Forge Mixins

build.gradle

buildscript {
    repositories {
        jcenter()
        maven {
            name = 'forge'
            url = 'https://files.minecraftforge.net/maven'
        }
        maven {

            url = 'http://repo.spongepowered.org/maven'
        }
    }
    dependencies {
        classpath 'net.minecraftforge.gradle:ForgeGradle:2.3-SNAPSHOT'
        classpath 'org.spongepowered:mixingradle:0.6-SNAPSHOT'
    }
}

apply plugin: 'net.minecraftforge.gradle.forge'
apply plugin: 'org.spongepowered.mixin'

version = MODVERSION
group = com.MODID
archivesBaseName = MODID

sourceCompatibility = 1.8
targetCompatibility = 1.8

sourceSets {
    main {
        ext.refMap = 'mixins.MODID.refmap.json'
    }
}

processResources {
    // this will ensure that this task is redone when the versions change.
    inputs.property 'version', 1.12.2-14.23.4.2705
    inputs.property 'mcversion', stable_39

    // replace stuff in mcmod.info, nothing else
    from(sourceSets.main.resources.srcDirs) {
        include 'mcmod.info'

        // replace version and mcversion
        expand 'version': project.version, 'mcversion': project.minecraft.version
    }

    // copy everything else, thats not the mcmod.info
    from(sourceSets.main.resources.srcDirs) {
        exclude 'mcmod.info'
    }

    rename '(.+_at.cfg)', 'META-INF/$1'
}

minecraft {
    version = 1.12.2-14.23.4.2705
    runDir = 'run'

    // the mappings can be changed at any time, and must be in the following format.
    // snapshot_YYYYMMDD   snapshot are built nightly.
    // stable_#            stables are built at the discretion of the MCP team.
    // Use non-default mappings at your own risk. they may not always work.
    // simply re-run your setup task after changing the mappings to update your workspace.
    mappings = stable_39
    // makeObfSourceJar = false // an Srg named sources jar is made by default. uncomment this to disable.

    def args = [
            '-Dfml.coreMods.load=com.MODID.asm.MODCoremod',
            '-Dmixin.hotSwap=true',
            '-Dmixin.checks.interfaces=true'
    ]
    clientJvmArgs.addAll(args)
    serverJvmArgs.addAll(args)
}

configurations {
    embed
    implementation.extendsFrom(embed)
}

repositories {
    //mavenCentral()
    maven {
        url = 'http://repo.spongepowered.org/maven'
    }
}

dependencies {
    embed('org.spongepowered:mixin:0.7.11-SNAPSHOT') {
        transitive = false
    }
}

jar {
    from(configurations.embed.collect { it.isDirectory() ? it : zipTree(it) }) {
        exclude 'LICENSE.txt', 'META-INF/MANIFSET.MF', 'META-INF/maven/**', 'META-INF/*.RSA', 'META-INF/*.SF'
    }

    manifest.attributes(
            'FMLCorePluginContainsFMLMod': true,
            'ForceLoadAsMod': true,
            'TweakClass': 'org.spongepowered.asm.launch.MixinTweaker',
            'MixinConfigs': 'mixins.MODID.json',
            'FMLCorePlugin': 'com.MODID.asm.MODCoremod'
    )
}

src\main\java\com\MODID\asm\MODCoremod.java

package com.MODID.asm;

import net.minecraftforge.fml.relauncher.IFMLLoadingPlugin;
import org.spongepowered.asm.launch.MixinBootstrap;
import org.spongepowered.asm.mixin.Mixins;

import javax.annotation.Nullable;
import java.util.Map;

@IFMLLoadingPlugin.SortingIndex(CHOOSE_A_RANDOM_NUMBER)
public class MODCoremod implements IFMLLoadingPlugin {
	public MODCoremod() {
		MixinBootstrap.init();
		Mixins.addConfiguration("mixins.MODID.json");
	}
	
	@Override
	public String[] getASMTransformerClass() {
		return new String[0];
	}
	
	@Override
	public String getModContainerClass() {
		return null;
	}
	
	@Nullable
	@Override
	public String getSetupClass() {
		return null;
	}
	
	@Override
	public void injectData(Map<String, Object> data) {}
	
	@Override
	public String getAccessTransformerClass() {
		return null;
	}
}

src\main\resources\mixins.MODID.json

{
  "package": "com.MODID.asm.mixin",
  "refmap": "mixins.MODID.refmap.json",
  "target": "@env(DEFAULT)",
  "minVersion": "0.6",
  "compatibilityLevel": "JAVA_8",
  "mixins": [
    "MixinEntityPlayer"
  ]
}