Simple packet listener for 1.17.1

Example.java

@RequiredArgsConstructor
public class Example extends PacketListener {

    private static final java.lang.Class<?> CLAZZ;

    static {
        try {
            CLAZZ = java.lang.Class.forName("net.minecraft.network.protocol.game.PacketPlayInUseEntity$d");
        } catch (ClassNotFoundException e) {
            throw new IllegalArgumentException(e);
        }
    }

    private final OmgCoolPlugin plugin;

    @Override
    public void onReceive(PacketEvent event) {
        Player player = event.getPlayer();
        if (!(event.getPacket() instanceof PacketPlayInUseEntity packet) || !plugin.getPacketListenerPlayers().contains(player))
            return;

        Bukkit.getScheduler().runTask(plugin, () -> {
            try {
                World world = player.getWorld();
                net.minecraft.world.entity.Entity entity = packet.a(((CraftWorld) world).getHandle());
                if (entity == null)
                    return;

                Field bField = packet.getClass().getDeclaredField("b");
                bField.setAccessible(true);
                Object object = bField.get(packet);

                if (!CLAZZ.isInstance(object))
                    return;

                Field aField = object.getClass().getDeclaredField("a");
                aField.setAccessible(true);
                EnumHand hand = (EnumHand) aField.get(object);

                if (hand != EnumHand.a)
                    return;

                Configuration.getNPC(entity.getBukkitEntity()).ifPresent(npc -> Bukkit.getPluginManager().callEvent(new PlayerInteractEntityEvent(player, entity.getBukkitEntity())));
            } catch (Exception e) {
                e.printStackTrace();
            }
        });
    }
}

PacketEvent.java

@Data
public class PacketEvent {

    private final Packet<?> packet;
    private final Player player;
    private boolean cancelled;
}

PacketHandler.java

@RequiredArgsConstructor
@Getter
public class PacketHandler extends ChannelDuplexHandler {

    private final PacketInjector injector;
    private final Player player;

    @Override
    public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
        if (!(msg instanceof Packet<?> packet)) {
            super.write(ctx, msg, promise);
            return;
        }

        PacketEvent event = new PacketEvent(packet, player);
        injector.callSend(event);
        if (!event.isCancelled())
            super.write(ctx, msg, promise);
    }

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        if (!(msg instanceof Packet<?> packet)) {
            super.channelRead(ctx, msg);
            return;
        }

        PacketEvent event = new PacketEvent(packet, player);
        injector.callReceive(event);
        if (!event.isCancelled())
            super.channelRead(ctx, msg);
    }

PacketInjector.java

public class PacketInjector {

    private final List<PacketListener> listeners = new ArrayList<>();

    public void addPlayer(Player player) {
        ChannelPipeline pipeline = ((CraftPlayer) player).getHandle().b.a.k.pipeline();
        if (pipeline.get("PacketInjector") == null)
            pipeline.addBefore("packet_handler", "PacketInjector", new PacketHandler(this, player));
    }

    public void removePlayer(Player player) {
        ChannelPipeline pipeline = ((CraftPlayer) player).getHandle().b.a.k.pipeline();
        if (pipeline.get("PacketInjector") != null)
            pipeline.remove("PacketInjector"); //baka
    }

    public void addListener(PacketListener listener) {
        listeners.add(listener);
    }

    public void callReceive(PacketEvent event) {
        for (PacketListener listener : listeners)
            listener.onReceive(event);
    }

    public void callSend(PacketEvent event) {
        for (PacketListener listener : listeners)
            listener.onSend(event);
    }
}

PacketListener.java

public abstract class PacketListener {

    /**
     * Called when a packet is received from the client, when the packet is SENT TO THE SERVER
     * @param event the event
     */
    public void onReceive(PacketEvent event) {
    }

    /**
     * Called when a packet is sent to the client, when the packet is SENT TO THE PLAYER
     * @param event the event
     */
    public void onSend(PacketEvent event) {
    }
}