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

You might also be interested in MeCab, which is not 100% Java (JNI binding) but considered by many as more performant:
http://code.google.com/p/mecab/source/browse/#svn%2Ftrunk%2Fmecab%2Fjava