In the Apache Shiro project I was able to fix a few issues which only
occured on the OpenJ9 VM.
The OpenJ9 VM is the only big Java VM implementation next to hotspot.
All other VMs (or most other relevant VMs) are hotspot forks.
This is the only VM that actually differs from the OpenJDK builds
INFRA is using at the moment. For example, while all APIs are
implemented by both VMs, OpenJDK-Hotspot also added some extras, e.g.
alias for cryptography stuff (which was invalid but worked as an
alias). That made shiro work on all hotspot VMs, but not on OpenJ9.
As maven should work on all VMs, I would like to see OpenJ9 added to
the build matrix to prevent such issues. All other hotspot VMs will
most likely behave more similar to OpenJDK-hotspot than OpenJ9, as it
comes with a different memory layout, other powerful debug options,
other garbage collectors etc. No G1, no ZGC, etc.
What makes this easier is that OpenJ9 only has exactly ONE current
vendor: The Eclipse foundation's "Adopt" (adoptopenjdk.net  aka
Eclipse Adoptium’s  OpenJ9 ).
I requested support for those VMs at Infra  and with it an update
of the wiki, which currently does not have a vendor column (like:
Oracle, Zulu, Adopt, etc.) nor a column for the VM implementation
(hotspot vs OpnJ9).
I hope you like this idea. The OpenJ9 VM uses very little memory and I
like it for its powerful debug options.
For Apache Shiro, this helped us to discover another similar bug where
my patch did not catch an alias for a cipher, which was not required
for JVMs to be implemented.