private static void init() {
    mqttDir = IoTDBDescriptor.getInstance().getConfig().getMqttDir();
    logger.info("mqttDir: {}", mqttDir);

    try {
      makeMqttPluginDir();
      buildMqttPluginMap();
    } catch (IOException e) {
      logger.error("MQTT PayloadFormatManager init() error.", e);
    }
  }
PayloadFormatManager loads payload formatter from SPI services.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/protocol/mqtt/PayloadFormatManager.java