@ChannelHandler.Sharable
public class ClientLogin2Handler extends SimpleChannelInboundHandler<LoginResBean> {
    //1.构造函数私有化，避免创建实体
    private ClientLogin2Handler(){}
    //2.定义一个静态全局变量
    public static ClientLogin2Handler instance=null;
    //3.获取实体方法
    public static ClientLogin2Handler getInstance(){
        if(instance==null){
            synchronized (ClientLogin2Handler.class){
                if(instance==null){
                    instance=new ClientLogin2Handler();
                }
            }
        }
        return instance;
    }

    protected void channelRead0(
        ChannelHandlerContext channelHandlerContext, 
        LoginResBean loginResBean) throws Exception {

        //具体业务代码，参考之前
    }
}

@ChannelHandler.Sharable
public class ClientMsgHandler extends SimpleChannelInboundHandler<MsgResBean> {
    //1.构造函数私有化，避免创建实体
    private ClientMsgHandler(){}
    //2.定义一个静态全局变量
    public static ClientMsgHandler instance=null;
    //3.获取实体方法
    public static ClientMsgHandler getInstance(){
        if(instance==null){
            synchronized (ClientMsgHandler.class){
                if(instance==null){
                    instance=new ClientMsgHandler();
                }
            }
        }
        return instance;
    }

    protected void channelRead0(
        ChannelHandlerContext channelHandlerContext, 
        MsgResBean msgResBean) throws Exception {

        //具体业务代码，参考之前
    }
}

.handler(new ChannelInitializer<SocketChannel>() {
    @Override
    public void initChannel(SocketChannel ch) {
        //1.拆包器
        ch.pipeline().addLast(new LengthFieldBasedFrameDecoder(Integer.MAX_VALUE,5,4));
        //2.解码器
        ch.pipeline().addLast(new MyDecoder());
        //3.登录Handler，使用单例获取
        ch.pipeline().addLast(ClientLogin2Handler.getInstance());
        //4.消息发送Handler,使用单例获取
        ch.pipeline().addLast(ClientMsgHandler.getInstance());
        //5.编码器
        ch.pipeline().addLast(new MyEncoder());
    }
});

//1.定义一个集合存放数据（真实项目可以存放数据库或者redis缓存），这样数据比较安全。
private List<Map<Integer,String>> datas=new ArrayList<Map<Integer,String>>();

//2.服务端推送消息
private void pushMsg(MsgReqBean bean,Channel channel){
    Integer touserid=bean.getTouserid();
    Channel c=map.get(touserid);

    if(c==null){//对方不在线
        //2.1存放到list集合
        Map<Integer,String> data=new HashMap<Integer, String>();
        data.put(touserid,bean.getMsg());
        datas.add(data);

        //2.2.给消息“发送人”响应
        MsgResBean res=new MsgResBean();
        res.setStatus(1);
        res.setMsg(touserid+">>>不在线");
        channel.writeAndFlush(res);

    }else{//对方在线
        //2.3.给消息“发送人”响应
        MsgResBean res=new MsgResBean();
        res.setStatus(0);
        res.setMsg("发送成功);
        channel.writeAndFlush(res);

        //2.4.给接收人推送消息
        MsgRecBean res=new MsgRecBean();
        res.setFromuserid(bean.getFromuserid());
        res.setMsg(bean.getMsg());
        c.writeAndFlush(res);
    }
}

private void login(LoginReqBean bean, Channel channel){
    Channel c=map.get(bean.getUserid());
    LoginResBean res=new LoginResBean();
    if(c==null){
        //1.添加到map
        map.put(bean.getUserid(),channel);
        //2.给通道赋值
        channel.attr(AttributeKey.valueOf("userid")).set(bean.getUserid());
        //3.登录响应
        res.setStatus(0);
        res.setMsg("登录成功");
        res.setUserid(bean.getUserid());
        channel.writeAndFlush(res);

        //4.根据user查找是否有尚未推送消息
        //思路：根据userid去lists查找.......

    }else{
        res.setStatus(1);
        res.setMsg("该账户目前在线");
        channel.writeAndFlush(res);
    }
}

public class ServerChatGroupHandler extends ChannelInboundHandlerAdapter {
    //映射关系
    private static Map<Integer, Channel> map=new HashMap<Integer, Channel>();

    //连接断开，触发该事件
    @Override
    public void channelInactive(ChannelHandlerContext ctx) throws Exception {
        //1.获取Channel
        Channel channel=ctx.channel();

        //2.从map里面，根据Channel找到对应的userid
        Integer userid=null;
        for(Map.Entry<Integer, Channel> entry : map.entrySet()){
            Integer uid=entry.getKey();
            Channel c=entry.getValue();
            if(c==channel){
                userid=uid;
            }
        }
        //3.如果userid不为空，则需要做以下处理
        if(userid!=null){
            //3.1.删除映射
            map.remove(userid);
            //3.2.移除标识
            ctx.channel().attr(AttributeKey.valueOf("userid")).remove();
        }
    }
}

public class ServerChatGroupHandler extends ChannelInboundHandlerAdapter {
    //映射关系
    private static Map<Integer, Channel> map=new HashMap<Integer, Channel>();

    //连接断开，触发该事件
    @Override
    public void channelInactive(ChannelHandlerContext ctx) throws Exception {
        //1.获取Channel绑定的userid
        Object userid=channel.attr(AttributeKey.valueOf("userid")).get();

        //2.如果userid不为空
        if(userid!=null){
            //1.删除映射
            map.remove(userid);
            //2.移除标识
            ctx.channel().attr(AttributeKey.valueOf("userid")).remove();
        }
    }
}