How to share boilerplate Kotlin configuration across multiple Gradle projects?

Question

The typical Kotlin configuration in a Gradle project is very boilerplate, and I'm looking for a way of abstracting it out into an external build script so that it can be reused.

I have a working solution (below), but it feels like a bit of a hack as the kotlin-gradle-plugin doesn't work out of the box this way.

It's messy to apply any non-standard plugin from an external script as you can't apply the plugin by id, i.e.

apply plugin: 'kotlin' will result in Plugin with id 'kotlin' not found.

The simple (well, usually) workaround is to apply by the fully qualified classname of the plugin, i.e.

apply plugin: org.jetbrains.kotlin.gradle.plugin.KotlinPluginWrapper

which in this case throws a nice little exception indicating that the plugin probably wasn't meant to be called this way:

Failed to determine source cofiguration of kotlin plugin. 
Can not download core. Please verify that this or any parent project
contains 'kotlin-gradle-plugin' in buildscript's classpath configuration.

So I managed to hack together a plugin (just a modified version of the real plugin) which forces it to find the plugin from the current buildscript.

kotlin.gradle

buildscript {
    ext.kotlin_version = "1.0.3"
    repositories {
        jcenter()
    }
    dependencies {
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
    }
}

dependencies {
    compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
    compile "org.jetbrains.kotlin:kotlin-reflect:$kotlin_version"
}

apply plugin: CustomKotlinPlugin
import org.jetbrains.kotlin.gradle.plugin.CleanUpBuildListener
import org.jetbrains.kotlin.gradle.plugin.KotlinBasePluginWrapper
import org.jetbrains.kotlin.gradle.plugin.KotlinPlugin
import org.jetbrains.kotlin.gradle.tasks.KotlinTasksProvider

/**
 * Wrapper around the Kotlin plugin wrapper (this code is largely a refactoring of KotlinBasePluginWrapper).
 * This is required because the default behaviour expects the kotlin plugin to be applied from the project,
 * not from an external buildscript.
 */
class CustomKotlinPlugin extends KotlinBasePluginWrapper {

    @Override
    void apply(Project project) {
        // use String literal as KOTLIN_COMPILER_ENVIRONMENT_KEEPALIVE_PROPERTY constant isn't available
        System.setProperty("kotlin.environment.keepalive", "true")

        // just use the kotlin version defined in this script
        project.extensions.extraProperties?.set("kotlin.gradle.plugin.version", project.property('kotlin_version'))

        // get the plugin using the current buildscript
        def plugin = getPlugin(this.class.classLoader, project.buildscript)
        plugin.apply(project)

        def cleanUpBuildListener = new CleanUpBuildListener(this.class.classLoader, project)
        cleanUpBuildListener.buildStarted()
        project.gradle.addBuildListener(cleanUpBuildListener)
    }

    @Override
    Plugin<Project> getPlugin(ClassLoader pluginClassLoader, ScriptHandler scriptHandler){
        return new KotlinPlugin(scriptHandler, new KotlinTasksProvider(pluginClassLoader));
    }
}

This can then be applied in any project (i.e. apply from: "kotlin.gradle") and you're up and running for Kotlin development.

It works, and I haven't had any issues yet, but I'm wondering if there is a better way? I'm not really keen on merging in changes to the plugin every time there's a new version of Kotlin.

Mysticism

Answer 1

Check out the nebula-kotlin-plugin. It seems very close to what you're trying to achieve there.

Fanlight

Well that's hilarious - I'm already using a handful of Netflix's nebula plugins (project, release, etc)....the kotlin plugin didn't exist last time I looked. I will give this a try, thanks! – Mysticism

Answer 2

The problem here is that there is a known gradle bug about the inability to apply plugins by id from init scripts. That's why you need to use fully qualified class name as a workaround.

E.g. I have the following in the init script and it works:

apply plugin: org.jetbrains.kotlin.gradle.plugin.KotlinPlatformJvmPlugin

By the way, I created a gradle plugin for preparing custom gradle distributions with common setup defined in init script - custom-gradle-dist. It works perfectly for my projects, e.g. a build.gradle for a library project looks like this (this is a complete file, all repository, apply plugin, dependencies etc setup is defined in the init script):

dependencies {
    compile 'org.springframework.kafka:spring-kafka'
}

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