Kotlin support for Minecraft Forge 1.8-11.14.1.1371 or above

The basic workflow for using the Adapter in your project is;

1. Either shade (and repack) the adapter jar or manually copy the adapter + helpers you want into your project
2. Shade (via a Gradle plugin or with ForgeGradle trickery) the version of Kotlin you want
3. Hack away

These instructions are for FG 2.1+.

We require the Kotlin and Forge plugins to be enabled in build.gradle:

buildscript {
	[...]
	dependencies {
		[...]
		classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:1.0.1"
	}
	[...]
}

repositories {
	[...]
	// For Kotlin lang adapter
	maven {
		name 'Tethys'
		url 'http://tethys.drakon.io/maven'
	}
}

apply plugin: 'kotlin'
apply plugin: 'net.minecraftforge.gradle.forge'
[...]

This technique requires tinkering with your build.gradle file.

As a second warning: Doing this wrong could break other Kotlin-based mods!

Now the warnings are out the way, we're going to use a technique called shading combined with repacking. Built jars of the adapter do not come with Kotlin installations. This means your mod must include its own, repackaged version.

First, we need to add a new configuration:

configurations {
    shade
    compile.extendsFrom shade
}

Then on the lines where you would normally define compile in the main dependencies block for Kotlin, replace compile with shade - below is an example dependency block (for Kotlin 1.0.1.) If you need the kotlin-reflect library, add that in the same way (but be aware this'll add >1MB to your jar size!)

dependencies {
	shade 'org.jetbrains.kotlin:kotlin-stdlib:1.0.1'
	shade 'io.drakon.forge:kotlin-adapter:1.0.1+1.8.9'
}

Next, tell Gradle what to do with this new shade configuration inside the jar {} block:

jar {
	[...]
	// Shading
    configurations.shade.each { dep ->
        from(project.zipTree(dep)){
            exclude 'META-INF', 'META-INF/**'
        }
    }
	[...]
}

Finally, ask ForgeGradle to add some extra rules for reobfuscation, to avoid namespace issues with other Kotlin versions that may be on the classpath:

reobf {
    jar {
	    // Kotlin+Adapter shading
	    extraLines += [ "PK: kotlin your/package/here/repack/kotlin" ]
	    extraLines += [ "PK: org/jetbrains/annotations your/package/here/repack/annotations" ]
	    extraLines += [ "PK: io/drakon/forge/kotlin your/package/here/repack/adapter" ]
	}
}

The destination package (your/package/here/repack for this example) can be any package you think won't clash, either with your code or anybody else's packages. As in the example, package paths use forward slash (/) in PK lines.

Your @Mod implementation must be an object. There is no reason to make it a class and the adapter will not work if you do so.

The Kotlin Adapter is licensed under the ISC License (see LICENSE), which is just a modernised version of the MIT License.

Kotlin is property of Jetbrains. Look at their site for licensing bumf.