emq共享訂閱

原文鏈接：https://www.cnblogs.com/lasdaybg/p/11419711.html

1 共享訂閱

多個客戶端訂閱了同一個主題，發佈者發佈主題時，每個客戶端都會同時收到這個主題的消息。在客戶端集羣部署的場景下會出現消息重複處理的問題。
EMQ支持共享訂閱，多個客戶端訂閱了同一個主題，發佈者發佈主題時，只有其中一個客戶端接收到消息。
共享訂閱有兩種方式：
(1)共享訂閱：訂閱前綴$queue/ 多個客戶端訂閱了$queue/topic，發佈者發佈到topic，則只有一個客戶端會接收到消息。
(2)分組訂閱：訂閱前綴$share/<group>/ 多組客戶端訂閱了$queue/group1/topic、$queue/group2/topic…，發佈者發佈到topic，則消息會發布到每個group中，但是每個group中只有一個客戶端會接收到消息。

2 Java客戶端實現共享訂閱

開發時發現，使用eclipse paho java客戶端時，無法處理共享訂閱。訂閱$queue/topic能夠訂閱成功，並且跟蹤代碼能看到emq也把消息轉發到了客戶端，但是客戶端丟棄掉了。
解決方法就是重寫mqtt的回調函數，實現MqttCallback接口。

實現MqttCallback接口的代碼如下：

package com.emqtest.emqtest;
 
 
import java.util.HashMap;
import java.util.Map;
 
import org.eclipse.paho.client.mqttv3.IMqttDeliveryToken;
import org.eclipse.paho.client.mqttv3.IMqttMessageListener;
import org.eclipse.paho.client.mqttv3.MqttCallback;
import org.eclipse.paho.client.mqttv3.MqttMessage;
import org.eclipse.paho.client.mqttv3.MqttTopic;
 
public class SharedSubCallbackRouter implements MqttCallback {
    private Map<String, IMqttMessageListener> topicFilterListeners;
 
    public SharedSubCallbackRouter(Map<String, IMqttMessageListener> topicFilterListeners) {
        this.topicFilterListeners = topicFilterListeners;
    }
 
    public void addSubscriber(String topicFilter, IMqttMessageListener listener) {
        if (this.topicFilterListeners == null) {
             this.topicFilterListeners = new HashMap<>();
        }
        this.topicFilterListeners.put(topicFilter, listener);
    }
 
    @Override
    public void connectionLost(Throwable cause) {
 
    }
 
    @Override
    public void messageArrived(String topic, MqttMessage message) throws Exception {
        for (Map.Entry<String, IMqttMessageListener> listenerEntry : topicFilterListeners.entrySet()) {
            String topicFilter = listenerEntry.getKey();
            if (isMatched(topicFilter, topic)) {
                listenerEntry.getValue().messageArrived(topic, message);
            }
        }
    }
 
    @Override
    public void deliveryComplete(IMqttDeliveryToken token) {
 
    }
 
    /**
     * Paho topic matcher does not work with shared subscription topic filter of emqttd
     * https://github.com/eclipse/paho.mqtt.java/issues/367#issuecomment-300100385
     * <p>
     * http://emqtt.io/docs/v2/advanced.html#shared-subscription
     *
     * @param topicFilter the topicFilter for mqtt
     * @param topic       the topic
     * @return boolean for matched
     */
    private boolean isMatched(String topicFilter, String topic) {
        if (topicFilter.startsWith("$queue/")) {
            topicFilter = topicFilter.replaceFirst("\\$queue/", "");
        } else if (topicFilter.startsWith("$share/")) {
            topicFilter = topicFilter.replaceFirst("\\$share/", "");
            topicFilter = topicFilter.substring(topicFilter.indexOf('/'));
        }
        return MqttTopic.isMatched(topicFilter, topic);
    }
}

創建emq連接代碼如下：

mqttClient = new MqttClient("tcp://localhost:1883", "MqttClient");
mqttClient.connect();
Map<String, IMqttMessageListener> listeners = new HashMap<>();
IMqttMessageListener emqListener = new EmqListener();
listeners.put("$queue/testmqtt", emqListener);
mqttClient.setCallback(new SharedSubCallbackRouter(listeners));
mqttClient.subscribe("$queue/testmqtt", new EmqListener());

還要再寫一個實現IMqttMessageListener接口的Emq消息處理類：

@Component
public class EmqListener implements IMqttMessageListener {
 
  @Override
  public void messageArrived(String topic, MqttMessage message) throws Exception {
    try {
      System.out.println("topic: " + topic);
    } catch (Exception e) {
      e.printStackTrace();
    }
  }
}

轉載自：https://www.cnblogs.com/lasdaybg/p/11419711.html