EntityKnockbackByEntityEvent is called twice if triggered by a player doing a strong knockback attack.

[r6653116]

Expected behavior

EntityKnockbackByEntityEvent would be called a single time every time an entity was hit.

Observed/Actual behavior

EntityKnockbackByEntityEvent is called twice if it is triggered by a player who does a sprinting attack.

Steps/models to reproduce

Create a Paper plugin, register an EntityKnockbackByEntity listener and broadcast a debug message every time it runs.

Punch an entity like a mob or an animal without sprinting. It should broadcast the debug message once.

Now punch the entity again while sprinting. It'll now broadcast the debug message twice.

Plugin and Datapack List

knockbackEventTest

Paper version

This server is running Paper version 1.21.3-29-master@be886cf (2024-11-15T09:07:14Z) (Implementing API version 1.21.3-R0.1-SNAPSHOT)
You are running the latest version
Previous version: 1.21.3-25-1ef4c0e (MC: 1.21.3)

Other

No response

[AfonsoMendoncaJacinto]

My colleague and I were looking forward to reproducing this bug and further our investigation of it. However, we cannot seem to reproduce the bug. I would like to request some guidance in the step of creating the plugin. More specifically whether the class EntityKnockbackByEntityEvent needs an explicit declaration of the getHandlers() or getHandlerList() methods, taking into account the fact that the event classes that extend Event do not inherit these methods from said class, according to Paper’s website. While EntityKnockbackEvent possesses the referred methods, EntityKnockbackByEntityEvent lacks declarations for them. Is this supposed to happen? Documentation on Paper’s guide clearly states that entities lacking the getHandlerList() method cannot be listened to.

References:
https://docs.papermc.io/paper/dev/event-listeners
https://docs.papermc.io/paper/dev/custom-events

[electronicboy]

It walks up the class hierarchy to look for the handler list to use

…

On Fri, 28 Feb 2025 at 22:26, Afonso Mendonça Jacinto < ***@***.***> wrote: My colleague and I were looking forward to reproducing this bug and further our investigation of it. However, we cannot seem to reproduce the bug. I would like to request some guidance in the step of creating the plug-in. More specifically whether the class EntityKnockbackByEntityEvent needs an explicit declaration of the getHandlers() or getHandlerList() methods, taking into account the fact that the event classes that extend Event do not inherit these methods from said class, according to Paper’s website. While EntityKnockbackEvent possesses the referred methods, EntityKnockbackByEntityEvent lacks declarations for them. Is this supposed to happen? Documentation on Paper’s guide clearly states that entities lacking the getHandlerList() method cannot be listened to. Reference: https://docs.papermc.io/paper/dev/event-listeners — Reply to this email directly, view it on GitHub <#11622 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAJMAZCPTRUXJIG6TAU6O3L2SDPDBAVCNFSM6AAAAABR5NIEUCVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDMOJRGY2TCNBQGE> . You are receiving this because you are subscribed to this thread.Message ID: ***@***.***> [image: AfonsoMendoncaJacinto]*AfonsoMendoncaJacinto* left a comment (PaperMC/Paper#11622) <#11622 (comment)> My colleague and I were looking forward to reproducing this bug and further our investigation of it. However, we cannot seem to reproduce the bug. I would like to request some guidance in the step of creating the plug-in. More specifically whether the class EntityKnockbackByEntityEvent needs an explicit declaration of the getHandlers() or getHandlerList() methods, taking into account the fact that the event classes that extend Event do not inherit these methods from said class, according to Paper’s website. While EntityKnockbackEvent possesses the referred methods, EntityKnockbackByEntityEvent lacks declarations for them. Is this supposed to happen? Documentation on Paper’s guide clearly states that entities lacking the getHandlerList() method cannot be listened to. Reference: https://docs.papermc.io/paper/dev/event-listeners — Reply to this email directly, view it on GitHub <#11622 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAJMAZCPTRUXJIG6TAU6O3L2SDPDBAVCNFSM6AAAAABR5NIEUCVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDMOJRGY2TCNBQGE> . You are receiving this because you are subscribed to this thread.Message ID: ***@***.***>

[AfonsoMendoncaJacinto]

Thank you for your feedback, @electronicboy. I ended up replicating the bug with no issues whatsoever. I apologize for the initial confusion. Upon confirmation that the bug persists in version 1.21.4, my colleague and I will proceed to investigate matters further.

[TonytheMacaroni]

This is just because the attack does actually knockback the entity twice. Dealing damage does knockback (see net.minecraft.world.entity.LivingEntity#hurtServer). Separately, mobs & players can deal additional knockback via the minecraft:attack_knockback attribute (see net.minecraft.world.entity.player.Player#attack and net.minecraft.world.entity.Mob#doHurtTarget) - this extra knockback is also dealt when players are sprinting, since sprinting increases attack knockback for players by 1.

These are two separate sources of knockback, so the event will be called twice.

[AfonsoMendoncaJacinto]

What OP seems to be suggesting is that rather than the event being called twice, the expected behaviour would be that the event is called only once, only with increased force, compared to non-sprinting knockback events. A knockback event being called twice loses part of its' meaning and takes value away from the "knockback" attribute.

[electronicboy]

WHich is great, but, the server causes a knockback twice, there isn't any real ways of mitigating that without breaking behavior or trying to 'collect' it later, which is generally overly invasive and breaky at the same time

[AfonsoMendoncaJacinto]

Is the following theoretical scenario impossible?

---

When calling the EntityKnockbackByEntityEvent, since it inherits from EntityKnockbackEvent, it has the getCause() method, so it is possible to check what causes the Event. If it returns either SWEEP_ATTACK or ENTITY_ATTACK, and since EntityKnockbackEvent implements Cancellable, could we cancel this knockback event? Since we already have the damage knockback. Only issue I can see, for now, is the amount of knockback. @electronicboy said that it was incremented when a player sprints, so I assume it will be associated with the event we pretend to cancel, and not with the damage knockback, so I fear the amount of knockback might become too little.