This is Forge Mod Loader, or FML for short, by cpw.
More information can be found at https://github.com/MinecraftForge/FML/wiki
It is a clean reimplementation of a mod loading system for client and server.
It can be installed on its own, or as part of Minecraft Forge.
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About Forge Mod Loader
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Environments and compatibility
==============================
FML covers two main environments: client and server. All environments share
the main mod loading code base, with additional varying hooks based on the
specific environment.
There are some very visible changes to the client when FML is installed.
There are some bukkit compatibility hooks available, consult MCPC+, now known as Cauldron
for more information.
Minecraft Forge in all cases bundles FML as it's modloading technology of choice
because FML is open source, freely distributable, and can be easily updated by
contributors through github.
Notable integrations
====================
Optifine has FML compatibility. It varies from Optifine release to release, however
in general it will work well alongside an FML or
Minecraft Forge installation. FML will detect and ensure the good operation of
Optifine (you can see it in your client as an additional data line on the
bottom left).
Mod information
===============
FML exposes the mod information through a mod list visible on the main screen as
well as some small branding enhancements. For full data mods need to provide an
information file. This file is a standard format so hopefully tools providing
launch capabilities can also leverage this content.
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Binary installation information
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If you have downloaded a binary zip file you can install it as follows (client
or server):
Installation
============
To install on a server, simply execute the FML or Forge jar file, with a copy of
[email protected][email protected] placed in the same directory. FML will launch
it's patched copy.
To install on a client, run the installer by executing java -jar