ThreadWard

A small, surgical fix for a real parallel mod-loading crash.

What it does

ThreadWard wraps the backing maps of EntityRenderers and ItemProperties in thread-safe implementations via Mixin. This resolves a race condition that occurs when mods register renderers or item property overrides in parallel during FMLClientSetupEvent — which, in large modpacks (1000+ mods), can throw an intermittent ArrayIndexOutOfBoundsException and crash the game on startup.

If you've ever had your modpack crash on a different mod every time you launch it, with an ArrayIndexOutOfBoundsException somewhere inside EntityRenderers or ItemProperties, this is almost certainly the fix you need.

Why this exists

NeoForge dispatches FMLClientSetupEvent to mods in parallel across multiple threads for faster startup. Most of the time this is safe — but a handful of internal Minecraft/NeoForge collections were never designed to be written to concurrently. With enough mods registering renderers/item properties at the same time, you eventually lose the race and crash. The usual workaround is forcing ForkJoinPool.common.parallelism=1, which works but throws away the performance benefit of parallel loading entirely.

ThreadWard fixes the actual unsafe collections instead, so parallel mod loading stays fast and safe.

Compatibility

• Minecraft: 1.21.1
• Mod Loader: NeoForge (built and tested against 21.1.236)
• Dependencies: None
• Side: Client
• Should be compatible with essentially any modpack — this mod does not add content, change gameplay, or touch anything outside the two specific collections it patches.

Installation

Drop the jar in your mods folder. That's it — no configuration, nothing to set up. It works silently in the background.

FAQ

Will this conflict with anything? Very unlikely. ThreadWard only replaces the internal storage of two specific vanilla/NeoForge registries with thread-safe equivalents — it doesn't remove, override, or change any mod's actual content or behavior.

Do I need this if my pack isn't crashing? No harm in having it installed either way, but if you're not seeing intermittent ArrayIndexOutOfBoundsException crashes tied to EntityRenderers or ItemProperties, you probably don't need it.

Can I use this in my modpack? Yes — licensed under MIT, use it freely in any modpack, public or private.

Credits

Built to fix a real crash encountered in a 1300+ mod kitchen-sink modpack. Sometimes the best fixes are the small, targeted ones.