Q: How to compile with Java 9?

[nikos]

Compiling and running the demo with Java 8 is fine, but as soon as Java 9 I encounter the following error:

./gradlew bootRun

> Configure project :
e: Supertypes of the following classes cannot be resolved. Please make sure you have the required dependencies in the classpath:
    class org.gradle.api.Project, unresolved supertypes: Comparable<Project>
    class org.gradle.api.plugins.ExtensionAware, unresolved supertypes: Object
    class org.gradle.api.plugins.PluginAware, unresolved supertypes: Object
    class groovy.lang.Closure, unresolved supertypes: Cloneable, Runnable, Serializable
    class groovy.lang.GroovyObjectSupport, unresolved supertypes: Object
    class groovy.lang.GroovyObject, unresolved supertypes: Object
    class groovy.lang.GroovyCallable, unresolved supertypes: Callable<V>
    class org.gradle.api.Action, unresolved supertypes: Object
    class org.gradle.api.NamedDomainObjectFactory, unresolved supertypes: Object
    class org.gradle.api.initialization.dsl.ScriptHandler, unresolved supertypes: Object
    class org.gradle.api.NamedDomainObjectList, unresolved supertypes: List<T>
    class org.gradle.api.DomainObjectCollection, unresolved supertypes: Collection<T>
    class org.gradle.util.Configurable, unresolved supertypes: Object
    class org.gradle.api.artifacts.dsl.DependencyHandler, unresolved supertypes: Object



FAILURE: Build failed with an exception.

* What went wrong:
A problem occurred configuring root project 'spring-boot-kotlin-demo'.
> Could not open cache directory 7ooj87our40tvw5ni3o9390ur (/Users/niko/.gradle/caches/4.2/gradle-kotlin-dsl/7ooj87our40tvw5ni3o9390ur).
   > Internal error: unable to compile script, see log for details

my environment looks like:

./gradlew --version

WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by org.codehaus.groovy.reflection.CachedClass (file:/Users/niko/.gradle/wrapper/dists/gradle-4.2-bin/cl9xe1l8k8ophj72k8rv6xbu7/gradle-4.2/lib/groovy-all-2.4.11.jar) to method java.lang.Object.finalize()
WARNING: Please consider reporting this to the maintainers of org.codehaus.groovy.reflection.CachedClass
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release

------------------------------------------------------------
Gradle 4.2
------------------------------------------------------------

Build time:   2017-09-20 14:48:23 UTC
Revision:     5ba503cc17748671c83ce35d7da1cffd6e24dfbd

Groovy:       2.4.11
Ant:          Apache Ant(TM) version 1.9.6 compiled on June 29 2015
JVM:          9 (Oracle Corporation 9+181)
OS:           Mac OS X 10.11.6 x86_64

[dsebastien]

I've got the same issue with my maven-based spring boot 2 (M7)/kotlin project and indeed it's an issue with Java 9 only.