Mixin Glossary (DRAFT)

This page aims to address terms, acronyms, abbreviations, expressions, etc. and define them within as broad a scope as possible without instructing how to use the associated tools. It is complementary to actual documentation on how to use the tools and subsystem of Mixin and should only be used to get a quick, general idea of what a term means in the context of Mixin usage and learning. This glossary should also be taken with a grain of salt, as much as feedback will try to be used to make the definitions as accurate as possible, it can be as unreliable as any Wiki page.
It is advised to read the Introduction to Mixins (WIP) page.

Seeing as this page is likely to change a lot with time as terms, both official and common usage ones, may vary over time and need many amendments as both Mixin and associated libraries evolve and get used in different contexts; it feels wise to add a section on best practices for contributing to this page in the long run.

If you wish to contribute it is appreciated to use references to external sources when relevant, preferably the official Mixin Wiki, the Mixin javadoc and the official MixinExtras Wiki. If not, base your definitions on feedback from experienced devs and be willing to be scrutinized.
Due to the amount of potential areas that could be edited, it would also be appreciated to always specify which definitions you're adding or editing in your edit summary messages. If necessary, split your edit into different smaller edits to be able to more clearly indicate which definitions were modified.

When you are doubtful on a term's definition, add a FIXME above the relevant text with text in italics until a more confident definition can be found. Definitions should generally take the form of itemized lists with the term above the list's first items. Synonyms should be grouped together. Footnotes references describing the same resource and so on should use the exact same text so that they both redirect to the same footnote.

TODO notes should always describe what should be added there and should ideally be in italics

Example definition:

1. First def
   1. Sub-def
   • Note on First def
2. Second def
   • Note on Second def

You can reference another definition on the page by referencing the definition's header: text displayed for the link, for example:
The bytecode segment - [[#bytecode|The bytecode segment]]
The ASM definition - [[#asm|The ASM definition]]

This is under heavy construction and probably contains inaccuracies, likely redundancies, certainly inefficient definitions or may be missing important definitions.

1. Unnamed method that can be defined within a method body(see Enclosing Method), consisting of parameters followed by a method body or expression. Variables of a functional interface type, such as Java's Consumer<T>, may also be used to hold a lambda function to be passed as an argument or stored as a variable later. Lambdas targeted separately from the rest of the encapsulating class's methods, as they are compiled into separate synthetic methods in Bytecode.

• For a more thorough technical definition, see the official Oracle Documentation.

1. A nameless inner class declared and instantiated a single time. Anonymous classes may be declared within a method's body, such as to assign its instance to a local variable. As inner classes, anonymous classes must be targeted separately from the outer class when mixing into it.

1. The method containing the relevant subject, such as an anonymous class or a lambda method.

1. The class containing the relevant subject, often an inner class such as an anonymous class.

1. When used as a single term in the form of a proper noun, references the Mixin Subsystem as a whole. Mixin is a subsystem with the primary purpose of allowing a developer to describe modifications to be made at runtime using code. See Introduction to Mixins (WIP)
2. “a Mixin” or “a Mixin Class” refers to classes or interfaces that are annotated with @Mixin, and will be merged with a target class as a means to transform it. Non-accessor Mixins will not be available at runtime due to the merging process.

1. A class with contents being targeted by the relevant Mixin Class's transformations.

1. A method within a target class being targeted by transformations from a Mixin class.

1. In the context of Mixin, merging most of members and new interface implementations from the Mixin class into the target class's bytecode.

1. Reference to usages of @Shadow, an annotation used to simulate the presence of fields and methods from the target class to reference them. Shadowing avoids needing to cast a this to the target class for any and all fields.

1. For Mixins, references the practice of casting a this instance of the Mixin class to Object and then to the target class. This is used mainly to pass the this instance of the target class as an argument. It is advised to use Shadows to reference the target's fields, and to extend the target's parents to access the parents' fields.

1. Reference to an Accessor Mixin, or in other words an interface Mixin composed purely of @Accessor and @Invoker Mixins.
2. Reference to the @Accessor annotation and its usages. An accessor is used to create public getter/setter fields for fields that would otherwise be immutable or inaccessible.

1. Reference to the @Invoker annotation and its usages. An Invoker is used to call methods that would otherwise be inaccessible. It is mainly used in the context of Accessor Mixins.

1. A companion library for the Mixin subsystem. MixinExtras focuses on giving more versatile, precise and compatible injectors and general Mixin usage utility. MixinExtras is bundled with Fabric since Loader version 0.15; MixinExtras 0.5.0 has been bundled with Fabric since Loader version 0.17.0; See the MixinExtras Wiki.

1. Expressions referring to using Mixin to modify a target. For instance, “mixing into ServerLevel” would mean using a Mixin to modify the ServerLevel class.

1. References the IntelliJ Minecraft Development Plugin. It is very frequently used for Mixin development due to the added convenience, autocompletion and error detection it provides. See the plugin's home page.

1. Java framework used by Mixin and other tools to modify and manipulate JVM bytecode at runtime.
2. In general programming contexts, may refer to Assembly.

1. Bytecode is a set of less readable, computer-oriented instructions, which code running on the Java Virtual Machine will be compiled into. Mixin works with bytecode as opposed to “Source” code, both for the Mixins to be applied and the code to target, “source” code being the decompiled/pre-compilation code, and as such, whilst source code can be used to infer bytecode, one should always prioritize bytecode for precise targeting or for certain targets which may not be reliably in the same position or order in the bytecode compared to a decompilation. See Reading the Minecraft source's relevant section on bytecode.

1. A class signature encodes type information about a class or interface declaration. It describes any type parameters of the class or interface, and lists its direct superclass and direct superinterfaces, if any. A type parameter is described by its name, followed by any class bound and interface bounds.
2. A method signature encodes the name, modifiers and type information about a method declaration. It describes any type parameters of the method, the types of any formal parameters, the return type, and the types of any exceptions declared in the method's throws clause.
3. A field signature encodes the name and the type of a field, formal parameter, local variable, or record component declaration.

1. A field descriptor represents the type of a field, parameter, local variable, or value.
   • See the below table for possible field descriptors.

1. A method descriptor contains zero or more parameter descriptors, representing the types of parameters that the method takes, and a return descriptor, representing the type of the value that the method returns. Note this does not include the name.
2. For example the descriptor of

   Object method(int i, double d, Thread t) {...}

   is

   (IDLjava/lang/Thread;)Ljava/lang/Object;

1. In Java, synthetic members and constructs refers to constructs added by the Java compiler into the compiled bytecode that do not have a source code equivalent. For instance, an anonymous class is given synthetic final fields which are used to be able to reference values of the enclosing method the anonymous class is declared in.
   • Synthetic members may be targeted by Mixin, such as with @Shadow or an Accessor but will require carefully reading the relevant bytecode.

1. Also known as “callback injectors”. Used through the form of annotations such as @Inject decorating a “handler” method. Upon the handler being merged, Mixin will use information specified within the injector's annotation to inject a set of “callback” instructions that will call back to the handler at a specific point.
   • Typically only refers to injectors that can chain and do not remove the targeted operations, which excludes redirectors such as @Redirect.

1. In Mixins, a method decorated by an injector annotation and then merged into the target class. It is then invoked/called within “callback” instructions that will be injected by Mixin based on the annotation's information.

1. A series of injected instructions invoking the associated handler method based on that method's injection point definition. This can include but is not limited to, adding a single method call, creating an early return instruction, wrapping a method call.

1. An argument passed to an injector allowing to interact with the callback instructions. Notably, it may be “cancelled” to create an early return by making the Inject cancellable.
   • If the target method returns a value, the injector will instead be passed a CallbackInfoReturnable<R> where R is the return type.
   • See the JavaDocs for the CallbackInfo and CallbackInfoReturnable classes.

1. An injection point is the primary value used by Mixin to determine where to insert the callback instructions for a given injector. Injection points are typically passed in the @At annotation's value field. Certain injection points are self-sufficient, such as “HEAD” as it will simply try and target the earliest possible point in the target method, whilst others may necessitate more discriminators to have a specific target like “INVOKE” which will need to specify the method whose call should be targeted. Injection points are used to search for specific Bytecode operations within the boundaries of the target method.
   1. The injection point “reference” refers to the documentation of Mixin's built-in injection points. See the Mixin Wiki's injection point reference page.
2. In some cases, injection point may refer to the specific point an injector's callback instructions will be inserted, resulting both from an injection point and other discriminators.

1. Injector from MixinExtras, used primarily to replace or modify operations such as method calls, field get/set operations, etc.
   • See the official Wiki page

1. Injector from MixinExtras, used to modify the resultant value of a very wide range of expressions and operations.
   • See the official Wiki page

1. Injector from Mixinextras, used to modify the return value of a method in a compatible fashion.
   • See the official Wiki page

1. (Verb) To use a Mixin injector to add to or modify existing operations.
2. Reference to the @Inject injector, used to insert a method call to the handler at a desired point.

1. (General programming) Value confined to the relevant scope.
   1. When working with Mixins, the relevant scope will be the target method and its locals.
2. Reference to the @Local annotation from MixinExtras, used to be able to capture the values of locals within a handler method's parameters.
   • See the official Wiki article

1. The process of using @Local to capture one or more local variables within an injector's parameters.

1. (Verb) In the context of injectors, means to cancel the remaining operations of the target method.
2. (Annotation) Reference to the @Cancellable annotation from MixinExtras or a usage of it. Used to be able to cancel a target method without needing to use @Inject
   • See the official Wiki article

1. @Share annotation from MixinExtras and its usages. Used to share values between handler methods injecting into the same target method.
   • See the official Wiki article

Outside of Injection Points, the Mixin toolchain provides different features that may be used to more precisely target a specific element of a target method matching the injection point. This is essential for making an injector less “brittle”, and making more precise changes which are more compatible. Note that different discriminators may be combined, such as using both an expression and a slice for the same injector if needed.

1. A “brittle” injector or targeting refers to a way of targeting a given element in a method in a manner that is unreliable or particularly likely to break with an update affecting an unrelated function.

1. Reference to the @Slice annotation and its usages. Slicing consists of defining a range within the target method in which to look for valid injection points.

1. Added in MixinExtras 0.5.0, a feature consisting of injection point “MIXINEXTRAS:EXPRESSION” and associated tools, such as @Expression and @Definition. Used to allow injectors to target a very wide range of instructions in a target method by using java-like expressions closer to a source code representation. MixinExtras 0.5.0 is bundled with Fabric Loader 0.17.0 and later.
   1. To “use an expression” may refer to using MixinExtras Expressions to target a certain piece of target code.
   • See the official wiki articles and tutorial.

1. An IDE feature added by the [#mcdev|MCDev plugin]] for IntelliJ, used to generate and display a set of flowcharts representing a class's bytecode as expressions. Used to match, debug and write Expressions.

1. In the context of expressions, a “definition” is used to refer to @Definition annotation usages. Definitions are used to declare what individual identifiers within an expression correspond to in code, such as types, methods, or variables.

1. In the context of Expressions, a wildcard – represented by the ? character – can be used instead of a defined identifier or a series of identifiers. Wildcards are able to stand in for any identifier and will match mostly anything that would otherwise be matched by an identifier in that part of the expression, and are useful to deliberately leave a part of an expression ambiguous.

1. A discriminator which, between all valid targets once other filters and discriminators are applied, uses a zero-indexed value to select one of the remaining targets for the injector to target.
   • Ordinals are generally a more brittle and less recommended discriminator for targeting using injectors. Using slices or expressions is recommended instead in cases where an ordinal would be only used as a disambiguation tool between different targets for an injection point.