BlockDispenseEvent is called twice since 26.3 for most blocks #14415

Expected behavior

BlockDispenseEvent is only called once when a dispenser throws out a block

Observed/Actual behavior

When a block is dispensed from a dispenser, it used to go through the usual DefaultDispenseItemBehavior.
With the Sulfur Cubes that changed for most blocks (DispenserBlock line 121):

} else if (itemStack.is(ItemTags.SULFUR_CUBE_SWALLOWABLE)) {
    return SulfurCubeBlockDispenseItemBehavior.INSTANCE;
}

SulfurCubeBlockDispenseItemBehavior#execute (vanilla) falls back to super.execute if no Sulfur Cube took the item:

@Override
protected ItemStack execute(final BlockSource source, final ItemStack dispensed) {
    return dispenseBlock(source.level(), source.pos().relative(source.state().getValue(DispenserBlock.FACING)), dispensed, source, this) // Paper - Call BlockDispenseEvent
        ? dispensed
        : super.execute(source, dispensed);
}

Paper calls handleBlockDispenseEvent at the start of dispenseBlock, before it checks whether a Sulfur Cube is in front:

ItemStack result = org.bukkit.craftbukkit.event.CraftEventFactory.handleBlockDispenseEvent(source, pos, dispensed, currentBehavior);

If there is none, dispenseBlock returns false and vanilla falls back to super.execute, which fires the event again in DefaultDispenseItemBehavior.spawnItem.

The same applies to TNT: the TNT behavior also calls SulfurCubeBlockDispenseItemBehavior.dispenseBlock first and then fires its own BlockDispenseEvent. The TNT is only primed once, but the event is called twice for a single dispense.

sequenceDiagram
    autonumber
    participant D as DispenserBlock
    participant S as SulfurCubeBlockDispenseItemBehavior
    participant DD as DefaultDispenseItemBehavior (super)
    participant F as CraftEventFactory
    participant L as Plugin Listener

    D->>D: getDefaultDispenseMethod(itemStack)
    Note over D: itemStack.is(SULFUR_CUBE_SWALLOWABLE)<br/>→ SulfurCubeBlockDispenseItemBehavior.INSTANCE
    D->>S: dispense(source, stack)
    S->>S: execute(source, stack)
    S->>S: dispenseBlock(level, pos, stack, source, this)
    S->>F: handleBlockDispenseEvent(source, pos, stack, this)
    F->>L: BlockDispenseEvent (1st call)
    L-->>F: not cancelled
    F-->>S: null
    S->>S: getEntitiesOfClass(SulfurCube, AABB(pos))
    Note over S: no Sulfur Cube in front<br/>→ dispenseBlock returns false
    S->>DD: super.execute(source, stack)
    DD->>DD: spawnItem(level, stack, 6, direction, position, source)
    DD->>L: BlockDispenseEvent (2nd call)
    L-->>DD: not cancelled
    DD->>DD: level.addFreshEntity(itemEntity)

Steps/models to reproduce

Watch the BlockDispenseEvent with a event listener and see two logs for the same dispense.

Plugin and Datapack List

Paper version

This server is running Paper version 26.3-209-main@03af8b4 (2026-10-20T21:49:44Z) (Implementing API version 26.3.build.209-stable)

Other

No response