Fabric Wiki

This page is read only. You can view the source, but not change it. Ask your administrator if you think this is wrong.
====== Registering Mixins ======

==== Introduction ====

In this tutorial, you will learn how to register your Mixins through the ''%%resources/fabric.mod.json%%''.

Inside your ''%%resources%%'' folder is where your ''%%fabric.mod.json%%'' should be. 

Use this link to view the Fabric Example Mod's resources folder: [[https://github.com/FabricMC/fabric-example-mod/tree/master/src/main/resources|Fabric Example Mod Resources]]

Inside your ''%%fabric.mod.json%%'' is where you define where Fabric should look for your ''mixins.json''.

==== Register Mixins with Fabric ====

To register a mixin you have to tell Fabric where to look.
To tell Fabric where to look you need to add elements to the ''mixins'' array inside ''fabric.mod.json''

<code json [enable_line_numbers="true"]>
{
  "mixins": [
    "modid.mixins.json"
  ]
}
</code>

Providing a String ''%%"<modid>.mixins.json"%%'' inside the mixins array tells Fabric to load the mixins defined inside the file ''<modid>.mixins.json''.

==== Register Mixins ====

In the previous section, you learned about registering your ''<modid>.mixins.json'' files.

We still have to define which mixins to load and where these mixins are located.

A typical ''mixins.json'' file for 1.21.x development might look like this:

<code json>
{
  "required": true,
  "minVersion": "0.8",
  "package": "net.fabricmc.example.mixin",
  "compatibilityLevel": "JAVA_21",
  "mixins": [],
  "client": [
    "TitleScreenMixin"
  ],
  "server": [],
  "injectors": {
    "defaultRequire": 1
  },
  "overwrites": {
    "requireAnnotations": true
  }
}
</code>

The **4** main fields you should worry about when getting started with mixins are the ''package'' field, and the ''mixins'', ''client'', ''server'' arrays.

  * The ''package'' field defines which folder (package) to find the Mixins in. That package and its subpackages should be dedicated to Mixins, and nothing else.
  * The ''mixins'' array defines which classes should be loaded on both the **physical** client and **physical** server. Mixin Classes should be referenced by their file/class name when registered in your config. The naming convention is ''<TargetClass>Mixin'' for most Mixins and ''<TargetClass>Accessor'' for [[tutorial:mixin_accessors|Mixin Accessors & Invokers]]. Some may also prefer using the ''I'' prefix for interfaces, but it is not standardized.
    * The ''client'' array defines which classes should be loaded only on the **physical** client. A physical, dedicated server will never interact with client-exclusive classes. So Mixins targeting client-sided classes should go in this field.
    * The ''server'' array defines which classes should be loaded only on the **physical** server.

Following that logic: ''net.fabricmc.example.mixin.TitleScreenMixin'' is the mixin class that will be loaded on the client.


The other fields' rough function should also be kept in mind to understand when to change them from their defaults, and to understand what happens when another mod you're inspecting changes them:

  * The ''required'' field holds a boolean value that corresponds to whether or not Mixin should crash if this config or its Mixins raise errors during application.
  * ''minVersion'' defines the minimum Mixin version that must be present and used to apply the config's Mixins.
  * ''compatibilityLevel'' defines the minimum compatibility level that the Mixin sub system must be set to in order to apply the config's Mixins. It should generally be set to the Java level used by your MC version unless you specifically have a reason not to (e.g. ''JAVA_17'' for 1.20.1, ''JAVA_21'' for 1.21.x) Mixin will try to set itself to the highest version found in that field across the different configs it interacts with during launch.
  * ''injectors'' holds a group of fields that define the settings for injector Mixins. 
    * The main one to keep in mind is ''defaultRequire'', which is typically 1 in templates' configs, but is effectively 0 if unspecified. This setting controls the minimum amount of targets injectors under this config must match by default to raise an error. By setting this to 0, you effectively make injectors fail soft by default. Injector annotations such as ''@Inject'' have an optional ''require'' parameter that overrides the config setting.
  * ''overwrites'' holds a group of fields that define the settings for overwrite Mixins.
    * The main one to keep in mind is ''requireAnnotations'' which holds a boolean value that, when set to true, makes Overwrite methods always require a corresponding annotation to overwrite the target class method. There is hardly ever a reason to set it to false.

There are other fields that can optionally be added, most of which only serve specific purposes that are outside of the scope of a general explanation. There is, however, one that is relevant for MixinExtras:
  * ''mixinExtras'', similarly to ''injectors'' holds a group of fields that serve as settings for MixinExtras specific behavior. Mainly, it allows to set up a separate ''minVersion'' value for MixinExtras. Doing this is required for setting up Expressions introduced in MixinExtras 0.5.0 (see the [[https://github.com/LlamaLad7/MixinExtras/wiki/Expressions-Setup|dedicated page on MixinExtras' wiki]]), but it also sets the version MixinExtras should behave as for your mod's purposes. It is //recommended// that you fill this field yourself even if you don't use Expressions to lessen the chances of issues arising from MixinExtras trying to behave as the wrong version when interacting with your mod.


:!: It is fine, especially if you are a beginner, to not necessarily understand the above fields fully. You should ask in the Fabric or SpongePowered Discord about clarifications on any of the concepts or fields you do not understand from this page. It may help improve your overall Mixin comprehension, and will help you have a more complete understanding of the Mixin subsystem.

----
For further reading, check out the [[https://jenkins.liteloader.com/view/Other/job/Mixin/javadoc/index.html|Mixin Javadoc]], given specifics here can vary based on version, it may be best to read the JavaDoc from your specific build. (On IntelliJ IDEa) you can middle click on a field in the settings to be taken to the corresponding class that handles that field. See [[tutorial:reading_mc_code|Reading the Minecraft source]] for general setup for reading external source code, including both MC and libraries, APIs or subsystems like Mixin.