Fabric Loom, or just Loom for short, is a Gradle plugin for development of mods in the Fabric ecosystem. Loom provides utilities to install Minecraft and mods in a development environment so that you can link against them with respect to Minecraft obfuscation and its differences between distributions and versions. It also provides run configurations for use with Fabric Loader, Mixin compile processing and utilities for Fabric Loader's jar-in-jar system.

• migrateMappings: Migrates the current source to the specified mappings. See migratemappings.
• remapJar: Produces a jar containing the remapped output of the jar task. Also appends any included mods for jar-in-jar. Called when running build.
• genSources: Decompile the minecraft jar using the default decompiler (CFR).
• downloadAssets: Downloads the asset index and asset objects for the configured version of Minecraft into the user cache.
• genEclipseRuns: Installs Eclipse run configurations and creates the run directory if it does not already exist.
• vscode: Generates or overwrites a Visual Studio Code launch.json file with launch configurations in the .vscode directory and creates the run directory if it does not already exist.
• remapSourcesJar: Only exists if an AbstractArchiveTask sourcesJar exists. Remaps the output of the sourcesJar task in place.
• runClient: A JavaExec task to launch Fabric Loader as a Minecraft client.
• runServer: A JavaExec task to launch Fabric Loader as a Minecraft dedicated server.

When setting up a multi-project build that depenends on another loom project you should use the namedElements configuration when depending on the other project. By default a projects “outputs” are remapped to intermediary names. The namedElements configuration contains the project ouputs that have not been remapped.

dependencies {
	implementation project(path: ":name", configuration: "namedElements")
}

For years a common source of server crashes has been from mods accidentally calling client only code when installed on a server. The latest loom and loader verions provide an option to require all client code to be moved into its own sourceset. This is done to provide a compile-time guarantee against calling client only Minecraft code or client only API on the server. A single jar file that works on both the client and server is still built from the two sourcesets.

The following snippet from a build.gradle file shows how you can enable this for your mod. As your mod will now be split across two sourcesets, you will need to use the new DSL to define your mods sourcesets. This enables Fabric Loader to group your mods classpath together. This is also useful for some other complex multi-project setups.

loom {
	splitEnvironmentSourceSets()

	mods {
        	modid {
            		sourceSet sourceSets.main
            		sourceSet sourceSets.client
        	}
	}
 }

loom {
	// Set the access widener path, see https://fabricmc.net/wiki/tutorial:accesswideners
	accessWidenerPath = file("src/main/resources/modid.accesswidener")

	// Add additional log4j config files.
	log4jConfigs.from(file("log4j.xml"))

	// When enabled the output archives will be automatically remapped.
	remapArchives = true
	// When enabled the -dev jars in the *Elements configurations will be replaced by the remapped jars
	setupRemappedVariants = true
	// When enabled transitive access wideners will be applied from dependencies.
	enableTransitiveAccessWideners = true
	// When enabled log4j will only be on the runtime classpath, forcing the use of SLF4j.
	runtimeOnlyLog4j = false

	// When set only server related features and jars will be setup.
	serverOnlyMinecraftJar()
	// When set the minecraft jar will be split into common and clientonly. Highly experimental, fabric-loader does not support this option yet.
	splitMinecraftJar()

	// Used to configure existing or new run configurations
	runs {
		client {
			// Add a VM arg
			vmArgs "-Dexample=true"
			// Add a JVM property
			property("example", "true")
			// Add a program arg
			programArg "--example"
			// Add an environment variable
			environmentVariable("example", "true")
			// The environment (or side) to run, usually client or server.
			environment = "client"
			// The full name of the run configuration, i.e. 'Minecraft Client'. By default this is determined from the base name.
			configName = "Minecraft Client"
			// The default main class of the run configuration. This will be overridden if using a mod loader with a fabric_installer.json file.
			defaultMainClass = ""
			// The run directory for this configuration, relative to the root project directory.
			runDir = "run"
			// The sourceset to run, commonly set to sourceSets.test
			source = sourceSets.main
			// When true a run configuration file will be generated for IDE's. By default only set to true for the root project.
			ideConfigGenerated = true

			// Configure run config with the default client options.
			client()

			// Configure run config with the default server options.
			server()
		}

		// Example of creating a basic run config for tests
		testClient {
			// Copies settings from another run configuration.
			inherit client

			configName = "Test Minecraft Client"
			source = sourceSets.test
		}
	}

	// Configure all run configs to generate ide run configurations. Useful for sub projects.
	runConfigs.configureEach {
		ideConfigGenerated = true
	}

	// Used to configure mixin options or apply to additional source sets.
	mixin {
		// When disabled tiny remapper will be used to remap Mixins instead of the AP. Experimental.
		useLegacyMixinAp = true
		// Set the default refmap name
		defaultRefmapName = "example.refmap.json"

		// See https://github.com/FabricMC/fabric-loom/blob/dev/0.11/src/main/java/net/fabricmc/loom/api/MixinExtensionAPI.java for options to add additional sourcesets
	}

	// Configure or add new decompilers
	decompilers {
		// Configure a default decompiler, either cfr or fernflower
		cfr {
			// Pass additional options to the decompiler
			options += [
				key: "value"
			]
			// Set the amount of memory in meagabytes used when forking the JVM
			memory = 4096
			// Set the maximum number of threads that the decompiler can use.
			maxThreads = 8
		}
	}

	interfaceInjection {
		// When enabled injected interfaces from dependecies will be applied.
		enableDependencyInterfaceInjection = true
	}

	// Splits the Minecraft jar and incoming dependencies across the main (common) and client only sourcesets.
	// This provides compile time safety for accessing client only code.
	splitEnvironmentSourceSets()

	// This mods block is used group mods that are made up of multiplue classpath entries.
	mods {
		modid {
			// When using split sources you should add the main and client sourceset
			sourceSet sourceSets.main
			sourceSet sourceSets.client
		}
	}

	// Create modExampleImplementation and related configurations that remap mods.
	createRemapConfigurations(sourceSets.example)
}

remapJar {
	// Set the input jar for the task, also valid for remapSourcesJar
	inputFile = file("example.jar")
	// Set the source namespace, also valid for remapSourcesJar
	sourceNamespace = "named"
	// Set the target namespace, also valid for remapSourcesJar
	targetNamespace = "intermediary"
	// Add additional jar files to the remap classpath, also valid for remapSourcesJar
	classpath.from file("classpath.jar")

	// Add a nested mod jar to this task, the include configuration should be used for maven libraries and mods.
	nestedJars.from file("nested.jar")
	// When enabled nested jars will be included with the output jar.
	addNestedDependencies = true
}

dependencies {
	// Set the minecraft version.
	minecraft "com.mojang:minecraft:1.18.1"

	// Use mappings from maven.
	mappings "net.fabricmc:yarn:1.18.1+build.22:v2"

	// Use the offical mojang mappings
	mappings loom.officialMojangMappings()

	// Layered mappings using official mojang mappings and parchment.
	mappings loom.layered() {
		officialMojangMappings()
		// Use parchment mappings. NOTE: Parchment maven must be manually added. (https://maven.parchmentmc.org)
		parchment("org.parchmentmc.data:parchment-1.17.1:2021.09.05@zip")
	}

	// Remap a mod from maven and apply to gradle's implementation configuration
	// (Minor detail: it's not exactly applied *to* the configuration, but a clone of it intended for mod dependencies)
	modImplementation "net.fabricmc.fabric-api:fabric-api:0.46.2+1.18"

	// Remap a mod from maven and apply to gradle's api configuration
	modApi "net.fabricmc.fabric-api:fabric-api:0.46.2+1.18"

	// Remap a mod from maven and apply to gradle's compileOnly configuration
	modCompileOnly "net.fabricmc.fabric-api:fabric-api:0.46.2+1.18"

	// Remap a mod from maven and apply to gradle's compileOnlyApi configuration
	modCompileOnlyApi "net.fabricmc.fabric-api:fabric-api:0.46.2+1.18"

	// Remap a mod from maven and apply to gradle's runtimeOnly configuration
	modRuntimeOnly "net.fabricmc.fabric-api:fabric-api:0.46.2+1.18"

	// Remap a mod from maven and apply to loom's localRuntime configuration.
	// Behaves like runtimeOnly but is not exposed in to dependents. A bit like testRuntimeOnly but for mods.
	modLocalRuntime "net.fabricmc.fabric-api:fabric-api:0.46.2+1.18"

	// Include a mod jar in the remapped jar. None transitive.
	include "example:example-mod:1.1.1"

	// Include a none mod library jar in the remapped jar. A dummy mod will be generated. None transitive.
	include "example:example-lib:1.1.1"

	// Helper to aid with depending on a specific fabric api version.
	modImplementation fabricApi.module("fabric-api-base", "0.46.2+1.18")

	// Depend on a loom sub project by using the namedElements configuration.
	implementation project(path: ":name", configuration: "namedElements")
}

Loom and/or gradle can sometimes fail due to corrupted cache files. Running ./gradlew build –refresh-dependencies will force gradle and loom to re-download and recreate all of the files. This may take a few minutes but is usually quite successful with resolving cache related issues.

Loom is designed to work out of the box by simply setting up a workspace in the user's IDE of choice. It does quite a few things behind the scenes to create a development environment with Minecraft:

1. Downloads the client and server jar from official channels for the configured version of Minecraft.
2. Merges the client and server jar to produce a merged jar with @Environment and @EnvironmentInterface annotations.
3. Downloads the configured mappings.
4. Remaps the merged jar with intermediary mappings to produce an intermediary jar.
5. Remaps the merged jar with yarn mappings to produce a mapped jar.
6. Optional: Decompiles the mapped jar to produce a mapped sources jar and linemap, and applies the linemap to the mapped jar.
7. Adds dependencies of Minecraft.
8. Downloads Minecraft assets.
9. Processes and includes mod-augmented dependencies (and optionally extracts and remaps nested JARs).

• ${GRADLE_HOME}/caches/fabric-loom: The user cache, a cache shared by all Loom projects for a user. Used to cache Minecraft assets, jars, merged jars, intermediary jars and mapped jars.
• .gradle/loom-cache: The root project persistent cache, a cache shared by a project and its subprojects. Used to cache remapped mods as well as generated included mod JARs.
• build/loom-cache: The root project build cache.
• **/build/loom-cache: The (sub)project build cache.

• minecraft: Defines the version of Minecraft to be used in the development environment.
• mappings: Defines the mappings to be used in the development environment.
• modCompile, modImplementation, modApi and modRuntime: Augmented variants of compile, implementation, api and runtime for mod dependencies. Will be remapped to match the mappings in the development environment and has any nested JARs removed. Nested JARs can optionally be extracted and remapped.
• include: Declares a dependency that should be included as a jar-in-jar in the remapJar output. This dependency configuration is not transitive. For non-mod dependencies, Loom will generate a mod JAR with a fabric.mod.json using the name as the mod ID and the same version.

• Applies the following plugins: java, eclipse and idea.
• Adds the following Maven repositories: Fabric https://maven.fabricmc.net/, Mojang https://libraries.minecraft.net/ and Maven Central.
• Configures the idea extension to exclude directories .gradle, build, .idea and out, to download javadocs sources and to inherit output directories.
• Configures the idea task to be finalized by the genIdeaWorkspace task.
• Configures the eclipse task to be finalized by the genEclipseRuns task.
• If an .idea folder exists in the root project, downloads assets (if not up-to-date) and installs run configurations in .idea/runConfigurations.
• Adds net.fabricmc:fabric-mixin-compile-extensions and its dependencies with the annotationProcessor dependency configuration.
• Configures all non-test JavaCompile tasks with configurations for the Mixin annotation processor.
• Configures the remapJar task to output a JAR with the same name as the jar task output, then adds a “dev” classifier to the jar task.
• Configures the remapSourcesJar task to process the sourcesJar task output if the task exists.
• Adds the remapJar task and the remapSourcesJar task as dependencies of the build task.
• Configures the remapJar task and the remapSourcesJar task to add their outputs as archives artifacts when executed.
• For each MavenPublication (from the maven-publish plugin):
  • Manually appends dependencies to the POM for mod-augmented dependency configurations, provided the dependency configuration has a Maven scope.

All run configurations have the run directory ${projectDir}/run and the VM argument -Dfabric.development=true. The main classes for run configurations is usually defined by a fabric-installer.json file in the root of Fabric Loader's JAR file when it is included as a mod dependency, but the file can be defined by any mod dependency. If no such file is found, the main classes defaults to net.fabricmc.loader.launch.knot.KnotClient and net.fabricmc.loader.launch.knot.KnotServer.

The client run configuration is configured with –assetsIndex and –assetsDir program arguments pointing to the loom cache directory containing assets and the index file for the configured version of Minecraft. When running on OSX, the “-XstartOnFirstThread” VM argument is added.