UPDATE: I just created Jakaroma, its kanji transliteration is much more accurate than Kakasi-java so please use it instead, thanks! Also open source.

Kakasi-java
Convert Japanese kanji into romaji
See also http://kakasi.namazu.org

Originally written by Tomoyuki Kawao
Forked from the last code found at http://blog.kenichimaehashi.com/?article=13048363750
If you know any more recent version, please let me know by email nicolas.raoul at gmail

To build just run: ant

Usage:

   java -Dkakasi.home=. -jar lib/kakasi.jar [-JH | -JK | -Ja] [-HK | -Ha] [-KH | -Ka]
    [-i<input-encoding>] [-o<output-encoding>]
    [-p] [-f] [-c] [-s] [-b]
    [-r{hepburn|kunrei}] [-C | -U] [-w]
    [dictionary1 [dictionary2 [,,,]]]
Character Set Conversions:
 -JH: kanji to hiragana
 -JK: kanji to katakana
 -Ja: kanji to romaji
 -HK: hiragana to katakana
 -Ha: hiragana to romaji
 -KH: katakana to hiragana
 -Ka: katakana to romaji
Options:
 -i: input encoding
 -o: output encoding
 -p: list all readings (with -J option)
 -f: furigana mode (with -J option)
 -c: skip whitespace chars within jukugo
 -s: insert separate characters
 -b: output buffer is not flushed when a newline character is written
 -r: romaji conversion system
 -C: romaji Capitalize
 -U: romaji Uppercase
 -w: wakachigaki mode

Example:

java  -Dkakasi.home=. -jar lib/kakasi.jar -Ja
国際財務報告基準
kokusaizaimuhoukokukijun

Original documentation (in Japanese): http://nicolas-raoul.github.com/kakasi-java