# OrderedShutdownHooks.java

    /*
    ** Module   : OrderedShutdownHooks.java
    ** Abstract : Manages registered shutdown hooks in a prioritized order during JVM shutdown.
    **
    ** Copyright (c) 2024-2025, Golden Code Development Corporation.
    **
    ** -#- -I- --Date-- ---------------------------------Description----------------------------------
    ** 001 ICP 20240722 First version.
    ** 002 GBB 20250403 Refactoring, removing misleadingly named method getInstance.
    */
    /*
    ** This program is free software: you can redistribute it and/or modify
    ** it under the terms of the GNU Affero General Public License as
    ** published by the Free Software Foundation, either version 3 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 Affero General Public License for more details.
    **
    ** You may find a copy of the GNU Affero GPL version 3 at the following
    ** location: https://www.gnu.org/licenses/agpl-3.0.en.html
    **
    ** Additional terms under GNU Affero GPL version 3 section 7:
    **
    **   Under Section 7 of the GNU Affero GPL version 3, the following additional
    **   terms apply to the works covered under the License.  These additional terms
    **   are non-permissive additional terms allowed under Section 7 of the GNU
    **   Affero GPL version 3 and may not be removed by you.
    **
    **   0. Attribution Requirement.
    **
    **     You must preserve all legal notices or author attributions in the covered
    **     work or Appropriate Legal Notices displayed by works containing the covered
    **     work.  You may not remove from the covered work any author or developer
    **     credit already included within the covered work.
    **
    **   1. No License To Use Trademarks.
    **
    **     This license does not grant any license or rights to use the trademarks
    **     Golden Code, FWD, any Golden Code or FWD logo, or any other trademarks
    **     of Golden Code Development Corporation. You are not authorized to use the
    **     name Golden Code, FWD, or the names of any author or contributor, for
    **     publicity purposes without written authorization.
    **
    **   2. No Misrepresentation of Affiliation.
    **
    **     You may not represent yourself as Golden Code Development Corporation or FWD.
    **
    **     You may not represent yourself for publicity purposes as associated with
    **     Golden Code Development Corporation, FWD, or any author or contributor to
    **     the covered work, without written authorization.
    **
    **   3. No Misrepresentation of Source or Origin.
    **
    **     You may not represent the covered work as solely your work.  All modified
    **     versions of the covered work must be marked in a reasonable way to make it
    **     clear that the modified work is not originating from Golden Code Development
    **     Corporation or FWD.  All modified versions must contain the notices of
    **     attribution required in this license.
    */
    package com.goldencode.p2j.main;

    import com.goldencode.p2j.util.logging.CentralLogger;

    import java.util.*;
    /**
     * This class ensures that registered hooks are run in a specified order of priority when the JVM shuts down.
     * It uses a singleton pattern to maintain a single instance and a list of hooks that are sorted and executed
     * based on their priority.
     * The shutdown hooks are executed automatically when the JVM shuts down.
     */
    public class OrderedShutdownHooks
    {
        /** Logger. */
        private static final CentralLogger LOG = CentralLogger.get(OrderedShutdownHooks.class);

        /** Singleton instance */
        private static Thread shutdownHook = null;

        /** List of hooks to be handled at server shutdown */
        private static final List<ShutdownHook> shutdownHooks = new ArrayList<>();

        /**
         * Class representing a shutdown hook. Each shutdown hook has a priority level
         * that determines the order of execution, with lower priority values executed first.
         * It also includes a {@link Runnable} action that defines the task to be performed during shutdown.
         */
        private static class ShutdownHook
        {
            /** The priority level of the shutdown hook */
            int priority;

            /** The action to be executed during shutdown */
            Runnable action;

            /**
             * Constructs a new ShutdownHook with the specified priority level and action.
             *
             * @param   priority
             *          The priority level of the shutdown hook.
             * @param   action
             *          The action to be executed during shutdown.
             */
            ShutdownHook(int priority, Runnable action)
            {
                this.priority = priority;
                this.action = action;
            }

            /**
             * Compare two instances.
             *
             * @param    obj
             *           An object to compare with.
             *
             * @return   <code>true</code> if the two are identical.
             */
            @Override
            public boolean equals(Object obj)
            {
                if (this == obj) return true;
                if (obj == null || getClass() != obj.getClass())
                {
                    return false;
                }
                ShutdownHook that = (ShutdownHook) obj;
                return priority == that.priority && Objects.equals(action, that.action);
            }

            /**
             * Calculate hash code
             *
             * @return hash code
             */
            @Override
            public int hashCode()
            {
                return Objects.hash(priority, action);
            }
        }

        /**
         * Private constructor to prevent instantiation from outside the class.
         */
        private OrderedShutdownHooks()
        {
        }

        /**
         * Registers a new shutdown hook with a specified priority.
         * The hooks will be executed in ascending order of their priority values.
         *
         * @param   priority
         *          The priority of the shutdown hook.
         * @param   action
         *          The {@link Runnable} action to be executed during shutdown.
         */
        public static synchronized void registerShutdownHook(int priority, Runnable action)
        {
            if (shutdownHook == null)
            {
                shutdownHook = new Thread(OrderedShutdownHooks::executeShutdownHooks);
                Runtime.getRuntime().addShutdownHook(shutdownHook);
            }
            shutdownHooks.add(new ShutdownHook(priority, action));
        }

        /**
         * Deregisters a shutdown hook with a specified priority.
         *
         * @param   priority
         *          The priority of the shutdown hook.
         * @param   action
         *          The {@link Runnable} action to be executed during shutdown.
         */
        public static synchronized void deregisterShutdownHook(int priority, Runnable action)
        {
            shutdownHooks.remove(new ShutdownHook(priority, action));
        }

        /**
         * Executes all registered shutdown hooks in the order of their priority.
         * Hooks with lower priority values are executed first.
         */
        private static synchronized void executeShutdownHooks()
        {
            shutdownHooks.stream()
                    .sorted(Comparator.comparingInt(hook -> hook.priority))
                    .forEach(hook ->
                    {
                        try
                        {
                            hook.action.run();
                        }
                        catch (Exception e)
                        {
                            LOG.severe("Error while running shutdown hook.", e);
                        }
                    });
        }
    }