使用 Java 開發 MQTT 客戶端

本章將使用Java連接MQTT客戶端，實現發佈消息，訂閱消息，接收消息

引入相關依賴

本例中使用 Maven 來管理依賴的庫文件，打開 pom.xml，加入以下的 JAR 依賴，等待完成相關 JAR 包的下載。

<dependency>
    <groupId>org.eclipse.paho</groupId>
    <artifactId>org.eclipse.paho.client.mqttv3</artifactId>
    <version>1.2.0</version>
</dependency>

完整例子

package paho_demo;

import java.text.MessageFormat;

import org.eclipse.paho.client.mqttv3.IMqttDeliveryToken;
import org.eclipse.paho.client.mqttv3.MqttCallback;
import org.eclipse.paho.client.mqttv3.MqttClient;
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.persist.MemoryPersistence;

public class Demo {
    public static void main(String[] args) {
        String broker = "tcp://localhost:1883";
        String clientId = "JavaSample";
        //Use the memory persistence
        MemoryPersistence persistence = new MemoryPersistence();

        try {
            MqttClient sampleClient = new MqttClient(broker, clientId, persistence);
            MqttConnectOptions connOpts = new MqttConnectOptions();
            connOpts.setCleanSession(true);
            System.out.println("Connecting to broker:" + broker);
            sampleClient.connect(connOpts);
            System.out.println("Connected");
            //建立連接
            String topic = "demo/topics";
            System.out.println("Subscribe to topic:" + topic);
            //訂閱消息，topic：主題名稱
            sampleClient.subscribe(topic);
            sampleClient.setCallback(new MqttCallback() {
            	//接收消息
                public void messageArrived(String topic, MqttMessage message) throws Exception {
                    String theMsg = MessageFormat.format("{0} is arrived for topic {1}.", new String(message.getPayload()), topic);
                    System.out.println(theMsg);
                }

                public void deliveryComplete(IMqttDeliveryToken token) {
                }

                public void connectionLost(Throwable throwable) {
                }
            });


            String content = "Message from MqttPublishSample";
            int qos = 2;
            System.out.println("Publishing message:" + content);
            //發佈消息，topic：主題名稱，MqttMessage：消息內容
            MqttMessage message = new MqttMessage(content.getBytes());
            message.setQos(qos);
            sampleClient.publish(topic, message);
            System.out.println("Message published");

        } catch (MqttException me) {
            System.out.println("reason" + me.getReasonCode());
            System.out.println("msg" + me.getMessage());
            System.out.println("loc" + me.getLocalizedMessage());
            System.out.println("cause" + me.getCause());
            System.out.println("excep" + me);
            me.printStackTrace();
        }
    }
}

參考文檔：https://docs.emqx.io/tutorial/v4/cn/client_dev/java.html