如何使用java编写MQTT的客户端和服务端

您可以使用 Eclipse Paho 提供的 Java 客户端库来编写 MQTT 客户端和服务端。以下是使用 Eclipse Paho 的示例代码：

客户端：

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

public class MqttClientSample {
    public static void main(String[] args) {
        String broker = "tcp://localhost:1883";
        String clientId = "JavaSample";
        MemoryPersistence persistence = new MemoryPersistence();

        try {
            MqttClient mqttClient = new MqttClient(broker, clientId, persistence);
            MqttConnectOptions connOpts = new MqttConnectOptions();
            connOpts.setCleanSession(true);
            System.out.println("Connecting to broker: " + broker);
            mqttClient.connect(connOpts);
            System.out.println("Connected");

            String topic = "test";
            String content = "Hello World";
            int qos = 2;
            System.out.println("Publishing message: " + content);
            MqttMessage message = new MqttMessage(content.getBytes());
            message.setQos(qos);
            mqttClient.publish(topic, message);
            System.out.println("Message published");

            mqttClient.disconnect();
            System.out.println("Disconnected");
            System.exit(0);
        } catch (MqttException e) {
            System.out.println("Reason: " + e.getReasonCode());
            System.out.println("Message: " + e.getMessage());
            System.out.println("Loc: " + e.getLocalizedMessage());
            System.out.println("Cause: " + e.getCause());
            System.out.println("Excep: " + e);
            e.printStackTrace();
        }
    }
}

服务端：

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

public class MqttServerSample {
    public static void main(String[] args) {
        String broker = "tcp://localhost:1883";
        String clientId = "JavaServer";
        MemoryPersistence persistence = new MemoryPersistence();

        try {
            MqttClient mqttClient = new MqttClient(broker, clientId, persistence);
            MqttConnectOptions connOpts = new MqttConnectOptions();
            connOpts.setCleanSession(true);
            System.out.println("Connecting to broker: " + broker);
            mqttClient.connect(connOpts);
            System.out.println("Connected");

            mqttClient.setCallback(new MqttCallback() {
                public void connectionLost(Throwable cause) {
                    System.out.println("Connection lost");
                }

                public void messageArrived(String topic, MqttMessage message) throws Exception {
                    System.out.println("Message received: " + new String(message.getPayload()));
                }

                public void deliveryComplete(IMqttDeliveryToken token) {
                    System.out.println("Delivery complete");
                }
            });

            mqttClient.subscribe("test");
            System.out.println("Subscribed to topic: test");

            while (true) {
                // do nothing
            }
        } catch (MqttException e) {
            System.out.println("Reason: " + e.getReasonCode());
            System.out.println("Message: " + e.getMessage());
            System.out.println("Loc: " + e.getLocalizedMessage());
            System.out.println("Cause: " + e.getCause());
            System.out.println("Excep: " + e);
            e.printStackTrace();
        }
    }
}