用java写一段mqtt调用代码

### 回答1：
这是一段基本的Java代码，用于调用MQTT服务：

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

public class MqttPublisher {
  public static void main(String[] args) {
    String topic = "test";
    String content = "Hello MQTT";
    int qos = 2;
    String broker = "tcp://localhost:1883";
    String clientId = "JavaPublisher";
    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");
      System.out.println("Publishing message: " + content);
      MqttMessage message = new MqttMessage(content.getBytes());
      message.setQos(qos);
      client.publish(topic, message);
      System.out.println("Message published");
      client.disconnect();
      System.out.println("Disconnected");
      System.exit(0);
    } 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);
      me.printStackTrace();
    }
  }
}

请注意，在使用该代码之前，您需要将其导入Eclipse Paho MQTT客户端库。

### 回答2：
以下是一个使用Java编写的MQTT调用代码示例：

import org.eclipse.paho.client.mqttv3.IMqttDeliveryToken;
import org.eclipse.paho.client.mqttv3.MqttCallback;
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.MqttMessage;
import org.eclipse.paho.client.mqttv3.persist.MemoryPersistence;

public class MQTTClient {

  // 定义MQTT服务器地址和端口号
  private static final String BROKER = "tcp://mqtt.example.com:1883";
  // 定义客户端ID
  private static final String CLIENT_ID = "mqtt_client_example";

  public static void main(String[] args) {
    try {
      // 创建MqttClient实例
      MqttClient mqttClient = new MqttClient(BROKER, CLIENT_ID, new MemoryPersistence());

      // 配置MqttConnectOptions
      MqttConnectOptions connOpts = new MqttConnectOptions();
      connOpts.setCleanSession(true); // 设置为非持久化会话
      connOpts.setAutomaticReconnect(true); // 设置自动重新连接

      // 设置回调函数
      mqttClient.setCallback(new MqttCallback() {
        @Override
        public void connectionLost(Throwable cause) {
          System.out.println("连接丢失!");
        }

        @Override
        public void messageArrived(String topic, MqttMessage message) throws Exception {
          System.out.println("收到消息: " + new String(message.getPayload()));
        }

        @Override
        public void deliveryComplete(IMqttDeliveryToken token) {
          // 消息发送成功后的回调
        }
      });

      // 连接到MQTT服务器
      mqttClient.connect(connOpts);

      // 订阅主题
      mqttClient.subscribe("topic/example");

      // 发布消息
      String messageContent = "Hello, MQTT!";
      MqttMessage message = new MqttMessage(messageContent.getBytes());
      message.setQos(1); // 设置消息质量为1（最多一次）
      mqttClient.publish("topic/example", message);

      // 断开与MQTT服务器的连接
      mqttClient.disconnect();

    } catch (MqttException e) {
      e.printStackTrace();
    }
  }
}

上述代码使用Eclipse Paho MQTT库实现了一个简单的MQTT客户端。在代码中创建了一个MqttClient实例，并设置连接选项，包括服务器地址和客户端ID。然后，通过设置MqttCallback来处理连接丢失、收到消息和消息发送成功后的回调。接着，通过调用mqttClient.connect()方法连接到MQTT服务器，并使用mqttClient.subscribe()方法订阅主题。然后，创建一个MqttMessage实例并使用mqttClient.publish()方法发布消息。最后，通过mqttClient.disconnect()方法断开与MQTT服务器的连接。请注意，上述代码是一个简单示例，实际应用中可能会根据需求进行进一步的处理和错误处理。

### 回答3：
以下是用Java写的一个简单的MQTT调用代码：

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

public class MQTTClient {
    private static final String BROKER = "tcp://localhost:1883";
    private static final String TOPIC = "test/topic";
    private static final String CLIENT_ID = "mqtt-client";

    public static void main(String[] args) {
        MemoryPersistence persistence = new MemoryPersistence();

        try {
            MqttClient mqttClient = new MqttClient(BROKER, CLIENT_ID, persistence);
            MqttConnectOptions connOpts = new MqttConnectOptions();
            connOpts.setCleanSession(true);

            mqttClient.connect(connOpts);

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

                @Override
                public void messageArrived(String topic, MqttMessage message) throws Exception {
                    System.out.println("Received message: " + message.toString());
                }

                @Override
                public void deliveryComplete(IMqttDeliveryToken token) {
                    System.out.println("Message Delivered!");
                }
            });

            mqttClient.subscribe(TOPIC);

            String payload = "Hello, MQTT!";
            MqttMessage mqttMessage = new MqttMessage(payload.getBytes());
            mqttClient.publish(TOPIC, mqttMessage);

            mqttClient.disconnect();
        } catch (MqttException e) {
            e.printStackTrace();
        }
    }
}

这段代码使用了Eclipse Paho MQTT客户端库，首先创建一个`MqttClient`对象，指定了MQTT代理地址和客户端ID。然后设置连接选项，包括设置`CleanSession`为`true`，以及连接回调函数。

接下来，连接到MQTT代理并设置回调函数，当连接断开、收到消息或消息发送完成时，回调函数会被调用。

然后，订阅主题`test/topic`，并发布一条消息到主题上。

最后，断开与MQTT代理的连接。

以上是一个简单的MQTT调用代码，主要包括连接、订阅、发布等操作。根据实际需求，你可以根据这个例子进行修改和扩展。