Fix NoClassDefFoundError: CommandProtocol$1 on Paper plugin classloader

src/main/java/com/comphenix/protocol/CommandProtocol.java

@@ -120,25 +120,9 @@ private void performUpdate(final CommandSender sender, UpdateType type, final bo
             return;
         }
 
-        // Perform on an async thread
-        Runnable notify = new Runnable() {
-            @Override
-            public void run() {
-                if (command) {
-                    sender.sendMessage(ChatColor.YELLOW + "[ProtocolLib] " + updater.getResult());
-                    String remoteVersion = updater.getRemoteVersion();
-                    if (remoteVersion != null) {
-                        sender.sendMessage(ChatColor.YELLOW + "Remote version: " + remoteVersion);
-                        sender.sendMessage(ChatColor.YELLOW + "Current version: " + plugin.getDescription().getVersion());
-                    }
-                } else if (updater.shouldNotify() || config.isDebug()) {
-                    sender.sendMessage(ChatColor.YELLOW + "[ProtocolLib] " + updater.getResult());
-                }
-
-                updater.removeListener(this);
-                updateFinished();
-            }
-        };
+        // Top-level class instead of anonymous inner class so that PaperPluginClassLoader
+        // can resolve the .class entry when BukkitScheduler dispatches it on the main thread.
+        Runnable notify = new UpdateNotifyRunnable(sender, command, updater, config, plugin, this);
         updater.start(type);
         updater.addListener(notify);
     }

src/main/java/com/comphenix/protocol/UpdateNotifyRunnable.java

@@ -0,0 +1,69 @@
+/**
+ *  ProtocolLib - Bukkit server library that allows access to the Minecraft protocol.
+ *  Copyright (C) 2012 Kristian S. Stangeland
+ *
+ *  This program is free software; you can redistribute it and/or modify it under the terms of the
+ *  GNU General Public License as published by the Free Software Foundation; either version 2 of
+ *  the License, or (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
+ *  without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ *  See the GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License along with this program;
+ *  if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
+ *  02111-1307 USA
+ */
+package com.comphenix.protocol;
+
+import org.bukkit.ChatColor;
+import org.bukkit.command.CommandSender;
+import org.bukkit.plugin.Plugin;
+
+import com.comphenix.protocol.updater.Updater;
+
+/**
+ * Callback invoked on the main thread after an asynchronous update check completes.
+ * <p>
+ * Extracted as a top-level class (instead of an anonymous inner class) so that
+ * {@code PaperPluginClassLoader} can always resolve its {@code .class} entry when
+ * {@link org.bukkit.scheduler.BukkitScheduler#runTask} dispatches it back onto the
+ * main server thread — the same pattern used by {@code SpigotUpdateRunnable}.
+ */
+final class UpdateNotifyRunnable implements Runnable {
+
+    private final CommandSender sender;
+    private final boolean command;
+    private final Updater updater;
+    private final ProtocolConfig config;
+    private final Plugin plugin;
+    private final CommandProtocol owner;
+
+    UpdateNotifyRunnable(CommandSender sender, boolean command,
+                         Updater updater, ProtocolConfig config,
+                         Plugin plugin, CommandProtocol owner) {
+        this.sender = sender;
+        this.command = command;
+        this.updater = updater;
+        this.config = config;
+        this.plugin = plugin;
+        this.owner = owner;
+    }
+
+    @Override
+    public void run() {
+        if (command) {
+            sender.sendMessage(ChatColor.YELLOW + "[ProtocolLib] " + updater.getResult());
+            String remoteVersion = updater.getRemoteVersion();
+            if (remoteVersion != null) {
+                sender.sendMessage(ChatColor.YELLOW + "Remote version: " + remoteVersion);
+                sender.sendMessage(ChatColor.YELLOW + "Current version: " + plugin.getDescription().getVersion());
+            }
+        } else if (updater.shouldNotify() || config.isDebug()) {
+            sender.sendMessage(ChatColor.YELLOW + "[ProtocolLib] " + updater.getResult());
+        }
+
+        updater.removeListener(this);
+        owner.updateFinished();
+    }
+}