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, PacketBuffer> responsible for encoding the message into the provided PacketBuffer
• The fourth parameter is a Function<PacketBuffer, MSG> responsible for decoding the message from the provided PacketBuffer
• 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(PacketBuffer) still satisfies BiConsumer<MSG, PacketBuffer>, the MSG simply becomes the implicit first argument.

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)
        EntityPlayerMP sender = ctx.get().getSender(); // the client that sent this packet
        // do stuff
    });
    ctx.get().setPacketHandled(true);
}

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

Alert

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 instead using IThreadListener.addScheduledTask.

Simply call ctx.get().enqueueWork(Runnable), which will call the given Runnable on the main thread at the next opportunity.

Alert

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 tile entities. When accessing blocks and tile entities in unloaded areas of the world, 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 tile entities if world.isBlockLoaded(pos) is true.

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, entityPlayerMP.connection.getNetworkManager(), 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(playerMP), new MyMessage());

// Send to all players tracking this chunk
INSTANCE.send(PacketDistributor.TRACKING_CHUNK.with(chunk), 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.