifyCode;
      }
docs: 生成随机验证码文件,并返回验证码值
  @param w
  @param h
  @param outputFile
  @param verifySize
  @return
  @throws IOException