MQTT协议因低延迟、效率高在工业物联网领域使用的频率特别高。前文介绍了如何用代码发送MQTT消息，本文在前文的基础上实现MQTT消息的订阅接收。
操作步骤：

1. 引入相关的依赖

    org.springframework.boot
    spring-boot-starter-integration


    org.springframework.integration
    spring-integration-mqtt



    org.projectlombok
    lombok
    true

1. 在application.yml配置MQTT服务器信息

server:
  port: 8090

mqtt:
  host: tcp://127.0.0.1:1883
  clientinid: mqttinId
  clientoutid: mqttoutid
  topic: virus
  qoslevel: 1
  #MQTT 认证
  username: xxx
  password: xxx
  timeout: 10000
  #20s
  keepalive: 20

1. 配置MQTT消息推送配置

package com.favccxx.mqtt.config;

import lombok.extern.slf4j.Slf4j;
import org.eclipse.paho.client.mqttv3.MqttConnectOptions;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.integration.annotation.IntegrationComponentScan;
import org.springframework.integration.annotation.ServiceActivator;
import org.springframework.integration.channel.DirectChannel;
import org.springframework.integration.core.MessageProducer;
import org.springframework.integration.mqtt.core.DefaultMqttPahoClientFactory;
import org.springframework.integration.mqtt.core.MqttPahoClientFactory;
import org.springframework.integration.mqtt.inbound.MqttPahoMessageDrivenChannelAdapter;
import org.springframework.integration.mqtt.support.DefaultPahoMessageConverter;
import org.springframework.messaging.Message;
import org.springframework.messaging.MessageChannel;
import org.springframework.messaging.MessageHandler;
import org.springframework.messaging.MessagingException;

@Slf4j
@Configuration
@IntegrationComponentScan
public class MQTTReceiveConfig {

    @Value("${mqtt.username}")
    private String username;

    @Value("${mqtt.password}")
    private String password;

    @Value("${mqtt.host}")
    private String hostUrl;

    @Value("${mqtt.clientinid}")
    private String clientId;

    @Value("${mqtt.topic}")
    private String defaultTopic;

    @Value("${mqtt.timeout}")
    private int completionTimeout ;   //连接超时

    @Bean
    public MqttConnectOptions getReceiverMqttConnectOptions(){
        MqttConnectOptions mqttConnectOptions=new MqttConnectOptions();
        mqttConnectOptions.setCleanSession(true);
        mqttConnectOptions.setConnectionTimeout(10);
        mqttConnectOptions.setKeepAliveInterval(90);
        mqttConnectOptions.setAutomaticReconnect(true);
        mqttConnectOptions.setUserName(username);
        mqttConnectOptions.setPassword(password.toCharArray());
        mqttConnectOptions.setServerURIs(new String[]{hostUrl});
        mqttConnectOptions.setKeepAliveInterval(2);
        return mqttConnectOptions;
    }
    @Bean
    public MqttPahoClientFactory mqttClientFactory() {
        DefaultMqttPahoClientFactory factory = new DefaultMqttPahoClientFactory();
        factory.setConnectionOptions(getReceiverMqttConnectOptions());
        return factory;
    }

    //接收通道
    @Bean
    public MessageChannel mqttInputChannel() {
        return new DirectChannel();
    }

    //配置client,监听的topic
    @Bean
    public MessageProducer inbound() {
        MqttPahoMessageDrivenChannelAdapter adapter =
                new MqttPahoMessageDrivenChannelAdapter(clientId+"_inbound", mqttClientFactory(),
                        defaultTopic);
        adapter.setCompletionTimeout(completionTimeout);
        adapter.setConverter(new DefaultPahoMessageConverter());
        adapter.setQos(1);
        adapter.setOutputChannel(mqttInputChannel());
        return adapter;
    }

    //通过通道获取数据
    @Bean
    @ServiceActivator(inputChannel = "mqttInputChannel")
    public MessageHandler handler() {
        return new MessageHandler() {
            @Override
            public void handleMessage(Message message) throws MessagingException {
                log.info("主题：{}，消息接收到的数据：{}", message.getHeaders().get("mqtt_receivedTopic"), message.getPayload());
            }
        };
    }

}