用Netty-Mqtt-Client实现Mqtt客户端发布消息和订阅消息的核心Java代码，要注释说明

好的，以下是使用Netty-Mqtt-Client实现Mqtt客户端发布消息和订阅消息的核心Java代码，带注释说明：

import io.netty.buffer.Unpooled;
import io.netty.handler.codec.mqtt.*;
import io.netty.handler.codec.mqtt.MqttQoS;
import io.netty.handler.codec.mqtt.MqttPublishMessage;
import io.netty.handler.codec.mqtt.MqttSubscribeMessage;
import io.netty.handler.codec.mqtt.MqttUnsubscribeMessage;
import io.netty.handler.codec.mqtt.MqttMessageBuilders.*;

// 创建一个Mqtt客户端类
public class MqttClient {

    private final String clientId; // 客户端ID
    private final String serverHost; // 服务器主机名
    private final int serverPort; // 服务器端口号
    private final String username; // 用户名
    private final String password; // 密码
    private final int keepAlive; // 心跳间隔时间

    private EventLoopGroup group; // Netty线程组
    private MqttClientInitializer initializer; // Netty客户端初始化器
    private Channel channel; // Netty通道

    // 构造方法，初始化Mqtt客户端配置
    public MqttClient(String clientId, String serverHost, int serverPort, String username, String password, int keepAlive) {
        this.clientId = clientId;
        this.serverHost = serverHost;
        this.serverPort = serverPort;
        this.username = username;
        this.password = password;
        this.keepAlive = keepAlive;
    }

    // 连接服务器
    public void connect() {
        group = new NioEventLoopGroup(); // 创建Netty线程组
        initializer = new MqttClientInitializer(clientId, username, password, keepAlive); // 创建Netty客户端初始化器
        Bootstrap bootstrap = new Bootstrap(); // 创建Netty客户端启动器
        bootstrap.group(group)
                .channel(NioSocketChannel.class)
                .remoteAddress(serverHost, serverPort)
                .handler(initializer);

        try {
            ChannelFuture future = bootstrap.connect().sync(); // 连接服务器，同步等待连接完成
            if (future.isSuccess()) { // 连接成功
                channel = future.channel(); // 获取Netty通道
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    // 断开连接
    public void disconnect() {
        if (channel != null && channel.isActive()) {
            channel.close(); // 关闭Netty通道
        }
        if (group != null) {
            group.shutdownGracefully(); // 关闭Netty线程组
        }
    }

    // 发布消息
    public void publish(String topic, String message, MqttQoS qos) {
        MqttFixedHeader header = new MqttFixedHeader(MqttMessageType.PUBLISH, false, qos, false, 0);
        MqttPublishVariableHeader variableHeader = new MqttPublishVariableHeader(topic, 0);
        ByteBuf payload = Unpooled.buffer();
        payload.writeBytes(message.getBytes());
        MqttPublishMessage publishMessage = new MqttPublishMessage(header, variableHeader, payload);
        channel.writeAndFlush(publishMessage); // 发送Mqtt PUBLISH消息
    }

    // 订阅主题
    public void subscribe(String topic, MqttQoS qos) {
        MqttFixedHeader header = new MqttFixedHeader(MqttMessageType.SUBSCRIBE, false, MqttQoS.AT_LEAST_ONCE, false, 0);
        MqttMessageIdVariableHeader variableHeader = MqttMessageIdVariableHeader.from(1);
        MqttTopicSubscription topicSubscription = new MqttTopicSubscription(topic, qos);
        MqttSubscribePayload payload = new MqttSubscribePayload(Arrays.asList(topicSubscription));
        MqttSubscribeMessage subscribeMessage = new MqttSubscribeMessage(header, variableHeader, payload);
        channel.writeAndFlush(subscribeMessage); // 发送Mqtt SUBSCRIBE消息
    }

    // 取消订阅主题
    public void unsubscribe(String topic) {
        MqttFixedHeader header = new MqttFixedHeader(MqttMessageType.UNSUBSCRIBE, false, MqttQoS.AT_LEAST_ONCE, false, 0);
        MqttMessageIdVariableHeader variableHeader = MqttMessageIdVariableHeader.from(1);
        MqttUnsubscribePayload payload = new MqttUnsubscribePayload(Arrays.asList(topic));
        MqttUnsubscribeMessage unsubscribeMessage = new MqttUnsubscribeMessage(header, variableHeader, payload);
        channel.writeAndFlush(unsubscribeMessage); // 发送Mqtt UNSUBSCRIBE消息
    }
}

以上代码中，我们创建了一个MqttClient类，该类通过Netty-Mqtt-Client实现了Mqtt客户端发布消息和订阅消息的功能。具体实现细节如下：

- connect()方法：连接Mqtt服务器，其中我们通过Netty创建了一个NioEventLoopGroup线程组、一个MqttClientInitializer客户端初始化器和一个Bootstrap客户端启动器，并将它们配置好后发起连接请求；
- disconnect()方法：断开Mqtt服务器连接，关闭Netty通道和线程组；
- publish()方法：发布Mqtt消息，其中我们使用了MqttFixedHeader、MqttPublishVariableHeader、ByteBuf和MqttPublishMessage等Netty-Mqtt-Client提供的类来构建Mqtt PUBLISH消息，并通过Netty通道将其发送给服务器；
- subscribe()方法：订阅Mqtt主题，其中我们使用了MqttFixedHeader、MqttMessageIdVariableHeader、MqttTopicSubscription、MqttSubscribePayload和MqttSubscribeMessage等Netty-Mqtt-Client提供的类来构建Mqtt SUBSCRIBE消息，并通过Netty通道将其发送给服务器；
- unsubscribe()方法：取消订阅Mqtt主题，其中我们使用了MqttFixedHeader、MqttMessageIdVariableHeader、MqttUnsubscribePayload和MqttUnsubscribeMessage等Netty-Mqtt-Client提供的类来构建Mqtt UNSUBSCRIBE消息，并通过Netty通道将其发送给服务器。