SimpleChannel

SimpleChannel is the name given to the packet system that revolves around the SimpleChannel class. Using this system is by far the easiest way to send custom data between clients and the server.

Getting Started

First you need to create your SimpleChannel object. We recommend that you do this in a separate class, possibly something like ModidPacketHandler. Create your SimpleChannel as a static field in this class, like so:

private static final String PROTOCOL_VERSION = "1";
public static final SimpleChannel INSTANCE = NetworkRegistry.newSimpleChannel(
    new ResourceLocation("mymodid", "main"),
    () -> PROTOCOL_VERSION,
    PROTOCOL_VERSION::equals,
    PROTOCOL_VERSION::equals
);

The first argument is a name for the channel. The second argument is a Supplier<String> returning the current network protocol version. The third and fourth arguments respectively are Predicate<String> checking whether an incoming connection protocol version is network-compatible with the client or server, respectively. Here, we simply compare with the PROTOCOL_VERSION field directly, meaning that the client and server PROTOCOL_VERSIONs must always match or FML will deny login.

Protocol Versions

If your mod does not require the other side to have a specific network channel, or to be a Forge instance at all, you should take care that you properly define your version compatibility checkers (the Predicate<String> parameters) to handle additional "meta-versions" (defined in NetworkRegistry) that can be received by the version checker. These are:

• ABSENT - if this channel is missing on the other endpoint. Note that in this case, the endpoint is still a Forge endpoint, and may have other mods.
• ACCEPTVANILLA - if the endpoint is a vanilla (or non-Forge) endpoint.

Returning false for both means that this channel must be present on the other endpoint. If you just copy the code above, this is what it does. Note that these values are also used during the list ping compatibility check, which is responsible for showing the green check / red cross in the multiplayer server select screen.

Registering Packets

Next, we must declare the types of messages that we would like to send and receive. This is done using the INSTANCE.registerMessage method, which takes 5 parameters.

• The first parameter is the discriminator for the packet. This is a per-channel unique ID for the packet. We recommend you use a local variable to hold the ID, and then call registerMessage using id++. This will guarantee 100% unique IDs.
• The second parameter is the actual packet class MSG.
• The third parameter is a BiConsumer<MSG, FriendlyByteBuf> responsible for encoding the message into the provided FriendlyByteBuf
• The fourth parameter is a Function<FriendlyByteBuf, MSG> responsible for decoding the message from the provided FriendlyByteBuf
• The final parameter is a BiConsumer<MSG, Supplier<NetworkEvent.Context>> responsible for handling the message itself

The last three parameters can be method references to either static or instance methods in Java. Remember that an instance method MSG.encode(FriendlyByteBuf) still satisfies BiConsumer<MSG, FriendlyByteBuf>, the MSG simply becomes the implicit first argument.

class MessagePacket {
   public void encoder(FriendlyByteBuf buffer) {
      // Write to buffer
   }

   public static MessagePacket decoder(FriendlyByteBuf buffer) {
      // Create packet from buffer data
   }

   public void messageConsumer(Supplier<NetworkEvent.Context> ctx) {
      // Handle message
   }
}

// Original direct registration syntax
INSTANCE.registerMessage(id, MessagePacket.class,
   MessagePacket::encoder,
   MessagePacket::decoder,
   MessagePacket::messageConsumer);
   // Consumer here must use enqueueWork and setPacketHandled

// New builder-based definition
INSTANCE.messageBuilder(MessagePacket.class, id)
   .encoder(MessagePacket::encoder)
   .decoder(MessagePacket::decoder)
   .consumerMainThread(MessagePacket::messageConsumer)
   .add();
// You can use consumerMainThread or consumerNetworkThread.
// If you use consumerMainThread, the builder will take care of the enqueueWork and setPacketHandled.
// With consumerNetworkThread, you can return a value instead of calling setPacketHandled.

Handling Packets

There are a couple things to highlight in a packet handler. A packet handler has both the message object and the network context available to it. The context allows access to the player that sent the packet (if on the server), and a way to enqueue threadsafe work.

public static void handle(MyMessage msg, Supplier<NetworkEvent.Context> ctx) {
    ctx.get().enqueueWork(() -> {
        // Work that needs to be threadsafe (most work)
        ServerPlayer sender = ctx.get().getSender(); // the client that sent this packet
        // do stuff
    });
    ctx.get().setPacketHandled(true);
}

Packets sent from the server to the client should be handled in another class and wrapped via DistExecutor#unsafeRunWhenOn.

// In Packet class
public static void handle(MyClientMessage msg, Supplier<NetworkEvent.Context> ctx) {
    ctx.get().enqueueWork(() ->
        // Make sure it's only executed on the physical client
        DistExecutor.unsafeRunWhenOn(Dist.CLIENT, () -> () -> ClientPacketHandlerClass.handlePacket(msg, ctx))
    );
    ctx.get().setPacketHandled(true);
}

// In ClientPacketHandlerClass
public static void handlePacket(MyClientMessage msg, Supplier<NetworkEvent.Context> ctx) {
    // Do stuff
}

Note the presence of #setPacketHandled, which used to tell the network system that the packet has successfully completed handling.

Common Packet Handling Pitfalls

Know that packets are by default handled on the network thread.

That means that your handler can not interact with most game objects directly. Forge provides a convenient way to make your code execute on the main thread through the supplied NetworkEvent$Context.

Simply call NetworkEvent$Context#enqueueWork(Runnable), which will call the given Runnable on the main thread at the next opportunity.

Be defensive when handling packets on the server.

A client could attempt to exploit the packet handling by sending unexpected data.
A common problem is vulnerability to arbitrary chunk generation. This typically happens when the server is trusting a block position sent by a client to access blocks and block entities. When accessing blocks and block entities in unloaded areas of the level, the server will either generate or load this area from disk, then promply write it to disk. This can be exploited to cause catastrophic damage to a server's performance and storage space without leaving a trace.

To avoid this problem, a general rule of thumb is to only access blocks and block entities if Level#hasChunkAt is true.

Register encoders on the physical client, as well as the physical server.

If you only register an encoder for a clientbound packet on the physical server, your mod will likely crash or present unintended behaviour in single-player worlds. Forge will happily send packets that have no encoder defined, without a warning or error message! They will be encoded as a 256-long byte buffer filled only with null bytes, by default.

Sending Packets

Sending to the Server

There is but one way to send a packet to the server. This is because there is only ever *one* server the client can be connected to at once. To do so, we must again use that SimpleChannel that was defined earlier. Simply call INSTANCE.sendToServer(new MyMessage()). The message will be sent to the handler for its type, if one exists.

Sending to Clients

Packets can be sent directly to a client using the SimpleChannel: HANDLER.sendTo(MSG, serverPlayer.connection.getConnection(), NetworkDirection.PLAY_TO_CLIENT). However, this can be quite inconvenient. Forge has some convenience functions that can be used:

// Sending to one player
INSTANCE.send(PacketDistributor.PLAYER.with(serverPlayer), new MyMessage());

// Send to all players tracking this level chunk
INSTANCE.send(PacketDistributor.TRACKING_CHUNK.with(levelChunk), new MyMessage());

// Sending to all connected players
INSTANCE.send(PacketDistributor.ALL.noArg(), new MyMessage());

There are additional PacketDistributor types available, check the documentation on the PacketDistributor class for more details.