<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.integration</groupId>
        <artifactId>spring-integration-mqtt</artifactId>
    </dependency>
</dependencies>

import org.eclipse.paho.client.mqttv3.MqttConnectOptions;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.integration.annotation.ServiceActivator;
import org.springframework.integration.channel.DirectChannel;
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.outbound.MqttPahoMessageHandler;
import org.springframework.integration.mqtt.support.DefaultPahoMessageConverter;
import org.springframework.messaging.MessageChannel;
import org.springframework.messaging.MessageHandler;

@Configuration
public class MqttConfig {

    @Bean
    public MqttPahoClientFactory mqttClientFactory() {
        DefaultMqttPahoClientFactory factory = new DefaultMqttPahoClientFactory();
        MqttConnectOptions options = new MqttConnectOptions();
        options.setServerURIs(new String[]{"${spring.mqtt.url}"});
        options.setUserName("${spring.mqtt.username}");
        options.setPassword("${spring.mqtt.password}".toCharArray());
        factory.setConnectionOptions(options);
        return factory;
    }

    @Bean
    public MessageChannel mqttInputChannel() {
        return new DirectChannel();
    }

    @Bean
    public MqttPahoMessageDrivenChannelAdapter inbound() {
        MqttPahoMessageDrivenChannelAdapter adapter =
                new MqttPahoMessageDrivenChannelAdapter("${spring.mqtt.client-id}", mqttClientFactory(),
                        "${spring.mqtt.default-topic}");
        adapter.setCompletionTimeout(5000);
        adapter.setConverter(new DefaultPahoMessageConverter());
        adapter.setQos(1);
        adapter.setOutputChannel(mqttInputChannel());
        return adapter;
    }

    @Bean
    @ServiceActivator(inputChannel = "mqttInputChannel")
    public MessageHandler handler() {
        return message -> {
            System.out.println("Received message: " + message.getPayload());
        };
    }

    @Bean
    public MessageChannel mqttOutputChannel() {
        return new DirectChannel();
    }

    @Bean
    @ServiceActivator(inputChannel = "mqttOutputChannel")
    public MessageHandler mqttOutbound() {
        MqttPahoMessageHandler messageHandler =
                new MqttPahoMessageHandler("${spring.mqtt.client-id}-publisher", mqttClientFactory());
        messageHandler.setAsync(true);
        messageHandler.setDefaultTopic("${spring.mqtt.default-topic}");
        return messageHandler;
    }
}

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.messaging.MessageChannel;
import org.springframework.messaging.support.GenericMessage;
import org.springframework.stereotype.Service;

@Service
public class MqttMessageSender {

    @Autowired
    private MessageChannel mqttOutputChannel;

    public void sendMessage(String message) {
        mqttOutputChannel.send(new GenericMessage<>(message));
    }
}

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class MqttController {

    @Autowired
    private MqttMessageSender mqttMessageSender;

    @GetMapping("/send")
    public String sendMessage(@RequestParam String message) {
        mqttMessageSender.sendMessage(message);
        return "Message sent: " + message;
    }
}