Packet-sending code.

/**
 * Packet sent to players upon joining the server, for sending registry information as loaded or otherwise held by the
 * server.
 */
public class PlayerJoinPacket implements IMessage
{
    /**
     * The packet handler, for copying config information from the server into the client.
     */
    public static class Handler implements IMessageHandler<PlayerJoinPacket, IMessage>
    {
        @Override
        public IMessage onMessage(PlayerJoinPacket message, MessageContext ctx)
        {
            ArcaneBooks.instance.registries.spellEffects.loadFromString(message.unparsedSpellEffects);
            return null;
        }

        /*

        Version of the above method for Minecraft v1.8.

        @Override
        public IMessage onMessage(PlayerJoinPacket message, MessageContext ctx)
        {
            IThreadListener mainThread = Minecraft.getMinecraft();

            mainThread.addScheduledTask(new Runnable()
            {
                @Override
                public void run()
                { ArcaneBooks.instance.registries.spellEffects.fillFromString(message.unparsedSpellEffects); }
            });
            return null;
        }

        */
    }

    String unparsedSpellEffects = null;

    @Override
    public void fromBytes(ByteBuf buf)
    { unparsedSpellEffects = ByteBufUtils.readUTF8String(buf); }

    @Override
    public void toBytes(ByteBuf buf)
    { ByteBufUtils.writeUTF8String(buf, ArcaneBooks.instance.registries.spellEffects.toString()); }
}



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// In the main mod class.

    @Mod.EventHandler
    public void PreInitializationEvent(FMLPreInitializationEvent event)
    {
        packetChannel = NetworkRegistry.INSTANCE.newSimpleChannel("ArcaneBooks");
        packetChannel.registerMessage(PlayerJoinPacket.Handler.class, PlayerJoinPacket.class, 1, Side.CLIENT);
        FMLCommonHandler.instance().bus().register(new PlayerJoinServerListener());
    }



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/**
 * Listens to players logging into the server, so that the server can send the player a packet containing registry
 * information/contents as loaded or otherwise held by the server.
 */
public class PlayerJoinServerListener
{
    @SubscribeEvent
    public void onPlayerJoin(PlayerLoggedInEvent event)
    {
        boolean runningOnServer = !DimensionManager.getWorld(0).isRemote;

        if(runningOnServer)
            ArcaneBooks.instance.packetChannel.sendTo(new PlayerJoinPacket(), (EntityPlayerMP)event.player);
    }
}