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Fix NoClassDefFoundError: CommandProtocol$1 on Paper plugin classloader - #3673
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dmulloy2 merged 1 commit intoOct 6, 2026
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…ader When BukkitScheduler.runTask() dispatches the update-notify Runnable back onto the main thread, CraftTask.run() executes it under the server's classloader rather than ProtocolLib's PaperPluginClassLoader. Anonymous inner classes compiled as Outer\.class entries are not resolvable through the server classloader, causing: java.lang.NoClassDefFoundError: com/comphenix/protocol/CommandProtocol\ The fix follows the pattern already applied in commit dda284c for SpigotUpdateRunnable: extract the anonymous Runnable from performUpdate() into a named top-level package-private class (UpdateNotifyRunnable) so the runtime always loads a single .class entry through the plugin's own classloader.
dmulloy2
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Oct 6, 2026
dmulloy2
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October 6, 2026 22:00
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Problem
When ProtocolLib's update checker runs on Paper 1.19.3+ (and Purpur), the following exception is thrown repeatedly:
java.lang.NoClassDefFoundError: com/comphenix/protocol/CommandProtocol$1
at CommandProtocol.performUpdate(CommandProtocol.java:124)
at CommandProtocol.checkVersion(CommandProtocol.java:94)
at ProtocolLib.checkUpdates(ProtocolLib.java:524)
Caused by: java.lang.ClassNotFoundException: com.comphenix.protocol.CommandProtocol$1
at PaperPluginClassLoader.loadClass(...)
Root cause: performUpdate() creates an anonymous Runnable (compiled as
CommandProtocol$1.class) and registers it as an update listener.
SpigotUpdateRunnable dispatches it back to the main thread via
BukkitScheduler.runTask(). CraftTask.run() executes it under the server's
classloader instead of ProtocolLib's PaperPluginClassLoader — the anonymous
Outer$N class entry is not resolvable through the server classloader.
Fix
Extract the anonymous Runnable into a named top-level package-private class
UpdateNotifyRunnable — the same pattern applied in dda284c for
SpigotUpdateRunnable ("top-level class so the runtime always loads a single
.class entry"). CommandProtocol.java was not updated at that time.
No API changes. Behaviour is identical.