jamesfredley commented on PR #15541:
URL: https://github.com/apache/grails-core/pull/15541#issuecomment-4200650677
## Micronaut 5.0.0-M1 - Per-Module JDK 21 Attempt
### What We Tried
Per the suggested approach of using `ext.javaVersion = 21` with a Gradle
toolchain in `grails-micronaut/build.gradle`:
```groovy
ext.javaVersion = 21
java {
toolchain {
languageVersion = JavaLanguageVersion.of(21)
}
}
```
### Dependency Resolution
With `micronautPlatformVersion=5.0.0-M1`, all Micronaut dependencies resolve
correctly:
- `micronaut-inject-groovy:5.0.0-M20` ✓
- `micronaut-spring-boot-starter:6.0.0-M1` ✓
- All transitives resolve ✓
### Compilation Failure
The Micronaut AST transforms (`InjectTransform`) are compiled as JDK 21
bytecode (class file version 65.0). These transforms run inside the Groovy
compiler at compile time, meaning:
1. `ext.javaVersion = 21` sets `--release 21` (target output) but doesn't
change the compiler JVM
2. Even with `java.toolchain.languageVersion = 21`, the build machine needs
JDK 21 installed
3. Error: `UnsupportedClassVersionError:
io/micronaut/ast/groovy/InjectTransform has been compiled by a more recent
version of the Java Runtime (class file version 65.0)`
The local build environment only has JDK 17. CI has JDK 21 available (tested
in matrix), so this would work in CI but not for local development on JDK 17.
### Path Forward
The per-module toolchain approach IS technically correct - it just requires
JDK 21 to be installed. Options:
1. **Enable Gradle toolchain auto-provisioning** to download JDK 21
automatically (requires configuring a toolchain repository in `settings.gradle`)
2. **Require JDK 21 for contributors** working on the `grails-micronaut`
module
3. **Wait for Micronaut 4.x Spring Boot 4 backport** (if it happens)
4. **Bump Grails baseline to JDK 21** (biggest change, enables full
Micronaut 5 support)
The toolchain auto-provisioning (option 1) is the most practical path -
Gradle can automatically download JDK 21 when needed for this specific module
while the rest of the build continues on JDK 17.
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