The Loader element represents the web
  application class loader that will be used to load Java
  classes and resources for your web application.  Such
  a class loader must follow the requirements of the Servlet
  Specification, and load classes from the following locations:
  
  - From the /WEB-INF/classesdirectory inside your
      web application.
- From JAR files in the /WEB-INF/libdirectory
      inside your web application.
- From resources made available by Catalina to all web
      applications globally.
A Loader element MAY be nested inside a Context
  component.  If it is not included, a default Loader configuration will be
  created automatically, which is sufficient for most requirements.
  For a more in-depth description of the class loader hierarchy
  that is implemented by Catalina, see the ClassLoader HowTo.
    The description below uses the variable name $CATALINA_BASE to refer the
    base directory against which most relative paths are resolved. If you have
    not configured Tomcat for multiple instances by setting a CATALINA_BASE
    directory, then $CATALINA_BASE will be set to the value of $CATALINA_HOME,
    the directory into which you have installed Tomcat.