Project Zomboid: Note on Java Mods and Modding

Project Zomboid has a large and varied modding community, with everything from small system tweaks to full overhauls. The Indie Stone thanks the modders who make and maintain them, big or small.

Recently, some mods have changed how Project Zomboid plays altogether. They can do that because they are written in Java, the game’s core coding language. With Java-based loaders and frameworks, modders can alter the game at a fundamental level.

That freedom carries risks, both for your game and potentially for your computer.

Download Mods Responsibly

Only download and run mods from sources and authors you trust. That applies to every mod, whatever language it is written in.

Java mods usually need manual installation. They may ask you to download files from external websites or install a special loader. In general, they want more from you than a click on Subscribe in the Steam Workshop.

Java is also the language of Project Zomboid itself, so Java mods can change the game’s code directly. That makes radical changes possible. It also means a Java mod could be used to create security vulnerabilities.

Lua mods stay inside the confines of the game’s code. The Indie Stone offers a comparison. A Lua mod repaints your house and adds new wallpaper. A Java mod builds a new house nearby, possibly without you knowing, and that house could have an unseen back door.

Most Java mods are not necessarily malicious, and installing one does not automatically put your computer at risk. Still, Java mods call for a different level of trust. You are letting third-party code run on your machine, which goes beyond adding game content.

The studio strongly encourages you to look for mods that are open source or allow reproducible builds, meaning anyone can reproduce the code and get the same mod. Either one adds transparency, but neither guarantees a mod is safe. For the same reason, The Indie Stone recommends that Java modders release open source mods or allow their builds to be reproduced.

The Steam Workshop and similar distribution platforms have systems to flag and report mods. Popular mods have usually been installed by many community members, which is reassuring. It is still no guarantee of safety.

If you install mods from other sources, check whether the source code is public and whether the distributed files can be independently verified. If neither is true, be very careful.

Understanding our Modding support

The Indie Stone officially supports Lua-based modding for Project Zomboid. Java-based mods are not officially supported. That includes third-party loaders, frameworks, and any modification that alters the game’s underlying implementation.

Project Zomboid is written in Java, but its internal code is not a supported modding interface. Any mod in any language can break after a game update. Java mods are more exposed because they rely on internal code that keeps changing as the game develops. When new updates ship, Java-based mods will be broken by them, without warning.

The studio needs to keep its resources on the core game. For Java-based mods, that means it cannot do the following:

• Technical support: The team will not help with technical problems involving Java mods.
• Compatibility fixes: The team will not resolve conflicts between Java mods and other mods.
• Development time: The team will not dedicate development resources to Java mods.

Bug reports are investigated only when the bug appears in an un-modded game save. If you hit a problem with any mod, Java or Lua, reproduce it without mods installed before you submit a report. Reports that cannot be reproduced without mods will not be considered for investigation. Even a reproducible report does not guarantee an investigation or a fix.

The policy is not a judgement on individual mods, their creators, or players who use them. The Indie Stone says it needs the policy to test, support, and maintain Project Zomboid at a reasonable level.

Enjoy Modding, with the above in mind

The Indie Stone wants the community to keep experimenting, creating, and finding new ways to play, and it appreciates the effort that goes into making mods.

If you use Java-based mods, the studio asks that you understand the risks. Those mods may break after game updates, and the studio’s support for them is limited.

If you are thinking about making a mod or want more detail, read the Modding Policy.

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