Trial Spawners will not activate/spawn mobs if gamerule "doMobSpawning" is set to "false"

Confirmed for 1.20.3 pre-3

Confirmed for 1.20.4

Confirmed for 23w51b

I think a good solution to this (if possible), would be adding a new gamerule which controls if spawners(both normal and trial versions) can spawn mobs or not. This would separate the function of spawners from the "doMobSpawning' gamerule and allow them to work independently from each other.

Confirmed for 24w13a

Confirmed for 24w14a.

I personally use the doMobSpawning rule to disable natural mob spawning in non-peaceful modes, and have it so only spawners can spawn mobs, as it's a more intentional seeking out of mobs in an otherwise isolated feeling environment.  But when I went to the Trial Chambers, I was surprised to see the Trial Spawners not responding.

This relates to MC-170191, which is Works As Intended.

Confirmed for 1.20.6 and 24w18a.

confirmed for 24w19b

will further note that as of 24w18a, if there is any relation to this behavior, the particles inside the trial spawner and its sounds are receptive/active which are more in line with the normal monster spawners.  this was not the case in any version prior.  mobs still do not spawn as one would expect.

confirmed for 1.21 pre-release 1

confirmed for 1.21 rc1

The check is performed in net.minecraft.world.level.block.entity.trialspawner.TrialSpawner:

public boolean canSpawnInLevel(Level world) {
        return this.overridePeacefulAndMobSpawnRule
            || world.getDifficulty() != Difficulty.PEACEFULL && world.getGameRules().getBoolean(GameRules.RULE_DOMOBSPAWNING);
    }

From what I can tell, the mob inside isn't even necessarily inside when the gamerule is false. I was just trying to make a custom one and even though it shows a mob inside, running /data on the block shows nothing inside. Switching to true makes the spinning mob disappear. Going from true (/data shows mob inside) to false also then shows nothing inside. So, I don't know if it's entirely that they don't spawn when set to false, but they delete their mob contents when false.

Actually, as I was typing this I was testing more. The spawn_potentials field seems intact through gamerule state swaps which is what the vanilla ones use, but the spawn_data field is deleted, which is what I was using for my custom one. I don't know if that's a different bug or related.

I have been working on a large Trial Chambers themed map several months. Trial spawners are one of the last things to implement and I put it late on my list of things to do, not expecting it to be difficult. However, this bug is preventing me from completing and releasing my map, which I have poured many days into! Please fix this bug!!!

A simple quick fix solution for anyone else whos making a map in creative mode, and I don't know why I didn't think of this before: Keep the mob spawning gamerule on true. Use the /fillbiome command to change the biome of the area you don't want mobs to spawn in to deep dark. Mobs spawning from trial spawners will spawn even in the deep dark biome.

Confirmed for 1.21.2 and 1.21.3

This issue makes Trial Spawners a lot less useful for map-makers, and it is not consistent with the behavior of regular Monster Spawners. Really hoping this will get fixed soon!

Confirmed for 24w44a

I support WillChill's idea to add a new gamerule to control Monster Spawners and Trial Spawners separately from natural mob spawning.

The doMobSpawning gamerule would not affect any of the Spawners, and the new gamerule could be named spawnerBlocksEnabled (similarly to commandBlocksEnabled), or even doMobSpawningFromSpawners.

Can confirm in 24w44a. This seems to be caused by the following in net.minecraft.world.level.block.entity.trialspawner.TrialSpawner#canSpawnInLevel:

public boolean canSpawnInLevel(ServerLevel serverLevel) {
    if (this.overridePeacefulAndMobSpawnRule) {
        return true;
    }
    if (serverLevel.getDifficulty() == Difficulty.PEACEFUL) {
        return false;
    }
    return serverLevel.getGameRules().getBoolean(GameRules.RULE_DOMOBSPAWNING);
}

When the doMobSpawning gamerule is set to false, this method will return false and will cause the trial spawner to stay in the waiting_for_players state

This is inconsistent behaviour and is currently affecting map makers negatively, please fix this soon!

Can confirm for 24w45a

Confirmed for 24w46a and 1.21.4 Pre-Release 1

Please fix this for the 1.21.4, trial spawners could be so useful for map makers !!

Confirmed for 1.21.4, please fix this soon !

Confirmed for 1.21.4 and 25w02a

Please fix issue for all versions. This problem makes trial spawners hardly usable for mapmaking.

Can confirm for 25w03a

Can confirm for 25w04a, 25w05a and 25w06a.

Confirmed for 25w09a

Please fix this issue ! It’s so useful for map makers, and yet it has been more than 6 months and this is still blocking a lot of people from doing what they want !

Can confirm 1.21.5