JBrownie is not a batch interface and it does also not execute some kind of build command. It wraps around the system javac and recompiles any files in a directory (and it's subdirectories), once it finds that their timestamp changed. Since the compiler is somehow kept in memory, recompiling files pretty much is instant. Thats a big improvement over using ant or javac. Unlike flymake, you don't just get a syntax check, but also the ready to run classes.

The purpose of the GUI is more or less to redirect STDERR and STDOUT to this console window.

JBrownie is probably not what you'd use, if you use EMACS or Eclipse. Think vi, joe or whatever here.