import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelPipeline;
import io.netty.channel.socket.SocketChannel;
public class ServerAdapterInitializer extends ChannelInitializer<SocketChannel> {

    @Override
    protected void initChannel(SocketChannel channel) throws Exception {
        ChannelPipeline pipeline = channel.pipeline();

        pipeline.addLast("encoder", new Encoder());//Кодирует
        pipeline.addLast("decoder", new Decoder());//Расшифровывает

        pipeline.addLast("handler", new ServerAdapterHandler());// ServerAdapterHandler() будет принимать все пакеты от клиентов.
    }
}

import com.sun.xml.internal.ws.api.message.Packet;
import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandlerContext;
import io.netty.handler.codec.MessageToByteEncoder;

public class Encoder extends MessageToByteEncoder<Packet> {

    @Override
    protected void encode(ChannelHandlerContext channelHandlerContext, Packet packet, ByteBuf byteBuf) throws Exception {

    }
}