Spigot 1.21.1 NMS (net.minecraft.server) packet listener

README.md

Obviously getting the NMS is a pain and the way that I have obtained it is by opening the bootstrap spigot (the one you use to run your spigot server) as a zip. Once opened go to META-INF/libraries and extract the spigot-api-..-R.-SNAPSHOT.jar, and the spigot-..-R*.*-SNAPSHOT.jar. Also extract all of the netty libraries from the same path. Use these extracted jars as libraries in your plugin.

With all of that you should be able to listen for incoming packets with spigot.

PacketListener.java

public class MyEventHandler implements Listener {

    @EventHandler
    public void onJoin(PlayerJoinEvent event) throws NoSuchFieldException, IllegalAccessException {
        Player player = event.getPlayer();
        //Craft Player (NMS varient of player)
        EntityPlayer nmsPlayer = ((CraftPlayer) player).getHandle();

        //Get connection
        ServerCommonPacketListenerImpl connection = nmsPlayer.c;

        //Get Network Manager field (Protected)
        Field f = ServerCommonPacketListenerImpl.class.getDeclaredField("e");
        f.setAccessible(true);

        NetworkManager networkManager = (NetworkManager) f.get(connection);

        //Get Netty Channel field (Private)
        f = networkManager.getClass().getDeclaredField("n");
        f.setAccessible(true);

        Channel channel = (Channel) f.get(networkManager);

        //Add listener
        ChannelDuplexHandler duplexHandler = new ChannelDuplexHandler(){
            @Override
            public void channelRead(ChannelHandlerContext context, Object object)throws Exception {
                if(object instanceof PacketPlayInUseEntity){
                    PacketPlayInUseEntity packet = (PacketPlayInUseEntity) object;

                    Field f = packet.getClass().getDeclaredField("b");
                    f.setAccessible(true);

                    int z = (int) f.get(packet);
                    System.out.println("INTERACT-Z: "+z);
                }

                super.channelRead(context, object);
            }
        };

        channel.pipeline().addBefore("packet_handler", player.getName(), duplexHandler);
    }
}