Error Codes › java.lang.ClassNotFoundException

java.lang.ClassNotFoundException

COAL-010 · Class not found (ClassNotFoundException)

The same underlying problem as NoClassDefFoundError - a missing or version-mismatched mod or library - reported through a different Java exception.

What it looks like in the log

Caused by: java.lang.ClassNotFoundException: com.example.somelib.SomeClass
	at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:641)

What it means

A class was requested by name and the loader could not find it. For server troubleshooting this is the same situation as COAL-009 - the difference is only in how the lookup was made, and it very often appears as the Caused by: line underneath one.

How to fix it

Fix it as you would COAL-009: work out what the missing class belongs to, then install the mod or library that provides it, at a version matching your Minecraft version and loader.

A package under net/minecraft/client or org/lwjgl: a client-only mod is on the server.
A recognisable library name: install that library mod.
A mod you just updated: it likely needs a newer version of one of its dependencies.