ConcurrentModificaitionException Spawning Entity and Strange Classloader Issue

Drosophila · December 13, 2021

Mappings: official mojang mappings, version 1.16.5

Forge version: 1.16.5-36.2.0

I've been trying to replace vanilla zombies with a custom class to play around with their AI. I've experienced a couple strange issues so far:

First, simply trying to replace zombies that spawn with a new zombie produces a strange classpath error. I have the following utility functions:

Spoiler

private final Map<CreatureEntity, BlockPos> spawnQ = new HashMap<>();

private synchronized boolean addZombieIfReady(Map.Entry<CreatureEntity, BlockPos> creatureEntityBlockPosEntry) {
        CreatureEntity zombie = creatureEntityBlockPosEntry.getKey();
        BlockPos blockPos = creatureEntityBlockPosEntry.getValue();
        World world = zombie.level;
        if (world.isLoaded(blockPos)) {
            try {
                LOGGER.info("Attempting to add zombie");
                world.addFreshEntity(zombie);
            } catch (Exception e) {
                LOGGER.error("Could not spawn zombie: " + e.getMessage());
            }
            return true;
        } else {
            return false;
        }
    }

    public boolean addZombieIfReady(CreatureEntity zombie) {
        return addZombieIfReady(new AbstractMap.SimpleImmutableEntry<>(
                zombie,
                new BlockPos(zombie.getPosition(1.0f))
        ));
    }

I wrote them to avoid attempting to spawn in entities in chunks that aren't finished loading. Then I use the following logic to override vanilla spawn behavior:

Spoiler

@SubscribeEvent(priority = EventPriority.HIGHEST)
    public void replaceVanillaZombies(EntityJoinWorldEvent event) {
        if (!event.getWorld().isClientSide()) {
            Entity entity = event.getEntity();

            if (entity instanceof ZombieEntity) {
                ZombieEntity zombie = (ZombieEntity) entity;
                /* EntitySmartZombie smartZombie = new EntitySmartZombie(event.getWorld()); */
                CreatureEntity smartZombie = new ZombieEntity(EntityType.ZOMBIE, event.getWorld());
                smartZombie.setPos(zombie.getX(), zombie.getY(), zombie.getZ());
                smartZombie.setYHeadRot(zombie.getYHeadRot());
                if (!addZombieIfReady(smartZombie)) {
                    spawnQ.put(smartZombie, new BlockPos(
                            smartZombie.getPosition(1.0f)
                    ));
                }
                event.setCanceled(true);
            }
        }
    }

    @SubscribeEvent(priority = EventPriority.NORMAL)
    public void spawnInLoadedChunks(TickEvent.WorldTickEvent tickEvent) {
        if (tickEvent.world.isClientSide()) return;
        spawnQ.entrySet().removeIf(this::addZombieIfReady);
    }

Here is the tail of the log file when the client crashes:

Spoiler

[13Dec2021 12:46:27.790] [Server thread/INFO] [com.noone.smartzombies.SmartZombies/]: Attempting to add zombie
[13Dec2021 12:46:27.790] [Server thread/INFO] [com.noone.smartzombies.SmartZombies/]: Attempting to add zombie
[13Dec2021 12:46:27.790] [Server thread/INFO] [com.noone.smartzombies.SmartZombies/]: Attempting to add zombie
[13Dec2021 12:46:27.790] [Server thread/INFO] [com.noone.smartzombies.SmartZombies/]: Attempting to add zombie

[about 1000 more of these]

[13Dec2021 12:46:27.790] [Server thread/INFO] [com.noone.smartzombies.SmartZombies/]: Attempting to add zombie
[13Dec2021 12:46:28.262] [Server thread/FATAL] [net.minecraftforge.common.ForgeMod/]: Preparing crash report with UUID 6701b212-ea13-43ca-aa8c-7383b5e2742d
[13Dec2021 12:46:28.266] [Server thread/ERROR] [net.minecraft.server.MinecraftServer/]: Encountered an unexpected exception
net.minecraft.crash.ReportedException: Exception ticking world
   at net.minecraft.server.MinecraftServer.tickChildren(MinecraftServer.java:855) ~[forge:?]
   at net.minecraft.server.MinecraftServer.tickServer(MinecraftServer.java:787) ~[forge:?]
   at net.minecraft.server.integrated.IntegratedServer.tickServer(IntegratedServer.java:78) ~[forge:?]
   at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:642) ~[forge:?]
   at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:232) ~[forge:?]
   at java.lang.Thread.run(Thread.java:748) [?:1.8.0_292]
Caused by: java.lang.BootstrapMethodError: java.lang.LinkageError: loader constraint violation: loader (instance of sun/misc/Launcher$AppClassLoader) previously initiated loading for a different type with name "org/apache/logging/log4j/util/MessageSupplier"
   at net.minecraftforge.eventbus.EventBus.handleException(EventBus.java:316) ~[eventbus-4.0.0.jar:?]
   at net.minecraftforge.eventbus.EventBus.post(EventBus.java:307) ~[eventbus-4.0.0.jar:?]
   at net.minecraftforge.eventbus.EventBus.post(EventBus.java:283) ~[eventbus-4.0.0.jar:?]
   at net.minecraft.world.server.ServerWorld.addEntity(ServerWorld.java:813) ~[forge:?]
   at net.minecraft.world.server.ServerWorld.addFreshEntity(ServerWorld.java:754) ~[forge:?]
   at java.util.stream.Streams$StreamBuilderImpl.forEachRemaining(Streams.java:419) ~[?:1.8.0_292]
   at java.util.stream.Streams$ConcatSpliterator.forEachRemaining(Streams.java:742) ~[?:1.8.0_292]
   at java.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:647) ~[?:1.8.0_292]
   at net.minecraft.world.IServerWorld.addFreshEntityWithPassengers(IServerWorld.java:10) ~[forge:?]
   at net.minecraft.world.spawner.WorldEntitySpawner.spawnCategoryForPosition(WorldEntitySpawner.java:178) ~[forge:?]
   at net.minecraft.world.spawner.WorldEntitySpawner.spawnCategoryForChunk(WorldEntitySpawner.java:124) ~[forge:?]
   at net.minecraft.world.spawner.WorldEntitySpawner.spawnForChunk(WorldEntitySpawner.java:110) ~[forge:?]
   at net.minecraft.world.server.ServerChunkProvider.lambda$tickChunks$5(ServerChunkProvider.java:364) ~[forge:?]
   at java.util.ArrayList.forEach(ArrayList.java:1259) ~[?:1.8.0_292]
   at net.minecraft.world.server.ServerChunkProvider.tickChunks(ServerChunkProvider.java:351) ~[forge:?]
   at net.minecraft.world.server.ServerChunkProvider.tick(ServerChunkProvider.java:326) ~[forge:?]
   at net.minecraft.world.server.ServerWorld.tick(ServerWorld.java:333) ~[forge:?]
   at net.minecraft.server.MinecraftServer.tickChildren(MinecraftServer.java:851) ~[forge:?]
   ... 5 more
Caused by: java.lang.LinkageError: loader constraint violation: loader (instance of sun/misc/Launcher$AppClassLoader) previously initiated loading for a different type with name "org/apache/logging/log4j/util/MessageSupplier"
   at java.lang.ClassLoader.defineClass1(Native Method) ~[?:1.8.0_292]
   at java.lang.ClassLoader.defineClass(ClassLoader.java:756) ~[?:1.8.0_292]
   at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142) ~[?:1.8.0_292]
   at java.net.URLClassLoader.defineClass(URLClassLoader.java:468) ~[?:1.8.0_292]
   at java.net.URLClassLoader.access$100(URLClassLoader.java:74) ~[?:1.8.0_292]
   at java.net.URLClassLoader$1.run(URLClassLoader.java:369) ~[?:1.8.0_292]
   at java.net.URLClassLoader$1.run(URLClassLoader.java:363) ~[?:1.8.0_292]
   at java.security.AccessController.doPrivileged(Native Method) ~[?:1.8.0_292]
   at java.net.URLClassLoader.findClass(URLClassLoader.java:362) ~[?:1.8.0_292]
   at java.lang.ClassLoader.loadClass(ClassLoader.java:418) ~[?:1.8.0_292]
   at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:352) ~[?:1.8.0_292]
   at java.lang.ClassLoader.loadClass(ClassLoader.java:351) ~[?:1.8.0_292]
   at net.minecraftforge.eventbus.EventBus.handleException(EventBus.java:316) ~[eventbus-4.0.0.jar:?]
   at net.minecraftforge.eventbus.EventBus.post(EventBus.java:307) ~[eventbus-4.0.0.jar:?]
   at net.minecraftforge.eventbus.EventBus.post(EventBus.java:283) ~[eventbus-4.0.0.jar:?]
   at net.minecraft.world.server.ServerWorld.addEntity(ServerWorld.java:813) ~[forge:?]
   at net.minecraft.world.server.ServerWorld.addFreshEntity(ServerWorld.java:754) ~[forge:?]
   at java.util.stream.Streams$StreamBuilderImpl.forEachRemaining(Streams.java:419) ~[?:1.8.0_292]
   at java.util.stream.Streams$ConcatSpliterator.forEachRemaining(Streams.java:742) ~[?:1.8.0_292]
   at java.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:647) ~[?:1.8.0_292]
   at net.minecraft.world.IServerWorld.addFreshEntityWithPassengers(IServerWorld.java:10) ~[forge:?]
   at net.minecraft.world.spawner.WorldEntitySpawner.spawnCategoryForPosition(WorldEntitySpawner.java:178) ~[forge:?]
   at net.minecraft.world.spawner.WorldEntitySpawner.spawnCategoryForChunk(WorldEntitySpawner.java:124) ~[forge:?]
   at net.minecraft.world.spawner.WorldEntitySpawner.spawnForChunk(WorldEntitySpawner.java:110) ~[forge:?]
   at net.minecraft.world.server.ServerChunkProvider.lambda$tickChunks$5(ServerChunkProvider.java:364) ~[forge:?]
   at java.util.ArrayList.forEach(ArrayList.java:1259) ~[?:1.8.0_292]
   at net.minecraft.world.server.ServerChunkProvider.tickChunks(ServerChunkProvider.java:351) ~[forge:?]
   at net.minecraft.world.server.ServerChunkProvider.tick(ServerChunkProvider.java:326) ~[forge:?]
   at net.minecraft.world.server.ServerWorld.tick(ServerWorld.java:333) ~[forge:?]
   at net.minecraft.server.MinecraftServer.tickChildren(MinecraftServer.java:851) ~[forge:?]
   ... 5 more

This seemed strange to me, stranger still is that swapping the zombie out for a pig (changing EntityZombie to EntityPig) produces an entirely different issue- a ConcurrentModificationException. Code change:

Spoiler

...
CreatureEntity smartZombie = new PigEntity(EntityType.PIG, event.getWorld());
                smartZombie.setPos(zombie.getX(), zombie.getY(), zombie.getZ());
                smartZombie.setYHeadRot(zombie.getYHeadRot());
                if (!addZombieIfReady(smartZombie)) {
                    spawnQ.put(smartZombie, new BlockPos(
                            smartZombie.getPosition(1.0f)
                    ));
                }
                event.setCanceled(true);
...

Exception:

Spoiler

[13Dec2021 13:02:27.385] [Server thread/DEBUG] [net.minecraftforge.fml.network.FMLHandshakeHandler/FMLHANDSHAKE]: Handshake complete!
[13Dec2021 13:02:27.438] [Netty Local Client IO #0/INFO] [net.minecraftforge.fml.network.NetworkHooks/]: Connected to a modded server.
[13Dec2021 13:02:27.482] [Server thread/INFO] [net.minecraftforge.common.AdvancementLoadFix/]: Using new advancement loading for net.minecraft.advancements.PlayerAdvancements@d32f59b
[13Dec2021 13:02:27.499] [Server thread/INFO] [net.minecraft.server.management.PlayerList/]: Dev[local:E:7f6b99bc] logged in with entity id 341 at (564.6999999880791, 70.01771364921112, -701.437238548806)
[13Dec2021 13:02:27.538] [Server thread/INFO] [net.minecraft.server.MinecraftServer/]: Dev joined the game
[13Dec2021 13:02:27.785] [Server thread/INFO] [net.minecraft.server.integrated.IntegratedServer/]: Saving and pausing game...
[13Dec2021 13:02:27.797] [Server thread/INFO] [net.minecraft.server.MinecraftServer/]: Saving chunks for level 'ServerLevel[New World]'/minecraft:overworld
[13Dec2021 13:02:27.911] [Render thread/INFO] [net.minecraft.advancements.AdvancementList/]: Loaded 7 advancements
[13Dec2021 13:02:28.851] [Server thread/INFO] [net.minecraft.server.MinecraftServer/]: Saving chunks for level 'ServerLevel[New World]'/minecraft:the_nether
[13Dec2021 13:02:28.852] [Server thread/INFO] [net.minecraft.server.MinecraftServer/]: Saving chunks for level 'ServerLevel[New World]'/minecraft:the_end
[13Dec2021 13:02:28.854] [Server thread/DEBUG] [net.minecraftforge.fml.FMLWorldPersistenceHook/WP]: Gathering id map for writing to world save New World
[13Dec2021 13:02:28.858] [Server thread/DEBUG] [net.minecraftforge.fml.FMLWorldPersistenceHook/WP]: ID Map collection complete New World
[13Dec2021 13:02:35.070] [Thread-1/DEBUG] [net.minecraftforge.fml.config.ConfigFileTypeHandler/CONFIG]: Config file forge-server.toml changed, sending notifies
[13Dec2021 13:02:35.071] [Thread-1/DEBUG] [net.minecraftforge.common.ForgeConfig/FORGEMOD]: Forge config just got changed on the file system!
[13Dec2021 13:02:37.905] [Server thread/INFO] [com.noone.smartzombies.SmartZombies/]: Attempting to add zombie
[13Dec2021 13:02:37.906] [Server thread/FATAL] [net.minecraft.util.concurrent.ThreadTaskExecutor/]: Error executing task on Chunk source main thread executor for minecraft:overworld
java.util.concurrent.CompletionException: java.util.ConcurrentModificationException
   at java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:273) ~[?:1.8.0_292]
   at java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:280) ~[?:1.8.0_292]
   at java.util.concurrent.CompletableFuture.uniApply(CompletableFuture.java:618) ~[?:1.8.0_292]
   at java.util.concurrent.CompletableFuture$UniApply.tryFire(CompletableFuture.java:591) ~[?:1.8.0_292]
   at java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:456) ~[?:1.8.0_292]
   at net.minecraft.world.chunk.ChunkTaskPriorityQueueSorter.lambda$null$1(ChunkTaskPriorityQueueSorter.java:44) ~[forge:?]
   at net.minecraft.util.concurrent.ThreadTaskExecutor.doRunTask(ThreadTaskExecutor.java:136) ~[forge:?]
   at net.minecraft.world.server.ServerChunkProvider$ChunkExecutor.doRunTask(ServerChunkProvider.java:519) ~[forge:?]
   at net.minecraft.util.concurrent.ThreadTaskExecutor.pollTask(ThreadTaskExecutor.java:109) ~[forge:?]
   at net.minecraft.world.server.ServerChunkProvider$ChunkExecutor.pollTask(ServerChunkProvider.java:527) ~[forge:?]
   at net.minecraft.world.server.ServerChunkProvider.pollTask(ServerChunkProvider.java:272) ~[forge:?]
   at net.minecraft.server.MinecraftServer.pollTaskInternal(MinecraftServer.java:722) ~[forge:?]
   at net.minecraft.server.MinecraftServer.pollTask(MinecraftServer.java:711) ~[forge:?]
   at net.minecraft.util.concurrent.ThreadTaskExecutor.managedBlock(ThreadTaskExecutor.java:119) ~[forge:?]
   at net.minecraft.server.MinecraftServer.waitUntilNextTick(MinecraftServer.java:697) ~[forge:?]
   at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:646) ~[forge:?]
   at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:232) ~[forge:?]
   at java.lang.Thread.run(Thread.java:748) [?:1.8.0_292]
Caused by: java.util.ConcurrentModificationException
   at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:911) ~[?:1.8.0_292]
   at java.util.ArrayList$Itr.next(ArrayList.java:861) ~[?:1.8.0_292]
   at com.google.common.collect.Iterators$3.next(Iterators.java:174) ~[guava-21.0.jar:?]
   at net.minecraft.world.server.ChunkManager.lambda$null$25(ChunkManager.java:593) ~[forge:?]
   at com.mojang.datafixers.util.Either.lambda$mapLeft$0(Either.java:162) ~[datafixerupper-4.0.26.jar:?]
   at com.mojang.datafixers.util.Either$Left.map(Either.java:38) ~[datafixerupper-4.0.26.jar:?]
   at com.mojang.datafixers.util.Either.mapLeft(Either.java:162) ~[datafixerupper-4.0.26.jar:?]
   at net.minecraft.world.server.ChunkManager.lambda$protoChunkToFullChunk$26(ChunkManager.java:569) ~[forge:?]
   at java.util.concurrent.CompletableFuture.uniApply(CompletableFuture.java:616) ~[?:1.8.0_292]
   ... 15 more
[13Dec2021 13:02:37.918] [Server thread/ERROR] [net.minecraft.server.MinecraftServer/]: Encountered an unexpected exception
java.util.concurrent.CompletionException: java.util.ConcurrentModificationException
   at java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:273) ~[?:1.8.0_292]
   at java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:280) ~[?:1.8.0_292]
   at java.util.concurrent.CompletableFuture.uniApply(CompletableFuture.java:618) ~[?:1.8.0_292]
   at java.util.concurrent.CompletableFuture$UniApply.tryFire(CompletableFuture.java:591) ~[?:1.8.0_292]
   at java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:456) ~[?:1.8.0_292]
   at net.minecraft.world.chunk.ChunkTaskPriorityQueueSorter.lambda$null$1(ChunkTaskPriorityQueueSorter.java:44) ~[forge:?]
   at net.minecraft.util.concurrent.ThreadTaskExecutor.doRunTask(ThreadTaskExecutor.java:136) ~[forge:?]
   at net.minecraft.world.server.ServerChunkProvider$ChunkExecutor.doRunTask(ServerChunkProvider.java:519) ~[forge:?]
   at net.minecraft.util.concurrent.ThreadTaskExecutor.pollTask(ThreadTaskExecutor.java:109) ~[forge:?]
   at net.minecraft.world.server.ServerChunkProvider$ChunkExecutor.pollTask(ServerChunkProvider.java:527) ~[forge:?]
   at net.minecraft.world.server.ServerChunkProvider.pollTask(ServerChunkProvider.java:272) ~[forge:?]
   at net.minecraft.server.MinecraftServer.pollTaskInternal(MinecraftServer.java:722) ~[forge:?]
   at net.minecraft.server.MinecraftServer.pollTask(MinecraftServer.java:711) ~[forge:?]
   at net.minecraft.util.concurrent.ThreadTaskExecutor.managedBlock(ThreadTaskExecutor.java:119) ~[forge:?]
   at net.minecraft.server.MinecraftServer.waitUntilNextTick(MinecraftServer.java:697) ~[forge:?]
   at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:646) ~[forge:?]
   at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:232) ~[forge:?]
   at java.lang.Thread.run(Thread.java:748) [?:1.8.0_292]
Caused by: java.util.ConcurrentModificationException
   at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:911) ~[?:1.8.0_292]
   at java.util.ArrayList$Itr.next(ArrayList.java:861) ~[?:1.8.0_292]
   at com.google.common.collect.Iterators$3.next(Iterators.java:174) ~[guava-21.0.jar:?]
   at net.minecraft.world.server.ChunkManager.lambda$null$25(ChunkManager.java:593) ~[forge:?]
   at com.mojang.datafixers.util.Either.lambda$mapLeft$0(Either.java:162) ~[datafixerupper-4.0.26.jar:?]
   at com.mojang.datafixers.util.Either$Left.map(Either.java:38) ~[datafixerupper-4.0.26.jar:?]
   at com.mojang.datafixers.util.Either.mapLeft(Either.java:162) ~[datafixerupper-4.0.26.jar:?]
   at net.minecraft.world.server.ChunkManager.lambda$protoChunkToFullChunk$26(ChunkManager.java:569) ~[forge:?]
   at java.util.concurrent.CompletableFuture.uniApply(CompletableFuture.java:616) ~[?:1.8.0_292]
   ... 15 more
[13Dec2021 13:02:37.925] [Server thread/FATAL] [net.minecraftforge.common.ForgeMod/]: Preparing crash report with UUID 44c1762c-5bcc-45ba-9929-4a161ed06430

I tried synchronizing the addZombieIfReady method (even knowing the event handlers will be run within the same thread) and that didn't change the result. What are the causes of the two exceptions and why does swapping the zombie out for a pig change behavior so much?

Edited December 13, 2021 by Drosophila

Drosophila · December 14, 2021

12 hours ago, diesieben07 said:

You don't need to replace the entire entity to modify the AI.

Ah, because goalSelector and targetSelector are both public. The clean way to do this would be to update them in the spawn event handler?

12 hours ago, diesieben07 said:

Why is this synchronized?

It shouldn't be. I did that because I was confused by the concurrent modification exception and new to forge. I wanted to confirm that the handlers were not somehow firing in parallel.

12 hours ago, diesieben07 said:

This Map is a giant memory leak.

Ah yep I should probably just keep reusing the same block position object and updating its coordinates, then it could just be a List of entity objects that have a much longer lifetime.

12 hours ago, diesieben07 said:

You listen for EntityJoinWorldEvent and check if a zombie spawned. If so, you cancel the event and spawn a zombie. This triggers your event handler. You cancel the event and spawn a zombie. This triggers your event handler. You cancel the event and spawn a zombie. This goes on forever until the JVM tells you to stop being silly.

Oops. Thanks. That was pretty silly.

12 hours ago, diesieben07 said:

You cannot spawn entities during EntityJoinWorldEvent usually. Check out ForgeInternalHandler#onEntityJoinWorld for how to do it properly.

So actually I should be submitting the task to the work queue that corresponds to the logical side calling the event handler like this?

ThreadTaskExecutor<Runnable> executor = LogicalSidedProvider.WORKQUEUE.get(event.getWorld().isClientSide ? LogicalSide.CLIENT : LogicalSide.SERVER);
executor.tell(new TickDelayedTask(0, () -> event.getWorld().addFreshEntity(newEntity)));

Does that mean that an EntityJoinWorldEvent will never fire for an entity in a chunk that hasn't loaded yet?

Drosophila · December 14, 2021

3 minutes ago, diesieben07 said:

Yes it will, which is precisely why you need this.

Will the TickDelayedTask not execute until the junk has loaded? Or do I still need extra logic to queue up entities and periodically check to see if their positions are finished loading?

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