gradle fails to compile java classes using kotlin class - gradle

Question

I have a gradle 4.1 multiproject containing a "projectA" containing 2 subfolders "api" and "implementation".
The multiproject uses kotlin and java-library plugins defined in the subprojects section of the main build.gradle.
The implementation project avec a API dependency to :projectA:api
In the api folder I have kotlin and java files inside 'src/main/java' and in the implementation project I'm creating a new instance of a kotlin class from the API.
Inside Intellij Idea, I don't have any compilation errors ; but when I compile the whole project using gradle I have an error: cannot find symbol. It is as if the compileJava doesn't have access to the folder kotlin-classes.
Inside the build/kotlin-classes, I see my file.class
The class file is on build/classes dir also
Details of the error :
Task :projectA:api:compileKotlin
Using kotlin incremental compilation
Task :projectA:implementation:compileJava
(...) error: cannot find symbol (the import fails)
Update 1 : removing java-library solved my problem

Answer 1

This is a known issue of the java-library plugin: when used in a project with another JVM language (Kotlin, Scala, Groovy etc.) , it does not register the classes of the other language so that the dependent projects get them as they consume the classes.
Fortunately, it has a workaround as well. Adapted to Kotlin, it would look like:
configurations {
apiElements {
outgoing.variants.getByName('classes').artifact(
file: compileKotlin.destinationDir,
type: ArtifactTypeDefinition.JVM_CLASS_DIRECTORY,
builtBy: compileKotlin)
}
}
If you use Kapt1, it's file: compileKotlinAfterJava.destinationDir, and for Gradle versions lower than 4.0 use builtBy: copyMainKotlinClasses instead.
This issue is also tracked in the Kotlin issue tracker: KT-18497, follow that issue to see when it's fixed on the Kotlin Gradle plugin side, so that the above workaround will be no more necessary.