Gradle Kotlin DSL: Define Kotlin version in unique place

What I just stumbled upon was using Kotlin classes ins my build.gradle.kts.

I had to:

• create a module called buildSrc with src/main/kotlin and a build.gradle.kts in its root.
• (obsolete) include("buildSrc") in settings.gradle.kts

The buildSrc/build.gradle.kts is very minimal:

plugins {
    `kotlin-dsl`
}

repositories {
    jcenter()
}

In buildSrc/src/main/kotlin I've added a Config.kt

const val GROUP_ID = "my-company"
const val VERSION = "0.1.0-SNAPSHOT"

const val POM_NAME = "my-library-name"
const val POM_DESCRIPTION = "A library doing stuff."
const val POM_URL = "https://github.com/${GROUP_ID}/${POM_NAME}/"
const val POM_SCM_URL = POM_URL
const val POM_SCM_CONNECTION = "scm:git:git://github.com/${GROUP_ID}/${POM_NAME}.git"
const val POM_SCM_DEV_CONNECTION = "scm:git:ssh://[email protected]/${GROUP_ID}/${POM_NAME}.git"
const val POM_LICENCE_NAME = "The Apache Software License, Version 2.0"
const val POM_LICENCE_URL = "http://www.apache.org/licenses/LICENSE-2.0.txt"
const val POM_LICENCE_DIST = "repo"
const val POM_DEVELOPER_ID = "me"
const val POM_DEVELOPER_NAME = "meeee"
const val POM_DEVELOPER_EMAIL = "[email protected]"

And a Dependencies.kt

@file:Suppress("MemberVisibilityCanBePrivate")

object Jvm {
    const val version = "1.8"
}

object Kotlin {
    const val version = "1.3.50"
    const val stdlibJdk8 = "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$version"
    const val jvmId = "jvm"
    const val kaptId = "kapt"
}

object MavenPublish {
    const val id = "maven-publish"
}

object Arrow {
    const val version = "0.10.1"
    const val core = "io.arrow-kt:arrow-core:$version"
    const val syntax = "io.arrow-kt:arrow-syntax:$version"
    const val optics = "io.arrow-kt:arrow-optics:$version"
    const val fx = "io.arrow-kt:arrow-fx:$version"
    const val meta = "io.arrow-kt:arrow-meta:$version"
}

object Versions {
    const val version = "0.27.0"
    const val versions = "com.github.ben-manes:gradle-versions-plugin:$version"
    const val id = "com.github.ben-manes.versions"
}

So I could use it in my root build.gradle.kts like

import org.jetbrains.kotlin.gradle.tasks.KotlinCompile

plugins {
    kotlin(Kotlin.jvmId) version Kotlin.version
    kotlin(Kotlin.kaptId) version Kotlin.version
    id(Versions.id) version Versions.version
    id(MavenPublish.id)
}

group = GROUP_ID
version = VERSION

repositories {
    mavenCentral()
    jcenter()
}

dependencies {
    implementation(Kotlin.stdlibJdk8)
    implementation(Arrow.core)
    implementation(Arrow.syntax)
    kapt(Arrow.meta)
}

tasks.withType<KotlinCompile> {
    kotlinOptions.jvmTarget = Jvm.version
}

publishing {
    publications {
        create<MavenPublication>("mavenJava") {
            @Suppress("UnstableApiUsage")
            pom {
                name.set(POM_NAME)
                description.set(POM_DESCRIPTION)
                url.set(POM_URL)
                licenses {
                    license {
                        name.set(POM_LICENCE_NAME)
                        url.set(POM_LICENCE_URL)
                        distribution.set(POM_LICENCE_DIST)
                    }
                }
                developers {
                    developer {
                        id.set(POM_DEVELOPER_ID)
                        name.set(POM_DEVELOPER_NAME)
                        email.set(POM_DEVELOPER_EMAIL)
                    }
                }
                scm {
                    connection.set(POM_SCM_CONNECTION)
                    developerConnection.set(POM_SCM_DEV_CONNECTION)
                    url.set(POM_SCM_URL)
                }
            }
        }
    }
}

I am quite happy with this, but when it comes down to automatically increment the version I may fall back to maintain it in the gradle.properties.

---

Edit: It is no longer necessary (and allowed) to add buildSrc to the settings.gradle.kts, instead it will automatically get picked up if present.

There's a workaround available, which searches the version defined for the kotlin plugin and assignes this one to the outer variable. The following demonstrates this:

val kotlinVersion: String? by extra {
    buildscript.configurations["classpath"]
            .resolvedConfiguration.firstLevelModuleDependencies
            .find { it.moduleName == "kotlin-gradle-plugin" }?.moduleVersion
}

plugins {
    kotlin("jvm").version("1.2.30")
    //more
}

The variable kotlinVersion can then be used in the dependencies without further trouble.

In later versions of Gradle you no longer need to specify the version of your kotlin(stdlib|reflect|test) dependencies, the Kotlin plugin will automatically configure them for you.

As for extracting the dependency to a single place, there are two main patterns:

• define the constants you want to share in an object within buildSrc/src/main/kotlin/ and use that object in your build script, code from buildSrc is available to the whole script including the plugins block

• use a system property, you can define a system property in gradle.properties by prefixing its name with systemProp. and you can access system properties via System.getProperties(), for example:

  // build.gradle.kts
  plugins {
    val kotlinVersion by System.getProperties()
    println("Kotlin version is $kotlinVersion")
  }
  
  // gradle.properties
  systemProp.kotlinVersion=1.2.20
  

You can extract the version from the plugin class:

import org.jetbrains.kotlin.gradle.plugin.KotlinPluginWrapper

plugins {
    kotlin("jvm") version "1.2.0"
}

val kotlinVersion = plugins.getPlugin(KotlinPluginWrapper::class.java).kotlinPluginVersion