java开发MQTT客户端服务端

Java开发MQTT客户端服务端需要使用MQTT协议的Java库，例如Eclipse Paho或者Mosquitto等。以下是一个基本的MQTT客户端和服务端的代码示例：

MQTT客户端：

import org.eclipse.paho.client.mqttv3.*;

public class MqttClientExample {

    public static void main(String[] args) {

        String brokerUrl = "tcp://localhost:1883";
        String clientId = "mqtt-client";
        String topic = "test/topic";
        int qos = 2;

        try {
            MqttClient mqttClient = new MqttClient(brokerUrl, clientId);
            mqttClient.connect();

            MqttMessage message = new MqttMessage();
            message.setPayload("Hello, MQTT!".getBytes());
            message.setQos(qos);
            mqttClient.publish(topic, message);

            mqttClient.disconnect();
        } catch (MqttException e) {
            e.printStackTrace();
        }
    }
}

MQTT服务端：

import org.eclipse.paho.server.mqttv3.*;

public class MqttServerExample {

    public static void main(String[] args) {

        int port = 1883;
        MemoryPersistence persistence = new MemoryPersistence();

        try {
            MqttServer mqttServer = new MqttServer(port, persistence);
            mqttServer.start();

            mqttServer.subscribe("test/topic", (topic, message) -> {
                System.out.println("Received message: " + new String(message.getPayload()));
            });

            Thread.sleep(5000);

            mqttServer.stop();
        } catch (MqttException | InterruptedException e) {
            e.printStackTrace();
        }
    }
}

这个示例使用了Eclipse Paho库来实现MQTT客户端和服务端。MQTT客户端连接到MQTT服务端并发布一条消息，MQTT服务端订阅了该主题并打印出接收到的消息。