Horse Spawners cannot have SpawnData or SpawnPotentials or they crash

Can confirm this bug for MC 1.6.2. It is very unfortunate, too. It is impossible to spawn a horse with a saddle or armor. Additionally you cannot spawn a horse with specific attributes, which would benefit custom maps greatly. (same speed, no jump/very high jump, etc.)

http://imgur.com/a/DrCFf

In the 1.6 snapshots this didn't crash the game.

Even if you spawn a horse riding a snowball the game crashes.

Examining the console spam made me realize something interesting: the game fails to createEntityByName on horse altogether - that spam is so large because the game's continuously trying to create a horse for the spawner and failing to do so. This is why horse spawners have an empty cage even if they don't have SpawnData and don't cause crashes!

The cause, unsurprisingly, is within the EntityHorse class itself:

Client> Caused by: java.lang.NullPointerException
Client> 	at rr.cI(SourceFile:448)
Client> 	at rr.cH(SourceFile:444)
Client> 	at rr.<init>(SourceFile:140)
Client> 	... 17 more

The problem isn't that the game doesn't know what to spawn - it's that DrZhark's horses can't be instantiated this way. The InvocationTargetException means that the constructor itself throws an exception. A Minecraft Coder Pack snippet of the immediate cause of this failure:

private void func_110232_cE()
    {
        if (!this.worldObj.isRemote)
        {
            this.func_110251_o(this.field_110296_bG.getStackInSlot(0) != null);

            if (this.func_110259_cr())
            {
                this.func_110236_r(this.func_110260_d(this.field_110296_bG.getStackInSlot(1)));
            }
        }
    }

Apparently, worldObj is null when this line is called - and what a surprise!

public Entity func_98281_h()
    {
        if (this.field_98291_j == null)
        {
            Entity var1 = EntityList.createEntityByName(this.getEntityNameToSpawn(), (World)null);
            var1 = this.func_98265_a(var1);
            this.field_98291_j = var1;
        }

        return this.field_98291_j;
    }

The base cause of this issue is that horses are the only mob which cannot be created in a null world! The spawner attempts to construct an entity for the spawnercage, and a null world is provided because this entity is not intended to be "real". It fails because Horses don't follow the null-world-is-okay standard that other mobs follow, and thus the spawnercage entity is null. This, in turn, causes a crash if SpawnData is present, because the game then makes the assumption that the spawnercage entity is not null.

Thus, you get a crash identical to those seen by people who try creating spawners with invalid EntityIDs, despite the cause being very, very different.

I noticed something in the code: if Riding is present, the game will attempt to create a fake entity for the ridden entity as well, but if it fails, it gracefully continues instead of crashing. This means it is possible to trick the game into spawning custom horses, but the developer console is still spammed with InvocationTargetExceptions.

This is a good workaround for the time being, but there's one troubling consequence: such a spawner follows the entity cap of the rider (a very useful trick 99% of the time, I might add, so please don't change this behavior!). This means that, if you use/abuse this trick to get your horse spawners to work, they will not stop spawning regardless of how many horses you have. This'll be a bit of a problem for a horse stable I was working on, but I suppose it's better than nothing.

This renders it impossible to create a custom horse spawner in vanilla Minecraft.

Working as intended.

I did the tests myself, I really can't spawn a vanilla horse with spawners, the world just crashes.
Yet Grum confirms "Working as intended", is the crash really intentional? If yes, may I ask why?

The ticket you're posting on only applies to horse spawners. That's why this is closed; Grum's statement only applies to horse spawners with SpawnData/SpawnPotentials. You're making a Witch spawner.

For your spawner, you made the first equipment array the wrong length. Correct it to this, and no crash occurs:

setblock ~ ~1 ~ minecraft:mob_spawner 0 replace {EntityId:Witch,SpawnData:{Equipment:[{},{},{},{},
{id:276,Count:1}
]},SpawnPotentials:[{Type:Witch,Weight:1,Properties:{Equipment:[{},{},{},{},
{id:46,Count:1}
]}}]}

Also, witches use their weapon slot to store potions they drink. Don't be surprised if that item doesn't appear at all - Dinnerbone said it's just beyond what's currently possible with witch NBT.

EDIT: actually, I can't get it to crash even if I use the same tags you used.

i've used the same tag's many times before an simply changed the monster and its hasnt crashed, but for some reason this time it did and it's a permanent crash with that map chunk, i was hoping the crash report would enlighten to what/why it was occuring. The witches and/or zombies when spawned are holding the diamond sword np's, but i didnt expect them to drop the sword but only the normal drop's they have.. This is why its perplexing as ive been using that style tag with no side effects untill now.

@Kumasasa if you read this,

MC-43567 is NOT a dupe of this.

This is related to invalid parameters,MC-43567 is related to bad characters!

@Professor Epic Anonomus: Changed the resolution of MC-43567

Seems to be fixed in 1.8.5, 1.8.6 (and maybe earlier)

Command

/setblock ~ ~1 ~ mob_spawner 0 replace {Delay:702s,EntityId:"EntityHorse",MaxNearbyEntities:6s,MaxSpawnDelay:800s,MinSpawnDelay:200s,RequiredPlayerRange:16s,SpawnCount:4s,SpawnPotentials:[{Properties:{},Type:"EntityHorse",Weight:1}],SpawnRange:4s}