Java实现MQTT连接

MQTT是一种基于发布/订阅模式的轻量级通信协议，常用于物联网中设备与服务器之间的通信。Java实现MQTT连接需要使用Eclipse Paho MQTT客户端库。

以下是一个简单的Java程序示例，用于连接到MQTT代理并发布消息：

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

public class MQTTClient {
    public static void main(String[] args) {
        String broker = "tcp://localhost:1883";
        String clientId = "JavaClient";
        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/topic";
            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");
        } catch (MqttException me) {
            System.out.println("reason: " + me.getReasonCode());
            System.out.println("msg: " + me.getMessage());
            System.out.println("loc: " + me.getLocalizedMessage());
            System.out.println("cause: " + me.getCause());
            System.out.println("excep: " + me);
        }
    }
}

在这个例子中，我们使用`MqttClient`类来创建一个MQTT客户端。`MqttConnectOptions`类用于设置连接选项，例如设置清除会话标志（`setCleanSession(true)`）。接着，我们使用`mqttClient.connect(connOpts)`方法连接到MQTT代理，并使用`mqttClient.publish(topic, message)`方法发布一条消息。最后，我们使用`mqttClient.disconnect()`方法断开连接。

请确保已经添加Eclipse Paho MQTT客户端库到您的Java项目中，否则编译代码时将会出现错误。