java连接MQTT服务器(gradle整合MQTT)

可以使用Eclipse Paho MQTT客户端库来实现Java连接MQTT服务器。

以下是使用gradle构建的示例代码：

1. 在build.gradle文件中添加以下依赖项：

dependencies {
    implementation 'org.eclipse.paho:org.eclipse.paho.client.mqttv3:1.2.3'
}

2. 编写Java代码：

import org.eclipse.paho.client.mqttv3.MqttClient;
import org.eclipse.paho.client.mqttv3.MqttConnectOptions;
import org.eclipse.paho.client.mqttv3.MqttException;
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");

            // 订阅主题
            String topic = "test/topic";
            int qos = 2;
            System.out.println("Subscribing to topic: " + topic);
            client.subscribe(topic, qos);
            System.out.println("Subscribed");

            // 发布消息
            String content = "Hello from Java MQTT client";
            System.out.println("Publishing message: " + content);
            client.publish(topic, content.getBytes(), qos);

            // 断开连接
            client.disconnect();
            System.out.println("Disconnected");
        } catch (MqttException e) {
            System.out.println("Exception: " + e.getMessage());
            e.printStackTrace();
        }
    }
}

此代码创建一个MqttClient实例并连接到指定的MQTT服务器。它还订阅一个主题并发布一条消息。在使用时，请更改broker和topic变量以适应您的环境。