DataFixerUpper

DataFixerUpper, often abbreviated as DFU, is a library developed and used by Mojang for handling world upgrades and version changes in Java Edition. It provides utilities for data fixing, such as Functions with more inputs (3, 4... 16), and serialization, such as codecs, used by various elements across the codebase. The library helps developers update and migrate data entries such as ids and the format of how data is stored.

com.mojang.serialization — contains codec and helpers for codecs and serialization in general.

com.mojang.datafixers — contains the utilities used for data fixing.