给我写一个MQTT的JAVA案例

好的，这是一个使用 JAVA 语言编写的 MQTT 客户端的示例代码：

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;
import org.eclipse.paho.client.mqttv3.persist.MemoryPersistence;

public class MqttPublishSample {

  public static void main(String[] args) {

    String topic = "MQTT Examples";
    String content = "Message from MqttPublishSample";
    int qos = 2;
    String broker = "tcp://iot.eclipse.org:1883";
    String clientId = "JavaSample";
    MemoryPersistence persistence = new MemoryPersistence();

    try {
      MqttClient sampleClient = new MqttClient(broker, clientId, persistence);
      MqttConnectOptions connOpts = new MqttConnectOptions();
      connOpts.setCleanSession(true);
      System.out.println("Connecting to broker: " + broker);
      sampleClient.connect(connOpts);
      System.out.println("Connected");
      System.out.println("Publishing message: " + content);
      MqttMessage message = new MqttMessage(content.getBytes());
      message.setQos(qos);
      sampleClient.publish(topic, message);
      System.out.println("Message published");
      sampleClient.disconnect();
      System.out.println("Disconnected");
      System.exit(0);
    } 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();
    }
  }
}

在上面的代码中，我们使用了 Eclipse Paho 库来连接到 MQTT 服务器，并使用该库发布一条消息。

首先，我们需要导入所需的类：`MqttClient`、`MqttConnectOptions`、`MqttException` 和 `MqttMessage`。

然后，我们创建了一个名为 `sampleClient` 的 `MqttClient` 对象，