mqtt keepAlive原理详见【https://www.emqx.com/zh/blog/mqtt-keep-alive】

# 下面开始写测试代码

【pom.xml】

<dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-web</artifactId><version>2.3.12.RELEASE</version>
</dependency>
<dependency><groupId>org.eclipse.paho</groupId><artifactId>org.eclipse.paho.client.mqttv3</artifactId><version>1.2.2</version>
</dependency>
<dependency><groupId>com.alibaba.fastjson2</groupId><artifactId>fastjson2</artifactId><version>2.0.49</version>
</dependency>

【MyDemo7MqttCallback.java】

package com.chz.myMqttV3.demo7;@Slf4j
public class MyDemo7MqttCallback implements MqttCallbackExtended {private MqttClient client;private MqttConnectOptions options;private String[] topics;public MyDemo7MqttCallback(MqttClient client, MqttConnectOptions options, String[] topics){this.client = client;this.options = options;this.topics = topics;}@SneakyThrows@Overridepublic void connectionLost(Throwable throwable) {log.error("connectionLost", throwable);while (!client.isConnected()) {log.info("emqx重新连接....................................................");client.connect(options);Thread.sleep(1000);}}@Overridepublic void messageArrived(String topic, MqttMessage message) throws Exception {log.info("messageArrived: topic={}, message={}", topic, new String(message.getPayload()));}@SneakyThrows@Overridepublic void deliveryComplete(IMqttDeliveryToken token) {if( token!=null ){MqttMessage message = token.getMessage();String topic = token.getTopics()==null ? null : Arrays.asList(token.getTopics()).toString();String str = message==null ? null : new String(message.getPayload());log.info("deliveryComplete: topic={}, message={}", topic, str);} else {log.info("deliveryComplete: null");}}@SneakyThrows@Overridepublic void connectComplete(boolean reconnect, String serverURI) {log.info("connectComplete: reconnect={}, serverURI={}", reconnect, serverURI);if( topics.length > 0 ){int[] qosArr = new int[topics.length];Arrays.fill(qosArr, 2);MyDemo7MqttMessageListener[] listeners = new MyDemo7MqttMessageListener[topics.length];Arrays.fill(listeners, new MyDemo7MqttMessageListener());client.subscribe(topics, qosArr, listeners);}}
}

【MyDemo7MqttMessageListener.java】

package com.chz.myMqttV3.demo7;@Slf4j
public class MyDemo7MqttMessageListener implements IMqttMessageListener
{@Overridepublic void messageArrived(String topic, MqttMessage message) {log.info("messageArrived: topic={}, message={}", topic, new String(message.getPayload()));}
}

【MyDemo7MqttClient1Test.java】

package com.chz.myMqttV3.demo7;public class MyDemo7MqttClient1Test
{public static void main(String[] args) throws  MqttException {MqttConnectOptions options = new MqttConnectOptions();options.setUserName("admin");options.setPassword("public".toCharArray());options.setCleanSession(true);options.setAutomaticReconnect(true);options.setConnectionTimeout(20);options.setKeepAliveInterval(10);MqttClient client = new MqttClient("tcp://192.168.44.228:1883", "MyDemo7MqttClient1Test", new MemoryPersistence());client.setCallback(new MyDemo7MqttCallback(client, options, new String[]{"device/#"}));client.connect(options);}
}

【MyDemo7MqttSenderTest.java】

package com.chz.myMqttV3.demo7;public class MyDemo7MqttSenderTest
{public static void main(String[] args) throws UnknownHostException, MqttException, InterruptedException {MqttConnectOptions options = new MqttConnectOptions();options.setUserName("admin");options.setPassword("public".toCharArray());options.setCleanSession(true);options.setAutomaticReconnect(true);options.setConnectionTimeout(20);options.setKeepAliveInterval(10);// 这里设置遗嘱消息，当broker认为本client断联时会将遗嘱消息发出，可以用来确认borker是否认为本client已经断联options.setWill("device/1", "I am MyDemo7MqttSenderTest, I am dead!!!".getBytes(), 1, false);MqttClient client = new MqttClient("tcp://192.168.44.228:1883", "MyDemo7MqttSenderTest", new MemoryPersistence());client.setCallback(new MyDemo7MqttCallback(client, options, new String[]{}));client.connect(options);for( int i=0; true; i++ ){String topic = "device/1";MqttMessage mqttMessage = new MqttMessage();mqttMessage.setRetained(true);String msg = "I am MyDemo7MqttSenderTest, at node [192.168.44.228:1883]:" + i;mqttMessage.setPayload(msg.getBytes(StandardCharsets.UTF_8));client.publish(topic, mqttMessage);System.out.println("send: " + msg);Thread.sleep(1000L);    // 测试运行的时候在这里加入断点，卡住MqttClient内部自动发的PINGREQ消息}}
}

# 下面开始进行测试

1、启动【MyDemo7MqttClient1Test】。
2、在【MyDemo7MqttSenderTest】的【sleep】那一句加入断点，然后运行【MyDemo7MqttSenderTest】

等一段时间，可以看到遗嘱消息【I am MyDemo7MqttSenderTest, I am dead!!!】发出来了