TinyProtocol fixed for 1.17+

BukkitReflection.java

public class BukkitReflection {
    public static ServerPlayer getServerPlayer(Player p) {
        var m = Reflection.getMethodCached("{obc}.entity.CraftPlayer", "getHandle");
        return invoke(m, p);
    }
    private static <T> T invoke(java.lang.reflect.Method m, Object instance, Object... args) {
        try {
            return (T) m.invoke(instance, args);
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }
}

Reflection.java

import java.lang.reflect.*;
import java.security.AccessController;
import java.security.PrivilegedAction;

// Use Kristian's (Original author of TinyProtocol) Reflection class, it's good and has everything you'll need
public class Reflection {
    public static < T extends AccessibleObject > T setAccessible(T object, boolean access) {
        AccessController.doPrivileged((PrivilegedAction)() - > {
            object.setAccessible(access);
            return null;
        });
        return object;
    }
}

TinyProtocol.java

import com.google.common.collect.Lists;
import com.google.common.collect.MapMaker;
import com.mojang.authlib.GameProfile;
import io.netty.channel.*;
import net.blackofworld.sneakybastard.Utils.BukkitReflection;
import net.blackofworld.sneakybastard.Utils.Reflection;
import net.minecraft.network.protocol.login.ClientboundGameProfilePacket;
import net.minecraft.server.MinecraftServer;
import net.minecraft.server.network.ServerConnectionListener;
import net.minecraft.server.network.ServerGamePacketListenerImpl;
import org.bukkit.entity.Player;
import org.bukkit.event.EventHandler;
import org.bukkit.event.EventPriority;
import org.bukkit.event.HandlerList;
import org.bukkit.event.Listener;
import org.bukkit.event.player.PlayerJoinEvent;
import org.bukkit.event.server.PluginDisableEvent;
import org.bukkit.plugin.Plugin;
import org.bukkit.scheduler.BukkitRunnable;

import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Modifier;
import java.util.*;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.logging.Level;

/**
 * Represents a very tiny alternative to ProtocolLib.
 * <p>
 * It now supports intercepting packets during login and status ping (such as OUT_SERVER_PING)!
 *
 * @author Kristian
 */
public abstract class TinyProtocol {
    private static final AtomicInteger ID = new AtomicInteger(0);

    private static Field getNetworkMarkers = null;
    private static Field getChannels = null;

    protected volatile boolean closed;
    protected Plugin plugin;
    // Speedup channel lookup
    private Map < String, Channel > channelLookup = new MapMaker().weakValues().makeMap();
    private Listener listener;
    // Channels that have already been removed
    private Set < Channel > uninjectedChannels = Collections.newSetFromMap(new MapMaker().weakKeys().makeMap());
    // List of network markers
    private List < Object > networkManagers;
    // Injected channel handlers
    private List < Channel > serverChannels = Lists.newArrayList();
    private ChannelInboundHandlerAdapter serverChannelHandler;
    private ChannelInitializer < Channel > beginInitProtocol;
    private ChannelInitializer < Channel > endInitProtocol;
    // Current handler name
    private String handlerName;
    static {
        var fields = ServerConnectionListener.class.getDeclaredFields();
        for (var field: fields) {
            var mod = field.getModifiers();
            if (Modifier.isFinal(mod) && Modifier.isPrivate(mod) && field.getType().equals(List.class)) {
                getChannels = field;
                Reflection.setAccessible(getChannels, true);
                continue;
            } else if (getChannels != null && Modifier.isFinal(mod) && field.getType().equals(List.class)) {
                getNetworkMarkers = field;
                Reflection.setAccessible(getNetworkMarkers, true);
                break;
            }
        }
    }

    /**
     * Construct a new instance of TinyProtocol, and start intercepting packets for all connected clients and future clients.
     * <p>
     * You can construct multiple instances per plugin.
     *
     * @param plugin - the plugin.
     */
    public TinyProtocol(final Plugin plugin) {
        this.plugin = plugin;

        // Compute handler name
        this.handlerName = getHandlerName();

        // Prepare existing players
        registerBukkitEvents();

        try {
            try {
                registerChannelHandler();
                registerPlayers(plugin);
            } catch (InvocationTargetException e) {
                e.printStackTrace();
            } catch (IllegalAccessException e) {
                e.printStackTrace();
            }
        } catch (IllegalArgumentException ex) {
            // Damn you, late bind
            new BukkitRunnable() {
                @Override
                public void run() {
                    try {
                        registerChannelHandler();
                        registerPlayers(plugin);
                    } catch (InvocationTargetException e) {
                        e.printStackTrace();
                    } catch (IllegalAccessException e) {
                        e.printStackTrace();
                    }
                }
            }.runTask(plugin);
        }
    }

    private void createServerChannelHandler() {
        // Handle connected channels
        endInitProtocol = new ChannelInitializer < Channel > () {

            @Override
            protected void initChannel(Channel channel) throws Exception {
                try {
                    // This can take a while, so we need to stop the main thread from interfering
                    synchronized(networkManagers) {
                        // Stop injecting channels
                        if (!closed) {
                            channel.eventLoop().submit(() - > injectChannelInternal(channel));
                        }
                    }
                } catch (Exception e) {}
            }

        };

        // This is executed before Minecraft's channel handler
        beginInitProtocol = new ChannelInitializer < Channel > () {

            @Override
            protected void initChannel(Channel channel) throws Exception {
                channel.pipeline().addLast(endInitProtocol);
            }

        };
        serverChannelHandler = new ChannelInboundHandlerAdapter() {

            @Override
            public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
                Channel channel = (Channel) msg;
                // Prepare to initialize the channel
                channel.pipeline().addFirst(beginInitProtocol);
                //channel.pipeline().addFirst(customPacketHandler); // TODO: work on this
                ctx.fireChannelRead(msg);
            }

        };
    }

    /**
     * Register bukkit events.
     */
    private void registerBukkitEvents() {
        listener = new Listener() {

            @EventHandler(priority = EventPriority.LOWEST)
            public final void onPlayerJoin(PlayerJoinEvent e) throws InvocationTargetException, IllegalAccessException {
                if (closed)
                    return;

                Channel channel = getChannel(e.getPlayer());

                // Don't inject players that have been explicitly uninjected
                if (!uninjectedChannels.contains(channel)) {
                    injectPlayer(e.getPlayer());
                }
            }

            @EventHandler
            public final void onPluginDisable(PluginDisableEvent e) throws InvocationTargetException, IllegalAccessException {
                if (e.getPlugin().equals(plugin)) {
                    close();
                }
            }

        };

        plugin.getServer().getPluginManager().registerEvents(listener, plugin);
    }

    @SuppressWarnings("unchecked")
    private void registerChannelHandler() throws InvocationTargetException, IllegalAccessException {
        ServerConnectionListener serverConnection = MinecraftServer.getServer().getConnection();
        boolean looking = true;

        // We need to synchronize against this list
        networkManagers = (List < Object > ) getNetworkMarkers.get(serverConnection);
        createServerChannelHandler();

        // Find the correct list, or implicitly throw an exception
        for (int i = 0; looking; i++) {
            List < Object > list = (List < Object > ) getChannels.get(serverConnection);

            for (Object item: list) {
                if (!(item instanceof ChannelFuture))
                    break;

                // Channel future that contains the server connection
                Channel serverChannel = ((ChannelFuture) item).channel();

                serverChannels.add(serverChannel);
                serverChannel.pipeline().addFirst(serverChannelHandler);
                looking = false;
            }
        }
    }

    private void unregisterChannelHandler() {
        if (serverChannelHandler == null)
            return;

        for (Channel serverChannel: serverChannels) {
            final ChannelPipeline pipeline = serverChannel.pipeline();

            // Remove channel handler
            serverChannel.eventLoop().execute(new Runnable() {

                @Override
                public void run() {
                    try {
                        pipeline.remove(serverChannelHandler);
                    } catch (NoSuchElementException e) {
                        // That's fine
                    }
                }

            });
        }
    }

    private void registerPlayers(Plugin plugin) throws InvocationTargetException, IllegalAccessException {
        for (Player player: plugin.getServer().getOnlinePlayers()) {
            injectPlayer(player);
        }
    }

    /**
     * Invoked when the server is starting to send a packet to a player.
     * <p>
     * Note that this is not executed on the main thread.
     *
     * @param receiver - the receiving player, NULL for early login/status packets.
     * @param channel  - the channel that received the packet. Never NULL.
     * @param packet   - the packet being sent.
     * @return The packet to send instead, or NULL to cancel the transmission.
     */
    public Object onPacketOutAsync(Player receiver, Channel channel, Object packet) {
        return packet;
    }

    /**
     * Invoked when the server has received a packet from a given player.
     * <p>
     * Use {@link Channel#remoteAddress()} to get the remote address of the client.
     *
     * @param sender  - the player that sent the packet, NULL for early login/status packets.
     * @param channel - channel that received the packet. Never NULL.
     * @param packet  - the packet being received.
     * @return The packet to recieve instead, or NULL to cancel.
     */
    public Object onPacketInAsync(Player sender, Channel channel, Object packet) {
        return packet;
    }

    /**
     * Send a packet to a particular player.
     * <p>
     * Note that {@link #onPacketOutAsync(Player, Channel, Object)} will be invoked with this packet.
     *
     * @param player - the destination player.
     * @param packet - the packet to send.
     */
    public void sendPacket(Player player, Object packet) throws InvocationTargetException, IllegalAccessException {
        sendPacket(getChannel(player), packet);
    }

    /**
     * Send a packet to a particular client.
     * <p>
     * Note that {@link #onPacketOutAsync(Player, Channel, Object)} will be invoked with this packet.
     *
     * @param channel - client identified by a channel.
     * @param packet  - the packet to send.
     */
    public void sendPacket(Channel channel, Object packet) {
        channel.pipeline().writeAndFlush(packet);
    }

    /**
     * Pretend that a given packet has been received from a player.
     * <p>
     * Note that {@link #onPacketInAsync(Player, Channel, Object)} will be invoked with this packet.
     *
     * @param player - the player that sent the packet.
     * @param packet - the packet that will be received by the server.
     */
    public void receivePacket(Player player, Object packet) throws InvocationTargetException, IllegalAccessException {
        receivePacket(getChannel(player), packet);
    }

    /**
     * Pretend that a given packet has been received from a given client.
     * <p>
     * Note that {@link #onPacketInAsync(Player, Channel, Object)} will be invoked with this packet.
     *
     * @param channel - client identified by a channel.
     * @param packet  - the packet that will be received by the server.
     */
    public void receivePacket(Channel channel, Object packet) {
        channel.pipeline().context("encoder").fireChannelRead(packet);
    }

    /**
     * Retrieve the name of the channel injector, default implementation is "tiny-" + plugin name + "-" + a unique ID.
     * <p>
     * Note that this method will only be invoked once. It is no loger necessary to override this to support multiple instances.
     *
     * @return A unique channel handler name.
     */
    protected String getHandlerName() {
        return plugin.getName() + "-" + ID.incrementAndGet();
    }

    /**
     * Add a custom channel handler to the given player's channel pipeline, allowing us to intercept sent and received packets.
     * <p>
     * This will automatically be called when a player has logged in.
     *
     * @param player - the player to inject.
     */
    public void injectPlayer(Player player) throws InvocationTargetException, IllegalAccessException {
        injectChannelInternal(getChannel(player)).player = player;
    }

    /**
     * Add a custom channel handler to the given channel.
     *
     * @param channel - the channel to inject.
     * @return The intercepted channel, or NULL if it has already been injected.
     */
    public void injectChannel(Channel channel) {
        injectChannelInternal(channel);
    }

    /**
     * Add a custom channel handler to the given channel.
     *
     * @param channel - the channel to inject.
     * @return The packet interceptor.
     */
    private PacketInterceptor injectChannelInternal(Channel channel) {
        try {
            PacketInterceptor interceptor = (PacketInterceptor) channel.pipeline().get(handlerName);

            // Inject our packet interceptor
            if (interceptor == null) {
                interceptor = new PacketInterceptor();
                channel.pipeline().addBefore("packet_handler", handlerName, interceptor);
                uninjectedChannels.remove(channel);
            }

            return interceptor;
        } catch (IllegalArgumentException e) {
            // Try again
            return (PacketInterceptor) channel.pipeline().get(handlerName);
        }
    }

    /**
     * Retrieve the Netty channel associated with a player. This is cached.
     *
     * @param player - the player.
     * @return The Netty channel.
     */
    public Channel getChannel(Player player) throws IllegalAccessException, InvocationTargetException {
        Channel channel = channelLookup.get(player.getName());

        // Lookup channel again
        if (channel == null) {

            ServerGamePacketListenerImpl connection = BukkitReflection.getServerPlayer(player).connection;
            channelLookup.put(player.getName(), channel = connection.connection.channel);
        }

        return channel;
    }

    /**
     * Uninject a specific player.
     *
     * @param player - the injected player.
     */
    public void uninjectPlayer(Player player) throws InvocationTargetException, IllegalAccessException {
        uninjectChannel(getChannel(player));
    }

    /**
     * Uninject a specific channel.
     * <p>
     * This will also disable the automatic channel injection that occurs when a player has properly logged in.
     *
     * @param channel - the injected channel.
     */
    public void uninjectChannel(final Channel channel) {
        // No need to guard against this if we're closing
        if (!closed) {
            uninjectedChannels.add(channel);
        }

        // See ChannelInjector in ProtocolLib, line 590
        channel.eventLoop().execute(() - > channel.pipeline().remove(handlerName));
    }

    /**
     * Determine if the given player has been injected by TinyProtocol.
     *
     * @param player - the player.
     * @return TRUE if it is, FALSE otherwise.
     */
    public boolean hasInjected(Player player) throws InvocationTargetException, IllegalAccessException {
        return hasInjected(getChannel(player));
    }

    /**
     * Determine if the given channel has been injected by TinyProtocol.
     *
     * @param channel - the channel.
     * @return TRUE if it is, FALSE otherwise.
     */
    public boolean hasInjected(Channel channel) {
        return channel.pipeline().get(handlerName) != null;
    }

    /**
     * Cease listening for packets. This is called automatically when your plugin is disabled.
     */
    public final void close() throws InvocationTargetException, IllegalAccessException {
        if (!closed) {
            closed = true;

            // Remove our handlers
            for (Player player: plugin.getServer().getOnlinePlayers()) {
                uninjectPlayer(player);
            }

            // Clean up Bukkit
            HandlerList.unregisterAll(listener);
            unregisterChannelHandler();
        }
    }

    /**
     * Channel handler that is inserted into the player's channel pipeline, allowing us to intercept sent and received packets.
     *
     * @author Kristian
     */
    private final class PacketInterceptor extends ChannelDuplexHandler {
        // Updated by the login event
        public volatile Player player;

        @Override
        public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
            // Intercept channel
            final Channel channel = ctx.channel();
            handleLoginStart(channel, msg);

            try {
                msg = onPacketInAsync(player, channel, msg);
            } catch (Exception e) {
                plugin.getLogger().log(Level.SEVERE, "Error in onPacketInAsync().", e);
            }

            if (msg != null) {
                super.channelRead(ctx, msg);
            }
        }

        @Override
        public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
            try {
                msg = onPacketOutAsync(player, ctx.channel(), msg);
            } catch (Exception e) {
                plugin.getLogger().log(Level.SEVERE, "Error in onPacketOutAsync().", e);
            }

            if (msg != null) {
                super.write(ctx, msg, promise);
            }
        }

        private void handleLoginStart(Channel channel, Object packet) throws IllegalAccessException {
            if (ClientboundGameProfilePacket.class.isInstance(packet)) {
                GameProfile profile = ((ClientboundGameProfilePacket) packet).getGameProfile();
                channelLookup.put(profile.getName(), channel);
            }
        }
    }
}