In very ancient days, there used to be a way to "stop" or "kill" a thread, though this is has been deprecated due to it allowing system instability. I noticed though, it's possible to "cancel" a running future. The quotes are there because I don't know how the thread is treated at an OS level.
For my education, What's the difference between canceling a Future, effectively canceling the thread, and stopping the thread? Why is cancelling okay, but stopping/killing a thread in the old days bad?
Cancelling tells the Future its result is no longer desired (if it hasn't already completed), and lets it stop cleanly, whereas Thread.stop() kills the underlying native thread and releases all its monitors. The thread doesn't get a chance to wrap up what it was doing or put anything into a known, good state.
The difference is, one sends a message, but lets the recipient respond to the message on its own terms, and the other just forcefully stops it, effectively instantly. (Letting the recipient respond in its own way can even mean that the job will continue to execute, since different implementations of Future may handle cancellation differently, but any result will still be ignored.)
It's like the difference between putting out the red flag in an auto race, telling a car it needs to come in to the pit stop on the next lap, versus suddenly putting a wall in the middle of the racetrack right in front of the car, which it then crashes into.