我正在尝试使用

Eclipse Paho在Java中的MQTT客户端上实现某些功能.目标是订阅主题,并且当收到消息时,客户端发送关于另一主题的另一消息.

这看起来很容易,但我有一个奇怪的问题,我无法解决.这是我的代码：

import org.eclipse.paho.client.mqttv3.IMqttDeliveryToken;

import org.eclipse.paho.client.mqttv3.IMqttToken;

import org.eclipse.paho.client.mqttv3.MqttCallback;

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.MqttClient;

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

public class MqttOperations implements MqttCallback {

MqttClient sampleClient;

MqttConnectOptions connOpts;

public MqttOperations() {

}

public static void main(String[] args) throws InterruptedException {

new MqttOperations().launchMqttClient();

}

public void launchMqttClient() throws InterruptedException {

try {

MemoryPersistence persistence = new MemoryPersistence();

sampleClient = new MqttClient("tcp://broker.mqttdashboard.com:1883", "iamaclient", persistence);

connOpts = new MqttConnectOptions();

connOpts.setCleanSession(true);

sampleClient.connect(connOpts);

sampleClient.subscribe("topic/example/ofmessage");

sampleClient.setCallback(this);

} 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();

}

}

@Override

public void connectionLost(Throwable cause) {

// TODO Auto-generated method stub

}

@Override

public void messageArrived(String topic, MqttMessage message) throws MqttException

{

System.out.println("Received: " + message.toString());

try{

System.out.println("Publishing message: i am the answer");

MqttMessage ans = new MqttMessage("i am the answer".getBytes());

ans.setQos(2);

sampleClient.publish("topic/example/ofanswer", ans);

System.out.println("Message published");

}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();

}

}

@Override

public void deliveryComplete(IMqttDeliveryToken token) {

}

}

问题是,这个程序只运行一次.收到消息后,将发送此消息的答案,但似乎消息“消息已发布”从未显示在屏幕上,并且客户端不会收到任何其他消息.

我有这样的印象,即sampleClient.publish行(“topic / example / ofanswer”,ans);永远不会完成它的执行.

有谁知道它是怎么来的,请问如何解决我的问题？