The difference of Timer implementation in WebLogic 8.x and 9.x

eDocs of wl8.x http://edocs.bea.com/wls/docs81/jmx/timer.html
eDocs of wl9.x http://edocs.bea.com/wls/docs90/jmxinst/timer.html

In the 8.1, the timer uses a simple way to add and remove notification while 9.x uses a service to do schedule.