NoClassDefFoundError
The server throws NoClassDefFoundError (or ClassNotFoundException). At runtime, Java needed a class that is not on the classpath, typically a missing library or dependency that a plugin or mod relies on.
What does this error mean?
These errors mean a required class could not be found when it was needed. The code that uses it loaded fine, but its dependency is absent or incomplete, often a library that was never installed, a partial jar, or a version that no longer provides the class.
java.lang.NoClassDefFoundError / ClassNotFoundException
Most Common Causes
- A required library/dependency not installed.
- A plugin/mod missing a bundled or external dependency.
- A partial or corrupted jar lacking some classes.
- A version mismatch where a class was removed/renamed.
- A library present at the wrong version.
How To Diagnose
- Read the stack trace for the missing class and the plugin/mod that needed it.
- Identify which library provides that class.
- Check whether that dependency is installed and at the right version.
- Confirm the jar is complete (re-download if unsure).
Recommended Fixes
- Install the dependency
Add the missing library/dependency the plugin or mod requires. - Re-download the jar
Replace a partial/corrupt jar with a complete official download. - Match versions
Use dependency versions that still provide the referenced class. - Check the mod/plugin page
Install every required library listed by the mod or plugin.
Frequently Asked Questions
Both mean a class was missing at runtime. The cause and fix, a missing/incomplete dependency, are the same.
The stack trace names the missing class, look up which library provides it and install that.
A version change may have removed the class, or a dependency went missing, realign the versions.