code: public static String toUnicodeString(String s) {
      if (Strings.isNullOrEmpty(s)) return "";
      int length = s.length();
      StringBuilder sbd = new StringBuilder(length * 6);
      for (int i=0; i< length; i++) {
        char c = s.charAt(i);
        int hi = c >>> 8;
        int lo = c & 0x0ff;
        sbd.append('\\').append('u').append(HEX_DIGIT[hi]).append(HEX_DIGIT[lo]);
      }
      return sbd.toString();
    }
docs: 把所有字符都换成\\uxxxx的字符编码样式
  @param s 待编码字符
  @return 编码后的字符串