DefaultChannelGroup 是getty内部提供用于保存通道的一个group类，用于保存通道对象以备方便使用。

示例：

public class SimpleHandler extends SimpleChannelInboundHandler<MessageClass.Message> {
    //实例化一个group保存客户端连接
    DefaultChannelGroup defaultChannelGroup = new DefaultChannelGroup();
    @Override
    public void channelAdded(AioChannel aioChannel) {
        System.out.println("连接过来了");
        //把连接保存起来以备使用
        defaultChannelGroup.add(aioChannel);
        //可以通过AioChannel的channelId获取通道。比如与用户映射起来
        AioChannel tempChannel = defaultChannelGroup.find(aioChannel.getChannelId());
        tempChannel.writeAndFlush("123".getBytes());
    }
    @Override
    public void channelClosed(AioChannel aioChannel) {
        System.out.println("连接关闭了");
    }
    @Override
    public void channelRead0(AioChannel aioChannel, MessageClass.Message str) {
        System.out.println("读取消息:" + str.getId());
    }
    @Override
    public void exceptionCaught(AioChannel aioChannel, Throwable cause, PipelineDirection pipelineDirection) {
        System.out.println("出错了");
    }
}