【MQTT在java中的应用】

目录

• Java MqttServer应用的示例代码：
• 使用Java的MqttClient类实现基本功能的示例代码：

Java MqttServer应用的示例代码：

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

public class MqttServer {

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

        try {
            MqttClient mqttClient = new MqttClient(broker, clientId, persistence);

            MqttConnectOptions connectOptions = new MqttConnectOptions();
            connectOptions.setCleanSession(true);

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

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

                public void deliveryComplete(IMqttDeliveryToken token) {
                    // Not used in this example
                }
            });

            mqttClient.connect(connectOptions);
            mqttClient.subscribe("mytopic");

            System.out.println("MqttServer is running");

        } catch (MqttException e) {
            System.out.println("Error: " + e.getMessage());
        }
    }
}

这个示例代码使用了Eclipse Paho MQTT库来实现MqttServer。首先，创建了一个MqttClient对象，使用指定的broker和client ID。然后，我们设置了连接选项，并为MqttClient对象设置了回调函数。

回调函数由MqttCallback接口定义，用于处理连接丢失、消息到达和消息投递完成等事件。在回调函数中，我们简单地打印出了相应的信息。

接下来，连接到broker并订阅名为"mytopic"的主题。最后，我们打印出"MqttServer is running"表示MqttServer已经开始运行。

使用Java的MqttClient类实现基本功能的示例代码：

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

public class MqttClientExample {
    public static void main(String[] args) {
        String broker = "tcp://mqtt.eclipse.org:1883";
        String clientId = "JavaMqttClient";
        MemoryPersistence persistence = new MemoryPersistence();

        try {
            MqttClient client = new MqttClient(broker, clientId, persistence);
            MqttConnectOptions connOpts = new MqttConnectOptions();
            connOpts.setCleanSession(true);

            System.out.println("Connecting to broker: " + broker);
            client.connect(connOpts);
            System.out.println("Connected");

            String topic = "test/topic";
            int qos = 1;

            System.out.println("Subscribing to topic: " + topic);
            client.subscribe(topic, qos);

            System.out.println("Publishing message: Hello, MQTT!");
            String message = "Hello, MQTT!";
            MqttMessage mqttMessage = new MqttMessage(message.getBytes());
            mqttMessage.setQos(qos);
            client.publish(topic, mqttMessage);

            client.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("Received message: " + new String(message.getPayload()));
                }

                public void deliveryComplete(IMqttDeliveryToken token) {
                    System.out.println("Message delivered");
                }
            });

            // Wait for some time to receive messages
            Thread.sleep(5000);

            client.disconnect();
            System.out.println("Disconnected");
        } catch (MqttException | InterruptedException e) {
            e.printStackTrace();
        }
    }
}

在上述示例代码中，首先创建一个MqttClient对象，并设置要连接的broker地址、客户端ID和持久化实例。接下来，设置连接选项并连接到broker。然后，订阅一个主题并发布一条消息。最后，我们设置一个MqttCallback对象来处理连接丢失、消息到达和消息传递完成的事件。最后，通过调用disconnect()方法来断开连接。