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: Fabric Example Mod Resources

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

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

{
  "mixins": [
    "modid.mixins.json"
  ]
}

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

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:

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

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. The naming convention is <TargetClass>Mixin for most Mixins and I<TargetClass>Accessor for Mixin Accessors & Invokers.
  • 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 and its Mixins fail to apply or fail to find their targets. Injector Mixins have their own requirement value that precedes this.
• 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)
• injectors holds a group of fields that define the settings for injector Mixins.
  • The main one to keep in mind is defaultRequire, which is by default 1. This setting controls the amount of targets injectors must match by default to not crash. By setting this to 0, you effectively make injectors not required by default. Injector annotations such as @Inject have an optional require parameter that overrides the config setting. Both the injector-specific setting and the config setting precede the required field in the config.
• 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 dedicated page on MixinExtras' wiki)

It is fine, especially if you are a beginner, to not necessarily understand the above fields fully.

For further reading, check out the 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 Reading the Minecraft source for general setup for reading external source code, including both MC and libraries, APIs or subsystems like Mixin.