Upgrading to Java 21 LTS

[rtyley]

Base assumption: Project is already on Java 11

Overview

• Running Scala Steward on a repo at the Guardian #7 requires your project to build with Java 21
• You may want to complete Upgrading to Scala 2.13 #2 first - old project dependencies may use bytecode-manipulation libraries that don't yet support Java 21. Updating to Scala 2.13 will give you a better chance of having up-to-date dependencies that can handle Java 11.

Minimum requirements

• Scala: 2.12.18+, 2.13.11+, 3.3.1+
• sbt: 1.9.0+

Using a version of sbt that's too old while running under Java 21 will give an error like this:

error:
    bad constant pool index: 0 at pos: 48461
       while compiling: <no file>
          during phase: globalPhase=<no phase>, enteringPhase=<some phase>
       library version: version 2.12.16
      compiler version: version 2.12.16
    reconstructed args: -classpath /home/runner/.sbt/boot/scala-2.12.16/lib/scala-library.jar -Yrangepos

Tips

• Include "-release:21" in your build.sbt scalacOptions, to ensure that the project is built to support Java 21 or above.