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Fabric Tutorials



These pages are essential must-reads when modding with Fabric, and modding Minecraft in general, if you are new to modding, it is recommended you read the following.


Blocks and Block Entities

Data Generation

World Generation


These pages will guide you through Mojang's Brigadier library which allows you to create commands with complex arguments and actions.


These pages will guide you through using the many events included in Fabric API, and how to create your own events for you or other mods to use.



Mixins & ASM

These pages will guide you through the usage of SpongePowered's Mixin library, which is a highly complex topic. We recommend you read these pages thoroughly.



Contribute to Fabric


Mixin Injects


Injects allows you to place custom code at a specified position inside an existing method. For a working example, view the Practical Example category at the bottom of this page. The standard form of an inject is as shown:

private void injectMethod(METHOD ARGS, CallbackInfo info) {

The Injection Point Reference defines where the code inside the method body is injected inside the target method. The following table describes a few of the options:

Name Description
HEAD Top of the method
RETURN Before every return statement
INVOKE At a method call
TAIL Before the final return statement

In the case of injection points that reference statements or members, the target value can be set inside @At. Target value is specified using JVM bytecode descriptors.

Oracle defines the following field descriptors:

Descriptor Primitive Description
B byte signed byte
C char Unicode character code point in the Basic Multilingual Plane, encoded with UTF-16
D double double-precision floating-point value
F float single-precision floating-point value
I int signed integer
J long signed long integer
LClassName; reference an instance of ClassName
S short signed short
Z boolean true or false
[ reference one array dimension

A method descriptor is comprised of the method name, followed by a set of parentheses containing the parameter types, followed by the return type. A method defined in Java as Object m(int i, double[] d, Thread t) would have the method descriptor m(I[DLjava/lang/Thread;)Ljava/lang/Object;.

In the case that the return type is void, you need to use V (Void Descriptor Type) as the type (for example, void foo(String bar) would become foo(Ljava/lang/String;)V).

Generics' types are left out, as generics don't exist on runtime. So Pair<Integer, ? extends Task<? super VillagerEntity>‍> would become Lcom/mojang/datafixers/util/Pair.

@Inject methods always have a void return type. The method name does not matter and neither does the access modifier; using something that describes what the inject does is best. The target method's arguments are placed first in the method's header, followed by a CallbackInfo object. If the target method has a return type (T), CallbackInfoReturnable<T> is used instead of CallbackInfo.

Returning & Cancelling from Inject

To cancel or return early inside a method, use CallbackInfo#cancel or CallbackInfoReturnable<T>#setReturnValue(T). Note that cancel does not have to be called after setReturnValue. In both instances, cancellable will have to be set to true in the inject annotation:

@Inject(method = "...", at = @At("..."), cancellable = true)

Injecting into Constructors

To inject into a constructor, use <init>()V as the method target, with () containing the constructor argument descriptors. When injecting into constructors, @At must be set to either TAIL or RETURN. No other forms of injection are officially supported. Note that some classes have methods named init which are different from <init>. Don't get confused!

To inject into a static constructor, use <clinit> as the method name.

Practical Example

The following example injects a print statement at the top of TitleScreen#init (note: the method init is a normal method and not a constructor).

public class ExampleMixin {
	@Inject(at = @At("HEAD"), method = "init()V")
	private void init(CallbackInfo info) {
		System.out.println("This line is printed by an example mod mixin!");

For more information on this particular example, view its usage in the Fabric Example Mod repo.

tutorial/mixin_injects.txt · Last modified: 2022/08/05 19:19 by clomclem