Gradle Kotlin DSL: Creating additional jars

Sometimes you want to create more than one JAR file for your project. It could for example contain some source code or your compiled test classes. To do that in Gradle you first need create a task that inherits from the Gradle Jar task and will produce your JAR file:

"testJar"(Jar::class) {
    classifier = "tests"
    from(java.sourceSets["test"].output)
    dependsOn("testClasses")
}

Then you add the artifact itself:

artifacts {
    add("archives", tasks["testJar"])
}

Your build will now create an additional JAR file with the specified content which are the compiled class files for tests in this example.