Downgrading Spotless to pre-JDK16 versions breaks idempotency

[gmaltsev]

How to reproduce:

1. Set up a project running Spotless 5.14.1 and JDK 17.
2. Run ./gradlew clean spotlessApply.
3. Get an exception described in this issue.
4. Upgrade Spotless to 6.23.0.
5. Run the command from step 2 again.
6. Get no exception this time, as expected.
7. Downgrade back to 5.14.1.
8. Run the command from step 2 again.

Actual behaviour: the exception from step 3 is gone, build passes.

Expected behaviour: the same exception is back.

If you delete the ~/.gradle/daemon directory, it makes Spotless behave as expected again i.e. the exception is back.

This bug might create inconsistencies between local and CI environments if someone needs to downgrade Spotless for whatever reason.

Setup:
Gradle version: 8.4
Spotless version: 5.14.1
Operating system and version: Mac OS X 13.5.1 aarch64

Gradle config:

buildscript {
    ...
    dependencies {
        classpath 'com.diffplug.spotless:spotless-plugin-gradle:5.14.1'
        ...
    }
}

...

sourceCompatibility = 17
targetCompatibility = 17

spotless {
    java {
        googleJavaFormat('1.18.1')
    }
}

[nedtwigg]

5.14.1 was released 2021-07-06, there's no chance that we will publish a new patch release for 5.x. This is very interesting behavior, thank you for sharing it, but it's very unlikely that figuring this out will ever reach the top of our todo list.