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BlockDispenseEvent is called twice since 26.3 for most blocks

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
55/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Active
Tech stack
java
Domain
api, backend

Research direction

Start in DispenserBlock around line 121 and trace SulfurCubeBlockDispenseItemBehavior.execute into dispenseBlock and CraftEventFactory.handleBlockDispenseEvent. Check how the fallback through DefaultDispenseItemBehavior triggers the event, then reproduce with an event listener that logs each dispense. Done means one BlockDispenseEvent per dispense, with the sulfur cube behavior and fallback still working.

Written by the indexing model from the issue text.

Description

status: accepted version: 26.3
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

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