Called by the portlet container to indicate to a filter that it is
being taken out of service. This method is only called once all threads
within the filter's
doFilter
method have exited or
after a timeout period has passed.
After the portlet container calls this method, it will not call the
doFilter
method again on this instance of the filter.
This method gives the filter an opportunity to clean up any resources
that are being held (for example, memory, file handles, threads) and
make sure that any persistent state is synchronized with the
filter's current state in memory.