Register and unregisterable listeners in Bukkit.

EventUtils.java

package co.ryred.util.event;

import org.bukkit.event.Event;
import org.bukkit.event.HandlerList;
import org.bukkit.plugin.IllegalPluginAccessException;
import org.bukkit.plugin.Plugin;
import org.bukkit.plugin.RegisteredListener;

import java.lang.reflect.Method;
import java.util.Collection;
import java.util.Map;
import java.util.Set;

/**
 * Created by Cory Redmond on 07/02/2017.
 * [email protected]
 */
public class EventUtils {

    public static void registerEvents(UnregisterableListener listener, Plugin plugin) {
        if (!plugin.isEnabled()) {
            throw new IllegalPluginAccessException("Plugin attempted to register " + listener + " while not enabled");
        }

        listener.setEventHandlers(plugin.getPluginLoader().createRegisteredListeners(listener, plugin));
        if( listener.getEventHandlers() == null ) return;

        for (Map.Entry<Class<? extends Event>, Set<RegisteredListener>> entry : listener.getEventHandlers().entrySet()) {
            getEventListeners(getRegistrationClass(entry.getKey())).registerAll(entry.getValue());
        }

    }

    public static void unregisterEvents(UnregisterableListener listener, Plugin plugin) {
        if (!plugin.isEnabled()) {
            throw new IllegalPluginAccessException("Plugin attempted to register " + listener + " while not enabled");
        }

        if( listener.getEventHandlers() == null ) return;

        for (Map.Entry<Class<? extends Event>, Set<RegisteredListener>> entry : listener.getEventHandlers().entrySet()) {
            unregisterAll(getEventListeners(getRegistrationClass(entry.getKey())), entry.getValue());
        }

        listener.setEventHandlers(null);

    }

    private static void unregisterAll(HandlerList handlerList, Collection<RegisteredListener> registeredListeners) {
        registeredListeners.forEach(handlerList::unregister);
    }

    /**
     * Stolen from Bukkit.
     */
    private static HandlerList getEventListeners(Class<? extends Event> type) {
        try {
            Method method = getRegistrationClass(type).getDeclaredMethod("getHandlerList");
            method.setAccessible(true);
            return (HandlerList) method.invoke(null);
        } catch (Exception e) {
            throw new IllegalPluginAccessException(e.toString());
        }
    }

    /**
     * Stolen from Bukkit.
     */
    private static Class<? extends Event> getRegistrationClass(Class<? extends Event> clazz) {
        try {
            clazz.getDeclaredMethod("getHandlerList");
            return clazz;
        } catch (NoSuchMethodException e) {
            if (clazz.getSuperclass() != null
                    && !clazz.getSuperclass().equals(Event.class)
                    && Event.class.isAssignableFrom(clazz.getSuperclass())) {
                return getRegistrationClass(clazz.getSuperclass().asSubclass(Event.class));
            } else {
                throw new IllegalPluginAccessException("Unable to find handler list for event " + clazz.getName());
            }
        }
    }

}

UnregisterableListener.java

package co.ryred.util.event;

import lombok.AccessLevel;
import lombok.Getter;
import lombok.Setter;
import org.bukkit.event.Event;
import org.bukkit.event.Listener;
import org.bukkit.plugin.RegisteredListener;

import java.util.Map;
import java.util.Set;

/**
 * Created by Cory Redmond on 07/02/2017.
 * [email protected]
 */
public abstract class UnregisterableListener implements Listener {

    @Setter(AccessLevel.PACKAGE)// Lombok because it's godlike.
    @Getter(AccessLevel.PACKAGE)// Aww you aren't using Lombok.. Time for change.
    private transient Map<Class<? extends Event>, Set<RegisteredListener>> eventHandlers = null;

}