I want to implement replaygain in my Java based jukebox project. It should work with Ogg Vorbis, FLAC and MP3.

I know vorbisgain which only adds metadata in the Ogg Comments. I guess i just have to adjust the volume from this value. Right?
http://sjeng.org/vorbisgain.html

Then there is mp3gain, which actually changes some information in the frame-headers. Is it correct that these changes will work with any decoder? Does this project have a metadata-tag-only solution?
http://mp3gain.sourceforge.net/

What about LAME replay gain. How do I handle this?
http://wiki.hydrogenaudio.org/index.php?title=Replaygain

Is there other common used replaygain techniques?

You can see the XMMS decoder for details on how. In short, you scale the sample values, before converting/clipping to the final integers that are sent to the sound card.

It can do either, afaik. If it changes the frame headers, it will work with any decoder. Or you can tell it to only write APE tags, in which case it is very similar to the Ogg case.

Similar to the Ogg case. You'll need to extract the value(s) from the LAME Xing/Info header.

Foobar2000 writes the ReplayGain information to TXXX ID3V2 tags (format similar to VorbisGain). There is an ID3V2 frame for it too, but I don't know much about it.

how does one know it the metatag from mp3gain has'nt already been "executed" in the frames?