java.io.File.setReadOnly() for the obvious.
As for the rest, you will probably have to execute OS specific commands.

http://www.google.co.nz/search?q=cache:BEmy9qt1BtoJ:www.pittjug.org/pastmeetings/pittjug_javamail-carl50902.pdf+TransportEvent&hl=en
Haven't used it yet, so I can't comment too much on it's use.

Forgive my ignorance, I forget what the thread titles are when I'm working.
You want to restrict the textfield to only 3 chars right?
A possible solution would be to create a keyListener on the...

Parse the file as a string, then find char position of the second to last line return, substring from 0th char to 2nd last CR/LR (removing bottom line), and append this substring to the line you want...

Do you mean that the textfield should not allow a fourth char to be typed?

Or, that the String returned by getText() is rejected if over 3 chars?