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]

switch to JAVA_HOME_21_X64 instead

I'll try this week to add this behavior for jruby-10+ and jruby-head setup.

[eregon]

Another thought on this is tons of tools use JAVA_HOME so it might break some workflows to override JAVA_HOME.
Does JRuby accept another env var to find the JDK? If so it would be safer to set that, and we could just always set it, regardless of what JAVA_HOME is set to.

[headius]

We do not support running with a different JDK than is set up in JAVA_HOME. Doing so would introduce a very tricky situation if users compile code with one JDK and then run it with JRuby on another JDK. Our recommendation is that JRuby should run in an environment with JAVA_HOME set to the JDK you intend to run.

The only cases I can think of where this would impact users is if they are building other libraries for use with JRuby that can't be compiled on Java 21. In that case, they're just delaying the inevitable by depending on an old JDK for part of their toolchain and a newer JDK for JRuby.

[headius]

I need to proceed with forcing jruby-head installs to use Java 21. I am not familiar with this codebase, though.

@eregon Where would this code go? I believe all that's necessary is to detect JRuby version being "head" or "> 10" and change JAVA_HOME env to whatever is in JAVA_HOME_21_* as appropriate for the host platform.

There's a discussion here about selecting the appropriate environment variable for the current platform: https://github.com/actions/runner-images/discussions/9266

[headius]

Filed feature request: #718