java mqtt协议

import org.eclipse.paho.client.mqttv3.IMqttDeliveryToken;
import org.eclipse.paho.client.mqttv3.MqttCallback;
import org.eclipse.paho.client.mqttv3.MqttClient;
import org.eclipse.paho.client.mqttv3.MqttConnectOptions;
import org.eclipse.paho.client.mqttv3.MqttException;
import org.eclipse.paho.client.mqttv3.MqttMessage;

public class MqttExample {

    public static void main(String[] args) {
        String broker = "tcp://mqtt.example.com:1883";
        String clientId = "JavaSampleClient";
        String topic = "test/topic";
        String content = "Hello World";
        int qos = 2;
        String username = "user";
        String password = "password";

        try {
            // 创建MQTT客户端
            MqttClient client = new MqttClient(broker, clientId);

            // 设置连接选项
            MqttConnectOptions connOpts = new MqttConnectOptions();
            connOpts.setUserName(username);
            connOpts.setPassword(password.toCharArray());
            connOpts.setCleanSession(true);

            // 连接到MQTT代理
            System.out.println("Connecting to broker: " + broker);
            client.connect(connOpts);
            System.out.println("Connected");

            // 创建回调函数
            client.setCallback(new MqttCallback() {
                @Override
                public void connectionLost(Throwable cause) {
                    System.out.println("Connection lost");
                }

                @Override
                public void messageArrived(String topic, MqttMessage message) throws Exception {
                    System.out.println("Message arrived: " + new String(message.getPayload()));
                }

                @Override
                public void deliveryComplete(IMqttDeliveryToken token) {
                    System.out.println("Delivery complete");
                }
            });

            // 发布消息
            MqttMessage message = new MqttMessage(content.getBytes());
            message.setQos(qos);
            client.publish(topic, message);
            System.out.println("Message published");

            // 断开连接
            client.disconnect();
            System.out.println("Disconnected");
        } catch (MqttException me) {
            System.out.println("reason " + me.getReasonCode());
            System.out.println("msg " + me.getMessage());
            System.out.println("loc " + me.getLocalizedMessage());
            System.out.println("cause " + me.getCause());
            System.out.println("excep " + me);
            me.printStackTrace();
        }
    }
}

解释说明：

1. 引入依赖：此代码使用了Eclipse Paho库来实现MQTT协议。你需要确保项目中包含Paho库的依赖。

2. 创建MQTT客户端：通过MqttClient类创建一个MQTT客户端实例，指定Broker地址和客户端ID。

3. 设置连接选项：通过MqttConnectOptions类设置连接选项，如用户名、密码和是否清理会话。

4. 连接到MQTT代理：调用client.connect(connOpts)方法连接到MQTT代理。

5. 设置回调函数：通过client.setCallback()方法设置回调函数，用于处理连接丢失、消息到达和消息发送完成等事件。

6. 发布消息：创建并发布一条消息到指定的主题。

7. 断开连接：完成操作后，调用client.disconnect()方法断开与MQTT代理的连接。

8. 异常处理：捕获并打印可能发生的MQTT异常信息。