[物联网]Java如何接入MQTT协议

在Java中接入MQTT协议，你可以使用Eclipse的Paho客户端库。以下是如何使用Paho客户端库在Java中连接到MQTT代理（broker）并发送接收消息的简单示例。

首先，确保你的项目中包含了Paho客户端库。如果你使用Maven，可以在pom.xml中添加以下依赖：

<dependency>
    <groupId>org.eclipse.paho</groupId>
    <artifactId>org.eclipse.paho.client.mqttv3</artifactId>
    <version>1.2.5</version>
</dependency>

以下是Java代码示例：

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.IMqttDeliveryToken;
import org.eclipse.paho.client.mqttv3.MqttCallback;
 
public class MqttExample {
 
    public static void main(String[] args) {
        try {
            String broker = "tcp://YOUR_BROKER:1883";
            String clientId = "JavaClient";
            MqttClient sampleClient = new MqttClient(broker, clientId);
            MqttConnectOptions connOpts = new MqttConnectOptions();
            connOpts.setCleanSession(true);
            System.out.println("Connecting to broker: " + broker);
            sampleClient.connect(connOpts);
            System.out.println("Connected");
 
            // 订阅主题
            String topic = "test/java";
            int qos = 2;
            sampleClient.subscribe(topic, qos);
 
            // 设置回调处理接收到的消息
            sampleClient.setCallback(new MqttCallback() {
                public void messageArrived(String topic, MqttMessage message) throws Exception {
                    System.out.println("Message arrived: " + new String(message.getPayload()));
                }
 
                public void connectionLost(Throwable cause) {
                    System.out.println("Connection lost");
                }
 
                public void deliveryComplete(IMqttDeliveryToken token) {
                    System.out.println("Delivery complete");
                }
            });
 
            // 发布消息
            MqttMessage message = new MqttMessage("Hello, MQTT".getBytes());
            message.setQos(qos);
            sampleClient.publish(topic, message);
            System.out.println("Message published");
 
            // 断开连接
            sampleClient.disconnect();
            System.out.println("Disconnected");
            System.exit(0);
 
        } catch (MqttException me) {
            System.out.println("reason " + me.getReasonCode());