Access Transformers

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Access Transformers are Forge's native way of allowing you to access (read and write) functions that have a lower-than-ideal visibility, and/or removing finality.

Visibility

Here's a quick rundown of the visibility options offered by Java 16:

• public visible to all classes inside and outside its package
• protected visible only to classes inside the package and subclasses
• default visible only to classes inside the package
• private visible only to inside the class

Additionally, there are final variants of each of these, each represented by the phrase "-f" on the end of the access specifier.

How It Works

As Forge is loading, it scans the jar file for the META-INF/accesstransformers.cfg file. if it's found, it is parsed according to the specification:

NewAccessSpecifier ClassSignature MemberSignature # comment

If you're targetting a class itself, you may omit the Member Signature.
However, it is required if you're targetting a method or field inside the class.
MemberSignatures use the Java specification for parts.
Importantly, classes are in the form Lpath/to/Class;, method descriptors are name(parameters)return and V is void.

A couple random examples to understand the syntax:

public net.minecraft.world.level.chunk.PaletteResize # makes the PaletteResize class public
protected net.minecraft.client.gui.Gui f_168667_ # COLOR_WHITE
public net.minecraft.client.renderer.GameRenderer m_109128_(Lnet/minecraft/resources/ResourceLocation;)V # loadEffect

If a valid entry is found on a line, Forge will look into the bytecode of the file where that member is defined, and change its access to whatever you desire it to be.

This startup modification means accessing is O(1) for all accesses after this initial change, which makes it a great option for performance if you're looking at something a lot

Removing Finality

Access transformers can also be used to remove the final keyword on code elements. To do this, append -f to the access specifier.

An example of this in place:

public-f net.minecraft.world.level.chunk.PaletteResize # makes PaletteResize public

Note: You can also use this to add a final keyword via +f. This is never recommended however, and should be avoided at all costs unless absolutely necessary.

Using Access Transformers in your mod

Important

You should avoid using ATs if a private variable / function you're looking at has a getter or a setter. Variables are usually private for a reason.

Step 1: Uncomment the access transformer line in your build.gradle

Step 2: Create a new file called "accesstransformer.cfg" in the src/main/resources/META-INF/ folder.

Step 3: Refresh the Gradle project. This can be done natively in IDEA and Eclipse.

When not to use Access Transformers

• Unnecessary access transformations (e.g. using ATs to make something public when it's already public)