Entrypoints are declared in a mod's fabric.mod.json to expose parts of the code for usage by Fabric Loader or other mods. They are primarily used to make some code run during game initialization to initialize mods, though the entrypoint system has other uses as well. Entrypoints are loaded by language adapters, which will attempt to produce a Java object of a specified type using the name of the code object.

An entrypoint is exposed under some name, refers to some code object and must be based on a familiar entrypoint prototype. An entrypoint prototype defines the name (such as “main” or “client”) and the expected type of object that the entrypoint should refer to (such as the ModInitializer interface). An entrypoint prototype provider declares an entrypoint prototype and is responsible for accessing entrypoints, and can state how the they will be used. Fabric Loader provides some built-in entrypoint prototypes, while mods can also provide their own.

Entrypoints can be considered a more powerful implementation of Java Service Provider Interfaces.

A mod can declare any amount of entrypoints under different names in their fabric.mod.json. “main” is used to initialize the parts of a mod that are common to both a Minecraft client and a dedicated server, such as registry entries. “client” is used to initialize the parts of a mod that are reserved for clients only, such as registration of rendering-related objects.

{
  [...]
  "entrypoints": {
    "main": [
      "net.fabricmc.ExampleMod"
    ],
    "client": [
      "net.fabricmc.ExampleClientMod"
    ]
  }
  [...]
}

Caution: It is recommended to use separate classes for main, client and server entrypoints to avoid class loading issues. Consider the case where the same class is used for both a main and a client entrypoint. When launched on a dedicated server, even if the “client” entrypoint is never loaded, the class that contains the client initialization logic will be. Even if the client logic will never be executed, the act of merely loading the code may trigger class loading issues.

Fabric Loader provides four built-in entrypoint prototypes for mod initialization, some of which are designed to deal with initialization with respect to physical sides (see side). The main, client and server entrypoints are loaded and called early during the game's initialization, at a point where most but not all game systems are ready for modification. These entrypoints are typically used to bootstrap mods by registering registry objects, event listeners and other callbacks for doing things later.

• main: The first normal initialization entrypoint to run. Uses the type ModInitializer and will call onInitialize.
• client: Will run after main only in a physical client. Uses the type ClientModInitializer and will call onInitializeClient.
• server: Will run after main only in a physical server. Uses the type DedicatedServerModInitializer and will call onInitializeServer.
• preLaunch (not recommended for general use): The earliest possible entrypoint, which is called just before the game launches. Use with caution to not interfere with the game's initialization. Uses the type PreLaunchEntryPoint and will call onPreLaunch.

All main entrypoints are called before all client/server entrypoints. The exact time these entrypoints are called is unspecified and may vary between different versions of the game.

An entrypoint's code reference is turned into an instance of the entrypoint prototype's type. The most common way to make an entrypoint is to refer to a class which implements the expected type, but these code references can be made in multiple ways. Internally, a language adapter is responsible for interpreting the references and turning them into instances. The default language adapter is designed for Java code, and thus supports the following types of references:

• Class reference: ex. net.fabricmc.example.ExampleMod refers to the non-abstract class by this name. The class must have a public constructor with no arguments. The class must implement or extend the expected type. The resulting object is a new instance of the class.
• Method reference: ex. net.fabricmc.example.ExampleMod::method refers to a public method in that class by that name. If the method is nonstatic, a new instance of the class is constructed for the method to be called on, meaning the class must be non-abstract and also have a public constructor with no arguments. Methods can only be used for interface types. The method must accept the same arguments and have the same return type as the abstract method(s) in the interface. The resulting value is a proxy implementation of the interface, which will implement abstract methods by delegating to this method.
• Static field reference: ex. net.fabricmc.example.ExampleMod::field refers to the field in that class by that name. The field must be static and public. The the type of the field must be compatible with the expected type. The resuling value is the value of that field.

References to class members must be unambiguous, meaning the class must contain one and only one field or method with the targeted name. The language adapter cannot resolve methods overloads. In case of ambiguity, the entrypoint will fail to resolve.

Language adapters for other languages can be implemented by mods. fabric-language-kotlin provides a language adapter for Kotlin.

Mods can call each others' entrypoints for integration purposes. An entrypoint is loaded lazily when entrypoints for a specific entrypoint prototype are requested, which makes an entrypoint an excellent tool for optional mod integrations. A mod may become an entrypoint prototype provider by declaring that other mods should provide entrypoints based on an entrypoint prototype, often using a class or interface that the mod provides in its API. Mods can safely use this class or interface even if the provider is not installed (rendering the class or interface inaccessible) because entrypoints are loaded only on request. When the provider is not present, the entrypoint will simply be ignored.

Entrypoint instances can be accessed by calling FabricLoader#getEntrypointContainers(name, type). This returns a list of entrypoint containers. These containers contain the entrypoint instance and the mod container of the mod which provided the instance. This can be used by a mod to determine which mods have registered an entrypoint.

Entrypoint instances are memoized by their name and also their type. Using the same code reference for multiple entrypoints will result in multiple instances. Though highly absurd in practice, if getEntrypoints is called multiple times with the same name but different types, instances are constructed and memoized per type.

Fabric Loader does not have a concept of a load order or loading phases. Initializer entrypoints are the mechanism with which most mod loading is usually done, but whether or not an initializer has been called does not determine whether or not a mod can be considered to be “loaded”. Thus, it is unreasonable to expect that a mod has completed its modifications to the game after its initializers have been called. Additionally, the order in which entrypoints are called is mostly undefined and cannot be altered. The only guarantee is that a list of initializers in a fabric.mod.json file are called in the order in which they are declared. Fabric Loader does not provide multiple phases of initializers to work around the lack of order, either.

A common example is the expectation that mod A should be able to load after mod B because mod A will replace an object registered by mod B. Alternatively, mod C wants to be loaded before mod D because mod D will do something in response to a registration performed by mod C. This is cannot be done for two reasons:

1. Mod initializers are not required to represent a transition in a “mod loading lifecycle” so that after the initializer is called, all its registry objects are registered.
2. The order in which mod initializers are called is undefined, and cannot be influenced so that mod A's initializers are called after mod B's initializers, or so that mod C's initializers are called before mod D's initializers.

Leaving aside the missing guarantee of registration of all objects in initializers, one might argue that there should therefore be other entrypoints to perform “pre-initialization” and “post-initialization” so that there is a sense of order. This creates a multi-phase loading scheme, which in practice creates issues with the establishment of conventions for which operations are to be performed in which phase, uncertainty and lack of adherence to these conventions and outliers which do not fit.