概述
微消息隊列MQ for IoT在處理離線消息時，爲了簡化離線消息獲取機制，微消息隊列系統在客戶端成功建立連接並通過權限校驗後，會自動加載離線消息並下發到客戶端，但是實際在使用過程中會出現消費端啓動後遲遲無法獲取離線消息的問題，本文主要介紹延遲消息的發送與接收環節需要注意的問題。
協議相關
注意在使用SDK進行離線消息的發送過程中需要特別注意QoS和cleanSession兩個參數。
- QoS 指代消息傳輸的服務質量(主要針對發送端)
| 取值 | 1 | 2 | 3 |
|---|---|---|---|
| 意義 | 最多分發一次 | 最多分發一次 | 僅分發一次 |
- cleanSession 建立 TCP 連接後是否關心之前狀態(主要針對接收端)
true | false |
------- | ------- |
客戶端再次上線時，將不再關心之前所有的訂閱關係以及離線消息 | 客戶端再次上線時，還需要處理之前的離線消息，而之前的訂閱關係也會持續生效 |
爲了處理的方便，對於處理離線消息的情況，建議不論是發送端還是接收端，參數都設置爲:
QoS = 1
cleanSession = falseJava示例代碼
Send Code
import org.eclipse.paho.client.mqttv3.*;
import org.eclipse.paho.client.mqttv3.persist.MemoryPersistence;
import java.io.IOException;
import java.util.Date;
import static org.eclipse.paho.client.mqttv3.MqttConnectOptions.MQTT_VERSION_3_1_1;
public class MQTTSendMsg1 {
public static void main(String[] args) throws IOException {
final String broker ="tcp://******.mqtt.aliyuncs.com:1883";
final String acessKey ="******";
final String secretKey ="******";
final String topic ="******";
final String clientId ="GID_******@@@ClientID_device1";
String sign;
MemoryPersistence persistence = new MemoryPersistence();
try {
final MqttClient sampleClient = new MqttClient(broker, clientId, persistence);
final MqttConnectOptions connOpts = new MqttConnectOptions();
System.out.println("Connecting to broker: " + broker);
sign = MacSignature.macSignature(clientId.split("@@@")[0], secretKey);
connOpts.setUserName(acessKey);
connOpts.setServerURIs(new String[] { broker });
connOpts.setPassword(sign.toCharArray());
connOpts.setCleanSession(false);
connOpts.setKeepAliveInterval(90);
connOpts.setAutomaticReconnect(true);
connOpts.setMqttVersion(MQTT_VERSION_3_1_1);
sampleClient.setCallback(new MqttCallbackExtended() {
public void connectComplete(boolean reconnect, String serverURI) {
System.out.println("connect success");
//連接成功，需要上傳客戶端所有的訂閱關係
}
public void connectionLost(Throwable throwable) {
System.out.println("mqtt connection lost");
}
public void messageArrived(String topic, MqttMessage mqttMessage) throws Exception {
System.out.println("messageArrived:" + topic + "------" + new String(mqttMessage.getPayload()));
}
public void deliveryComplete(IMqttDeliveryToken iMqttDeliveryToken) {
System.out.println("deliveryComplete:" + iMqttDeliveryToken.getMessageId());
}
});
sampleClient.connect(connOpts);
for (int i = 0; i < 5; i++) {
try {
String scontent = new Date()+"MQTT Test body" + i;
//此處消息體只需要傳入 byte 數組即可，對於其他類型的消息，請自行完成二進制數據的轉換
final MqttMessage message = new MqttMessage(scontent.getBytes());
message.setQos(1);//設置離線消息的情況
System.out.println(i+" pushed at "+new Date()+" "+ scontent);
sampleClient.publish(topic+"/notice/", message);
} catch (Exception e) {
e.printStackTrace();
}
}
} catch (Exception me) {
me.printStackTrace();
}
}
}Receive Code
import org.eclipse.paho.client.mqttv3.*;
import org.eclipse.paho.client.mqttv3.persist.MemoryPersistence;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
public class MQTTRecvMsg {
public static void main(String[] args) {
final String broker ="tcp://******.mqtt.aliyuncs.com:1883";
final String acessKey ="******";
final String secretKey ="******";
final String topic ="******";
final String clientId ="GID_******@@@ClientID_device2";
String sign;
MemoryPersistence persistence = new MemoryPersistence();
try {
final MqttClient sampleClient = new MqttClient(broker, clientId, persistence);
final MqttConnectOptions connOpts = new MqttConnectOptions();
System.out.println("Connecting to broker: " + broker);
sign = MacSignature.macSignature(clientId.split("@@@")[0], secretKey);
final String[] topicFilters=new String[]{topic+"/notice/"};
final int[]qos={1};
connOpts.setUserName(acessKey);
connOpts.setServerURIs(new String[] { broker });
connOpts.setPassword(sign.toCharArray());
connOpts.setCleanSession(false);//設置確定是否繼續接受離線消息
connOpts.setKeepAliveInterval(90);
connOpts.setAutomaticReconnect(true);
final ExecutorService executorService = new ThreadPoolExecutor(1, 1, 0, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
sampleClient.setCallback(new MqttCallbackExtended() {
public void connectComplete(boolean reconnect, String serverURI) {
System.out.println("connect success");
//連接成功，需要上傳客戶端所有的訂閱關係
executorService.submit(new Runnable()
{
public void run()
{
try
{
sampleClient.subscribe(topicFilters, qos);
} catch(Exception me)
{
me.printStackTrace();
}
}
});
}
public void connectionLost(Throwable throwable) {
System.out.println("mqtt connection lost");
}
public void messageArrived(String topic, MqttMessage mqttMessage) throws Exception {
System.out.println("messageArrived:" + topic + "------" + new String(mqttMessage.getPayload()));
}
public void deliveryComplete(IMqttDeliveryToken iMqttDeliveryToken) {
System.out.println("deliveryComplete:" + iMqttDeliveryToken.getMessageId());
}
});
//客戶端每次上線都必須上傳自己所有涉及的訂閱關係，否則可能會導致消息接收延遲
sampleClient.connect(connOpts);
//每個客戶端最多允許存在30個訂閱關係，超出限制可能會丟棄導致收不到部分消息
sampleClient.subscribe(topicFilters,qos);
Thread.sleep(Integer.MAX_VALUE);
} catch (Exception me) {
me.printStackTrace();
}
}
}特別注意：
離線消息生成需要一定的時間，因爲推送的消息需要等待客戶端的 ack 超時纔會被判成離線消息，所以獲取離線消息一般也需要訂閱端等待一定的時間。