Javadoc
Initiates an orderly shutdown in which previously submitted
tasks are executed, but no new tasks will be accepted. If the
ExecuteExistingDelayedTasksAfterShutdownPolicy has
been set
false, existing delayed tasks whose delays
have not yet elapsed are cancelled. And unless the
ContinueExistingPeriodicTasksAfterShutdownPolicy has
been set
true, future executions of existing periodic
tasks will be cancelled.