When building this port with poudriere, it requires to build java/openjdk12. But building 2 OpenJDK ports is time-consuming for low-spec machine. So please consider providing bootstrap port for this one.
I don't intend to do a bootstrap port for openjdk13. I will definitely do one for openjdk17, which at the moment will be the next long term support release. I may do one earlier than that, but it depends a little on resources.
Can you not instruct poudriere to install openjdk12 from a package?