code: public static void appendHex(StringBuilder builder, byte b, boolean toLowerCase) {
  		final char[] toDigits = toLowerCase ? DIGITS_LOWER : DIGITS_UPPER;
  		
  		int high = (b & 0xf0) >>> 4;//高位
  		int low = b & 0x0f;//低位
  		builder.append(toDigits[high]);
  		builder.append(toDigits[low]);
  	}
docs: 将byte值转为16进制并添加到{@link StringBuilder}中
  @param builder {@link StringBuilder}
  @param b byte
  @param toLowerCase 是否使用小写
  @since 4.4.1