MessageFilter.java

public class MessageFilter implements Listener {
	private static final String
			HANDLER = "packet_handler",
			CUSTOM_HANDLER = "custom_packet_handler";
	private static final Field
			FIELD_COMPONENT,
			FIELD_CONNECTION,
			FIELD_NETWORK,
			FIELD_CHANNEL;
	private static final Method
			METHOD_TEXT,
			METHOD_HANDLE;
	private static final Class<?>
			CLASS_PACKET_CHAT;
	
	static {
		try {
			final String cb = Bukkit.getServer().getClass().getPackage().getName();
			final String nms = "net.minecraft.server." + cb.substring(23);
			final Class<?> chatComponent = Class.forName(nms + ".IChatBaseComponent");
			CLASS_PACKET_CHAT = Class.forName(nms + ".PacketPlayOutChat");
			FIELD_COMPONENT = Arrays.stream(CLASS_PACKET_CHAT.getDeclaredFields())
					.filter(field -> field.getType() == chatComponent)
					.findFirst().orElseThrow(IllegalStateException::new);
			METHOD_TEXT = chatComponent.getDeclaredMethod("toPlainText");
			METHOD_HANDLE = Class.forName(cb + ".CraftPlayer").getDeclaredMethod("getHandle");
			FIELD_CONNECTION = METHOD_HANDLE.getReturnType().getDeclaredField("playerConnection");
			FIELD_NETWORK = FIELD_CONNECTION.getType().getDeclaredField("networkManager");
			FIELD_CHANNEL = FIELD_NETWORK.getType().getDeclaredField("channel");
		} catch (Throwable reason) {
			throw new RuntimeException(reason);
		}
	}
	
	private final Predicate<String> filter;
	private final Map<Player, Closeable> injections = new IdentityHashMap<>();
	
	public MessageFilter(Predicate<String> filter) {
		this.filter = filter;
	}
	
	@EventHandler
	void onJoin(PlayerJoinEvent event) {
		try {
			final ChannelPipeline pipeline = ((Channel) FIELD_CHANNEL.get(
					FIELD_NETWORK.get(
							FIELD_CONNECTION.get(
									METHOD_HANDLE.invoke(event.getPlayer())
							)
					)
			)).pipeline();
			final ChannelHandler handler = new ChannelDuplexHandler() {
				@Override
				public void write(ChannelHandlerContext context, Object packet, ChannelPromise promise) throws Exception {
					if (packet.getClass() == CLASS_PACKET_CHAT)
						if (!filter.test(METHOD_TEXT.invoke(FIELD_COMPONENT.get(packet)).toString()))
							return;
					super.write(context, packet, promise);
				}
			};
			injections.put(event.getPlayer(), () -> pipeline.remove(handler));
		} catch (Throwable reason) {
			throw new RuntimeException(reason);
		}
	}
	
	@EventHandler
	void onQuit(PlayerQuitEvent event) {
		try {
			final Closeable injection = injections.remove(event.getPlayer());
			if (injection != null)
				injection.close();
		} catch (Throwable reason) {
			throw new RuntimeException(reason);
		}
	}
}