Java rules don't appear to be usable in OSS edition of buck2

I tried to use `java_binary()`. I was pretty quickly hit with an error about one of my provider fields (`JavaToolchainInfo.fat_jar`) being unset, so I looked into what it might be expecting, and found this.

https://github.com/facebook/buck2/blob/d458e16b57cf34853ff4b239cd609eabe235f257/prelude/java/java_binary.bzl#L40-L48

Whatever tool this is doesn't appear to be a standard JRE tool, and I suspect it's a Meta internal tool. Which means all of the Java rules don't work outside of Meta. Is my hunch correct about this, or is there a way to get this tool?