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RainAPI

Core library for RainJava featuring embedded ECJ compiler, Javassist bytecode tools, and ASM transformation pipeline. Enables dynamic Java compilation and class modification at runtime.

Forge
Loaders
Forge
Downloads on Modrinth
463
Followers on Modrinth
2
versions
3
Updated
February 27, 2026Feb 27
Published
in 2026
This is a library — other mods need it, and it adds nothing to the game by itself

At a glance

A library: other mods need it
Runs on Forge
For Minecraft 1.20.1
Needed in the game and on the server
Latest version — 1.0.2 from February 27, 2026
License MIT — free
Where it's needed
In gamerequiredOn serverrequired

Needed both in the game and on the server

Game versions1.20.1

RainJava API

Advanced Runtime Class Transformation Framework

A foundational library providing Mixin and Coremod interoperability layers, powered by enterprise-grade bytecode and compilation tools.

Overview

RainJava API establishes the critical infrastructure for runtime class transformation pipelines. It serves as a middleware bridge between dynamic compilation workflows and Minecraft's standard modification systems (Mixin & Coremod), ensuring bytecode-level compatibility without compromising game stability.

While designed as the prerequisite foundation for the upcoming RainJava ecosystem, this API functions independently as a robust toolkit for developers requiring deep class manipulation capabilities.

Embedded Libraries

This mod ships with zero-external-dependency versions of industry-standard tools:

  • Javassist — High-level bytecode manipulation library for runtime class engineering

  • ECJ (Eclipse Compiler for Java) — Full-featured Java compiler capable of runtime source-to-bytecode transformation

  • JavaParser — Abstract Syntax Tree (AST) analysis and code generation framework

These libraries are shaded and isolated to prevent conflicts with other mods.

Technical Scope

  • Transformation Bridge — Facilitates communication between runtime-generated class definitions and established bytecode modification frameworks

  • Mixin Compatibility Layer — Enables standardized injection patterns to recognize and process dynamically provisioned classes

  • Coremod Integration — Provides stable hooks for runtime class definitions to participate in Forge's transformation chain

  • Sandboxed Execution — Implements security boundaries for runtime code evaluation environments

For Modders

Install this API if you:

  • Plan to develop for or use the upcoming RainJava framework

  • Require embedded Javassist/ECJ/JavaParser without bundling them yourself

  • Need stable Mixin/Coremod interoperability for advanced bytecode operations

FAQ

How to install RainAPI?

Install Forge for your Minecraft version, download RainAPI for the same version and loader, put the file into the mods folder and launch the game with the loader profile.

Which Minecraft versions does RainAPI support?

RainAPI has builds for Minecraft 1.20.1.

Is RainAPI needed on the server?

Needed in the game and on the server

License and source
MIT Licensefree to use and share with credit to the author
Original — Modrinth↗Files unchanged, verified with SHA-512Is this your mod? For rights holders

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