Access Transformers

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 8:

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 MemberSignature # comment

A random example to understand the syntax:

public net.minecraft.util.palette.IResizeCallback # makes IResizeCallback public

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.util.palette.IResizeCallback # makes IResizeCallback 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)