Gradle "Could not initialize class org.codehaus.groovy..." - Fix JDK

Gradle’s embedded Groovy could not initialize because the JDK running Gradle is newer than that Gradle version supports. Strong module encapsulation in newer JDKs breaks the reflection Groovy relies on.

What this error means

Gradle fails during configuration with Could not initialize class org.codehaus.groovy.reflection.ReflectionCache (often an InaccessibleObjectException underneath). It happens before your build logic, on a too-new JDK.

gradle output

* What went wrong:
Could not initialize class org.codehaus.groovy.reflection.ReflectionCache
> Caused by: java.lang.reflect.InaccessibleObjectException: Unable to make
  ... accessible: module java.base does not "opens java.lang" to unnamed module

Common causes

Gradle version too old for the JDK

Older Gradle bundles an older Groovy that uses deep reflection. Newer JDKs enforce module encapsulation, so Groovy cannot initialize.

JAVA_HOME points at a too-new JDK

CI provisioned the newest JDK and Gradle picked it up to run on. The Gradle-runtime JDK, not your build target, is the problem.

How to fix it

Upgrade the Gradle wrapper

A newer Gradle bundles a Groovy compatible with current JDKs.

Terminal

./gradlew wrapper --gradle-version 8.8
# commit the updated gradle-wrapper.properties

Or run Gradle on a supported JDK

Keep your code targeting newer Java via toolchains, but run the Gradle process on a JDK this Gradle supports.

.github/workflows/ci.yml

- uses: actions/setup-java@v4
  with:
    distribution: temurin
    java-version: '17'   # JDK that runs Gradle

How to prevent it

Keep the Gradle wrapper current with the JDKs CI provisions.
Decouple the compile JDK from the Gradle-runtime JDK with toolchains.
Commit gradle-wrapper.properties so everyone runs the same Gradle.