1. Cancellation

2. Why we should terminate our threads cleanly?

4. NeverEnds (at least) two threads are created:
first thread is main thread, executes main() 
second thread is t
main() thread terminates almost immediately.
t never terminates - loops forever
program will never terminate
process only terminates when all of its threads have terminated
Runnable based thread terminates when  run() does

5. t.stop()? No JAVA interface to shutting down threads from the outside
no way to invoke t.stop() from main

6. t.stop()? assume Java allows one thread to stop another
assume while T1 calls T2.stop(), T2 holds some locks and is in the middle of changing the internal state of some object protected by these locks.
if locks are not released when T2 is stopped - deadlock in the future
if locks are immediately released the object which T2 was working on is in inconsistent state

7. introduce a new Exception?
introduce a new exception – ThreadDeath
calls to stop() throws ThreadDeath to the thread, interrupting the thread's current execution.
good in theory but impractical and dangerous:
ThreadDeath exception can be thrown anywhere! examine all code, take care of cleanup prior to terminating thread
ThreadDeath exception can be thrown even while we are cleaning after a previous ThreadDeath exception

8. How to terminate threads cleanly?
use a variable in Thread object that indicates someone requested this thread to stop
thread object checks this variable at regular intervals in run() method
when it is safe to do so
determine whether it accepts to stop (return from run) or not

9. How to terminate threads cleanly?
thread cancellation is a cooperative protocol:
one thread asks another one to stop
receiving thread can accept or reject request

11. How to interrupt long waits?
Threads may sometime be stopped for a long period of time. Threads may:
blocked waiting to enter synchronize block
sleep()
wait()
join()
perform IO

12. interrupted()
Sometimes, when we want to tell a thread T that it needs to stop
T may not notice our request for some time, since T's execution is currently stopped
interrupted status - internal flag to every thread :
interrupt() - flag may be set by any other thread
Thread.isInterrupted() - flag may be checked and cleared

13. interrupted flag If Thread T is in If thread, D calls T.interrupt():
Sleep()
wait()
join()
If thread, D  calls T.interrupt():
InterruptedException will be thrown to T
break from wait
interrupted status of T is cleared

14. interrupted flag
If  T is performing IO, using an interruptible channel:
channel will be closed
 T's interrupt status will be set
ClosedByInterruptException will be thrown to T

15. interrupted flag If T is waiting for a Selector to return
T's interrupt status will be set
call to the selector will return.

16. interrupted flag
If none of the previous conditions holds then 's interrupt status will be set.

17. interrupt mechanism is used in conjunction with the methods mentioned above