emq共享订阅

原文链接：https://www.cnblogs.com/lasdaybg/p/11419711.html

1 共享订阅

多个客户端订阅了同一个主题，发布者发布主题时，每个客户端都会同时收到这个主题的消息。在客户端集群部署的场景下会出现消息重复处理的问题。
EMQ支持共享订阅，多个客户端订阅了同一个主题，发布者发布主题时，只有其中一个客户端接收到消息。
共享订阅有两种方式：
(1)共享订阅：订阅前缀$queue/ 多个客户端订阅了$queue/topic，发布者发布到topic，则只有一个客户端会接收到消息。
(2)分组订阅：订阅前缀$share/<group>/ 多组客户端订阅了$queue/group1/topic、$queue/group2/topic…，发布者发布到topic，则消息会发布到每个group中，但是每个group中只有一个客户端会接收到消息。

2 Java客户端实现共享订阅

开发时发现，使用eclipse paho java客户端时，无法处理共享订阅。订阅$queue/topic能够订阅成功，并且跟踪代码能看到emq也把消息转发到了客户端，但是客户端丢弃掉了。
解决方法就是重写mqtt的回调函数，实现MqttCallback接口。

实现MqttCallback接口的代码如下：

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);
    }
}

创建emq连接代码如下：

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());

还要再写一个实现IMqttMessageListener接口的Emq消息处理类：

@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();
    }
  }
}

转载自：https://www.cnblogs.com/lasdaybg/p/11419711.html