1 / 17

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

junius
Télécharger la présentation

Cancellation

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related