> For the complete documentation index, see [llms.txt](https://ddozzi.gitbook.io/mixin-tutorial/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://ddozzi.gitbook.io/mixin-tutorial/how-to-make-a-mixin-client/4-or-creating-your-first-mixin.md).

# 4 | Creating your first mixin

Creating a mixin

### Creating our Mixin class

So, Minecraft is divided into several folders as shown below:

![a small portion of the many folders that Minecraft has](https://4250098061-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-MiA6dBhw40h7Hy8268O%2F-MiIl4jWleJYEg4zE2Qq%2F-MiIq8uYzvA5m9GmOgyu%2FScreen%20Shot%202021-08-29%20at%205.37.21%20PM.png?alt=media\&token=104c6c54-abe1-4522-b626-57f63aa130bc)

The class we want to modify is the Minecraft class, which is located inside the `client` folder Which means, we should make a `client` folder inside our mixins folder to house our mixin class

```
project
    -> src
        -> java
            -> GROUP_NAME
                -> mixins
                    -> client <-- Create this
```

Since we don't want to pollute Minecraft's namespace, we append the word `Mixin` to our mixin classes to avoid it clashing with Minecraft class names, which means, inside the client folder we just created, create a class by the name of `MinecraftMixin`or `MixinMinecraft` which will act as a mixin for the `Minecraft` class. An example is shown below:

![](https://4250098061-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-MiA6dBhw40h7Hy8268O%2F-MiIl4jWleJYEg4zE2Qq%2F-MiIm5NAMAnBwREa4zV1%2FScreen%20Shot%202021-08-29%20at%205.19.43%20PM.png?alt=media\&token=30379756-e6d0-477a-a8b1-5506bfdbaf9f)

### Creating the mixin

We need to tell the Mixin library what class `MixinMinecraft` is going mixin to. (a class that will add in it's own code to another class).

This is trivial, as we can use a simple annotation to denote that this is a mixin for the `Minecraft` class

```java
@Mixin(Minecraft.class)
public class MixinMinecraft {
    ...
}
```

Now that we have told `Mixin` that we are going to modify the `Minecraft` class's methods, we can actually modify a function, but before that, we need to understand the two main ways of doing it

You can use the `@Overwrite` annotation, this will completely overwrite the code in a function, which is a **destructive change**, but it can be done like so:

```java
@Overwrite
private void functionNameToOverwrite(int p1, String p2) {
    ...
}
```

{% hint style="warning" %}
Most of the time, you never want to use the `@Overwrite` annotation, as it will completely overwrite any pre-existing code.
{% endhint %}

However, since this is a destructive change, you may want to instead *inject* your code, which allows you to add code at either the `HEAD` of the function, which is the top of the function, or at the `RETURN` of the function, which is just before the return statement (or the end of the function), this is a bit more complex, as the `@Inject` annotation needs quite a bit more information.

You need to supply

1. The method name in a string
2. Where you want to inject at
3. The method parameters for the method to inject, and a `CallbackInfo` object with provides information about the function and allow you to handle certian things like cancel the function

```java
@Inject(method = "functionNameToInject", at = @At("RETURN"))
private void functionNameToInject(int otherParameters, CallbackInfo ci) {
    ...
}

```

Now that you know the basics of mixins, we can now attempt to write our first mixin

1. The class that we're injecting into, `Minecraft.java` has function `startGame` which is called when the game is started. This is the function we want to inject into.
2. We'll be injecting into the `HEAD` of the function, as that will allow our code to be ran before Minecraft has been initialized. We could inject into the `RETURN` of the function, but in this case, we want our code to be ran before Minecraft's initialziation.

We can now use our `@Inject` annotation to inject at the head of `startGame`

Since we're just starting to learn the basics of mixins, we'll just make it so that it just outputs something to the console.

This can be done as shown:

```java
@Inject(method = "startGame", at = @At("HEAD"))
private void init(CallbackInfo ci) {
    System.out.println("Hello, Mixins!");
}
```

{% hint style="info" %}
You might be wondering why our function is named `init` when we're clearly injecting into `startGame` . This is because we've already specified that we're injecting at the `startGame` function using the `@Inject` annotation -- so Mixin doesn't really care what the function is named.
{% endhint %}

### Mixin doesn't recognize our Injection

Remember the file that we created in[ 2 | Setting up the workspace](https://ddozzi.gitbook.io/mixin-tutorial/how-to-make-a-mixin-client/2-or-setting-up-the-workspace), by the name of `mixins.PROJECT_NAME.json`?

Well, we need to modify that json file to tell Mixin that `MixinMinecraft` is a mixin that does infact, exist

So, open `mixins.PROJECT_NAME.json`

```
project
    -> src
        -> resources
            -> mixins.PROJECT_NAME.json
```

It should look somewhat similar to this:

```javascript
{
  "required": true,
  "compatibilityLevel": "JAVA_8",
  "verbose": true,
  "package": "me.ddozzi.exampleclient.mixins",
  "refmap": "mixins.ExampleClient.refmap.json",
  "mixins": []
}
```

First get the relative path, from your `mixins` package to your class, so for us it should be `client.MixinMinecraft`, add that to the mixins array in the refmap file, like so:

```javascript
{
  "required": true,
  "compatibilityLevel": "JAVA_8",
  "verbose": true,
  "package": "me.ddozzi.exampleclient.mixins",
  "refmap": "mixins.ExampleClient.refmap.json",
  "mixins": [
      "client.MixinMinecraft"
  ]
}
```

### Finishing off

{% hint style="success" %}
You should now be able to run the client, and see our console output!
{% endhint %}

{% hint style="warning" %}
Getting a `FileNotFound` error? Go check your run configurations and make sure that your `--tweakClass` is set to the correct location.
{% endhint %}

![](https://4250098061-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-MiA6dBhw40h7Hy8268O%2F-MiIl4jWleJYEg4zE2Qq%2F-MiIvWSEtd3Uc2gP2vlW%2FScreen%20Shot%202021-08-29%20at%206.00.36%20PM.png?alt=media\&token=1c75aa58-5b8c-4062-b34d-a5940fd138c8)

{% hint style="success" %}
Woohoo! We can see our first mixin working!
{% endhint %}