ConcurrentModificationException - forge bug or mod bug?

[Tyron]

My Linux server occassionally crashes with a ConcurrentModificationException.

Details here: http://pastebin.com/KfvE7Vt3

 

I'm only using forge and my own mod on this Server and I have no idea whether this is a bug in forge or with my mod.

 

[Edit:] Ok apparently it happens during server join. Here is the fml logs: http://pastebin.com/38JCw45L

 

How can I debug this?

[TheGreyGhost]

Hi

 

Multithreading bugs are an absolute pain to debug.  You could try using an earlier version of Forge and seeing if the problem goes away.  Otherwise, you need to look carefully in your code (at.tyron.vintagecraft.WorldGen.Helper.WorldProviderVC ? ) to see if it's modifying the same object that triggers the error

        at net.minecraft.world.World.func_175650_b(World.java:3126)

        at net.minecraft.world.chunk.Chunk.func_76631_c(Chunk.java:952)

(I'm using a different forge version so those line numbers don't match mine)

 

It looks like you might be adding chunks, entities, or tileentities in the wrong thread.

 

-TGG

 

 

 

 

[Tyron]

Thank you for the reply TheGreyGhost, I now have a guess as to why this crash is happening.

 

1. I'm using EntityJoinWorldEvent to occasionally add another mob to a spawning mob

2. EntityJoinWorldEvent is also fired during chunk loading - which loops through the chunk entitylist -> probably gets modified due to my code adding another entity

 

I'm going to switch over to LivingSpawnEvent, that should probably fix the problem!

[coolAlias]

Here's my reply to another thread which may also prove useful to you, and you can also look up ConcurrentModificationException on Google to find many great explanations.

 

Basically, a CME usually happens with Collections when one thread is iterating over the collection and another thread tries to modify the same collection. Easy fix is to synchronize your iteration, or, if you are the one modifying it, use thread-safe operations e.g. Iterator.