...is difficult at best, and impossible most of the time.
UPDATE: ...CAN be very easy, IF you "specify the security domain" in NetBeans (and choose Manufacturer, for best results). All the problems mentioned below disappear, because the emulator stops asking questions. But if you want to test behavior on regular phone with no rights, you'll have to test on an actual phone - read on.
one: unlike most (if not all) phones, the emulator runs confirmation windows in the same thread as commandActions.
That means that if you try to do anything file-related from a commandAction, a security confirmation pops up and you can't dismiss it, because your command thread is (dead)locked waiting for the result of this confirmation.
Oh well, I made my file browser run the file commands in a background thread.
two: unlike all phones, the emulator wants a security confirmation for each and every method call on the FileConnection object. And it does not remember permissions. Therefore, listing a directory takes three confirmation clicks, and every time you want to re-read a file, another confirmation (which is likely to deadlock you, for reason one).
three: the emulator (at least version 2.5.2 in linux) creates a brand new tree structure for each run. And it appears that you can't choose what will appear in that structure. So if you want to test opening a specific file, you have to copy it into the proper location every time.
(although i remember that it behaved slightly differently on my notebook. i'll have to check that out too)
and all's good when the end's good, four: do not ever in your life try to "mount" parts of your filesystem into the emulator by linking them to the temporary filesystem root.
Upon exiting, the emulator happily descended into my directory and recursively deleted it. Fortunately, there wasn't anything irreplaceable, or i haven't noticed yet; but i lost a good hour of work.
Good to know, though, is that there is now a way to undelete files from an ext3 filesystem. Didn't help me, though; the software insisted on segfaulting. It's apparently a known problem with "large partitions on some 32bit systems".