Building a Kotlin + Java 9 project with Gradle

Question

I\'m fairly new to Gradle (and Java 9, to be honest), and I\'m trying to use Gradle to build a simple library project that is a mix of Java 9 and Kotlin. More in detail, there i

Answer 1

I am using the following gradle script where I put the module-info.java under src/module. It gets automatically included in the jar (without duplicates):

if (JavaVersion.current() >= JavaVersion.VERSION_1_9) {
    subprojects {
        def srcModule = "src/module"
        def moduleInfo = file("${project.projectDir}/$srcModule/module-info.java")
        if (moduleInfo.exists()) {

            sourceSets {
                module {
                    java {
                        srcDirs = [srcModule]
                        compileClasspath = main.compileClasspath
                        sourceCompatibility = '9'
                        targetCompatibility = '9'
                    }
                }
                main {
                    kotlin { srcDirs += [srcModule] }
                }
            }

            compileModuleJava.configure {
                dependsOn compileKotlin
                destinationDir = compileKotlin.destinationDir
                doFirst {
                    options.compilerArgs = ['--module-path', classpath.asPath,]
                    classpath = files()
                }
            }
            jar.dependsOn compileModuleJava
        }
    }
}

I won't update it any longer, have a look at https://github.com/robstoll/atrium/blob/master/build.gradle to see the current version in use.

Answer 2

The accepted answer did not work for me (atleast not the way it was presented), but this is what worked:

plugins {
    id "org.jetbrains.kotlin.jvm" version "1.3.50"
}

compileKotlin {
    doFirst {
        destinationDir = compileJava.destinationDir
    }
}

jar {
    duplicatesStrategy = DuplicatesStrategy.EXCLUDE
}

Doing it the way the accepted answer suggests led to me getting this error:

Directory '/path/to/project/build/classes/kotlin/main' specified for property 'compileKotlinOutputClasses' does not exist.

---

Gradle version: 5.6

Answer 3

Solved! It was sufficient to set the kotlin compilation dir to the same dir as Java:

compileKotlin.destinationDir = compileJava.destinationDir

It works now, both with the sources in the same tree or in different trees; but with a quirk: the jar task produces a jar with all the entries duplicated. I'll work on fix this, next.

Thanks to everyone!