1) do I have to take care about the file separators for cross-platform issues ? On Windows the file separator is different than Linux/Mac OS, of course.I think / works on any platform, but it is safer to use File.separator.
2) how can I get the default local directory at client side ?new File(".").getAbsolutePath()
3) is that a good idea to map a "ramdisk" to avoid the security checks related to the access of the local file system by the Applet to write/create/modify some files ? The idea is to write to this ramdisk the files instead of writing them to the local file system using FileOutputStream() -- I only need 1Go at most.No. Your applet is already signed, so you should be able to use the file system, as far as the normal (Windows) user rights allow. You have some problems right now, but they are solvable.