How to build JOSM jar from source?
- Check out the trunk from http://josm.openstreetmap.de/svn/trunk using subversion
- Run “ant” in the trunk directory. You don’t need any parameters, it defaults to the jar build
You now should find a josm-custom.jar in trunk/dist. I assume that you have Subversion and Ant installed.
If you aren’t interested in JOSM itself but want to develop plugins, look here.