Prevent NPE with rootLogger shorthand when no appenders are specified

[jhl221123]

This pull request addresses issue #3206, which could lead to a NullPointerException when using the shorthand notation for the root logger (e.g., rootLogger = INFO) in log4j2.properties without explicitly defining any appender references.

To resolve this, the static helper method LoggerConfig.getLevelAndRefs() has been updated to ensure that the LevelAndRefs container always holds a non-null list of AppenderRef objects. When no appenders are specified in the shorthand string, the method has been updated to return an empty list instead of null.

Checklist

• Base your changes on 2.x branch if you are targeting Log4j 2; use main otherwise
• ./mvnw verify succeeds (if it fails due to code formatting issues reported by Spotless, simply run ./mvnw spotless:apply and retry)
• Non-trivial changes contain an entry file in the src/changelog/.2.x.x directory
• Tests for the changes are provided
• Commits are signed (optional, but highly recommended)

[github-actions]

Job	Requested goals	Build Tool Version	Build Outcome	Build Scan®
build-macos-latest	clean install	3.9.8	✅
build-ubuntu-latest	clean install	3.9.8	✅
build-windows-latest	clean install	3.9.8	✅

Generated by gradle/develocity-actions

[ppkarwasz]

Looks good to me!

Can you add a changelog entry to the src/changelog/.2.x.x folder. The type of the entry should be fixed.

[jhl221123]

Thanks, @ppkarwasz! I've added the changelog entry. I appreciate your guidance and review!

[ppkarwasz]

I've added the changelog entry. I appreciate your guidance and review!

Thanks!

For some reason I don't fully understand the required GitHub Action workflows didn't start on your last commit.

• @vy any idea why?
• @jhl221123, could you try updating the branch again, e.g. git rebase -f origin/2.x; git push -f? Maybe it is a temporary problem and it will work for the next commit.