@Override
  public void start() throws Exception {
    throw new UnsupportedOperationException(PLACEHOLDER_ERROR_MSG);
  }
This class is a placeholder and should not be initialized. It represents an external Extractor
that uses the MQTT protocol to receive data. There is a real implementation in the server module
but cannot be imported here. The pipe agent in the server module will replace this class with the
real implementation when initializing the MQTTExtractor.
iotdb-core/node-commons/src/main/java/org/apache/iotdb/commons/pipe/agent/plugin/builtin/extractor/mqtt/MQTTExtractor.java