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Cancellation

Cancellation. Why we should terminate our threads cleanly?. 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

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Cancellation

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  1. Cancellation

  2. Why we should terminate our threads cleanly?

  3. 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

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

  5. 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 whenT2 is stopped - deadlock in the future • if locks are immediately released the object which T2 was working on is in inconsistent state

  6. introduce a new Exception? • introduce a new exception – ThreadDeath • calls to stop() throwsThreadDeathto 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 ThreadDeathexception

  7. 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

  8. 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

  9. 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

  10. 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

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

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

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

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

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

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