In case anyone wonders what this BeanShell stuff is about: it allows you to script the innards of the program without using a java compiler and without relaunching after modifications. The syntax is very much Java, so you can easily prototype new functionality and copy it to .java files later. An example FrostWire.bsh file can be found here:

FrostWire.bsh

If this file is put in ForstWire's preference folder it will be loaded upon startup and executed. Among other things it shows how to add a menu with custom commands that execute script functions.

Similar files can be loaded from the BeanShell console tab while the program is running. You can also type arbitrary statements here, for example print() statements to display variables in Java classes.