shutdownNow
Attempts to stop all actively executing tasks, halts the processing of waiting tasks, and returns a list of the tasks that were awaiting execution.
This method does not wait for actively executing tasks to terminate. Use .awaitTermination to do that.
There are no guarantees beyond best-effort attempts to stop processing actively executing tasks. For example, typical implementations will cancel via Thread.interrupt, so any task that fails to respond to interrupts may never terminate.
Return
list of tasks that never commenced execution
Throws
if a security manager exists and shutting down this ExecutorService may manipulate threads that the caller is not permitted to modify because it does not hold [ ]("modifyThread"), or the security manager's checkAccess method denies access.