When you store jar files in a single database and make those jar files available to that database, it is possible to make changes to jar files or to change the database jar "classpath" dynamically (without having to reboot).
That is, when you install or replace a jar file within an SQL statement or change the database jar "classpath" (the derby.database.classpath property), Derby is able to load the new classes right away without your having to reboot.