[MC-149777] Crash when loading world: java.util.ConcurrentModificationException when using Java 11 or above

When attempting to play attached world in Minecraft client, game may crash, then freeze. Doesn't always happen. Crash report is only printed in launcher, not saved.

1.14: [^crashreport.txt]

java.util.ConcurrentModificationException
	at java.base/java.util.HashMap.computeIfAbsent(HashMap.java:1134)
	at cje.b(SourceFile:67)
	at cje.a(SourceFile:57)
	at cff.a(SourceFile:98)
	at cfi.a(SourceFile:86)
	at java.base/java.util.stream.ReferencePipeline$4$1.accept(ReferencePipeline.java:212)
	at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1654)
	at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:484)
	at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:474)
	at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:913)
	at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at java.base/java.util.stream.IntPipeline.reduce(IntPipeline.java:496)
	at java.base/java.util.stream.IntPipeline.max(IntPipeline.java:459)
	at cfi.a(SourceFile:86)
	at cfd$c.a(SourceFile:175)
	at java.base/java.util.stream.ReferencePipeline$4$1.accept(ReferencePipeline.java:212)
	at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1654)
	at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:484)
	at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:474)
	at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:913)
	at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at java.base/java.util.stream.IntPipeline.reduce(IntPipeline.java:496)
	at java.base/java.util.stream.IntPipeline.max(IntPipeline.java:459)
	at cfd$c.a(SourceFile:176)
	at cfd$c.<init>(SourceFile:98)
	at cfd.a(SourceFile:280)
	at cet.a(SourceFile:25)
	at ces$a.a(SourceFile:89)
	at bwy.a(SourceFile:187)
	at bxc.d(SourceFile:44)
	at bxc.a(SourceFile:199)
	at us.a(SourceFile:461)
	at com.mojang.datafixers.util.Either$Left.map(Either.java:38)
	at us.a(SourceFile:455)
	at java.base/java.util.concurrent.CompletableFuture$UniCompose.tryFire(CompletableFuture.java:1072)
	at java.base/java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:478)
	at uu.b(SourceFile:57)
	at agk.e(SourceFile:83)
	at agk.a(SourceFile:123)
	at agk.run(SourceFile:91)
	at java.base/java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1426)
	at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:290)
	at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1020)
	at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1656)
	at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1594)
	at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:177)

Linked issues

is duplicated by 3

MC-182982 Exception while generating new chunk Resolved

MC-195596 Exception generating new chunk: java.util.ConcurrentModificationException Resolved

MC-225584 Server Crash Report Resolved

Attachments

20w21a_log.txt

crash-2020-04-04_20.04.06-server.txt

crashreport.txt

documentary.zip

pre1.txt

Comments 21

violine1101 2019-04-26T08:42:12Z

This might happen because you're using a newer version of Java than it was designed for. The Minecraft launcher currently recommends Java 8 Update 51. (Nevertheless, Minecraft should work with newer Java versions.)

laputan 2019-05-07T17:30:55Z

Still occurs with 1.14.1-pre1.

[media]

Maity 2020-02-01T15:03:18Z

Acual fix: https://github.com/modmuss50/Voyager/blob/1.0.0/src/main/java/me/modmuss50/voyager/mixin/StructureManagerMixin.java

[MOD] Greymagic27 2020-02-01T20:42:38Z

Maity, may I ask what that .java file does?

liach 2020-02-01T21:37:32Z

That should be treated as the code analysis and fix suggestion. It shows why the crash can happen (because worldgen and server threads use the structure manager hash map concurrently)

11 more comments

[MOD] Greymagic27 2020-02-02T12:39:58Z

Is this still an issue in 1.15.2?

Evan James Roscoe 2020-02-02T15:09:28Z

Greymagic27, Yes, I have problems loading worlds in both singleplayer and multiplayer, basically showing the same crash error.

modmuss50 2020-03-30T19:19:42Z

I will be using yarn names throught this comment:

I spent sometime looking into the best way of fixing this. I came to the conclusion that using Collections.synchronizedMap() is the ideal fix. The reason for using this over ConcurrentHashMap is HashMap allows null values, where as ConcurrentHashMap does not, this is important as in getStructureOrBlank a null map value is possible if the structure could not be found, synchronizedMap does not change the implimentation details of HashMap.

The mixin linked above is the old method, [StructureManagerMixin.java|https://github.com/modmuss50/Voyager/blob/master/src/main/java/me/modmuss50/voyager/mixin/StructureManagerMixin.java] shows how I achieved this using Collections.synchronizedMap()

liach 2020-03-30T22:31:35Z

Confirmed for 20w13b.

violine1101 2020-04-04T18:06:20Z

This just randomly happened to me in 1.15.2 in my vanilla survival world.

I was playing in the world for a while, went exploring, and while riding with a boat across an ocean, this just happened. I'm playing in the world since 1.14, and I'm using Java 12.

modmuss50 2020-04-09T01:06:03Z

Confirmed to still be an issue in 20w15a

LuxiKeks 2020-05-06T15:54:24Z

Still in 20w18a

modmuss50 2020-05-25T21:51:45Z

I have repoduced this in 20w21a, however it was only logged to console and didnt crash the game. I still believe the hard crash can still happen as the code appears untouched since it was last reproduced in 20w18a.

[media]

Sollace 2020-07-21T15:14:54Z

I know this exception. I've encountered it before multiple times. It's due to a change in Java 11 that makes it so HashMap.computeIfAbsent will throw this exception, either if the call is nested, or if something else interacts with the collection whilst that method is being called.

map.computeIfAbsent(keyOne, a -> {
      ....
      return map.computeIfAbsent(keyTwo, b -> {
      }); // throws
});

Only solution I know of is to switch to using something like a TreeMap instead, which does not have it's own implementation of computeIfAbsent.

Happyfungamer 2020-12-19T18:00:59Z

Does this affect the latest release 1.16.4?

LuxiKeks 2021-01-10T10:00:42Z

Yes it does.

Riley McLain 2021-03-07T22:06:30Z

This bug is caused mainly by bad programming that mojang refuses to fix even though it isn't hard to fix. As a workaround, you can use the fabric modloader with the fabric API installed, or you can use java 8.

i509VCB 2021-05-12T15:36:19Z

Given 21w19a now has a dependency on Java 16, I'd recommend a fix for this issue before release.

Zytron 2021-05-12T15:43:50Z

was gonna make my own bug report but I saw this and this is close enough,

servers for 21w19a crash when loading my world that was created in 21w18a

[media][media]

so if this is fixable by me (I assume I need to download Java 16?) how do I fix it and how do I upgrade to Java 16

It also wont start a fresh server with no worlds yet. Wont even generate the needed files such as the Eula or server properties. This is bad for me because I play 80% on servers so my friend can join when we want instead of having them visit me and use LAN.

EDIT: This was simply because I didn't have Java 16 installed.

MalbaCato 2021-05-12T16:20:37Z

this is not suited for the bugtracker, but as I assume this ticket is going to attract many of these questions, answer is go to https://adoptopenjdk.net and follow some simple installation prompts, make sure to download java version 16.

SPGoding 2021-05-12T17:12:39Z

@unknown, that is a different issue than this bug. You need to install Java 16 to be able to run the server after 21w19a, and I'd suggest you go to Community Support if you need further assistance.