Minecraft bukkit zombie-spawning error

StackOverflow https://stackoverflow.com/questions/23583624

  •  19-07-2023
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Question

I am writing a simple zombie-spawning plugin. The spawning is done by another thread. When I run the plugin, I get the following error after some time of playing:

[Server thread/ERROR]: Encountered an unexpected exception
 java.util.ConcurrentModificationException
 at java.util.HashMap$HashIterator.nextEntry(Unknown Source) ~[?:1.7.0_25]
 at java.util.HashMap$KeyIterator.next(Unknown Source) ~[?:1.7.0_25]
 at net.minecraft.server.v1_7_R1.EntityTracker.updatePlayers(EntityTracker.java:152) ~[craftbukkit.jar:git-Bukkit-1.7.2-R0.3-b3020jnks]
 at net.minecraft.server.v1_7_R1.MinecraftServer.u(MinecraftServer.java:646) ~[craftbukkit.jar:git-Bukkit-1.7.2-R0.3-b3020jnks]
 at net.minecraft.server.v1_7_R1.DedicatedServer.u(DedicatedServer.java:250) ~[craftbukkit.jar:git-Bukkit-1.7.2-R0.3-b3020jnks]
 at net.minecraft.server.v1_7_R1.MinecraftServer.t(MinecraftServer.java:545) ~[craftbukkit.jar:git-Bukkit-1.7.2-R0.3-b3020jnks]
 at net.minecraft.server.v1_7_R1.MinecraftServer.run(MinecraftServer.java:457) [craftbukkit.jar:git-Bukkit-1.7.2-R0.3-b3020jnks]
 at net.minecraft.server.v1_7_R1.ThreadServerApplication.run(SourceFile:617) [craftbukkit.jar:git-Bukkit-1.7.2-R0.3-b3020jnks]

This error continues to occur, but it occurs rarely and it seems random. My thoughts to this problem are that the spawning-thread is asynchroune to the EntityTracker, which leads to a exception because my thread modifies the entity-list while another part of bukkit tries to tell the players the positions of the monsters.
I don't have any idea what the exact error-source is, but I think I have to synchronize my thread with some part of bukkit. Does anyone have a solution for that? (And is there any way to set the items which a zombie will drop? The whole rotten flesh is starting to give me nightmares).

Thanks in advance.

Was it helpful?

Solution

Your stacktrace is saying that you are trying to modify your collection from another thread. You could either try to synchronize your methods or use something like a ConcurrentHashMap.

If you don't really need the modification to be on another thread, you could also use Bukkit's schedule facility to do the job :

private final Map<T> map = new HashMap<T>();
// ... stuff happens

final T value;
// work with your value

server.getScheduler().scheduleSyncDelayedTask(plugin, new Runnable() {
    public void run() {
        map.add(value);
    }
}, 0L);
// runs the run() method after 0 ticks on the main thread
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