A DESCRIPTION OF THE PROBLEM :
In some environments like a systemd service with DynamicUser=true, or in Docker, the NSS database isn't updated with useful information. In systemd querying the home directory returns "/" which is useless and in the past I've seen Docker environments return null, causing HotSpot to set user.home to "?". This can lead to very odd issues with directories name ? turning up all over the place as usually this value is passed straight to Path or File.

Some time ago I filed a bug against systemd asking them to put a more useful answer in NSS:

https://github.com/systemd/systemd/issues/19537

Unfortunately the maintainer doesn't want to fix this. His view is that everything on UNIX _except_ Java uses $HOME and that it's Java that needs to be fixed, as then a systemd unit can set HOME to something "meaningful" e.g. the state directory.

In the past, using $HOME for user.home has been deprioritized as being a potentially backwards incompatible change, presumably for weird UNIX environments where $HOME doesn't match the NSSDB even though $HOME is initialized from that db. So in this bug I propose a change that shouldn't have any compatibility issues - when NSS returns either "/" or NULL, fall back to reading $HOME and/or $STATE_DIRECTORY instead. Neither / nor ? are valid home directories on UNIX systems, thus presenting these values to an app is always wrong and can only break things.

This isn't a theoretical concern by the way. I'm currently dealing with some breakage of a Java app that runs in a systemd service because it tries to write to the root directory.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run this command on a Linux system with systemd and Java installed:

systemd-run -p DynamicUser=1 -p StateDirectory=xyz -p Environment=HOME=/var/lib/xyz -p WorkingDirectory=/var/lib/xyz -t /usr/bin/java -XshowSettings:properties --version | grep user.home



EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
It points to a writable directory.
ACTUAL -
user.home is set to /

CUSTOMER SUBMITTED WORKAROUND :
A mix of setting system properties by hand, changing your own code, etc, but user.home is used all over the place including in libraries so this gets painful fast.