Events

Events Skill — Paper

Purpose

Reference this skill when implementing event listeners on a Paper server. Covers event registration, priority ordering, cancellation, and thread-safety rules for all Bukkit/Paper events.

When to Use This Skill

• Adding a listener for any org.bukkit.event.* or io.papermc.paper.event.* event
• Deciding event priority for overlapping listeners
• Handling cancellable events correctly
• Working with async events (especially chat)

API Quick Reference

Class / Method	Purpose	Notes
Listener (interface)	Marker interface for event listener classes	Required on listener class
@EventHandler	Marks a method as an event handler	Method must have exactly one event parameter
EventPriority	Controls listener order: LOWEST → LOW → NORMAL → HIGH → HIGHEST → MONITOR	MONITOR = read-only, never cancel here
ignoreCancelled = true	Skip handler if event already cancelled	Recommended for most handlers
PluginManager#registerEvents(Listener, Plugin)	Register all @EventHandler methods in a listener	Call once in onEnable
HandlerList#unregisterAll(Plugin)	Unregister all listeners for a plugin	Called automatically on onDisable
Event#isCancelled()	Check if event is already cancelled
Cancellable#setCancelled(boolean)	Cancel the event	Prevents default action
AsyncChatEvent	Paper's modern async chat event	Replaces deprecated AsyncPlayerChatEvent

Code Pattern

package com.yourorg.myplugin.events;

import io.papermc.paper.event.player.AsyncChatEvent;
import net.kyori.adventure.text.Component;
import net.kyori.adventure.text.format.NamedTextColor;
import org.bukkit.entity.Player;
import org.bukkit.event.EventHandler;
import org.bukkit.event.EventPriority;
import org.bukkit.event.Listener;
import org.bukkit.event.player.PlayerJoinEvent;
import org.bukkit.event.player.PlayerQuitEvent;
import org.bukkit.plugin.java.JavaPlugin;

public class PlayerJoinListener implements Listener {

    private final JavaPlugin plugin;

    public PlayerJoinListener(JavaPlugin plugin) {
        this.plugin = plugin;
    }

    // Standard listener: NORMAL priority, skip if already cancelled
    @EventHandler(priority = EventPriority.NORMAL, ignoreCancelled = true)
    public void onPlayerJoin(PlayerJoinEvent event) {
        Player player = event.getPlayer();

        // Set custom join message using Adventure components
        event.joinMessage(Component.text("+ ")
            .color(NamedTextColor.GREEN)
            .append(Component.text(player.getName())
                .color(NamedTextColor.WHITE)));
    }

    @EventHandler(priority = EventPriority.NORMAL)
    public void onPlayerQuit(PlayerQuitEvent event) {
        Player player = event.getPlayer();

        event.quitMessage(Component.text("- ")
            .color(NamedTextColor.RED)
            .append(Component.text(player.getName())
                .color(NamedTextColor.WHITE)));
    }

    // Async chat event — DO NOT call Bukkit object mutations here
    @EventHandler(priority = EventPriority.NORMAL, ignoreCancelled = true)
    public void onChat(AsyncChatEvent event) {
        Player player = event.getPlayer();
        Component originalMessage = event.message();

        // Reading player data: safe (immutable snapshot)
        String name = player.getName();

        // Modifying the message: safe (event object, not Bukkit world state)
        event.message(Component.text("[Chat] ")
            .color(NamedTextColor.GRAY)
            .append(originalMessage));

        // NOT safe here: player.teleport(), world.dropItem(), etc.
        // If you must do Bukkit ops, schedule back to main thread:
        // Bukkit.getScheduler().runTask(plugin, () -> { ... });
    }
}

Registering the listener in your main class:

@Override
public void onEnable() {
    getServer().getPluginManager().registerEvents(new PlayerJoinListener(this), this);
}

Common Pitfalls

• Calling Bukkit methods from async event handlers: AsyncChatEvent fires on an async thread. Calling player.teleport(), world.dropItem(), or any world-mutating method will throw IllegalStateException or cause data corruption. Schedule back to main thread with Bukkit.getScheduler().runTask(plugin, runnable).

• Using MONITOR priority to cancel events: MONITOR is for read-only observation after all other plugins have processed the event. Cancelling at MONITOR is undefined behavior and may be ignored.

• Not using ignoreCancelled = true: Without it, your handler fires even when another plugin has already cancelled the event, potentially overriding that decision. Add ignoreCancelled = true unless you specifically need to handle already-cancelled events.

• Registering listeners multiple times: Calling registerEvents() twice on the same listener instance doubles all event firings. Register each listener exactly once in onEnable.

• Still using AsyncPlayerChatEvent: This event is deprecated in Paper 1.19+ and removed in some builds. Use AsyncChatEvent from io.papermc.paper.event.player instead.

Version Notes

• 1.21: AsyncChatEvent is the standard chat event. AsyncPlayerChatEvent is deprecated.
• 1.21.1: No breaking changes to event API vs 1.21.
• Both: PlayerMoveEvent fires very frequently (every tick a player is loaded). Use event.hasExplicitlyMoved() or event.hasChangedPosition() to filter out head rotations.

Related Skills

• player-events.md — Detailed PlayerJoinEvent, PlayerMoveEvent, PlayerInteractEvent reference
• world-events.md — Block and entity events
• custom-events.md — Creating and firing your own events
• async-events.md — Threading rules for async events
• ../commands/SKILL.md — Command registration