This would seem to be a simple task. After all, a web application which is dependent on a database being located somewhere on the disk can't store that information in the database itself. It needs some independent storage location. The obvious place to put this information is in the customised application somewhere. But after searching for it for hours I couldn't find it. No, it is in Tomcat in the /conf/Catalina/localhost directory. The file is called dv.xml (or jetspeed.xml for an uncustomised application).
The reason I wanted to know was that I wished to distribute the website and have it run on Windows. Obviously /tmp/jetspeed wouldn't be available. It would make more sense to store the database in the same folder as the website, and then have a single .bat file to start it all up. But I needed to alter the database location. Now I can.
Actually, I use a relative url. Instead of
/tmp/jetspeed/derby/productiondb I used:
../../jetspeed/derby/productiondb, and it worked. My jetspeed directory was in the same directory as apache-tomcat-6.0.20, and contains derby/productiondb.