RainAPI

RainAPI provides the infrastructure layer for dynamic class work: an embedded ECJ compiler turns source into bytecode at runtime, Javassist handles high-level bytecode edits, and ASM covers lower-level transformation. Together they let a mod compile and modify classes while the game is running, which the mod's own description positions as the groundwork for the upcoming RainJava framework.

A transformation bridge connects classes generated at runtime with the established bytecode modification frameworks, so they can be handled like any other class. A Mixin compatibility layer lets standard injection patterns recognize dynamically provisioned classes, and a Coremod integration places those class definitions in Forge's transformation chain. Sandboxed execution adds security boundaries for evaluating code at runtime.

RainJava requires RainAPI, so it is the dependency to install alongside RainJava. Its bundled tools are shaded and kept isolated so they do not clash with other mods, and a build with no outside dependencies is included. RainAPI runs on client and server, so both the server and every player need it. It is published for Forge only, for 1.20.1, under the MIT License, and it is not open source.