你的上上个回答中onDeliveryComplete函数没有被调用，请重新给我示例代码

当然，以下是一个简单的示例代码：

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

public class MqttSample {
    public static void main(String[] args) throws MqttException {
        String broker = "tcp://localhost:1883";
        String clientId = "JavaSample";
        MemoryPersistence persistence = new MemoryPersistence();

        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");

        String topic = "myTopic";
        String message = "Hello World!";
        MqttMessage mqttMessage = new MqttMessage(message.getBytes());
        mqttMessage.setQos(2);

        System.out.println("Publishing message: " + message);
        sampleClient.publish(topic, mqttMessage);
        System.out.println("Message published");

        sampleClient.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: " + message.toString());
            }

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

        sampleClient.disconnect();
        System.out.println("Disconnected");
    }
}

在上面的代码中，我们使用了 Paho MQTT Java 客户端库来建立连接并发布消息。当消息成功发布后，我们设置了一个回调函数，以确保消息已传递到 MQTT 代理服务器上。在 `deliveryComplete()` 回调函数中，我们可以检查消息是否成功传递。如果未成功传递，我们可以根据需要采取适当的措施。