java如何過濾無效的utf-8字符？

之前遇到過幾次nutch/solr報這樣的錯誤：Invalid UTF-8 character。原來1.3版本的nutch有Strip UTF-8 non-character codepoints的bug，在1.4就修復了。

bug鏈接：https://issues.apache.org/jira/browse/NUTCH-1016。

於是這裏把nutch裏如何過濾無效utf-8字符的代碼找出來給小夥伴們看看。直接上代碼了：

public static String stripNonCharCodepoints(String input) {
  StringBuilder retval = new StringBuilder();
  char ch;

  for (int i = 0; i < input.length(); i++) {
    ch = input.charAt(i);

    // Strip all non-characters http://unicode.org/cldr/utility/list-unicodeset.jsp?a=[:Noncharacter_Code_Point=True:]
    // and non-printable control characters except tabulator, new line and carriage return
    if (ch % 0x10000 != 0xffff && // 0xffff - 0x10ffff range step 0x10000
        ch % 0x10000 != 0xfffe && // 0xfffe - 0x10fffe range
        (ch <= 0xfdd0 || ch >= 0xfdef) && // 0xfdd0 - 0xfdef
        (ch > 0x1F || ch == 0x9 || ch == 0xa || ch == 0xd)) {

      retval.append(ch);
    }
  }

  return retval.toString();
}