How to serialize a class with non native fields in java with Gson?

Question

I am currently writing my own NPC class for a Spigot Plugin in Minecraft.

import com.mojang.authlib.GameProfile;
import com.mojang.authlib.properties.Property;
import net.minecraft.server.v1_16_R1.*;
import org.bukkit.Bukkit;
import org.bukkit.Location;
import org.bukkit.craftbukkit.v1_16_R1.CraftServer;
import org.bukkit.craftbukkit.v1_16_R1.CraftWorld;
import org.bukkit.craftbukkit.v1_16_R1.entity.CraftPlayer;
import org.bukkit.craftbukkit.v1_16_R1.util.CraftChatMessage;
import org.bukkit.entity.Player;
import org.bukkit.plugin.Plugin;
import org.bukkit.scheduler.BukkitRunnable;

import java.io.IOException;
import java.io.Serializable;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.util.ArrayList;
import java.util.List;
import java.util.UUID;

public class NPC extends EntityPlayer implements Serializable {

    private final int entityId;
    private Location location;
    private GameProfile gameprofile;

    private final List<Player> recipients;
    private Recipient recipient_type;

    private String display_name;
    private String tablist_name;
    private final DataWatcher dataWatcher;
    private final DataWatcherObject<Byte> object_entity_state;
    private final DataWatcherObject<String> object_customName;
    private final DataWatcherObject<Boolean> object_isSilent;
    private final DataWatcherObject<Boolean> object_hasGravity;
    private final DataWatcherObject<Boolean> object_isCustomNameVisible;

    private boolean isDestroyed;
    private final Plugin plugin;

    public NPC(String name, Location location, UUID uuid, WorldServer worldServer, Plugin plugin) {
        super(((CraftServer) Bukkit.getServer()).getServer(), worldServer, new GameProfile(uuid, name), new PlayerInteractManager(worldServer));
        this.plugin = plugin;
        this.display_name = name;
        this.tablist_name = name;
        this.recipient_type = Recipient.ALL;
        this.recipients = new ArrayList<>();
        this.entityId = (int) Math.ceil(Math.random() * 1000);
        this.gameprofile = new GameProfile(uuid, display_name);
        this.location = location;
        this.dataWatcher = new DataWatcher(null);
        this.dataWatcher.register(object_entity_state = new DataWatcherObject<>(0, DataWatcherRegistry.a), (byte) 0);
        this.dataWatcher.register(new DataWatcherObject<>(1, DataWatcherRegistry.b), 300);
        this.dataWatcher.register(object_customName = new DataWatcherObject<>(2, DataWatcherRegistry.d), "");
        this.dataWatcher.register(object_isCustomNameVisible = new DataWatcherObject<>(3, DataWatcherRegistry.i),false);
        this.dataWatcher.register(object_isSilent = new DataWatcherObject<>(4, DataWatcherRegistry.i), false);
        this.dataWatcher.register(object_hasGravity = new DataWatcherObject<>(5, DataWatcherRegistry.i), false);
        this.dataWatcher.register(new DataWatcherObject<>(9, DataWatcherRegistry.i), false);
        this.dataWatcher.register(new DataWatcherObject<>(6, DataWatcherRegistry.a), (byte) 0);
        this.dataWatcher.register(new DataWatcherObject<>(7, DataWatcherRegistry.c), 20.0F);
        this.dataWatcher.register(new DataWatcherObject<>(8, DataWatcherRegistry.b), 0);
        this.dataWatcher.register(new DataWatcherObject<>(10, DataWatcherRegistry.b), 0);
        this.dataWatcher.register(new DataWatcherObject<>(11, DataWatcherRegistry.c), 0.0F);
        this.dataWatcher.register(new DataWatcherObject<>(12, DataWatcherRegistry.b), 20);
        this.dataWatcher.register(new DataWatcherObject<>(13, DataWatcherRegistry.a), (byte) 127);
        this.dataWatcher.register(new DataWatcherObject<>(14, DataWatcherRegistry.a), (byte) 1);
        this.dataWatcher.register(new DataWatcherObject<>(15, DataWatcherRegistry.p), new NBTTagCompound());
        this.dataWatcher.register(new DataWatcherObject<>(16, DataWatcherRegistry.p), new NBTTagCompound());
    }

    public NPC(String name, Location location, Plugin plugin) {
        this(name, location, UUID.randomUUID(), ((CraftWorld) Bukkit.getWorlds().get(0)).getHandle(), plugin);
    }

    public NPC(String name, Location location, UUID uuid, Plugin plugin) {
        this(name, location, uuid, ((CraftWorld) Bukkit.getWorlds().get(0)).getHandle(), plugin);
    }

//    public Map<String, Object> serialize() {
//        Map<String, Object> result = new HashMap<>();
//        BeanInfo info = null;
//        try {
//            info = Introspector.getBeanInfo(this.getClass());
//        } catch (IntrospectionException e) {
//            e.printStackTrace();
//        }
//        for (PropertyDescriptor pd : info.getPropertyDescriptors()) {
//            Method reader = pd.getReadMethod();
//            if (reader != null) {
//                try {
//                    result.put(pd.getName(), reader.invoke(this));
//                } catch (IllegalAccessException | InvocationTargetException e) {
//                    e.printStackTrace();
//                }
//            }
//        }
//        return result;
//    }

    /*
     *  view all the packet receivers
     */
    public List<Player> getRecipients() {
        return this.recipients;
    }


    /*
     *  get the signature and skin from the gameprofile
     */
    public Property getSkin() {
        if (this.gameprofile.getProperties().isEmpty())
            return null;
        return (Property) this.gameprofile.getProperties().get("textures").toArray()[0];
    }

    /*
     * get npc id
     */
    public int getEntityId() {
        return entityId;
    }


    /*
     * get npc location
     */
    public Location getLocation() {
        return location;
    }


    /*
     * get npc gameprofile
     */
    public GameProfile getGameprofile() {
        return gameprofile;
    }

    /*
     * get all the packet receivers
     */
    public Recipient getRecipient_type() {
        return recipient_type;
    }


    /*
     * get npc displayname above head
     */
    public String getDisplay_name() {
        return display_name;
    }

    /*
     * get npc displayname in tablist
     */
    public String getTablist_name() {
        return tablist_name;
    }

    /*
     * check if npc is deleted
     */
    public boolean isDestroyed() {
        return isDestroyed;
    }

    /*
     * get plugin
     */
    public Plugin getPlugin() {
        return plugin;
    }


    /*
     *  add Player so it receives the update packets
     */
    public void addRecipient(Player p) {
        this.recipients.add(p);
    }


    /*
     *  Remove Player that receives the update packets
     */
    public void removeRecipient(Player p) {
        this.recipients.remove(p);
    }


    /*
     *  toggle if the packet receivers are all the players online or a specicif group.
     *  If set to ALL (online) then the set/get recipient will have no effect
     */
    public void setRecipientType(Recipient recipient_type) {
        this.recipient_type = recipient_type;
    }


    /*
     *  spawn the npc, this should be the last function after init.
     */
    public void spawn(boolean tablist, boolean fix_head) {
        PacketPlayOutNamedEntitySpawn packet = new PacketPlayOutNamedEntitySpawn();
        this.setField(packet, "a", this.entityId);
        this.setField(packet, "b", this.gameprofile.getId());
        this.setField(packet, "c", location.getX());
        this.setField(packet, "d", location.getY());
        this.setField(packet, "e", location.getZ());
        this.setField(packet, "f", fix_head ? (byte) ((int) location.getYaw() * 256.0F / 360.0F) : 0);
        this.setField(packet, "g", fix_head ? (byte) ((int) location.getPitch() * 256.0F / 360.0F) : 0);
//        this.setField(packet, "i", this.dataWatcher);
        this.addToTabList();
        this.sendPacket(packet);
        this.isDestroyed = false;


        //Delay is required to commit the tablist changes
        new BukkitRunnable() {

            @Override
            public void run() {
                if (!tablist) removeFromTabList();
            }
        }.runTaskLater(this.plugin, 5);

    }


    /*
     *  put items in inventory, see https://www.google.nl/search?q=bukkit+inventory+slots&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjr9v_FxvjaAhUFMuwKHQi7ALUQ_AUICigB&biw=1920&bih=974#imgrc=QUECAbUohgZxbM:
     *  for more info
     */
    public void setEquipment(EnumItemSlot slot, ItemStack item) {
        PacketPlayOutEntityEquipment packet = new PacketPlayOutEntityEquipment();
        this.setField(packet, "a", this.entityId);
        this.setField(packet, "b", slot);
        this.setField(packet, "c", item);
        this.sendPacket(packet);
    }


    /*
     *  set the name above the player
     */
    public void setDisplayNameAboveHead(String name) throws IOException {
        if(name.length() > 16) throw new IOException("Name cannot be longer than 16 chatacters.");
        this.display_name = name;
        this.reloadNpc();
    }


    /*
     *  set the name above the player and tablist.
     */
    public void setDisplayName(String name) throws IOException {
        this.setDisplayNameAboveHead(name);
        this.setTablistName(name);
    }


    /*
     *  set custom name in tablist
     */
    public void setTablistName(String name) {
        this.tablist_name = name;
        this.updateToTabList();
    }


    /*
     *  respawn the npc and refresh all comitted changes
     */
    public void reloadNpc() {
        this.updateProfile();
        if(!this.isDestroyed) {
            PacketPlayOutEntityDestroy packet = new PacketPlayOutEntityDestroy(this.entityId);
            this.sendPacket(packet);
            this.spawn(true, true);
        }
    }


    /*
     *  Update/Refresh the gameprofile that contains UUID, Name, Skin.
     */
    private void updateProfile() {
        Property skin = this.getSkin();
        this.gameprofile = new GameProfile(this.gameprofile.getId(), this.display_name);
        if (skin != null)
            this.setSkin(skin.getValue(), skin.getSignature());
    }


    /*
     *  set the texture and signature in the gameprofile, to submit it you must reload player.
     */
    public void setSkin(String texture, String signature) {
        this.gameprofile.getProperties().put("textures", new Property("textures", texture, signature));
    }



    /*
     *  remove npc from the recipient's tablist
     */
    public void removeFromTabList() {
        PacketPlayOutPlayerInfo packet = new PacketPlayOutPlayerInfo();
        PacketPlayOutPlayerInfo.PlayerInfoData data = packet.new PlayerInfoData(this.gameprofile, 0,
                EnumGamemode.NOT_SET, CraftChatMessage.fromString(tablist_name)[0]);
        List<PacketPlayOutPlayerInfo.PlayerInfoData> players = (List<PacketPlayOutPlayerInfo.PlayerInfoData>) getField(packet, "b");
        players.add(data);
        this.setField(packet, "a", PacketPlayOutPlayerInfo.EnumPlayerInfoAction.REMOVE_PLAYER);
        this.setField(packet, "b", players);
        this.sendPacket(packet);
    }

    /*
     *  update npc to the recipient's tablist
     */
    public void updateToTabList() {
        PacketPlayOutPlayerInfo packet = new PacketPlayOutPlayerInfo();
        PacketPlayOutPlayerInfo.PlayerInfoData data = packet.new PlayerInfoData(this.gameprofile, 0,
                EnumGamemode.NOT_SET, CraftChatMessage.fromString(tablist_name)[0]);
        List<PacketPlayOutPlayerInfo.PlayerInfoData> players = (List<PacketPlayOutPlayerInfo.PlayerInfoData>) getField(packet, "b");
        players.add(data);
        this.setField(packet, "a", PacketPlayOutPlayerInfo.EnumPlayerInfoAction.UPDATE_DISPLAY_NAME);
        this.setField(packet, "b", players);
        this.sendPacket(packet);
    }


    /*
     *  add npc from the recipient's tablist
     */
    public void addToTabList() {
        PacketPlayOutPlayerInfo packet = new PacketPlayOutPlayerInfo();
        PacketPlayOutPlayerInfo.PlayerInfoData data = packet.new PlayerInfoData(this.gameprofile, 0,
                EnumGamemode.NOT_SET, CraftChatMessage.fromString(tablist_name)[0]);
        List<PacketPlayOutPlayerInfo.PlayerInfoData> players = (List<PacketPlayOutPlayerInfo.PlayerInfoData>) getField(packet, "b");
        players.add(data);
        this.setField(packet, "a", PacketPlayOutPlayerInfo.EnumPlayerInfoAction.ADD_PLAYER);
        this.setField(packet, "b", players);
        this.sendPacket(packet);
    }


    /*
     * set player action, such as shift, onfire, ect recommending to use 'public void setAction(Action action)'.
     */
    public void setAction(byte action) {
        this.setMetaData(action);
    }


    /*
     * set player action, such as shift, onfire, ect.
     */
    public void setAction(Action action) {
        this.setMetaData(action.build());
    }


    /*
     * make sure the npc is near a bed or on it.
     */
//    public void setSleep(boolean state) {
//        if (state) {
//            Location bed = new Location(this.location.getWorld(), 0, 0, 0);
//            PacketPlayOutBed packet = new PacketPlayOutBed();
//            this.setField(packet, "a", this.entityId);
//            this.setField(packet, "b", new BlockPosition(bed.getX(), bed.getY(), bed.getZ()));
//
//            for (Player p : (this.recipient_type == Recipient.ALL ? Bukkit.getOnlinePlayers() : this.recipients)) {
//                ((CraftPlayer) p).sendBlockChange(bed, Material.BED_BLOCK, (byte) 0);
//                ((CraftPlayer) p).sendBlockChange(bed.add(0, 0, 1), Material.BED_BLOCK, (byte) 2);
//            }
//
//            this.sendPacket(packet);
//            this.teleport(location.clone().add(0, 0.3, 0), false);
//
//        } else {
//            this.setAnimation(NPCAnimation.LEAVE_BED);
//            this.teleport(location.clone().subtract(0, 0.3, 0), true);
//        }
//    }


    /*
     * delete the npc from the server.
     */
    public void destroy() {
        PacketPlayOutEntityDestroy packet = new PacketPlayOutEntityDestroy(this.entityId);
        this.removeFromTabList();
        this.sendPacket(packet);
        this.isDestroyed = true;
    }


    /*
     * set npc status such as die or hurt. I recommond to use method 'public void setStatus(NPCStatus status)'
     */
    public void setStatus(byte status) {
        PacketPlayOutEntityStatus packet = new PacketPlayOutEntityStatus();
        this.setField(packet, "a", this.entityId);
        this.setField(packet, "b", status);
        this.sendPacket(packet);
    }


    /*
     * set npc status such as die or hurt.
     */
    public void setStatus(NPCStatus status) {
        this.setStatus((byte) status.getId());
    }


    /*
     * set npc effect such as Night Vision or something else.
     */
    public void setEffect(MobEffect effect) {
        this.sendPacket(new PacketPlayOutEntityEffect(this.entityId, effect));
    }


    /*
     * set npc animation such as Swing arm ect. I recommend using method 'public void setAnimation(NPCAnimation animation)'
     */
    public void setAnimation(byte animation) {
        PacketPlayOutAnimation packet = new PacketPlayOutAnimation();
        this.setField(packet, "a", this.entityId);
        this.setField(packet, "b", animation);
        this.sendPacket(packet);
    }


    /*
     * set npc animation such as Swing arm ect.
     */
    public void setAnimation(NPCAnimation animation) {
        this.setAnimation((byte) animation.getId());
    }


    /*
     * teleport npc to different location
     */
    public void teleport(Location location, Boolean onGround) {
        PacketPlayOutEntityTeleport packet = new PacketPlayOutEntityTeleport();
        this.setField(packet, "a", this.entityId);
        this.setField(packet, "b", location.getX());
        this.setField(packet, "c", location.getY());
        this.setField(packet, "d", location.getZ());
        this.setField(packet, "e", (byte) location.getYaw());
        this.setField(packet, "f", (byte) location.getPitch());
        this.setField(packet, "g", onGround);
        this.sendPacket(packet);
        this.rotateHead(location.getPitch(), location.getYaw());
        this.location = location;
    }


    /*
     * rotate npc head to pitch and yaw.
     */
    public void rotateHead(float pitch, float yaw) {
        PacketPlayOutEntity.PacketPlayOutEntityLook packet = new PacketPlayOutEntity.PacketPlayOutEntityLook(this.entityId, getFixRotation(yaw), (byte) pitch, true);
        PacketPlayOutEntityHeadRotation packet_1 = new PacketPlayOutEntityHeadRotation();
        this.setField(packet_1, "a", this.entityId);
        this.setField(packet_1, "b", getFixRotation(yaw));
        this.sendPacket(packet);
        this.sendPacket(packet_1);
    }




    /*
     * These methods below are not usefull.
     */

    private <T> void setDataWatcherObject(DataWatcherObject<T> datawatcherobject, Object t0) {
        try {
            Method m = this.dataWatcher.getClass().getDeclaredMethod("registerObject", DataWatcherObject.class,
                    Object.class);
            m.setAccessible(true);
            m.invoke(this.dataWatcher, datawatcherobject, t0);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private void setMetaData(byte data) {
        this.setDataWatcherObject(this.object_entity_state, data);
        PacketPlayOutEntityMetadata packet = new PacketPlayOutEntityMetadata(this.entityId, this.dataWatcher, true);
        sendPacket(packet);
    }

    private byte getFixRotation(float yawpitch) {
        return (byte) ((int) (this.location.getYaw() * 256.0F / 360.0F));
    }

    private Object getField(Object obj, String field_name) {
        try {
            Field field = obj.getClass().getDeclaredField(field_name);
            field.setAccessible(true);
            return field.get(obj);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }

    private void setField(Object obj, String field_name, Object value) {
        try {
            Field field = obj.getClass().getDeclaredField(field_name);
            field.setAccessible(true);
            field.set(obj, value);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private void sendPacket(Packet<?> packet, Player player) {
        ((CraftPlayer) player).getHandle().playerConnection.sendPacket(packet);
    }

    private void sendPacket(Packet<?> packet) {
        for (Player p : (this.recipient_type == Recipient.ALL ? Bukkit.getOnlinePlayers() : this.recipients)) {
            this.sendPacket(packet, p);
        }

    }

    enum Recipient {
        ALL, LISTED_RECIPIENTS
    }

    enum NPCAnimation {

        SWING_MAIN_HAND(0),
        TAKE_DAMAGE(1),
        LEAVE_BED(2),
        SWING_OFFHAND(3),
        CRITICAL_EFFECT(4),
        MAGIC_CRITICAL_EFFECT(5);

        private final int id;

        NPCAnimation(int id) {
            this.id = id;
        }

        public int getId() {
            return id;
        }

    }

    enum NPCStatus {

        HURT(2), DIE(3);

        private final int id;

        NPCStatus(int id) {
            this.id = id;
        }

        public int getId() {
            return id;
        }
    }

    public static class Action {

        private boolean on_fire, crouched, sprinting, invisible, glowing, flying_elytra;
        private byte result = 0;

        public Action(boolean on_fire, boolean crouched, boolean sprinting, boolean invisible, boolean glowing,
                      boolean flying_elytra) {
            this.on_fire = on_fire;
            this.crouched = crouched;
            this.sprinting = sprinting;
            this.invisible = invisible;
            this.glowing = glowing;
            this.flying_elytra = flying_elytra;
        }

        public Action() {
        }

        public boolean isOn_fire() {
            return on_fire;
        }

        public Action setOn_fire(boolean on_fire) {
            this.on_fire = on_fire;
            return this;
        }

        public boolean isCrouched() {
            return crouched;
        }

        public Action setCrouched(boolean crouched) {
            this.crouched = crouched;
            return this;
        }

        public boolean isSprinting() {
            return sprinting;
        }

        public Action setSprinting(boolean sprinting) {
            this.sprinting = sprinting;
            return this;
        }

        public boolean isInvisible() {
            return invisible;
        }

        public Action setInvisible(boolean invisible) {
            this.invisible = invisible;
            return this;
        }

        public boolean isGlowing() {
            return glowing;
        }

        public Action setGlowing(boolean glowing) {
            this.glowing = glowing;
            return this;
        }

        public boolean isFlying_elytra() {
            return flying_elytra;
        }

        public Action setFlying_elytra(boolean flying_elytra) {
            this.flying_elytra = flying_elytra;
            return this;
        }

        public byte build() {
            result = 0;
            result = add(this.on_fire, (byte) 0x01);
            result = add(this.crouched, (byte) 0x02);
            result = add(this.sprinting, (byte) 0x08);
            result = add(this.invisible, (byte) 0x20);
            result = add(this.glowing, (byte) 0x40);
            result = add(this.flying_elytra, (byte) 0x80);
            return result;
        }

        private byte add(boolean condition, byte amount) {
            return result += (condition ? amount : 0x00);
        }
    }
}

I need this class to be serializable (preferably with Gson) to save the entities when the server shuts down. Currently I am doing this:

Gson gson = new Gson();
for (NPC npc : npcs) {
    System.out.println(gson.toJson(npc));
}

I am getting a Stack Overflow:

java.lang.StackOverflowError: null
        at com.google.gson.internal.$Gson$Types.resolve($Gson$Types.java:383) ~[patched_1.16.1.jar:git-Paper-44]
        at com.google.gson.internal.$Gson$Types.resolve($Gson$Types.java:378) ~[patched_1.16.1.jar:git-Paper-44]

The last two lines repeat a bunch of times until this comes:

at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.getBoundFields(ReflectiveTypeAdapterFactory.java:158) ~[patched_1.16.1.jar:git-Paper-44]
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory.create(ReflectiveTypeAdapterFactory.java:100) ~[patched_1.16.1.jar:git-Paper-44]
at com.google.gson.Gson.getAdapter(Gson.java:423) ~[patched_1.16.1.jar:git-Paper-44]

These lines also repeat. There is no output from the print statement, the NPC instances are intact and I can query their fields just fine.

I suppose Gson can not serialize the class properly because fields are not native and calls something recursively until the stack is fricked.

Can someone help me with this? :)

Answer 1

That can occur if an object A references an object B, which also references A. As an example using minecraft entities, this would look like:

PlayerA has a currentTarget field which references Creeper1, and Creeper1 has a lastDamageBy field of type Player referencing PlayerA. In this case, serializing Creeper1 will attempt to serialize PlayerA, which in turn will attempt to serialize Creeper1 again through it's currentTarget field. Eventually this will result in a StackOverflowException.

Look for this kind of cyclic references somewhere in your fields. Otherwise you can place a breakpoint in GSON code and check which exact field is causing the error.

Once you find the culprit, you can either remove that cyclic dependency, or use transient keyword or a Exclusion Strategy to define how or if deserialize/serialize those fields.

Answer 2

It could be caused by one of the fields that is non-serializable. Keep marking each as transient until you can successfully serialize the object, then you know which on is the culprit.