Nothing enforces the Gradle daemon JVM, so a pre-25 JDK fails with a cryptic KSP class-file error

[MariusVolkhart]

The build requires JDK 25, but nothing enforces the Gradle daemon's JVM, so any contributor whose java on PATH predates 25 gets a cryptic class-file-version error from an unrelated module instead of a clear "wrong JDK" message.

What happens

On a machine where the Gradle daemon runs JDK 21, ./gradlew build fails in :e2e-tests-micronaut:kspKotlin:

io/micronaut/kotlin/processing/.../TypeElementSymbolProcessorProvider has been compiled by a
more recent version of the Java Runtime (class file version 69.0), this version of the Java
Runtime only recognizes class file versions up to 65.0

Class file 69 = Java 25, 65 = Java 21. Micronaut 5.x baselines on Java 25 — verified by reading the bytecode header of TypeElementSymbolProcessorProvider.class in micronaut-inject-kotlin-5.1.12.jar (CA FE BA BE 00 00 00 45, 0x45 = 69).

Nothing about that message says "your JDK is too old", and it surfaces in a framework e2e module rather than anywhere the contributor was working.

Why the toolchain doesn't prevent it

buildSrc/src/main/kotlin/com/pkware/gradle/JavaConventionsPlugin.kt:26-30 declares the toolchain, and it is the only JVM-version declaration in the repo:

configure<JavaPluginExtension> {
  toolchain { languageVersion.set(JavaLanguageVersion.of(25)) }
}

That governs compilation and test JVMs. It does not govern the daemon. KSP2 is the default at the KSP version in use (com.google.devtools.ksp:2.3.11, no ksp.useKSP2 override), and it loads symbol processors inside the Kotlin compiler process rather than forking to the toolchain JVM — so the processor jar is loaded by the daemon's JDK, not the declared 25.

There is no gradle/gradle-daemon-jvm.properties and no org.gradle.java.home in gradle.properties, so the daemon inherits whatever java is on PATH.

Not a regression

CI is fine — .github/workflows/ci.yml sets java-version: 25 and ./gradlew build passes on main. And the Micronaut version is not the trigger: git log -S"micronaut = " -- gradle/libs.versions.toml shows the key was introduced in b0d1404, and every Renovate bump since (5.1.8 → 5.1.12) was already Java-25-targeted, since the major-5 baseline predates them all. The requirement has simply always been implicit.

Suggested fix

Add gradle/gradle-daemon-jvm.properties with toolchainVersion=25 so Gradle provisions or selects the right daemon JVM itself and fails with an actionable message otherwise. Worth stating the JDK 25 requirement in the README's prerequisites too — CLAUDE.md currently says "JDK 17+", which is now wrong.

Low priority: the workaround is one line (export JAVA_HOME=$(/usr/libexec/java_home -v 25)). It costs a new contributor an afternoon of misdirected debugging in an unrelated module, though.