無法接收已發布的消息以在mqtt paho上訂閱主題

Question

我正在使用paho發送和接收mqtt消息。 到目前為止，發送消息沒有問題。 我收到它們有問題。我的代碼是：

     package BenchMQTT;

     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.MqttException;
     import org.eclipse.paho.client.mqttv3.MqttMessage;
     import org.eclipse.paho.client.mqttv3.MqttClient;

     public class Test_A_2 implements MqttCallback {

     MqttClient clientR;
     MqttClient clientS;

     public Test_A_2() {
     }

     public static void main(String[] args) throws InterruptedException {
         long startTime = System.currentTimeMillis();
         new Test_A_2().doDemo();
         long endTime = System.currentTimeMillis();
     }

    public void doDemo() throws InterruptedException {
    try {   
    clientS = new MqttClient("tcp://mybroker:1883", "Sender");
    clientR = new MqttClient("tcp://mybroker:1883", "Reiever");
    clientR.connect();
    clientS.connect();
    MqttMessage message = new MqttMessage();

    String messagePayload = "qwertyuiopasdfghjklzxcvbnmqwertyuiopasdfghjk"
            + "lzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasdfghj"
            + "klzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasdfgh"
            + "jklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasdfg"
            + "hjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasd"
            + "fghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopas"
            + "dfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopa"
            + "sdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiop"
            + "asdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuio"
            + "pasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyui"
            + "opasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyu"
            + "iopasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwerty"
            + "uiopasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwert"
            + "nmqwertyuiop";

    clientR.subscribe("BenchMQTT");   
    clientR.setCallback(this);

    for(int i=0;i<10;i++)
    {
    message.setPayload((messagePayload)
            .getBytes());
    System.out.println(i);
    clientS.publish("BenchMQTT", message);
    }
    clientR.disconnect();   
    clientS.disconnect();
    clientR.close();   
    clientS.close();

   } catch (MqttException e)
    {
     System.out.println("ERROR");
    }
 }

     @Override
     public void connectionLost(Throwable cause) {
         // TODO Auto-generated method stub

     }

     @Override
     public void messageArrived(String topic, MqttMessage message)
     {
         System.out.println("Received: " + message.toString());
     }

     @Override
     public void deliveryComplete(IMqttDeliveryToken token) {

     }

     }

這發送和接收消息。

OUTPUT：

0
Received: 0
1
Received: 1
2
Received: 2
3
Received: 3
4
Received: 4
5
Received: 5
6
Received: 6
7
Received: 7
8
Received: 8
9
Received: 9

我想發送消息，然后收到它們。 有幫助嗎？ 預期產量：

0
1
2
3
4
5
6
7
8
9
Received: 0
Received: 1
Received: 2
Received: 3
Received: 4
Received: 5
Received: 6
Received: 7
Received: 8
Received: 9

Answer 1

這不是MQTT（或任何發布/訂閱消息）的工作原理，如果接收者連接到服務器，則消息將在發送時傳遞。

例外情況是，如果接收器連接並訂閱了大於0的QOS主題，則稍后斷開連接並重新連接（不使用干凈的會話標志集），那么已發布的QOS大於0的錯過消息將被傳遞在重新連接點。

另一種可能性是，如果消息已發布且保留標志設置為true，則只有發布到主題的最后一條消息將在接收客戶端訂閱時傳遞。

Answer 2

以下代碼執行您想要的操作，但它強制MQTT以不應該的方式運行。 消息隊列僅用於確保將所有消息傳遞到客戶端，即使它在一段時間內斷開連接，也始終會盡快傳遞消息。

 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;

 public class Test_A_2 implements MqttCallback {

 MqttClient clientR;
 MqttClient clientS;

 public Test_A_2() {
 }

 public static void main(String[] args) throws InterruptedException {
     long startTime = System.currentTimeMillis();
     new Test_A_2().doDemo();
     long endTime = System.currentTimeMillis();
 }

public void doDemo() throws InterruptedException {
try {   

    MqttConnectOptions options = new MqttConnectOptions();
    options.setCleanSession(false);

clientS = new MqttClient("tcp://localhost:1883", "Sender");
clientR = new MqttClient("tcp://localhost:1883", "Reiever");
clientR.connect(options);
clientS.connect();
clientR.setCallback(this);
clientR.subscribe("BenchMQTT",2);
MqttMessage message = new MqttMessage();

String messagePayload = "qwertyuiopasdfghjklzxcvbnmqwertyuiopasdfghjk"
        + "lzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasdfghj"
        + "klzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasdfgh"
        + "jklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasdfg"
        + "hjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasd"
        + "fghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopas"
        + "dfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopa"
        + "sdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiop"
        + "asdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuio"
        + "pasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyui"
        + "opasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyu"
        + "iopasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwerty"
        + "uiopasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwert"
        + "nmqwertyuiop";

clientR.disconnect();

for(int i=0;i<10;i++)
{
message.setPayload((messagePayload)
        .getBytes());
System.out.println(i);
message.setQos(2);
clientS.publish("BenchMQTT", message);
}



clientR.connect(options);
clientR.setCallback(this);
clientR.subscribe("BenchMQTT",2);

clientR.disconnect();   
clientS.disconnect();
clientR.close();   
clientS.close();

} catch (MqttException e)
{
 System.out.println("ERROR");
 e.printStackTrace();
}
}

 @Override
 public void connectionLost(Throwable cause) {
     // TODO Auto-generated method stub

 }

 @Override
 public void messageArrived(String topic, MqttMessage message)
 {
     System.out.println("Received: " + message.toString());
 }

 @Override
 public void deliveryComplete(IMqttDeliveryToken token) {

 }

 }