Proper Java module support in Forge mods

Way back in the Forge 1.17 days, work started for adding JPMS (Java Platform Module Support) to ModLauncher and ForgeModLoader. This has been used internally by Forge and some libraries for a while now, but mods (those with mods.toml specifically) have not been able to take advantage of it. As of Forge 1.21.1 and 1.21.3, this is now possible!

What is JPMS and what does it mean for modders?

JPMS is the Java Platform Module System, introduced in Java 9. It allows you to define modules, which are collections of packages and resources that can be exported or hidden from other modules. This allows for much more fine-tuned control over visibility, cleaner syntax for service declarations and support for sealed types across packages.

For example, you might have a mod with a module called `com.example.mod` that exports `com.example.mod.api` and `com.example.mod.impl` to other mods, but hides `com.example.mod.internal` from them. This would allow you to have a clean API for other mods to use, while keeping your internal implementation details hidden from IDE hints, helping prevent accidental usage of internals that might break without prior notice.

This is particularly useful if you'd like to use public records with module-private constructors or partially module-private record components, as you can create a sealed interface that only your record implements, having the interface be exported and the record hidden. It's also nice for declaring and using services, as you'll get compile-time errors from the Java compiler for typos and the like, rather than deferring to runtime errors. In more advanced cases, you can also have public methods that are only accessible to specific other modules -- handy if you want internal interactions between multiple of your own mods.

How do I bypass it?

We understand there may be drama in implementing a system that prevents mods from accessing each other's internals when necessary (like when a mod is abandoned or you need to fix a compat issue) -- after all, we are already modding a game that doesn't have explicit support for Java mods yet. We have already thought of this and are offering APIs from day one to selectively bypass module restrictions.

Let me be clear: Forge mods are not required to use JPMS. If you don't want to use it, you don't have to. The default behaviour is to have fully open, fully exported automatic modules.

In Java, you can use the `Add-Opens` and `Add-Exports` manifest attributes to selectively bypass module restrictions of other mods at launch time, and we've added explicit support for these when loading your Forge mods. At compile-time, you can use existing solutions such as the extra-java-module-info Gradle plugin to deal with non-modular dependencies and add extra opens and exports to other modules.

Here's an example on how to make the internal package `com.example.examplemod.internal` open to your mod in your build.gradle:

tasks.named('jar', Jar) {
    manifest {
        attributes([
            'Add-Opens'               : 'com.example.examplemod/com.example.examplemod.internal'
            'Specification-Title'     : mod_id,
            'Specification-Vendor'    : mod_authors
            // (...)
        ])
    }
}

With the above in your mod's jar manifest, you can now reflectively access the classes inside that internal package. Multiple entries are separated with a space, as per Java's official spec.

You can also use Add-Exports to directly call without reflection, however you'd need to use the Gradle plugin mentioned earlier to be able to compile. The syntax for Add-Exports is the same as Add-Opens, and instructions for the compile-time step with the Gradle plugin are detailed later in this post.

Remember to prefer the opens and exports keywords inside module-info.java for sources you control. The Add-Opens/Add-Exports attributes are only intended for forcing open other mods.

What else is new with module support?

Previously, the runtime module name was always forced to the first mod ID in your `mods.toml` file and all packages were forced fully open and exported. Module names are now distinguished from mod IDs, meaning the module name in your module-info.java can be different from the mod ID in your `mods.toml`. This allows you to have a more descriptive module name that doesn't have to be the same as your mod ID, however we strongly recommend including your mod ID as part of your module name to aid troubleshooting.

The `Automatic-Module-Name` manifest attribute is now also honoured, allowing you to specify a module name for your mod without needing to create a `module-info.java` file. This is particularly useful for mods that don't care about JPMS features but want to have a more descriptive module name and easier integration with other mods that do use JPMS.

How do I use it?

The first step is to create a `module-info.java` file in your mod's source directory. This file should be in the same package as your main mod class, and should look something like this:

open module com.example.examplemod {
    requires net.minecraftforge.eventbus;
    requires net.minecraftforge.fmlcore;
    requires net.minecraftforge.forge;
    requires net.minecraftforge.javafmlmod;
    requires net.minecraftforge.mergetool.api;
    requires org.slf4j;
    requires logging;
}

For now, we're leaving the whole module open to reflection, which is a good starting point. When we know we want to close something off, we can remove the open modifier from the module and open or export individual packages instead. Remember that you need to be open to Forge (module name net.minecraftforge.forge), otherwise it can't call your mod's constructor.

Next is fixing modules in Gradle. While Forge and Java support modules properly, Gradle does not put automatic modules on the module path by default, meaning that the logging module (from com.mojang:logging) is not found. To fix this, add the Gradle plugin and add a compile-time module definition for that Mojang library:

plugins {
  // (...)
  id 'org.gradlex.extra-java-module-info' version "1.9"
}

// (...)

extraJavaModuleInfo {
    failOnMissingModuleInfo = false
    automaticModule("com.mojang:logging", "logging")
}

The automatic module override specified in your build.gradle should match the runtime one to avoid errors. You can do the same for any library or mod dependency that is missing either a module-info or explicit Automatic-Module-Name, however be aware that you may need to update your mod once said library adds one.

That's all you need to get started with module support in your mods. You can learn more about modules and how to use them at dev.java.