apollo/docs/guide/plugins/01-what-is-a-plugin.md

What is a plugin?

Plugins in Apollo are a mechanism for extending the server with new functionality without having to change any of the core Java code. Every plugin is a straight forward Kotlin project with .plugin.kts files containing the plugin extension behaviour and optional Kotlin source files containing generic reusable code.

Plugin framework architecture

Under the hood, the Kotlin compiler transofmrs every .plugin.kts file into a class file that extends from @"org.apollo.game.plugin.kotlin.KotlinPluginScript", giving it access to the game @"org.apollo.game.model.World" and @"org.apollo.game.message.handler.MessageHandlerChainSet" the plugin framework is initialized with.