Tutorial: Making your first Mixin

This is meant to be complementary to the Introduction to Mixins page, it is not required but highly recommended to read it before this tutorial. This tutorial intends to hold a newcomer's hand through each step of creating a very elementary Mixin. This intends to create familiarity with basic Mixin notions and syntax, thus lessening the esoteric appearance Mixins have during their initial learning curve.
See Registering Mixins for setting up Mixins on a Fabric project if you intend to follow along with this.

This tutorial will be made using a 1.21.1 Fabric Project as reference. It is recommended to develop using IntelliJ IDEa Community Edition to be able to leverage the Minecraft Development plugin by demonwav, AKA MCDev. Yarn mappings will be used. If you wish to follow along with any other set of mappings, you may use a tool such as mappings.dev or Shedaniel.dev's mappings tool to translate between the Yarn mappings showcased and the ones in your development environment as needed.

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For this tutorial, we'll add a Logger call at the head of the loadWorld method in the Vanilla MinecraftServer class. This will be based on the ExampleMixin Mixin class present in the Fabric mod template at the time of writing, with some tweaks.

For this task, we'll use the injector @Inject, and explain the different elements as we build it. Mixin uses annotations to parse metadata that will be used to find where and how to specifically try and apply our changes.

It is important to note that we use @Inject specifically because we only need to insert a single method call at a given point. One must always consider which tool to use for the job, often it is not @Inject, and it may not even be right to use a Mixin in the first place.
The reason @Inject is the right tool here is because it injects a method call where specified, and that's all we need to do. It is as such the smallest, least intrusive change that can be made.

After following the necessary steps described in Registering Mixins to set up our mixins.json config, we'll create our class in our dedicated Mixin package. We'll call this class TutorialFirstMixin:

@Mixin(MinecraftServer.class)
abstract class TutorialFirstMixin {
 
}

It is conventional practice to name your Mixin classes the target class with Mixin appended as a suffix. Here, it would typically be named MinecraftServerMixin instead. This makes is very easy for someone who is looking through your project to know which Mixin classes have a given target. If you, for whatever reason, opt to not follow this convention, it is recommended to still name your Mixins after what they target in some way.

You'll notice we made the Mixin class abstract. This is a best practice when mixing into a class, as it avoids an accidental instantiation of the class, and removes the burden of needing to implement methods that may break the Mixin when implementing and extending the target's superclasses and interfaces.

Accordingly, we will also add this to our mixins.json config file:

{
...
    "mixins": [
        "TutorialFirstMixin"
    ],
...
}

And that's all there is to actually creating the Mixin Class, but it won't do anything on its own.

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We have a Mixin class, now let's use it to do something.

The first step, assuming nothing has caused a crash or gone to hell yet, is to create a “stub” method, AKA a method that's just a placeholder for our actual method. This will be the method we annotate with @Inject, and putting it there before the annotation allows us to not be bothered by the Annotations are not allowed here IDE error from having a lone annotation.

@Mixin(MinecraftServer.class)
abstract class TutorialFirstMixin {
 
    private void logOnWorldLoad() {
    }
}

Your Mixin injectors' methods, known as the handler, should always describe what it does rather than what it is. For example, in this tutorial it will be called logOnWorldLoad. We follow this convention to ease debugging as your handler name will appear in stack traces.

Next, we add the annotation:

abstract class TutorialFirstMixin {
 
    @Inject(method = "loadWorld")
    private void logOnWorldLoad() {
 
    }
}

This now specifies to Mixin that, when this method gets merged into MinecraftServer, we want to inject a call to it in the method loadWorld. Since there is only one method of that name in MinecraftServer, the name is enough, this would not be the case if the method had overloads. If you're using the Minecraft Development plugin, you should almost always leverage the plugin's autocomplete for a Mixin annotation's attributes to ensure accuracy.

Now, to specify where in the targeted method we want our injection to happen, we add something that may be a bit counterintuitive to some, an @At annotation as one of the arguments in the @Inject annotation:

abstract class TutorialFirstMixin {
 
    @Inject(method = "loadWorld", at = @At(value = "HEAD"))
    private void logOnWorldLoad() {
    }
}

Now, let's look closer at the annotations we have thus far. Think of them as holding a series of hierarchized instructions. A very simplified explanation would be:

• @Inject says that tells Mixin that it should inject a call to this method at specified point(s)
  • method = “loadWorld” narrows down the point of injection to within that target method
  • at = @At is used to specify what to look for in the targeted method(s)
    • value = “HEAD” specifies that Mixin should try to inject at the very start of the method at the point of injection.

Sidenote: one does not need to put value = if it is the only argument passed to @At, e.g. @At(“HEAD”) is valid.

Now, if you're following along with MCDev you'll likely notice that our handler method, logOnWorldLoad, does not have the correct signature for the target. In most cases, using MCDev's context action that quickly fixes it should work. @Inject handler methods must either contain none of the target method's parameters, or all of the target method's parameters in the same order – here there are none – followed by a CallbackInfo parameter, conventionally called ci, if injecting into a method of void return type. If we were injecting into a method that returns a value, it would instead be CallbackInfoReturnable<T> cir where T is the return type of the target method, such as a Boolean.

If we fix the signature and add a call to our mod's logger, the full class becomes:

@Mixin(MinecraftServer.class)
abstract class TutorialFirstMixin {
 
    @Inject(method = "loadWorld", at = @At(value = "HEAD"))
    private void logOnWorldLoad(CallbackInfo ci) {
        ExampleMod.LOGGER.info("MinecraftServer$loadWorld has started!")
    }
}

And that's it for making this Mixin!

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When the class you're targeting extends and/or implements parents, mimicking that on your Mixin class is an overall benefit, as it allows you to get all of the parent methods directly. For our example Mixin it'll look like:

abstract class TutorialFirstMixin extends ReentrantThreadExecutor<ServerTask> implements QueryableServer, ChunkErrorHandler, CommandOutput, AutoCloseable {
    public TutorialFirstMixin(String string) {
        super(string)
    }
 
    @Inject(method = "loadWorld", at = @At(value = "HEAD"))
    private void logOnWorldLoad(CallbackInfo ci) {
        ExampleMod.LOGGER.info("MinecraftServer$loadWorld has started!");
    }
}

The constructor is functionally irrelevant here, it's only there for the sake of extending without errors. One may notice that we didn't implement any methods, this is thanks to our class being abstract. Abstract classes do not need to implement methods from parents, allowing us to not have to handle all the overrides you would normally get.

Mixin provides debugging utilities, one of them is the ability to export the transformed classes once they're loaded. This allows to be able to specifically check what our Mixin has targeted and if it has applied where we wanted it to.
This section's information is detailed upon more specifically and quicker at the Exporting Mixin Classes page.

Exporting our Mixin would look like:

@Debug(export = true)
@Mixin(MinecraftServer.class)
abstract class TutorialFirstMixin extends ReentrantThreadExecutor<ServerTask> implements QueryableServer, ChunkErrorHandler, CommandOutput, AutoCloseable {
 
    @Inject(method = "loadWorld", at = @At(value = "HEAD"))
    private void logOnWorldLoad(CallbackInfo ci) {
        ExampleMod.LOGGER.info("MinecraftServer$loadWorld has started!")
    }
}

Setting a Mixin to be exported makes it so that once the targeted class has been transformed and has loaded (which could be anytime between the client starting and a world loading) Mixin will export a version of that now transformed class. It should be exported to the directory run/.mixin.out, and can be read using a decompiler or IDE equipped with one. You can also configure your development runs to export all Mixins by adding the following argument to your VM options: -Dmixin.debug.export=true

If you use the @Debug annotation, remember to disable it before building into a release-intended jar file.

One thing you will notice in an exported class is that your Mixin handler methods' names look odd when merged into the target class. This is the result of a process known as “name mangling” or just “mangling”. This process consists of procedurally modifying the merged member names such that they are unique. This is first and foremost for compatibility reasons, as two Mixins adding methods of the same name may cause a conflict even if the injectors target entirely different things. Injector methods are also given an identifier corresponding to the type of injector as part of mangling. For our case, @Inject has the prefix handler.

Breakpoints don't natively work with Mixin classes as they get merged at runtime. However, MCDev adds breakpoint support in Mixin Classes directly, allowing you to debug them by directly putting a breakpoint there, rather than having to try and anticipate your injector in the target class.

Logger calls can be very handy when trying to debug complex logical flows or just for general warning and error notifications. And whilst that may be useful for debugging, one should be particularly careful when using them anywhere that'll be called very frequently, for performance and log readability reasons. This applies just the same with Mixins, and one should in general not use loggers as their primary form of debugging.

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Assuming nothing blew up, you've completed your first functioning Mixin, and hopefully you learned a bit about how Mixin is used and broadly works along the way. What now?

Some principles to keep in mind are that Mixin is a very case-by-case tool, so this injector example was tailored to its specific use-case, but Mixin and MixinExtras provide a multitude of tools to use for different scenarios. Knowing which one to use comes down to knowing what effect you want to have in the code, and what is the most compatible and smallest change you can make to the target for your goal. You must also remember that Mixin is not always the solution, there are times where events or other modding framework tools outside of Mixins are already available, and you should prioritize using those rather than Mixins if you can afford to, as it is typically safer for compatibility.

For more thorough and formal intros to the topic, you should read through the Mixin Introduction page and especially its linked Wikis.
Namely, those are the Mixin Wiki and the MixinExtras Wiki. You may also find further information about specific tools in the relevant JavaDocs.

Other than that, the most important step from here is trial, error, and asking for help/information in the relevant support channels when stuck. This is an entirely expected part of the learning process, and even experienced devs ask questions about how to best approach an issue with Mixin.