This is simple to do, but not obvious from the docs.
First, find the mustache template you want to change.
Then make a folder named for the kind of generator you’re working on, and put the customization there.
When you invoke the generator CLI, use the -t argument with the folder you made – that’s all you have to do.
You only need to add files you change. The CLI calls a Java jar, and this path is added to the classpath prior to anything else, so anything not included in the folder will ultimately still be found.
generate -i openapi.yaml \
-g java \
-t ./java \
--additional-properties="<properties here>" \