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Cayde Dixon

Independent minecraft modder, developer and gamer. Email Twitter Google+ Github

Disclaimer: This is considered a massive hack, and is only really done for a proof-of-concept purpose. If you really want to do this in a more, supported mannerism, have a look at (Scala)[]

For those who follow my GitHub account, you may know of the ForgeGradle-Scripts repository that I have been working on with the kind help of Ntzrmtthihu777 where we have been compiling a series of scripts to facilitate a Minecraft modder’s term in the idea of a plug-and-play type system for making a build script allowing but more of that in another post at a later date.

One of the scripts in this system is 7.retrolambda.gradle which, using a few small hacks* allow us to use some of the basic Java 8 constructs such as the Lambda, for example:


Within the applications of java, this can allow for a much easier instance of inner-classes since it can implicitly override the Runnable class with just call like

new Thread(() -> {
    for (int i = 0; i < 10; i++) {
        System.out.println("Running! " + i);

As you can see, there is no need for even typing runnable as the Java compiler will automatically work out what is needed for these lambda functions and it will pass as a runnable to the Thread constructor.

So, how do we use this in ForgeGradle?

To use this in ForgeGradle we would initially think that just setting:

tasks.withType(JavaCompile) {
    sourceCompatibility = "1.8"
    targetCompatibility = "1.8"

Which, in any case outside of ForgeGradle is enough to say to Gradle anyway that we are targeting Java SE 8 byte code. Whereas in the case of FML, it has inner dependencies of ASM 4.0.1 and the earliest release to have Java 8 support in ASM is version 5.0.0.

From here, there is an older thing that we had to do in older versions for FML to allow for newer java classes to be ran, though nowadays it is no longer needed for forge due to a satisfactory ASM version.

So, do we just add in the dependencies to look at mavenCentral() and then apply our own dependency of compile 'org.ow2.asm:asm-debug-all:5.0.3'? Well, that is part of the solution, because another part of this hack, as I am calling it, also has a prioritization to the factor of compatibility, since when we end up running the gradle build and grabbing the generated jar files, it will not load in Minecraft, why? Well, to put it simply, ASM 4.0.1 as FML uses, does not understand it!

Getting it to work

After a lot of research I have found a nifty tool called Retrolambda and a gradle plugin for it, Gradle-Retrolambda after a bit of tinkering around with the plugin I have determined how to properly use this plugin when using ForgeGradle.

So, to get this working, with ForgeGradle, as explained in the Gradle Retrolambda github page, we need to add 'me.tatarka:gradle-retrolambda:2.2.3' to the buildscript classpath and then, after the apply plugin: 'forge' we need to add yet another plugin to be applied, retrolambda.

Notice: This is not needed if you have JAVA_HOME pointing towards a valid JDK8 install.

There are some more. Advanced configuration that you can do when applying the plugin which is documented at the GitHub page and it also assumes the JAVA6_HOME, JAVA7_HOME and JAVA8_HOME environment variables exist (you only need 1 of 6 OR 7) pointing towards a valid JDK install.