How can the MQTT framework be set up in Java?

There are multiple methods to build an MQTT framework in Java. One commonly used method is:

1. Add MQTT dependency: First, add MQTT dependency in the pom.xml file of the Java project, as shown below:

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

1. Create an MQTT client: Next, create an MQTT client in Java code as shown below:

import org.eclipse.paho.client.mqttv3.MqttClient;
import org.eclipse.paho.client.mqttv3.MqttException;

public class MQTTClient {
  private static final String BROKER = "tcp://mqtt.eclipse.org:1883";
  private static final String CLIENT_ID = "JavaMQTTClient";

  public static void main(String[] args) {
    try {
      MqttClient mqttClient = new MqttClient(BROKER, CLIENT_ID);
      
      // 连接到MQTT代理服务器
      mqttClient.connect();
      
      // 订阅主题
      String topic = "example/topic";
      mqttClient.subscribe(topic);
      
      // 发布消息
      String message = "Hello, MQTT!";
      mqttClient.publish(topic, message.getBytes(), 0, false);
      
      // 断开连接
      mqttClient.disconnect();
    } catch (MqttException e) {
      e.printStackTrace();
    }
  }
}

This is a simple MQTT client that connects to an MQTT broker, subscribes to a topic, publishes messages, and disconnects after completing the tasks.

It is important to note that the code above is just a simple example. In actual use, more exceptions may need to be handled and more parameters configured, such as setting usernames and passwords, setting QoS levels, etc. Additionally, callback functions can also be used to handle received messages and changes in connection status.