Java version for JRuby

[headius]

This should be an issue but the issue-filing requirements are incompatible with my situation.

I could find no documentation or code in the setup-ruby action that specifies or allows configuring the Java version used to run JRuby.

We would like to switch jruby-head builds to JRuby 10, which requires Java 21. JRuby 9.4, the current supported line, works fine on Java 21 as well, so I believe it would be safe to make setup-ruby always use Java 21... but I suspect it currently just defaults to whatever is the default Java on the runner.

If that is the case, JRuby users will be required to also configure the setup-java action or other OS-specific mechanisms (java-alternatives, etc) to update it. For example, the Ubuntu runners install Java 8, 11, 17, and 21, but the ubuntu-20.04 runners default to Java 11 and even the latest ubuntu-24.04 runners only default to Java 17.

My gut says that setup-ruby should depend on setup-java when running JRuby and pass through appropriate configs for version and distribution (defaulting to 21 and temurin or zulu perhaps).

[eregon]

It could be a feature request issue but this conversation is fine to track it.

Yes, setup-ruby currently just uses the default JAVA_HOME for simplicity.
Since there is a JAVA_HOME_21_X64 variable, at least on the Ubuntu runners you linked, I think that's reasonable to use when available.
Could you make a PR doing that?

I don't think it's really feasible nor desirable to "depend" on setup-java, it would be complicated, brittle and add a lot of dead code for non-JRuby.
But, I think it makes sense to support the case of users using setup-java first and then pickup that Java with setup-ruby for JRuby.
IOW I think ideally we would detect if JAVA_HOME is the default, if it is switch to JAVA_HOME_21_X64 instead if available, for all other cases do nothing.

[headius]

Ok, that makes sense. Users that want more control over the Java version being used can opt into setup-java, and otherwise it will use default installs. Perhaps we could still add config to specify the Java version to use on the default runners, rather than forcing everyone to 21? I know I said it should be ok to run 9.4 on 21, but I think it should also be configurable.

[headius]

I think it would be good to also include a bit of doco about this in the setup-ruby documentation, with a simple example.

[eregon]

Perhaps we could still add config to specify the Java version to use on the default runners, rather than forcing everyone to 21? I know I said it should be ok to run 9.4 on 21, but I think it should also be configurable.

What if people who want to configure it would just use setup-java to do so?
And of course we'd document that in the README.

We'd need a way to detect if the default JAVA_HOME was changed or not, or maybe whether setup-java has run, maybe it sets some env var or so we could check?