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Fabric Loader

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.



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

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.

Fabric Loader internals


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.

Class loading and transformation

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 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.

documentation/fabric_loader.txt · Last modified: 2020/02/22 00:55 by jamieswhiteshirt