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.

About Forge Mod Loader

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.

Binary installation information

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 minecraft_server-@[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 .jar. It will identify the location of you minecraft installation (this can be customized) and create a new profile "FML" there.

Forge Installation

This code also ships as a part of Minecraft Forge. You do not need to install it separately from your Minecraft Forge installation. Minecraft Forge contains the exact same code as this. You should not install FML if you are also installing Minecraft Forge.

Source installation information for modders

This code follows the Minecraft Forge installation methodology. It will apply some small patches to the vanilla MCP source code, giving you and it access to some of the data and functions you need to build a successful mod.

Note also that the patches are built against "unrenamed" MCP source code (aka srgnames) - this means that you will not be able to read them directly against normal code.

Source pack installation information:

Standalone source installation

To install this source code for development purposes, extract this zip file. It ships with a demonstration mod. Run "gradle setupDevWorkspace" to create a gradle environment primed with FML. Run gradle eclipse or gradle idea to create an IDE workspace of your choice. Refer to ForgeGradle for more information about the gradle environment

Forge source installation

MinecraftForge ships with this code and installs it as part of the forge installation process, no further action is required on your part.

For reference this is version @MAJOR@.@MINOR@.@REV@.@BUILD@ of FML for Minecraft version @MCVERSION@.