Cancellation and Shutdown - Java Concurrency in Practice - page 178

7.2.2. ExecutorService Shutdown
In Section 6.2.4 , we saw that ExecutorService offers two ways to shut down: graceful
shutdown with shutdown , and abrupt shutdown with shutdownNow . In an abrupt shut-
down, shutdownNow returns the list of tasks that had not yet started after attempting to
cancel all actively executing tasks.