A special mod for Project Zomboid, to allow loading of alternate mods written using the Java language. Since the game engine is written in Java, it opens up the possibility to implement new features that wouldn't be achievable through the usual Lua Modding API.
Download the zip file corresponding to your operating system. Extract the content to your ProjectZomboid install folder, something like C:\Program Files (x86)\Steam\steamapps\common\ProjectZomboid
Then create a custom Steam laucher for non-Steam games: