近期又接触到了新的一个东西MQTT，用本地环境模拟一下吧。

主要是用EMQ作为服务器，首先当然是去官网下载一个EMQ  下载 EMQX

我本地用的是windows版本，下载完后进去bin目录后有个emqx文件
用命令窗口输入emqx start 就启动了

这里主要是用mysql来鉴权，设置一下就好了，当然建表语句参考一下文档使用 MySQL 的密码认证 | EMQX 5.0 文档

 按规则自己创建账号密码，用工具测试一下吧

 服务器就没啥问题，那么剩下客户端，

我直接上代码啥也不说了

MqttService.class

package mqtt;

/**
 * @author xxx
 * @date 2022/11/3 16:31
 * @description
 */
public interface MqttService {
    /**
     * 发布
     * @param topic
     * @param data
     */
    public void sendToMqtt(String topic, String data);

    /**
     * 订阅
     * @param topics
     * @param qoss
     */
    public void subscribeFromMqtt(String[] topics,int[] qoss);

    /**
     *  初始化订阅主题
     */
    public void subscribeInit();



}

MqttServiceImpl.class

package mqtt.impl;


import mqtt.MqttService;
import org.eclipse.paho.client.mqttv3.*;
import org.springframework.beans.factory.InitializingBean;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Service;

import javax.net.SocketFactory;
import javax.net.ssl.SSLContext;
import javax.net.ssl.TrustManager;

/**
 * @author xxx
 * @date 2022/11/3 16:31
 * @description
 */
@Service
public class MqttServiceImpl implements MqttService,InitializingBean {


    @Value("${mqtt.username:xxx}")
    private String username;
    @Value("${mqtt.password:xxx}")
    private String password;
    @Value("${mqtt.serverURI:ssl://localhost:8883}")
    private String serverURI;
    @Value("${mqtt.clientId:xxx}")
    private String clientId;


    private MqttClient mqttClient;


    @Autowired
    private MyMqttCallback myMqttCallback;


    @Override
    public void sendToMqtt(String topic, String data) {
        MqttClient mqttClientPublish=init(clientId);
        try {
            MqttMessage message = new MqttMessage();
            message.setQos(1);
            message.setPayload(data.getBytes("UTF-8"));
            mqttClientPublish.publish(topic, message);
        }  catch (Exception e) {
            e.printStackTrace();
        }finally {
            try {
                mqttClientPublish.disconnect();//断开连接
                mqttClientPublish.close();//抛出异常时，不能仅断开连接，内部会创建连接池等资源造成内存不够用的情况，应用close释放资源
            } catch (MqttException e1) {
                e1.printStackTrace();
            }
        }
    }

    @Override
    public void subscribeFromMqtt(String[] topics,int[] qoss) {
        try {
            mqttClient.setCallback(myMqttCallback);
            mqttClient.subscribe(topics,qoss);
        } catch (MqttException e) {
            e.printStackTrace();
            try {
                mqttClient.disconnect();//断开连接
                mqttClient.close();//抛出异常时，不能仅断开连接，内部会创建连接池等资源造成内存不够用的情况，应用close释放资源
            } catch (MqttException e1) {
                e1.printStackTrace();
            }
        }
    }

    public MqttClient init(String clientIdPublish) {
        try {
            mqttClient = new MqttClient(serverURI, clientIdPublish);//使用随机值clientId解决集群问题，但是凭证生成密码使用同一个clientId
            // MQTT 连接选项
            MqttConnectOptions connOpts = new MqttConnectOptions();
            connOpts.setUserName(username);
            connOpts.setPassword(password.toCharArray());
            connOpts.setCleanSession(true);
            // 设置超时时间
            connOpts.setConnectionTimeout(10);
            // 设置会话心跳时间
            connOpts.setKeepAliveInterval(60);
            // 设置自动重连
            connOpts.setAutomaticReconnect(true);
            //忽略证书验证--单向认证不需要的
            connOpts.setHttpsHostnameVerificationEnabled(false);

            //ssl 连接 , 这里的 TrustManager 是自己实现的，没有去校验服务端的证书
            TrustManager[] trustAllCerts = new TrustManager[1];
            TrustManager tm = new MyTM();
            trustAllCerts[0] = tm;
            SSLContext sc = SSLContext.getInstance("SSL");
            sc.init(null, trustAllCerts, null);
            SocketFactory factory = sc.getSocketFactory();
            connOpts.setSocketFactory(factory);
            // 建立连接
            mqttClient.connect(connOpts);
            mqttClient.setTimeToWait(5000);//客户端设置好发送超时时间，防止无限阻塞
        }catch (Exception e){
            e.printStackTrace();
        }

        return mqttClient;
    }


    @Override
    public void afterPropertiesSet() throws Exception {
        //初始化mqtt客户端
        subscribeInit();
    }


    public void subscribeInit(){
        //订阅已有的主题
        String[] topics= {"/sys/test"};
        int[] qoss={0};
        this.subscribeFromMqtt(topics,qoss);
    }


}

MyMqttCallback.class

package mqtt.impl;

import mqtt.MqttService;
import org.eclipse.paho.client.mqttv3.*;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

/**
 * @author xxx
 * @date 2022/11/13 0:38
 * @description
 */

@Service
public class MyMqttCallback implements MqttCallbackExtended {
    @Autowired
    private MqttService mqttService;

    @Override
    public void connectionLost(Throwable throwable) {

    }

    @Override
    public void messageArrived(String topic, MqttMessage message) {
        //消息回调
    }

    @Override
    public void deliveryComplete(IMqttDeliveryToken iMqttDeliveryToken) {

    }

    @Override
    public void connectComplete(boolean b, String s) {
        try {
            //重新订阅主题
            mqttService.subscribeInit();
        }catch (Exception e){
            e.printStackTrace();
        }

    }
}

MyTM.class

package mqtt.impl;

import javax.net.ssl.TrustManager;
import javax.net.ssl.X509TrustManager;
import java.security.cert.CertificateException;
import java.security.cert.X509Certificate;

/**
 * @author xxx
 * @date 2022/11/5 17:28
 * @description  MyTM 是自己实现的认证管理类，里面并有校验服务端的证书就返回true,永久成功！
 */
public class MyTM implements TrustManager, X509TrustManager {
    @Override
    public X509Certificate[] getAcceptedIssuers() {
        return null;
    }

    public boolean isServerTrusted(X509Certificate[] certs) {
        return true;
    }

    public boolean isClientTrusted(X509Certificate[] certs) {
        return true;
    }

    @Override
    public void checkServerTrusted(X509Certificate[] certs, String authType)
            throws CertificateException {
        return;
    }

    @Override
    public void checkClientTrusted(X509Certificate[] certs, String authType)
            throws CertificateException {
        return;
    }
}

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>org.example</groupId>
    <artifactId>mqtt-win-new3</artifactId>
    <version>1.0-SNAPSHOT</version>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.4.1</version>
    </parent>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
            <version>2.4.1</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>2.0.22</version>
        </dependency>

        <dependency>
            <groupId>commons-codec</groupId>
            <artifactId>commons-codec</artifactId>
            <version>1.15</version>
        </dependency>
    </dependencies>
    
    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
                <configuration>
                    <mainClass>com.mqtt.win.DemoApplication</mainClass>
                </configuration>
            </plugin>
        </plugins>
    </build>

</project>

本次开发主要是为了对接设备，主要是项目启动的时候就要订阅对应的主题，所以客户端实例化启动是通过Bean的实例化完成后启动的客户端，Bean实现这个接口InitializingBean,还有客户端掉线重连后需要重新初始化订阅主题（因为cleansession设置了true）。

当然这个只是一个简单案例，实际情况肯定不止考虑这些，比如项目如果用的是集群，可以考虑用共享订阅，避免重复消费等。