利用netty-mqtt-client实现Mqtt消息的发布和订阅的java代码，并中文注释

以下是利用Netty-Mqtt-Client实现Mqtt消息发布和订阅的Java代码，并附有中文注释：

import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.SimpleChannelInboundHandler;
import io.netty.handler.codec.mqtt.*;
import io.netty.handler.timeout.IdleState;
import io.netty.handler.timeout.IdleStateEvent;
import io.netty.util.CharsetUtil;

import java.util.concurrent.TimeUnit;

public class MqttClientHandler extends SimpleChannelInboundHandler<MqttMessage> {

    private MqttClient mqttClient;

    public MqttClientHandler(MqttClient mqttClient) {
        this.mqttClient = mqttClient;
    }

    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        // 当连接建立成功后，发送连接消息
        MqttConnectMessage connectMessage = MqttMessageBuilders.connect()
                .clientId(mqttClient.getClientId())
                .protocolVersion(MqttVersion.MQTT_3_1_1)
                .build();
        ctx.writeAndFlush(connectMessage);
    }

    @Override
    public void channelRead0(ChannelHandlerContext ctx, MqttMessage mqttMessage) throws Exception {
        // 处理收到的消息
        switch (mqttMessage.fixedHeader().messageType()) {
            case CONNACK:
                // 当收到连接确认消息后，订阅主题
                MqttSubscribeMessage subscribeMessage = MqttMessageBuilders.subscribe()
                        .addSubscription(MqttQoS.AT_LEAST_ONCE, mqttClient.getTopic())
                        .messageId(mqttClient.getNextMessageId())
                        .build();
                ctx.writeAndFlush(subscribeMessage);
                break;
            case SUBACK:
                // 当收到订阅确认消息后，发送消息
                String message = "Hello from " + mqttClient.getClientId();
                MqttPublishMessage publishMessage = MqttMessageBuilders.publish()
                        .topicName(mqttClient.getTopic())
                        .qos(MqttQoS.AT_LEAST_ONCE)
                        .payload(message.getBytes(CharsetUtil.UTF_8))
                        .messageId(mqttClient.getNextMessageId())
                        .build();
                ctx.writeAndFlush(publishMessage);
                break;
            case PUBLISH:
                // 当收到消息时，输出消息内容
                MqttPublishMessage publish = (MqttPublishMessage) mqttMessage;
                String payload = publish.payload().toString(CharsetUtil.UTF_8);
                System.out.println("Received message: " + payload);
                break;
            default:
                break;
        }
    }

    @Override
    public void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception {
        // 当连接空闲时，发送心跳包
        if (evt instanceof IdleStateEvent) {
            IdleStateEvent event = (IdleStateEvent) evt;
            if (event.state() == IdleState.WRITER_IDLE) {
                MqttMessage pingReqMessage = MqttMessageBuilders.pingreq().build();
                ctx.writeAndFlush(pingReqMessage);
            }
        }
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        // 当出现异常时，输出异常信息
        cause.printStackTrace();
        ctx.close();
    }
}

其中，MqttClient是自定义的类，用于存储Mqtt连接相关信息，包括clientId和topic等。在channelActive方法中，发送连接消息；在channelRead0方法中，根据不同的消息类型处理收到的消息；在userEventTriggered方法中，当连接空闲时发送心跳包；在exceptionCaught方法中，处理异常。这里实现的是Mqtt发布和订阅功能，当连接建立成功后，会订阅指定的主题，然后发送一条消息，当收到消息时会输出消息内容。