需求场景：
智能家居网关(以下简称gateway)，需要和netty服务器通讯(以下简称netty)，netty和gateway之间需要保持长连接(换句话说，netty和gateway之间都会主动给对方发送消息)

碰到的问题:
netty作为服务器端如何主动的向gateway发送消息，我尝试当每个gateway连接到netty（TCP/IP）时使用一个map把该channelSocket的id和该channelSocket绑定在一起

    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        String uuid = ctx.channel().id().asLongText();
        GatewayService.addGatewayChannel(uuid, (SocketChannel)ctx.channel());
        System.out.println("a new connect come in: " + uuid);
    }

GatewayService其实就是一个ConcurrentHashMap

public class GatewayService {
    
    private static Map<String, SocketChannel> map = new ConcurrentHashMap<>();
    
    public static void addGatewayChannel(String id, SocketChannel gateway_channel){
        map.put(id, gateway_channel);
    }
    
    public static Map<String, SocketChannel> getChannels(){
        return map;
    }

    public static SocketChannel getGatewayChannel(String id){
        return map.get(id);
    }
    
    public static void removeGatewayChannel(String id){
        map.remove(id);
    }
}

我在服务器端尝试每间隔一段时间loop这个ConcurrentHashMap如果里面已经有绑定的channelSocket，就使用write方法向客户端发送消息

Runnable sendTask = new Runnable() {
            @Override
            public void run() {
                sendTaskLoop:
                    for(;;){
                        System.out.println("task is beginning...");
                        try{
                            Map<String, SocketChannel> map = GatewayService.getChannels();
                            Iterator<String> it = map.keySet().iterator();
                            while (it.hasNext()) {
                                String key = it.next();
                                SocketChannel obj = map.get(key);
                                System.out.println("channel id is: " + key);
                                System.out.println("channel: " + obj.isActive());
                                obj.writeAndFlush("hello, it is Server test header ping");
                            }
                        }catch(Exception e){break sendTaskLoop;}
                        try {
                            Thread.sleep(5000);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
            }
        };
        new Thread(sendTask).start();

理论上客户端应该是可以接受到我发送的消息，但是我观察了一下源代码，发现writeAndFlush这个方法最终会被handler触发，于是我又在handler中覆写了write方法

    @Override
    public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
        System.out.println("write handler");
        ctx.writeAndFlush(msg);
    }

可是最终结果客户端并没有收到任何消息，请问netty如何主动向客户端发送消息？