Fabric Loader is Fabric's lightweight mod loader. It provides the necessary tools to make Minecraft modifiable without depending on a specific version of the game. Game specific (and game version specific) hooks belong in Fabric API. It is possible to adapt Fabric Loader for many Java applications (for instance games like Slay the Spire and Starmade).
Fabric Loader has services to allow mods to have some code executed during initialization, to transform classes, declare and provide mod dependencies, all in a number of different environments.
For each Fabric Loader version, there is Javadoc available at
https://maven.fabricmc.net/docs/fabric-loader-[loader version]
For example, Fabric Loader 0.11.1's documentation is at https://maven.fabricmc.net/docs/fabric-loader-0.11.1/
A mod is a jar with a fabric.mod.json mod metadata file in its root declaring how it should be loaded. It primarily declares a mod ID and version as well as entrypoints and mixin configurations. The mod ID identifies the mod so that any mod with the same ID is considered to be the same mod. Only one version of a mod may be loaded at a time. A mod may declare other mods that it depends on or conflicts with. Fabric Loader will attempt to satisfy dependencies and load the appropriate versions of mods, or fail to launch otherwise.
Fabric Loader makes all mods equally capable of modifying the game. As an example, anything Fabric API does can be done by any other mod.
Mods are loaded both from the classpath and from the mods directory. They are expected to match the mappings in the current environment, meaning Fabric Loader will not remap any mods.
Nested JARs allow a mod to provide its own dependencies, so Fabric Loader can pick the best version matching the dependencies instead of requiring separate installation of dependencies. They also allow clean packaging of submodules, so each module can be used separately. Non-mod libraries can be repackaged as mods for nested JAR usage. A mod may bundle a number of other mods within its JAR. A nested JAR must itself also be a mod, which again can have nested JARs. Fabric Loader will load nested JARs while attempting to satisfy dependency constraints.
Nested JARs are not extracted, they are instead loaded in in-memory file system using jimfs. See the guidelines for how to use nested JARs effectively. Nested JARs must be declared by their paths relative to the containing JAR's root.
Fabric Loader has an entrypoint system, which is used by mods to expose parts of the code for usage by Fabric Loader or other mods. Fabric Loader uses it for mod initialization. Initializers are loaded and called early during the game's initialization which allows a mod to run some code to make its modifications. These entrypoints are typically used to bootstrap mods by registering registry objects, event listeners and other callbacks for doing things later.
Mixin allows mods to transform Minecraft classes and even mod classes, and is the only method of class transformation that Fabric Loader officially supports. A mod can declare its own mixin configuration which enables the use of Mixin.
Mixin was not specifically made for Fabric, so Fabric Loader uses a slightly modified version of Mixin. However, the documentation of the upstream version is still mostly valid. The modifications are mostly related to making it work without LegacyLauncher/LaunchWrapper.
Fabric Loader provides an API to determine names of classes, fields and methods with respect to the different environments that mods may be loaded in. This can be used to support reflection in any environment provided Fabric Loader has access to mappings to resolve the name.
When launched in a non-development environment, Fabric Loader will remap the Minecraft jar and realms client jar to intermediary names. Mods are expected to be mapped to intermediary, which will be compatible with this environment. The remapped jars are cached and saved in ${gameDir}/.fabric/remappedJars/${minecraftVersion} for re-use across launches.
Fabric Loader depends on a custom class loader to transform some classes at runtime. Classes belonging to a mod or Minecraft are loaded with a class loader that applies transformations to classes before they are loaded. Other classes, those belonging to other libraries, cannot be transformed. With Knot, these classes are delegated to the default classloader for isolation and performance.
Fabric Loader will perform side stripping on mod classes and Minecraft classes depending on the physical side that is launched. This involves completely removing classes, methods and fields annotated with @Environment annotations where the environment does not match. It also involves removing interface implementations on classes annotated with @EnvironmentInterface where the environment does not match. On Minecraft classes, this is used to simulate which classes and members that are available in the targeted runtime development environment. The annotation can be applied to mod classes to avoid class loading issues.
Package access hacks might be applied to Minecraft classes depending on the mappings in the current environment. With official (obfuscated) names and intermediary names, most classes are placed in the same package. However, Yarn mappings place classes in various packages which sometimes creates illegal access violations due to the access rules of protected and package-private members. Therefore, in a development environment where such access issues are expected to exist, Minecraft classes are transformed so that package-private and protected members are made public. Outside a development environment we know that the package structure is flat, so the package access hack is not needed. Note that this transformation is applied at runtime, which means it is not visible in the source.
A launcher (not to be confused with the game launcher) is something provides a method to use Fabric Loader in a Java process. A launcher must provide a few features to support Fabric Loader's functionality such as class transformation and dynamic class loading. Knot and LegacyLauncher/LaunchWrapper are the current supported launchers.
Knot is the default launcher included in Fabric Loader, designed specifically for Fabric Loader's features with support for modern versions of Java. Knot has the main classes net.fabricmc.loader.launch.knot.KnotClient and net.fabricmc.loader.launch.knot.KnotServer for clients and servers respectively.
When launching a server using Knot in a production environment, the net.fabricmc.loader.launch.server.FabricServerLauncher main class must be used, which is a main class that wraps the launch of KnotServer. It can be configured with the fabric-server-launcher.properties placed in the current working directory. The file has one property, serverJar, whose value is 'server.jar' by default, which is used to configure the path to the minecraft server jar.
Fabric Loader can also be launched with LegacyLauncher/LaunchWrapper using the tweakers net.fabricmc.loader.launch.FabricClientTweaker and net.fabricmc.loader.launch.FabricServerTweaker for clients and servers respectively. However, LegacyLauncher/LaunchWrapper support is currently outdated.
As stated above, Fabric Loader can be used outside of Minecraft to add mod loading for Java applications. This section will briefly summarize what do you need to do to integrate Fabric loader to your Java game/dedicated app. This was tested with Gradle 7 (specifically 7.3.1), Java 8 and IntelliJ IDEA 2022.3.1 (Community Edition).
First, you'll need to add Fabric Loader as a dependency, and its dependencies in you build.gradle:
repositories { // your other repositories... maven { url "https://repo.spongepowered.org/maven/" } maven { url "https://maven.fabricmc.net/" } } dependencies { // your dependencies... implementation "net.fabricmc:fabric-loader:0.13.0" implementation "net.fabricmc:access-widener:2.1.0" implementation "net.fabricmc:tiny-mappings-parser:0.2.2.14" implementation "com.google.guava:guava:21.0" implementation "com.google.code.gson:gson:2.8.7" implementation "org.ow2.asm:asm:9.2" implementation "org.ow2.asm:asm-analysis:9.2" implementation "org.ow2.asm:asm-commons:9.2" implementation "org.ow2.asm:asm-tree:9.2" implementation "org.ow2.asm:asm-util:9.2" }
If you've launched your app before through your own main(String[]), to incorporate Fabric Loader into your app, you will need to set its main class to use one of the Knot launchers. For client-side (or if you don't have client/server separation), specify net.fabricmc.loader.impl.launch.knot.KnotClient as your main class. For server-side only, specify net.fabricmc.loader.impl.launch.knot.KnotServer. You might ask, “but how will Fabric Loader know how to load my game?” That's where GameProvider comes into play.
GameProvider is an interface that lets Fabric Loader locate your game, configure Fabric launcher, and then finally launch the game. For Fabric Loader to instantiate your implementation of GameProvider, you need to define it as a service that can be found by ServiceLoader. In your project's resources, create META-INF/services/net.fabricmc.loader.impl.game.GameProvider file, and paste into it the full class name to your GameProvider. Something like this:
net.developer.app.fabric.AppGameProvider
This should allow Fabric Loader find your GameProvider. Now let's breakdown GameProvider interface.
For your game actually to launch, you need to carefully implement GameProvider in a specific way. Here is a breakdown of each method does:
getGameId() and getGameName(), self-explanatory, app's ID like epic_app, and app's name Epic App.getRawGameVersion() and getNormalizedGameVersion(), it's not entirely clear, but raw version can include git-hash, or any information about the game that developer may want to know (like v0.1.2+build17#f73acde), while normalized version seems to be for display (like v0.1.2).getBuiltinMods() allows you to provide built-in mod(s), which can be used by mods to target specific version, but it's completely optional, and you can just return Collections.emptyList().getEntryPoint(), full class name of your main app class. It seems to be used only in Minecraft.getLaunchDirectory() is a very important method. It should return where the app's resources folder is (like configs, data, saves, etc.). That's where Fabric Loader will create config and mods folder, and from which mods will be loaded from!isObfuscated() whether your app is obfuscated.requiresUrlClassLoader() it forces Fabric Loader to use a different compatibility class loader.isEnabled() whether your app is enabled. Just return true;locateGame(FabricLauncher, String[]) that's where you need to parse the arguments with net.fabricmc.loader.impl.util.Arguments, locate the game directory and return true; if game directory was found.initialize(FabricLauncher) here you can add extra configuration to FabricLauncher. Don't initialize your game here!getEntryPointTransformer() return a net.fabricmc.loader.impl.game.patch.GameTransformer that does extra modification on the app's jar. If you're setting up Fabric Loader to work with a separate app which you currently aren't developing in your development environment, then simply return a subclass of GameTransformer that returns null in its transform(String) method.unlockClassPath(FabricLauncher) is called after transformers (i.e. Mixin) were initialized and mods were detected and loaded (but not initialized). launch(ClassLoader) this is where you launch your app. Important: the app must be launched through reflection using the given ClassLoader, about that later.getArguments() simply return the Arguments instance you initialized in locateGame(FabricLauncher, String[])).getLaunchArguments(boolean) simply return this.getArguments.toArray().An example GameProvider would look like this:
public class AppGameProvider implements GameProvider { private Arguments arguments; private Path gameDirectory; private GameTransformer transformer = new AppGameTransformer(); @Override public String getGameId() { return "app"; } @Override public String getGameName() { return "App"; } @Override public String getRawGameVersion() { return App.FULL_VERSION; } @Override public String getNormalizedGameVersion() { return App.VERSION; } @Override public Collection<BuiltinMod> getBuiltinMods() { return Collections.emptyList(); } @Override public String getEntrypoint() { return "net.developer.app.App"; } @Override public Path getLaunchDirectory() { return this.gameDirectory; } @Override public boolean isObfuscated() { return false; } @Override public boolean requiresUrlClassLoader() { return false; } @Override public boolean isEnabled() { return true; } @Override public boolean locateGame(FabricLauncher launcher, String[] args) { this.arguments = new Arguments(); this.arguments.parse(args); this.gameDirectory = Paths.get(this.arguments.get("gameDirectory")); return Files.isDirectory(this.gameDirectory); } @Override public void initialize(FabricLauncher launcher) {} @Override public GameTransformer getEntrypointTransformer() { return this.transformer; } @Override public void unlockClassPath(FabricLauncher launcher) { for (Path path : this.jars) { launcher.addToClassPath(path); } } @Override public void launch(ClassLoader loader) { try { Class main = loader.loadClass(this.getEntrypoint()); Method method = main.getMethod("main", String[].class); method.invoke(null, (Object) this.arguments.toArray()); } catch (Exception e) { e.printStackTrace(); } } @Override public Arguments getArguments() { return this.arguments; } @Override public String[] getLaunchArguments(boolean sanitize) { return this.arguments.toArray(); } }
This class example assumes that argument –gameDirectory is the one that specifies where app's working directory is located. Adjust if needed.
Once you get Fabric Loading up and running in your app, you would see extra log in the console upon launching your app (if you did everything correctly):
[14:20:00] [INFO] [FabricLoader/GameProvider]: Loading App 0.1 (dev) with Fabric Loader 0.13.0 [14:20:00] [INFO] [FabricLoader/]: Loading 2 mods: - fabricloader 0.13.0 - java 8 [14:20:01] [WARN] [FabricLoader/ModRemap]: Runtime mod remapping disabled due to no fabric.remapClasspathFile being specified. You may need to update loom. [14:20:01] [INFO] [FabricLoader/Mixin]: SpongePowered MIXIN Subsystem Version=0.8.5 Source=file:/C:/Users/Developer/.gradle/caches/modules-2/files-2.1/org.spongepowered/mixin/0.8.5/9d1c0c3a304ae6697ecd477218fa61b850bf57fc/mixin-0.8.5.jar Service=Knot/Fabric Env=CLIENT [14:20:01] [INFO] [FabricLoader/Mappings]: Mappings not present!
That's a good sign. However, that's not all.
If you would try making a mod for your app either as a separate project or as a submodule in your project, there could be a couple of things that may not work either in development or in release. First thing is mods won't initialize. I.e. ModInitializer.onInitialize() won't be called. To fix that, simply call net.fabricmc.loader.impl.game.minecraft.Hooks.startClient(File, Object) (where File is game directory, and Object could be the main object of your game) somewhere after you initialized event busses, base registries, etc. so that mods could start modding your app.
Something like this:
public MainApp(App app) { super(); this.development = app.development; this.events.register(this); this.registerCore(this, app.gameDirectory); this.registerFactories(); this.registerFoundation(); this.registerMiscellaneous(); Hooks.startClient(app.gameDirectory, app); if (this.development) { this.watchDog = new WatchDog(this.assetsFolder); this.watchDog.register(this.textures); this.watchDog.register(this.models); this.watchDog.register(this.sounds); this.watchDog.start(); } }
And finally, even if you registered Mixins in your mod, they may load, but they won't be able to actually patch their target class. If you have following message in the log:
[12:20:05] [WARN] [FabricLoader/Knot]: * CLASS LOADER MISMATCH! THIS IS VERY BAD AND WILL PROBABLY CAUSE WEIRD ISSUES! * - Expected game class loader: net.fabricmc.loader.impl.launch.knot.KnotClassLoader@446a1e84 - Actual game class loader: null Could not find the expected class loader in game class loader parents!
Then it's the root of the issue. To fix this, you need to add your app's jar, or folder (in development) to FabricLauncher's class path. This could be done by looking in the class path for your jar/folder, and detecting some key files unique to your app. Here is what you can do:
public class AppGameProvider implements GameProvider { /* ... */ private List<Path> jars = new ArrayList<Path>(); private Set<String> jarFiles; public AppGameProvider() { this.jarFiles = new HashSet<String>(); this.jarFiles.add("net/developer/app/App.class"); this.jarFiles.add("net/developer/app/MainApp.class"); } /* ... */ @Override public boolean locateGame(FabricLauncher launcher, String[] args) { this.arguments = new Arguments(); this.arguments.parse(args); this.gameDirectory = Paths.get(this.arguments.get("gameDirectory")); for (Path path : launcher.getClassPath()) { if (this.isHostApp(path)) { this.jars.add(path); } } return Files.isDirectory(this.gameDirectory); } private boolean isHostApp(Path path) { if (Files.isDirectory(path)) { for (String string : this.jarFiles) { if (Files.exists(path.resolve(string))) { return true; } } return false; } try (ZipFile zip = new ZipFile(path.toFile())) { for (String string : this.jarFiles) { if (zip.getEntry(string) != null) { return true; } } } catch (Exception e) { e.printStackTrace(); } return false; } /* ... */ @Override public void unlockClassPath(FabricLauncher launcher) { for (Path path : this.jars) { launcher.addToClassPath(path); } } /* ... */ }
After detecting the app's jar(s)/folder(s) and adding them to FabricLauncher's class path, Mixins should work in both development and release environment after that.
If you're testing the mod from the submodule that has your app and Fabric Loader in the classpath, instead of compiling the mod and placing it into app's launch directory, for Fabric Loader to load your mod from classpath, you need to add -Dfabric.development=true to JVM flags when running it, then mod(s) from classpath would be loaded too.
For more reference check out Fabric Loader's source, net.fabricmc.loader.impl.game.minecraft.MinecraftGameProvider, and this GameProvider example.