JavaMail API getSubject(), subject has multiple "=?utf-8?B?~?=", how can I parse? - jakarta-mail

Question

My mail Subject is
Subject: =?utf-8?B?7IOI66Gc7Jq0IOyEpOusuOyhsOyCrOqwgCDsi5zsnpHrkJjs?=
=?utf-8?B?l4jsirXri4jri6QhIOydtCDquLDtmowg64aT7LmY7KeAIOuniOyEuOya?=
=?utf-8?B?lCE=?=
mimeMessage.getSubject() returns below:
The black diamonds are failed characters. And language is Korean.
And the below is correct subject:
I concatenated the raw data like below, MimeUtility.decodeText() returns good result.
(delete \r\n, delete inner "=?utf-8?B?" and "?=")
MimeUtility.decodeText(=?utf-8?B?7IOI66Gc7Jq0IOyEpOusuOyhsOyCrOqwgCDsi5zsnpHrkJjsl4jsirXri4jri6QhIOydtCDquLDtmowg64aT7LmY7KeAIOuniOyEuOyalCE=?=)
The result is:
How can I parse the subject which has multiple lines?

Answer 1

The problem is that the mailer that encoded this text encoded it incorrectly. What mailer was used to create this message?
The 16 bit Korean Unicode characters are converted to a stream of 8 bit bytes in UTF-8 format. The 8 bit bytes are then encoded using base64 encoding.
The MIME spec (RFC 2047) requires that each encoded word contain complete characters:
Each 'encoded-word' MUST represent an integral number of characters.
A multi-octet character may not be split across adjacent 'encoded-
word's.
In your example above, the bytes representing one of the Korean characters are split across multiple encoded words. Combining them into one encoded word, as you have done, allows the text to be decoded correctly.
This is a bug in the mailer that created the message and should be reported to the owner of that mailer.
Unfortunately, there's no good workaround in JavaMail for such a broken mailer.

Answer 2

I created function which decodes recursively 5 times.
/*
* Decodes 5 times encoded text with MimeUtility.decodeText()
*/
private String decode(String encoded) throws UnsupportedEncodingException {
String result = MimeUtility.decodeText(encoded);
int counter = 0;
while (result.contains("=?") && counter < 5) {
counter++;
String end = result.substring(result.indexOf("=?"));
result = result.substring(0, result.indexOf("=?")) + MimeUtility.decodeText(end);
}
return result;
}