Thanks for chiming in …
Finally figured it out curtesy of this post: java - How to use system.d service with SDKMan - Stack Overflow
Needed to add this line to /usr/lib/systemd/system/openhab.service in the [Service] section:
Environment="JAVA_HOME=/home/omr/.sdkman/candidates/java/21.0.6.crac-zulu"
My first time using sdkman so …
It is very login shell-centric so this is how to find what to put in JAVA_HOME for non login shells:
sdk current
java 21.0.6.crac-zulu
sdk use java 21.0.6.crac-zulu
Using java version 21.0.6.crac-zulu in this shell.
echo $JAVA_HOME
/home/omr/.sdkman/candidates/java/21.0.6.crac-zulu