I've used the TMP environment variable to control things like where gcc writes it's temporary files, but I can't seem to find an equivalent for java's createTempFile API.
Does such an environment variable exist?
Hmmm -- since this is handled by the JVM, I delved into the OpenJDK VM source code a little bit, thinking that maybe what's done by OpenJDK mimics what's done by Java 6 and prior. It isn't reassuring that there's a way to do this other than on Windows.
On Windows, OpenJDK's get_temp_directory() function makes a Win32 API call to GetTempPath(); this is how on Windows, Java reflects the value of the TMP environment variable.
On Linux and Solaris, the same get_temp_directory() functions return a static value of /tmp/.
I don't know if the actual JDK6 follows these exact conventions, but by the behavior on each of the listed platforms, it seems like they do.