Creating a large temporary file in a platform-agnostic way

Question

What's the best way of creating a large temporary file in Java, and being sure that it's on disk, not in RAM somewhere?

If I use

Path tempFile = Files.createTempFile("temp-file-name", ".tmp");

then it works fine for small files, but on my Linux machine, it ends up being stored in /tmp. On many Linux boxes, that's a tmpfs filesystem, backed by RAM, which will cause trouble if the file is large. The appropriate way of doing this on such a box is to put it in /var/tmp, but hard-coding that path doesn't seem very cross-platform to me.

Is there a good cross-platform way of creating a temporary file in Java and being sure that it's backed by disk and not by RAM?

Answer 1

There is no platform-independent way to determine free disk space. Actually there is not even a good platform-dependent way; things that happen are zfs filesystems (which may be compressing your data on the fly), directories that are being filled by other applications, or network shares that are simply lying to you.

I know of these options:

1. Assume that it is an operating concern. I.e. whoever uses the software should have an administrator who is aware of how much space is left on what device, and who expects to be able to explicitly configure the partition that should hold the data. I'd start considering this at several tens of GB, and prefer this at a few 100 GBs.
2. Assume it's really a temporary file. Document that the application needs xxx GB of temporary space (whatever rough estimate you can give them - my application says "needs ca. 100 GB for every automatic update that you keep on disk").
3. Abuse the user cache for the file. The XDG standard has $XDG_CACHE_HOME for the cache; the cache directory is supposed to be nice and big (take a look at the ~/.cache/ of anybody using a Linux machine). On Windows, you'd simply use %TEMP% but that's okay because %TEMP% is supposed to be big anyway.
   This gives the following strategy: Try environment variables, first XDG_CACHE_HOME (if it's nonempty, it's a Posix system with XDG conventions), then TMP (if it's nonempty, it's a Posix system and you don't have a better option than /tmp anyway), finally TEMP in case it's Windows.