JFFL
by wertygh
JFFL is a Forge library for changing Minecraft and other mods' classes at startup, using annotations instead of low-level bytecode code.
What it does
JFFL is a bytecode manipulation library built on Javassist. Developers apply patches declaratively through annotations that inject code into classes at startup, so they do not need to write ASM or Javassist operations by hand. The library is aimed at modders who want mixin-style patching on Forge.
Its feature set covers callback handlers, mapping and remapping support, and several injection points. Callbacks can be registered dynamically without generating synthetic methods. Detailed logs are written while patches apply, and modified classes can be dumped to a .jffl-dump directory for inspection. Because it works at the source-level Javassist layer, the output is easier to debug than raw bytecode.
JFFL is released for Forge only, with builds covering 1.20.1 – 1.20.6. Installation covers the server and each player's game. Files cover six release versions, with 1.0.4 for 1.20.6 as the latest file, and the code is open source under the Apache License 2.0.
Key features
Declarative annotations
Patches are written as annotations, so the injection target and behavior are declared in one place.
Callback handlers
Callbacks can be registered at runtime, without generating synthetic methods in the patched class.
Several injection points
Multiple injection points let a patch run at different positions within a target method.
Dump and logging output
Detailed logs record the patching process, and modified classes can be written to a .jffl-dump directory for inspection.
Compatibility
Latest build for each game version and loader.
Only full game releases are shown. This mod publishes no snapshot builds.
Use cases
Who it’s for: JFFL is for mod developers who need to alter Minecraft or other mods' classes on Forge and prefer declarative annotations to writing ASM or Javassist code directly. Players who only install mods have no direct use for it.
How it compares
JFFL is presented as an alternative to Mixin rather than a replacement for it, and the author recommends against applying both to the same class because of possible conflicts. Among the listed alternatives, RainAPI is client and server on Forge for 1.20.1 under the MIT License and is not open source. Dye is Forge only for 1.17 – 1.20.1, a client or server optional mod, and open source under the Apache License 2.0.
Questions
Does JFFL require a specific Forge version?
Does JFFL need to be installed on a server?
Can JFFL be used together with Mixin?
In the catalog
JFFL is one of 3,514 Library mods on ModsFinder, and one of 1,576 Library mods for Forge. 11,154 mods support Minecraft 1.20.6.
Similar mods
What the author says
JFFL is a bytecode manipulation library for Minecraft Forge based on Javassist, providing Mixin-like declarative patch injection capabilities. It allows developers to modify classes from Minecraft and other mods at runtime using annotations, without the need to write low-level ASM or Javassist code directly. Issues…
Official listing
- Modrinth page
- modrinth.com/mod/jffl
- Owner
- wertygh
- Published
- 2026-05-02
- Updated on Modrinth
- 2026-06-18
- Facts verified
- 2026-10-15