[MC-509] Exception in server tick loop :: ConcurrentModificationException Created: 25/Oct/12  Updated: 10/Dec/16  Resolved: 06/Dec/15

Status: Resolved
Project: Minecraft: Java Edition
Component/s: None
Affects Version/s: Minecraft 1.4.2, Minecraft 1.4.6, Minecraft 1.4.7, Minecraft 1.5, Minecraft 1.6.1, Minecraft 1.8
Fix Version/s: Minecraft 1.6.2

Type: Bug
Reporter: Anthony Martin Assignee: Unassigned
Resolution: Cannot Reproduce Votes: 1
Labels: crash

Issue Links:
Duplicate
is duplicated by MC-21813 Exception Ticking World Resolved
Relates
relates to MC-23105 Exception in Server when exiting Resolved
relates to MC-72390 Rcon is not thread-safe Resolved
relates to MC-111295 Crash: Exception ticking world 'java.... Resolved
relates to MC-5245 Exception ticking world: ssp unmodifi... Resolved
CHK:
Confirmation Status: Community Consensus
Game Mode: Survival

 Description   

It was not obvious what precipitated this crash. There were several cheaters attempting to fly, but they would get automatically kicked and nothing obvious seemed to go wrong after that. Then, out of nowhere, the server crashed.

An administrator did perform an RCON command right before the exception. This may or may not be related. The RCON command was:

give <player name> 394 16
---- Minecraft Crash Report ----
// Don't be sad, have a hug! <3

Time: 10/25/12 9:46 AM
Description: Exception in server tick loop

java.util.ConcurrentModificationException
        at java.util.HashMap$HashIterator.nextEntry(HashMap.java:894)
        at java.util.HashMap$KeyIterator.next(HashMap.java:928)
        at hz.a(SourceFile:90)
        at net.minecraft.server.MinecraftServer.r(SourceFile:721)
        at hg.r(SourceFile:233)
        at net.minecraft.server.MinecraftServer.q(SourceFile:661)
        at net.minecraft.server.MinecraftServer.run(SourceFile:595)
        at fp.run(SourceFile:818)

Relevant Details:
- Minecraft Version: 1.4.2
- Operating System: Mac OS X (x86_64) version 10.8.1
- Java Version: 1.7.0_07, Oracle Corporation
- Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
- Memory: 703529376 bytes (670 MB) / 3221225472 bytes (3072 MB) up to 3221225472 bytes (3072 MB)
- JVM Flags: 5 total; -Xmx3G -Xms3G -Xloggc:server_gc.log -XX:+UseG1GC -XX:+DisableExplicitGC
- AABB Pool Size: 24234 (1357104 bytes; 1 MB) allocated, 20708 (1159648 bytes; 1 MB) used
- Is Modded: Unknown (can't tell)
- Profiler Position: N/A (disabled)
- Vec3 Pool Size: 37661 (2109016 bytes; 2 MB) allocated, 19642 (1099952 bytes; 1 MB) used
- Player Count: 9 / 20
- World swim2 Players: 0 total; []
- World swim2 Chunk Stats: ServerChunkCache: 0 Drop: 0
- Type: Dedicated Server


 Comments   
Comment by Kumasasa [ 06/Dec/15 ]

No new reports of this kind since 1.8, but maybe this ticket got superseeded by MC-72390

Comment by Amn3zik [ 10/Oct/14 ]
---- Minecraft Crash Report ----
// Daisy, daisy...

Time: 10/10/14 21:23
Description: Exception in server tick loop

java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextNode(HashMap.java:1429)
	at java.util.HashMap$KeyIterator.next(HashMap.java:1453)
	at qn.a(SourceFile:166)
	at net.minecraft.server.MinecraftServer.z(SourceFile:615)
	at po.z(SourceFile:305)
	at net.minecraft.server.MinecraftServer.y(SourceFile:531)
	at net.minecraft.server.MinecraftServer.run(SourceFile:447)
	at java.lang.Thread.run(Thread.java:745)


A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- System Details --
Details:
	Minecraft Version: 1.8
	Operating System: Linux (amd64) version 2.6.32-042stab092.1
	Java Version: 1.8.0_20, Oracle Corporation
	Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
	Memory: 2289190112 bytes (2183 MB) / 5561982976 bytes (5304 MB) up to 10522722304 bytes (10035 MB)
	IntCache: cache: 0, tcache: 0, allocated: 13, tallocated: 95
	Profiler Position: N/A (disabled)
	Player Count: 21 / 80; 
	Is Modded: Unknown (can't tell)
	Type: Dedicated Server (map_server.txt)
Comment by Jim Rees [ 31/Oct/13 ]

I haven't seen this in a long time. Currently running 1.7.2. I suspect it is fixed.

Comment by Kumasasa [ 26/Jul/13 ]

Thanks for the feedback.

Comment by Anthony Martin [ 21/Jul/13 ]

I have not observed this situation for quite some time. I believe it was finally addressed in 1.6.

Comment by MC User [ 21/Mar/13 ]

// You should try our sister game, Minceraft!

Nice witty comment you got in the crash log .

Actually, confirmed.

Comment by Jason Watson [ 16/Mar/13 ]

+1 on a vanilla server

---- Minecraft Crash Report ----
// I'm sorry, Dave.

Time: 3/15/13 10:32 PM
Description: Exception ticking world

java.util.ConcurrentModificationException
        at java.util.HashMap$HashIterator.nextEntry(HashMap.java:894)
        at java.util.HashMap$KeyIterator.next(HashMap.java:928)
        at iy.b(SourceFile:196)
        at iz.b(SourceFile:122)
        at net.minecraft.server.MinecraftServer.r(SourceFile:454)
        at hz.r(SourceFile:255)
        at net.minecraft.server.MinecraftServer.q(SourceFile:397)
        at net.minecraft.server.MinecraftServer.run(SourceFile:331)
        at gp.run(SourceFile:573)


A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Stacktrace:
        at java.util.HashMap$HashIterator.nextEntry(HashMap.java:894)
        at java.util.HashMap$KeyIterator.next(HashMap.java:928)
        at iy.b(SourceFile:196)
        at iz.b(SourceFile:122)

-- Affected level --
Details:
        Level name: world
        All players: 1 total; [jc['(REDACTED)'/2xxxx6, l='world', x=-86.80, y=73.00, z=322.70]]
        Chunk stats: ServerChunkCache: 792 Drop: 334
        Level seed: 1290279039577655732
        Level generator: ID 00 - default, ver 1. Features enabled: true
        Level generator options: 
        Level spawn location: World: (-111,64,286), Chunk: (at 1,4,14 in -7,17; contains blocks -112,0,272 to -97,255,287), Region: (-1,0; contains chunks -32,0 to -1,31, blocks -512,0,0 to -1,255,511)
        Level time: 377270 game time, 127553 day time
        Level dimension: 0
        Level storage version: 0x04ABD - Anvil
        Level weather: Rain time: 49967 (now: false), thunder time: 112422 (now: false)
        Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Stacktrace:
        at net.minecraft.server.MinecraftServer.r(SourceFile:454)
        at hz.r(SourceFile:255)
        at net.minecraft.server.MinecraftServer.q(SourceFile:397)
        at net.minecraft.server.MinecraftServer.run(SourceFile:331)
        at gp.run(SourceFile:573)

-- System Details --
Details:
        Minecraft Version: 1.5
        Operating System: Linux (i386) version 2.6.31.5-127.fc12.i686.PAE
        Java Version: 1.7.0_10, Oracle Corporation
        Java VM Version: Java HotSpot(TM) Client VM (mixed mode, sharing), Oracle Corporation
        Memory: 1574450160 bytes (1501 MB) / 2075918336 bytes (1979 MB) up to 2075918336 bytes (1979 MB)
        JVM Flags: 2 total; -Xmx2G -Xms2G
        AABB Pool Size: 5973 (334488 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
        Suspicious classes: No suspicious classes found.
        IntCache: cache: 0, tcache: 0, allocated: 1, tallocated: 63
        Profiler Position: N/A (disabled)
        Vec3 Pool Size: 1481 (82936 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
        Player Count: 1 / 20; [jc['(REDACTED)'/2xxxx6, l='world', x=-86.80, y=73.00, z=322.70]]
        Is Modded: Unknown (can't tell)
        Type: Dedicated Server (map_server.txt)
Comment by Jim Rees [ 14/Mar/13 ]

I just got something similar in 1.5. Brand new world, no RCON. There were two of us on, but the trace says just one, so this might have happened just after the other guy disconnected. No mods, no DDoS, nothing special at all.

---- Minecraft Crash Report ----
// You should try our sister game, Minceraft!

Time: 3/13/13 7:56 PM
Description: Exception ticking world

java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextEntry(HashMap.java:894)
	at java.util.HashMap$KeyIterator.next(HashMap.java:928)
	at iy.b(SourceFile:196)
	at iz.b(SourceFile:122)
	at net.minecraft.server.MinecraftServer.r(SourceFile:454)
	at hz.r(SourceFile:255)
	at net.minecraft.server.MinecraftServer.q(SourceFile:397)
	at net.minecraft.server.MinecraftServer.run(SourceFile:331)
	at gp.run(SourceFile:573)


A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Stacktrace:
	at java.util.HashMap$HashIterator.nextEntry(HashMap.java:894)
	at java.util.HashMap$KeyIterator.next(HashMap.java:928)
	at iy.b(SourceFile:196)
	at iz.b(SourceFile:122)

-- Affected level --
Details:
	Level name: world
	All players: 1 total; [jc['LuckeyJim'/7448, l='world', x=-537.24, y=51.17, z=207.48]]
	Chunk stats: ServerChunkCache: 972 Drop: 125
	Level seed: 2106225363214519273
	Level generator: ID 00 - default, ver 1. Features enabled: true
	Level generator options: 
	Level spawn location: World: (-426,64,255), Chunk: (at 6,4,15 in -27,15; contains blocks -432,0,240 to -417,255,255), Region: (-1,0; contains chunks -32,0 to -1,31, blocks -512,0,0 to -1,255,511)
	Level time: 90082 game time, 90082 day time
	Level dimension: 0
	Level storage version: 0x04ABD - Anvil
	Level weather: Rain time: 15721 (now: false), thunder time: 13873 (now: false)
	Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Stacktrace:
	at net.minecraft.server.MinecraftServer.r(SourceFile:454)
	at hz.r(SourceFile:255)
	at net.minecraft.server.MinecraftServer.q(SourceFile:397)
	at net.minecraft.server.MinecraftServer.run(SourceFile:331)
	at gp.run(SourceFile:573)

-- System Details --
Details:
	Minecraft Version: 1.5
	Operating System: Linux (i386) version 3.7.10-1-ARCH
	Java Version: 1.7.0_15, Oracle Corporation
	Java VM Version: OpenJDK Client VM (mixed mode), Oracle Corporation
	Memory: 77103736 bytes (73 MB) / 185393152 bytes (176 MB) up to 778502144 bytes (742 MB)
	JVM Flags: 2 total; -Xms128M -Xmx768M
	AABB Pool Size: 5420 (303520 bytes; 0 MB) allocated, 4 (224 bytes; 0 MB) used
	Suspicious classes: No suspicious classes found.
	IntCache: cache: 0, tcache: 0, allocated: 3, tallocated: 63
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: 14605 (817880 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
	Player Count: 1 / 8; [jc['LuckeyJim'/7448, l='world', x=-537.24, y=51.17, z=207.48]]
	Is Modded: Unknown (can't tell)
	Type: Dedicated Server (map_server.txt)
Comment by Anthony Martin [ 30/Nov/12 ]

I have seen a great many ConcurrentModificationException spamming the console in 1.4.5 while the server is being DDoSed. Likely, this is a side effect of the garbage collector being overworked. This is one of those "tricky-to-re-produce" situations. I can't exactly ask someone to DDoS my server so I can test it.

Your call.

Comment by Tails [ 30/Nov/12 ]

Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.

Comment by Anthony Martin [ 25/Oct/12 ]

It appears that RCON commands do indeed have something to do with it, but only when the server has been up for quite a while.

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