Warn server owners of plugins using deprecated events. Fixes BUKKIT-2027

By: Wesley Wolfe <weswolf@aol.com>

paper-api/src/main/java/org/bukkit/plugin/java/JavaPluginLoader.java

@@ -8,6 +8,7 @@ import java.lang.reflect.Constructor;
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
 import java.net.URL;
+import java.util.Arrays;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.LinkedHashMap;
@@ -30,6 +31,7 @@ import org.bukkit.event.EventHandler;
 import org.bukkit.event.Listener;
 import org.bukkit.event.server.PluginDisableEvent;
 import org.bukkit.event.server.PluginEnableEvent;
+import org.bukkit.plugin.AuthorNagException;
 import org.bukkit.plugin.EventExecutor;
 import org.bukkit.plugin.InvalidDescriptionException;
 import org.bukkit.plugin.InvalidPluginException;
@@ -293,6 +295,24 @@ public class JavaPluginLoader implements PluginLoader {
                 eventSet = new HashSet<RegisteredListener>();
                 ret.put(eventClass, eventSet);
             }
+
+            for (Class<?> clazz = eventClass; Event.class.isAssignableFrom(clazz); clazz = clazz.getSuperclass()) {
+                // This loop checks for extending deprecated events
+                if (clazz.getAnnotation(Deprecated.class) != null) {
+                    plugin.getLogger().log(
+                            Level.WARNING,
+                            String.format(
+                                    "\"%s\" has registered a listener for %s on method \"%s\", but the event is Deprecated." +
+                                    " Server performance will be affected; please notify the authors %s.",
+                                    plugin.getDescription().getFullName(),
+                                    clazz.getName(),
+                                    method.toGenericString(),
+                                    Arrays.toString(plugin.getDescription().getAuthors().toArray())),
+                            new AuthorNagException(null));
+                    break;
+                }
+            }
+
             EventExecutor executor = new EventExecutor() {
                 public void execute(Listener listener, Event event) throws EventException {
                     try {