The Jar Scanner element represents the component that is
  used to scan the web application for JAR files. It is typically used during
  web application start to identify configuration files such as TLDs or
  web-fragment.xml files that must be processed as part of the web application
  initialisation.
  A Jar Scanner element MAY be nested inside a
  Context component.  If it is not included,
  a default Jar Scanner configuration will be created automatically, which
  is sufficient for most requirements.