code: public boolean cancel(SipApplicationSessionTimerTask expirationTimerTask) {
          boolean cancelled = ((StandardSasTimerTask)expirationTimerTask).cancel();
          // Purge is expensive when called frequently, only call it every now and then.
          // We do not sync the numCancelled variable. We dont care about correctness of
          // the number, and we will still call purge rought once on every 25 cancels.
          numCancelled++;
          if(numCancelled % 100 == 0) {
              super.purge();
          }
          return cancelled;
      }
docs: /* (non-Javadoc)
  @see org.mobicents.servlet.sip.startup.SipApplicationSessionTimerService#cancel(org.mobicents.servlet.sip.core.timers.SipApplicationSessionTimerTask)