Example Mod Installer for Forge in Groovy

modInstaller.groovy

import groovy.json.JsonSlurper

def mcDir = new File(System.getProperty('user.home') + '/.minecraft/')
def modsDir = new File(mcDir, 'mods')
def mods = [:]
def modsToDownload = args.toList()

def download = { String address, String path ->
    def file = new FileOutputStream(path)
    def out = new BufferedOutputStream(file)
    out << new URL(address).openStream()
    out.close()
}

def downloadMod = { String name ->
    def url = mods[name] as String
    def modFile = new File(modsDir, name + '.jar')
    if (modFile.exists()) {
        modFile.delete()
    }
    println 'Mod File: ' + modFile
    download url, modFile.absolutePath
}

def installForge = { ->
    def installerJar = new File(mcDir.absolutePath + '/forgeInstaller.jar')
    def installer = 'http://files.minecraftforge.net/minecraftforge/minecraftforge-installer-latest.jar'
    download installer, installerJar.absolutePath
    def classLoader = this.getClass().getClassLoader().rootLoader
    classLoader.addURL(installerJar.toURI().toURL())
    println 'Please install Forge with the Forge Installer when it is displayed.'
    sleep(3000)
    def installerClass = Class.forName('cpw.mods.fml.installer.SimpleInstaller', true, classLoader)
    installerClass.invokeMethod('main', new String[0])
}

def loadModList = { ->
    def modListFile = new File(mcDir.absolutePath + '/modlist.json')
    download 'https://gist.github.com/kaendfinger/6144135/raw/mods.json', modListFile.absolutePath
    mods = new JsonSlurper().parse(modListFile) as Map<String, String>
}

if (!modsDir.exists()) {
    modsDir.mkdirs()
}

println 'Installing Forge'

installForge()

loadModList()
println mods.size() + ' mods are on the mod list.'

if (modsToDownload.isEmpty()) {
    mods.keySet().each { String key ->
        println 'Installing ' + key
        downloadMod key
    }
} else {
    modsToDownload.each { String name ->
        println 'Installing ' + name
        downloadMod mods[name] as String
    }
}