Android JAVA连接MQTT，以及常见问题

本文介绍如何使用Android（JAVA）客户机连接MQTT服务器

第一步先引入MQTT

在 build.gradle 添加以下：

dependencies {
    implementation 'org.eclipse.paho:org.eclipse.paho.client.mqttv3:1.1.0'
}

接下来演示如何连接

创建MQTT客户机类

MqttConnect.class

public class MqttConnect {

  private String HOST = ContentValue.TCP_URL;
  private final String clientId = UUID.randomUUID().toString();
  private static MqttClient mqttClient;

  /**
   * 客户端connect连接mqtt服务器
   *
   * @param username 用户名
   * @param password 密码
   * @param mqttCallback 回调函数
   **/
  public void setMqttClient(String username, String password, MqttCallback mqttCallback)
      throws MqttException
  {
    MqttConnectOptions options = mqttConnectOptions(username, password);
    mqttClient.setCallback(mqttCallback);
    mqttClient.connect(options);
    if(mqttClient.isConnected()) {
      System.out.println("MQTT连接成功");
    }
  }

   /**
   * MQTT连接参数设置
   */
  private MqttConnectOptions mqttConnectOptions(String userName, String passWord)
      throws MqttException {
    mqttClient = new MqttClient(HOST, clientId, new MemoryPersistence());
    MqttConnectOptions options = new MqttConnectOptions();
    options.setUserName(userName);
    options.setPassword(passWord.toCharArray());
    options.setConnectionTimeout(60);
    options.setAutomaticReconnect(true);
    options.setCleanSession(false);
    return options;
  }

  /**
   * 关闭MQTT连接
   */
  public void close() throws MqttException {
    mqttClient.close();
    mqttClient.disconnect();
  }

  /**
   * 向某个主题发布消息 默认qos：1
   */
  public static void pub(String topic, String msg) throws MqttException {
    MqttMessage mqttMessage = new MqttMessage();
    mqttMessage.setPayload(msg.getBytes());
    MqttTopic mqttTopic = mqttClient.getTopic(topic);
    MqttDeliveryToken token = mqttTopic.publish(mqttMessage);
    token.waitForCompletion();
  }

  /**
   * 向某个主题发布消息
   *
   * @param topic: 发布的主题
   * @param msg: 发布的消息
   * @param qos: 消息质量    Qos：0、1、2
   */
  public static void pub(String topic, String msg, int qos) throws MqttException {
    MqttMessage mqttMessage = new MqttMessage();
    mqttMessage.setQos(qos);
    mqttMessage.setPayload(msg.getBytes());
    MqttTopic mqttTopic = mqttClient.getTopic(topic);
    MqttDeliveryToken token = mqttTopic.publish(mqttMessage);
    token.waitForCompletion();
  }

  /**
   * 订阅某一个主题 ，此方法默认的的Qos等级为：1
   *
   * @param topic 主题
   */
  public static void sub(String topic) throws MqttException {
    mqttClient.subscribe(topic);
  }

  /**
   * 订阅某一个主题，可携带Qos
   *
   * @param topic 所要订阅的主题
   * @param qos 消息质量：0、1、2
   */
  public void sub(String topic, int qos) throws MqttException {
    mqttClient.subscribe(topic, qos);
  }

}

创建MQTT消息回调类

MqttInitCallback.class

public class MqttInitCallback implements MqttCallback {

  /**
   * publish发布成功后会执行到这里
   */
  @Override
  public void deliveryComplete(IMqttDeliveryToken token) {
  }

  @Override
  public void connectionLost(Throwable throwable) {
    System.out.println("mqtt close...");
  }

  /**
   * subscribe订阅后得到的消息会执行到这里
   */
  @Override
  public void messageArrived(String topic, MqttMessage message) {
    System.out.println("topic-message:" + message.toString());
  }

}

到这里主要的两个函数也就完成了，分别为 MQTT客户机连接类以及 MQTT数据回调类

接下来是调用连接以及订阅监听消息

// 用户名
private String username = ContentValue.TCP_USERNAME;
// 密码
private String password = ContentValue.TCP_PASSWORD;
// MQTT客户机连接类
private MqttConnect server = new MqttConnect();
// MQTT回调函数
private MqttInitCallback initCallback = new MqttInitCallback();
//    MQTT连接
server.setMqttClient(username, password, initCallback);

// MQTT订阅  "/test"是主题 0是qos，一共可以设置0，1，2
server.sub("/test",0)