Add basic implementation.

api/src/main/java/com/velocitypowered/api/proxy/Player.java

@@ -1,6 +1,7 @@
 package com.velocitypowered.api.proxy;
 
 import com.velocitypowered.api.command.CommandSource;
+import com.velocitypowered.api.proxy.messages.ChannelMessageSource;
 import com.velocitypowered.api.server.ServerInfo;
 import com.velocitypowered.api.util.MessagePosition;
 import net.kyori.text.Component;
@@ -12,7 +13,7 @@ import java.util.UUID;
 /**
  * Represents a player who is connected to the proxy.
  */
-public interface Player extends CommandSource, InboundConnection {
+public interface Player extends CommandSource, InboundConnection, ChannelMessageSource {
     /**
      * Returns the player's current username.
      * @return the username

api/src/main/java/com/velocitypowered/api/proxy/ProxyServer.java

@@ -4,6 +4,7 @@ import com.velocitypowered.api.command.CommandSource;
 import com.velocitypowered.api.command.CommandManager;
 import com.velocitypowered.api.event.EventManager;
 import com.velocitypowered.api.plugin.PluginManager;
+import com.velocitypowered.api.proxy.messages.ChannelRegistrar;
 import com.velocitypowered.api.scheduler.Scheduler;
 import com.velocitypowered.api.server.ServerInfo;
 
@@ -101,4 +102,10 @@ public interface ProxyServer {
      * @return the scheduler instance
      */
     Scheduler getScheduler();
+
+    /**
+     * Gets the {@link ChannelRegistrar} instance.
+     * @return the channel registrar
+     */
+    ChannelRegistrar getChannelRegistrar();
 }

api/src/main/java/com/velocitypowered/api/proxy/ServerConnection.java

@@ -1,11 +1,12 @@
 package com.velocitypowered.api.proxy;
 
+import com.velocitypowered.api.proxy.messages.ChannelMessageSource;
 import com.velocitypowered.api.server.ServerInfo;
 
 /**
  * Represents a connection to a backend server from the proxy for a client.
  */
-public interface ServerConnection {
+public interface ServerConnection extends ChannelMessageSource {
     ServerInfo getServerInfo();
 
     Player getPlayer();

api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelIdentifier.java

@@ -4,4 +4,5 @@ package com.velocitypowered.api.proxy.messages;
  * Represents a kind of channel identifier.
  */
 public interface ChannelIdentifier {
+    String getId();
 }

api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelMessageSource.java

@@ -0,0 +1,4 @@
+package com.velocitypowered.api.proxy.messages;
+
+public interface ChannelMessageSource {
+}

api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelRegistrar.java

@@ -1,4 +1,11 @@
 package com.velocitypowered.api.proxy.messages;
 
+/**
+ * Represents an interface to register and unregister {@link MessageHandler} instances for handling plugin messages from
+ * the client or the server.
+ */
 public interface ChannelRegistrar {
+    void register(MessageHandler handler, ChannelIdentifier... identifiers);
+
+    void unregister(ChannelIdentifier... identifiers);
 }

api/src/main/java/com/velocitypowered/api/proxy/messages/ChannelSide.java

@@ -0,0 +1,6 @@
+package com.velocitypowered.api.proxy.messages;
+
+public enum ChannelSide {
+    FROM_SERVER,
+    FROM_CLIENT
+}

api/src/main/java/com/velocitypowered/api/proxy/messages/LegacyChannelIdentifier.java

@@ -40,4 +40,9 @@ public final class LegacyChannelIdentifier implements ChannelIdentifier {
     public int hashCode() {
         return Objects.hash(name);
     }
+
+    @Override
+    public String getId() {
+        return name;
+    }
 }

api/src/main/java/com/velocitypowered/api/proxy/messages/MessageHandler.java

@@ -0,0 +1,10 @@
+package com.velocitypowered.api.proxy.messages;
+
+public interface MessageHandler {
+    ForwardStatus handle(ChannelMessageSource source, ChannelSide side, byte[] data);
+
+    enum ForwardStatus {
+        FORWARD,
+        HANDLED
+    }
+}

api/src/main/java/com/velocitypowered/api/proxy/messages/MinecraftChannelIdentifier.java

@@ -61,4 +61,9 @@ public final class MinecraftChannelIdentifier implements ChannelIdentifier {
     public int hashCode() {
         return Objects.hash(namespace, name);
     }
+
+    @Override
+    public String getId() {
+        return namespace + ":" + name;
+    }
 }

proxy/src/main/java/com/velocitypowered/proxy/VelocityServer.java

@@ -21,6 +21,7 @@ import com.velocitypowered.proxy.config.VelocityConfiguration;
 import com.velocitypowered.proxy.connection.client.ConnectedPlayer;
 import com.velocitypowered.proxy.connection.http.NettyHttpClient;
 import com.velocitypowered.proxy.command.VelocityCommandManager;
+import com.velocitypowered.proxy.messages.VelocityChannelRegistrar;
 import com.velocitypowered.proxy.plugin.VelocityEventManager;
 import com.velocitypowered.proxy.protocol.util.FaviconSerializer;
 import com.velocitypowered.proxy.plugin.VelocityPluginManager;
@@ -84,6 +85,7 @@ public class VelocityServer implements ProxyServer {
     private Ratelimiter ipAttemptLimiter;
     private VelocityEventManager eventManager;
     private VelocityScheduler scheduler;
+    private VelocityChannelRegistrar channelRegistrar;
 
     private VelocityServer() {
         commandManager.register(new VelocityCommand(), "velocity");
@@ -137,6 +139,7 @@ public class VelocityServer implements ProxyServer {
         httpClient = new NettyHttpClient(this);
         eventManager = new VelocityEventManager(pluginManager);
         scheduler = new VelocityScheduler(pluginManager, Sleeper.SYSTEM);
+        channelRegistrar = new VelocityChannelRegistrar();
         loadPlugins();
 
         try {
@@ -304,4 +307,9 @@ public class VelocityServer implements ProxyServer {
     public VelocityScheduler getScheduler() {
         return scheduler;
     }
+
+    @Override
+    public VelocityChannelRegistrar getChannelRegistrar() {
+        return channelRegistrar;
+    }
 }

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/BackendPlaySessionHandler.java

@@ -1,6 +1,8 @@
 package com.velocitypowered.proxy.connection.backend;
 
 import com.velocitypowered.api.event.player.ServerConnectedEvent;
+import com.velocitypowered.api.proxy.messages.ChannelSide;
+import com.velocitypowered.api.proxy.messages.MessageHandler;
 import com.velocitypowered.proxy.VelocityServer;
 import com.velocitypowered.proxy.connection.client.ClientPlaySessionHandler;
 import com.velocitypowered.proxy.protocol.MinecraftPacket;
@@ -65,7 +67,11 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
                 return;
             }
 
+            MessageHandler.ForwardStatus status = VelocityServer.getServer().getChannelRegistrar().handlePluginMessage(
+                    connection, ChannelSide.FROM_SERVER, pm);
+            if (status == MessageHandler.ForwardStatus.FORWARD) {
                 connection.getPlayer().getConnection().write(pm);
+            }
         } else {
             // Just forward the packet on. We don't have anything to handle at this time.
             connection.getPlayer().getConnection().write(packet);

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ClientPlaySessionHandler.java

@@ -1,6 +1,8 @@
 package com.velocitypowered.proxy.connection.client;
 
 import com.velocitypowered.api.event.connection.DisconnectEvent;
+import com.velocitypowered.api.proxy.messages.ChannelSide;
+import com.velocitypowered.api.proxy.messages.MessageHandler;
 import com.velocitypowered.proxy.VelocityServer;
 import com.velocitypowered.proxy.protocol.MinecraftPacket;
 import com.velocitypowered.proxy.protocol.ProtocolConstants;
@@ -220,9 +222,13 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
             return;
         }
 
+        MessageHandler.ForwardStatus status = VelocityServer.getServer().getChannelRegistrar().handlePluginMessage(
+                player, ChannelSide.FROM_CLIENT, packet);
+        if (status == MessageHandler.ForwardStatus.FORWARD) {
             // We're going to forward on the original packet.
             player.getConnectedServer().getMinecraftConnection().write(packet);
         }
+    }
 
     public Set<String> getClientPluginMsgChannels() {
         return clientPluginMsgChannels;

proxy/src/main/java/com/velocitypowered/proxy/messages/VelocityChannelRegistrar.java

@@ -0,0 +1,48 @@
+package com.velocitypowered.proxy.messages;
+
+import com.google.common.base.Preconditions;
+import com.velocitypowered.api.proxy.messages.*;
+import com.velocitypowered.proxy.protocol.packet.PluginMessage;
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
+
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+
+public class VelocityChannelRegistrar implements ChannelRegistrar {
+    private static final Logger logger = LogManager.getLogger(VelocityChannelRegistrar.class);
+    private final Map<String, MessageHandler> handlers = new ConcurrentHashMap<>();
+
+    @Override
+    public void register(MessageHandler handler, ChannelIdentifier... identifiers) {
+        for (ChannelIdentifier identifier : identifiers) {
+            Preconditions.checkArgument(identifier instanceof LegacyChannelIdentifier || identifier instanceof MinecraftChannelIdentifier,
+                    "identifier is unknown");
+        }
+
+        for (ChannelIdentifier identifier : identifiers) {
+            handlers.put(identifier.getId(), handler);
+        }
+    }
+
+    public MessageHandler.ForwardStatus handlePluginMessage(ChannelMessageSource source, ChannelSide side, PluginMessage message) {
+        MessageHandler handler = handlers.get(message.getChannel());
+        if (handler == null) {
+            // Nothing we can do.
+            return MessageHandler.ForwardStatus.FORWARD;
+        }
+
+        try {
+            return handler.handle(source, side, message.getData());
+        } catch (Exception e) {
+            logger.info("Unable to handle plugin message on channel {} for {}", message.getChannel(), source);
+            // In case of doubt, do not forward the message on.
+            return MessageHandler.ForwardStatus.HANDLED;
+        }
+    }
+
+    @Override
+    public void unregister(ChannelIdentifier... identifiers) {
+
+    }
+}