JDK 25 compatibility: Drop JDK 8 support, JCTools 4.x, VarHandle fencing, CI modernization

[halotukozak]

Summary

• JCTools 3.3.0 → 4.0.5 — Updated the shaded JCTools dependency to the latest version
• VarHandle.fullFence() — Replaced direct sun.misc.Unsafe.fullFence() calls with VarHandle.fullFence() (JDK 9+) in FromMessagePassingQueue.scala and FromCircularQueue.scala
• CI modernization — Replaced deprecated olafurpg/setup-scala@v10 with actions/setup-java@v4 (Temurin), updated all actions to v4, runners to ubuntu-latest, added JDK 21 and 25 to jvm-tests matrix
• Drop JDK 8 support

Also includes infrastructure updates from bump-dependencies: SBT 1.10.7, Scala 2.12.20/2.13.16/3.3.5, sbt-tpolecat 0.5.3, plugin updates.

Test plan

• CI jvm-tests matrix passes for all 12 combinations (JDK [8, 11, 21, 25] × Scala [2.12, 2.13, 3.3])
• JDK 25 rows specifically pass — confirms sun.misc.Unsafe warnings are non-fatal
• JS tests pass on JDK 11
• MiMa binary compatibility checks pass

Note: The 40+ Java atomic files and shaded JCTools JAR still use sun.misc.Unsafe for CAS operations. On JDK 25 (JEP 498 Phase 2), these produce non-fatal warnings. On JDK 26+ (Phase 3), they will need migration to VarHandle — tracked as future work.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Updates Monix’s build/tooling and CI to improve compatibility with newer JDKs (incl. JDK 25), modernizes GitHub Actions, and bumps key build dependencies (including JCTools).

Changes:

• Bumped build tooling and plugins (SBT 1.10.7, sbt plugins, scalac/tpolecat configuration) and updated JCTools to 4.0.5.
• Replaced Unsafe.fullFence() usage with VarHandle.fullFence() in two JVM queue adapters.
• Modernized CI workflows (actions v4, ubuntu-latest, expanded JVM test matrix to JDK 8/11/21/25).

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
project/plugins.sbt	Updates sbt plugin versions; adds a dependency scheme override for geny.
project/build.properties	Bumps sbt version to 1.10.7.
monix-execution/shared/src/main/scala/monix/execution/CancelablePromise.scala	Suppresses an exhaustivity warning for a list destructure.
monix-execution/jvm/src/main/scala/monix/execution/internal/collection/queues/FromMessagePassingQueue.scala	Switches fencing implementation to VarHandle.fullFence().
monix-execution/jvm/src/main/scala/monix/execution/internal/collection/queues/FromCircularQueue.scala	Switches fencing implementation to VarHandle.fullFence().
build.sbt	Bumps versions (JCTools, plugins) and adjusts scalac/tpolecat options.
.jvmopts	Removes a deprecated JVM option (CMSClassUnloadingEnabled).
.gitignore	Ignores .planning/.
.github/workflows/build.yml	Modernizes actions/runners; expands JVM matrix and updates Scala versions.

Comments suppressed due to low confidence (1)

.github/workflows/build.yml:104

• Node.js 14 is end-of-life and may eventually stop being available on GitHub-hosted runners / receive security updates. Since this workflow is being modernized, consider bumping node-version to a supported LTS (e.g. 18 or 20) unless Scala.js tooling here explicitly requires 14.

      - name: Setup NodeJS
        uses: actions/setup-node@v4
        with:
          node-version: 14

---

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[mikkolaj]

Calling this MR JDK 25 compatibility is a bit misleading - current Monix version is compatible with JDK 25, but has some worrying warnings about Unsafe deprecation. You successfully switched from UNSAFE.fullFence() to VarHandle.fullFence(), but there are still lots of references to Unsafe in monix.execution.internal.atomic package.

Eliminating Unsafe usage is a valid thing to do but we have yet to decide how to do it. AtomicAny#safe(...) docs give some insight into why Unsafe was used in the first place:

    * NOTE that explicit usage of this builder is not usually necessary
    * because [[create]] can auto-detect whether the underlying platform
    * supports `sun.misc.Unsafe` and if it does, then its usage is
    * recommended, because the "safe" atomic instances have overhead.

Support for Java 7 Unsafe can be dropped like you did in another MR, but it still leaves some Unsafe leftovers in monix.execution.internal.atomic package. Two simplest ways to get rid of them seem to be:

• switching to their "safe" counterparts
• migrating from Unsafe toVarHandle

Both of these approaches might impact performance and we don't know the exact numbers. To decide which way to go we should compare benchmarks of all three variants - "safe", Unsafe and VarHandle. You might also want to check out this MR, which fixes some problems with Monix benchmarks.

[mikkolaj]

I'm not sure if I like this approach. Both methods still exist on JDK 25 and migration to threadId is trivial - I think we can stick with the deprecated method for now and switch to threadId when dropping support for JDK < 19. This way we'll avoid ugly reflection.