Add basic implementation.

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;
             }
 
-            connection.getPlayer().getConnection().write(pm);
+            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;
@@ -220,8 +222,12 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
             return;
         }
 
-        // We're going to forward on the original packet.
-        player.getConnectedServer().getMinecraftConnection().write(packet);
+        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() {

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) {
+
+    }
+}