Gradle conversion issue: Unsupported class file major version 69

[sergey-morenets]

Hi

We have Gradle project that uses JDK 25 and Gradle 9.3.1
We tried to migrate it to Mill however mill init command resulted in this error (I removed non-important parts of stack-trace):

converting Gradle build
Downloading https://services.gradle.org/distributions/gradle-8.11.1-bin.zip
..................................................................................................................................
org.gradle.tooling.BuildException: Could not fetch model of type 'BuildModel' using connection to Gradle distribution 'https://services.gradle.org/distributions/gradle-8.11.1-bin.zip'.
        at org.gradle.tooling.internal.consumer.ConnectionExceptionTransformer.transform(ConnectionExceptionTransformer.java:53)
        at org.gradle.tooling.internal.consumer.ResultHandlerAdapter.onFailure(ResultHandlerAdapter.java:42)
        at org.gradle.tooling.internal.consumer.async.DefaultAsyncConsumerActionExecutor$1$1.run(DefaultAsyncConsumerActionExecutor.java:68)
        at org.gradle.internal.concurrent.ExecutorPolicy$CatchAndRecordFailures.onExecute(ExecutorPolicy.java:64)
        at org.gradle.internal.concurrent.AbstractManagedExecutor$1.run(AbstractManagedExecutor.java:48)
        at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144)
        at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642)
        at java.base/java.lang.Thread.run(Thread.java:1583)
        at org.gradle.tooling.internal.consumer.BlockingResultHandler.getResult(BlockingResultHandler.java:46)
        at org.gradle.tooling.internal.consumer.DefaultModelBuilder.get(DefaultModelBuilder.java:50)
        at mill.main.gradle.MillGradleBuildGenMain$.$anonfun$9(MillGradleBuildGenMain.scala:69)
        at scala.util.Using$.resource(Using.scala:298)
        at mill.main.gradle.MillGradleBuildGenMain$.init(MillGradleBuildGenMain.scala:71)
        at mill.main.gradle.MillGradleBuildGenMain$.$anonfun$4(MillGradleBuildGenMain.scala:17)
        at mainargs.Invoker$.$anonfun$2(Invoker.scala:77)
        at mainargs.Result.flatMap(Result.scala:13)
        at mainargs.Result.flatMap$(Result.scala:7)
        at mainargs.Result$Success.flatMap(Result.scala:23)
        at mainargs.Invoker$.invoke0(Invoker.scala:77)
        at mainargs.Invoker$.invoke(Invoker.scala:86)
        at mainargs.Invoker$.invokeLocal$1(Invoker.scala:106)
        at mainargs.Invoker$.groupArgs$1$$anonfun$2(Invoker.scala:121)
        at mainargs.Result.flatMap(Result.scala:13)
        at mainargs.Result.flatMap$(Result.scala:7)
        at mainargs.Result$Success.flatMap(Result.scala:23)
        at mainargs.Invoker$.groupArgs$1(Invoker.scala:121)
        at mainargs.Invoker$.runMains(Invoker.scala:126)
        at mainargs.ParserForMethods.runRaw0(Parser.scala:291)
        at mainargs.ParserForMethods.runEither(Parser.scala:179)
        at mainargs.ParserForMethods.runOrExit(Parser.scala:73)
        at mill.main.gradle.MillGradleBuildGenMain$.main(MillGradleBuildGenMain.scala:17)
        at mill.main.gradle.MillGradleBuildGenMain.main(MillGradleBuildGenMain.scala)
Caused by: org.gradle.internal.exceptions.LocationAwareException: BUG! exception in phase 'semantic analysis' in source unit '_BuildScript_' Unsupported class file major version 69
        at org.gradle.initialization.exception.DefaultExceptionAnalyser.transform(DefaultExceptionAnalyser.java:97)
        at org.gradle.initialization.exception.DefaultExceptionAnalyser.collectFailures(DefaultExceptionAnalyser.java:63)
        at org.gradle.internal.concurrent.AbstractManagedExecutor$1.run(AbstractManagedExecutor.java:48)
Caused by: BUG! exception in phase 'semantic analysis' in source unit '_BuildScript_' Unsupported class file major version 69
        at org.gradle.groovy.scripts.internal.DefaultScriptCompilationHandler.compileScript(DefaultScriptCompilationHandler.java:160)
        at org.gradle.groovy.scripts.internal.DefaultScriptCompilationHandler.compileToDir(DefaultScriptCompilationHandler.java:112)
        at org.gradle.groovy.scripts.internal.BuildOperationBackedScriptCompilationHandler$2.run(BuildOperationBackedScriptCompilationHandler.java:54)
        at org.gradle.internal.operations.DefaultBuildOperationRunner$1.execute(DefaultBuildOperationRunner.java:29)
        at org.gradle.internal.operations.DefaultBuildOperationRunner$1.execute(DefaultBuildOperationRunner.java:26)
        at org.gradle.internal.operations.DefaultBuildOperationRunner$2.execute(DefaultBuildOperationRunner.java:66)
        at org.gradle.internal.operations.DefaultBuildOperationRunner$2.execute(DefaultBuildOperationRunner.java:59)
        at org.gradle.internal.buildtree.DefaultBuildTreeLifecycleController.lambda$fromBuildModel$2(DefaultBuildTreeLifecycleController.java:90)
        at org.gradle.internal.buildtree.DefaultBuildTreeLifecycleController.lambda$runBuild$4(DefaultBuildTreeLifecycleController.java:120)
        ... 75 more
Caused by: java.lang.IllegalArgumentException: Unsupported class file major version 69
        at groovyjarjarasm.asm.ClassReader.<init>(ClassReader.java:200)
        at groovyjarjarasm.asm.ClassReader.<init>(ClassReader.java:180)
        at groovyjarjarasm.asm.ClassReader.<init>(ClassReader.java:166)
        at groovyjarjarasm.asm.ClassReader.<init>(ClassReader.java:288)

We were surprised that Mill uses such old Gradle version (8.11.1) that released on October, 2024. It seems that using Gradle 9.x for migration could resolve this issue but the only article about automatic migration has no information about that: https://mill-build.org/mill/migrating/auto-migrating.html

[lihaoyi]

@ajaychandran could you take a look

[ajaychandran]

The error is thrown because using JDK 25 requires at least Gradle 9.1.0.

Mill uses the global Gradle installation to run a custom script to extract the build settings. No particular Gradle version is specified; the expectation is that the version used by the project is auto-selected.
@sergey-morenets Could you please provide the output for the following commands?

gradle --version
./gradlew --version

[sergey-morenets]

@ajaychandran Yes, sure.

Welcome to Gradle 9.3.1!

Here are the highlights of this release:
 - Test reporting improvements
 - Error and warning improvements
 - Build authoring improvements

For more details see https://docs.gradle.org/9.3.1/release-notes.html


------------------------------------------------------------
Gradle 9.3.1
------------------------------------------------------------

Build time:    2026-01-29 14:15:01 UTC
Revision:      44f4e8d3122ee6e7cbf5a248d7e20b4ca666bda3

Kotlin:        2.2.21
Groovy:        4.0.29
Ant:           Apache Ant(TM) version 1.10.15 compiled on August 25 2024
Launcher JVM:  25.0.2 (Oracle Corporation 25.0.2+10-LTS-69)
Daemon JVM:    C:\Program Files\Java\jdk-25.0.2 (no Daemon JVM specified, using current Java home)
OS:            Windows 10 10.0 amd64

[ajaychandran]

@sergey-morenets Is that the output for ./gradlew --version? Could you also share the output for the global Gradle installation (gradle --version)?

[sergey-morenets]

@ajaychandran I have Windows and don't use gradlew (only gradle).
What is interesting - Mill created .gradle folder in the root folder and 8.11.1 sub-folder there. If remove .gradle folder then Mill creates it again during conversion.
I checked all the files in the project - and found no mentioning of 8.11.1 Gradle version.

[sergey-morenets]

If I have gradlew.bat and gradle/wrapper/gradle-wrapper.properties then Mill uses exact Gradle version specified there:

distributionUrl=https\://services.gradle.org/distributions/gradle-9.3.1-bin.zip

[ajaychandran]

IIUC, when no Gradle wrapper is specified, the globally installed version of Gradle is used by default.
Does your system have more than one installation of Gradle?

[ajaychandran]

@sergey-morenets Could you remove the .gradle sub-folder and run gradle build? What Gradle version is downloaded in the .gradle sub-folder?

[sergey-morenets]

@ajaychandran After I removed .gradle folder and ran gradle build command Gradle created .gradle/9.3.1 folder. And in my PATH environment variable there's an entry to Gradle 9.3.1 installation folder.