MQTT协议，Java接入

要使用Java接入MQTT协议，你可以使用Eclipse的Paho客户端库。以下是一个简单的例子，展示了如何用Java连接到MQTT代理，订阅一个主题并发布一条消息。

首先，确保你的项目中包含了Paho客户端的依赖。如果你使用Maven，可以在pom.xml中添加以下依赖：

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

以下是Java代码示例：

import org.eclipse.paho.client.mqttv3.MqttClient;
import org.eclipse.paho.client.mqttv3.MqttException;
import org.eclipse.paho.client.mqttv3.MqttMessage;
import org.eclipse.paho.client.mqttv3.MqttPersistenceException;
import org.eclipse.paho.client.mqttv3.persist.MemoryPersistence;
 
public class MqttPubSub {
 
    public static void main(String[] args) {
        String broker = "tcp://your.broker.com:1883";
        String clientId = "JavaClient";
        String topic = "sampleTopic";
        int qos = 2;
        String content = "Hello MQTT";
        String userName = "yourUsername"; // 如果需要，设置用户名
        String password = "yourPassword"; // 如果需要，设置密码
 
        try {
            MqttClient sampleClient = new MqttClient(broker, clientId, new MemoryPersistence());
            MqttConnectOptions connOpts = new MqttConnectOptions();
            connOpts.setCleanSession(true);
            connOpts.setUserName(userName);
            connOpts.setPassword(password.toCharArray());
 
            System.out.println("Connecting to broker: " + broker);
            sampleClient.connect(connOpts);
            System.out.println("Connected");
 
            System.out.println("Publishing message: " + content);
            MqttMessage message = new MqttMessage(content.getBytes());
            message.setQos(qos);
            sampleClient.publish(topic, message);
 
            System.out.println("Subscribing to topic: " + topic);
            sampleClient.subscribe(topic);
 
            System.out.println("Waiting for messages. Press any key to exit");
        } 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("exiting application");
        } catch (MqttPersistenceException e) {
            e.printStackTrace();
        }
    }
}

确保替换your.broker.com:1883为你的MQTT代理的URL和端口，sampleTopic为你要发布和订