我先考一考你，你说这个字符串（ab一2?仯3?4）有多少个字符？
这时候你也许开始数了，123…9个，没错，确实我们肉眼确实是看到了9个，java代码如下：
有没有发现java判断出来的不是9，而是12！！！
那是因为这个字符串中有2个字符ab一2?仯3?4用黄色表示，实际上UTF-8已经满足不了，? 见下图：
java的String是使用UTF-16来表示的，U+0000-U+FFFF都ok，但是只要是下列这个区间的字符都无法用常规手段处理，5位内码的，用length都无法正确判断其长度
CJK Unified Ideographs Extension B (U+20000 through U+2A6DD)
CJK Unified Ideographs Extension C (U+2A700 through U+2B734)
CJK Unified Ideographs Extension D (U+2B740 through U+2B81D)
CJK Unified Ideographs Extension E (U+2B820 through U+2CEA1)
CJK Unified Ideographs Extension F (U+2CEB0 through U+2EBE0)
具体也可以查看官方资料 ，了解unicode，UTF-8，UTF-16，UTF-32的区别。
如下图，可以通过Character中的codePointCount来得到字符数量。
进入主题
jdk自带的Character类特别好用，汉字转内码，内码转汉字具体功能请参考下面的代码
public static String stringToCodePoints(String str) { StringBuilder stringBuilder = new StringBuilder(); str.codePoints().forEach(cp -> stringBuilder.append("u").append(Integer.toHexString(cp))); return stringBuilder.toString();}public static String codePointsToString(String codePoints) { StringBuilder stringBuilder = new StringBuilder(); for(String hexCodePoint : codePoints.split("\\u")){ if(StringUtils.isNotBlank(hexCodePoint)) { stringBuilder.append(codePointToString(Integer.parseInt(hexCodePoint, 16))); } } return stringBuilder.toString();}public static String codePointToString(int cp) { StringBuilder sb = new StringBuilder(); if (Character.isBmpCodePoint(cp)) { sb.append((char) cp); } else if (Character.isValidCodePoint(cp)) { sb.append(Character.highSurrogate(cp)); sb.append(Character.lowSurrogate(cp)); } else { sb.append(\'?\'); } return sb.toString();} 支持5位编码的在线转换工具
