Avoid synchronized in Device.isDisposed #74

[jukzi]

Device.isDisposed() is called much more frequently then dispose() and
should not be costly synchronized. The isDisposed() check in dispose()
is still synchronized so that still there will be no double dispose().

[vogella]

Is this change related to the removal of the synchronization or just better style?

[jukzi]

While it probably not any usual usecase to dispose() Device i wanted to make sure that disposed flag is updated as the very last statement during dispose(). One could also change it to be the very first statement- in any case if isDisposed() is checked in any other thread it has almost no meaning as it could be disposed in the next blink of an eye. So it does not really matter.

[vogella]

Same change needs to be done for SWT Mac and SWT GTK.

[SyntevoAlex]

Note that Device.dispose() is also synchronized. To my understanding, if one thread is disposing and the other is querying isDisposed(), the latter would wait and finally return disposed = true.

With new code, isDisposed() is no longer blocked and will return false even though the Device is already in the process of being disposed.

However, this new problem will probably not make things any worse. Because it's still wrong to call isDisposed() outside of lock and then expecting anything on the next line.

[jukzi]

However, this new problem will probably not make things any worse. Because it's still wrong to call isDisposed() outside of lock and then expecting anything on the next line.

Thats what i think too!

[jukzi]

Same change needs to be done for SWT Mac and SWT GTK.

done

[vogella]

However, this new problem will probably not make things any worse. Because it's still wrong to call isDisposed() outside of lock and then expecting anything on the next line.

Thats what i think too!

Calling isDisposed outside the main thread and expecting no change would be wrong IMHO with or without a lock on this method. So I agree the change is fine.

[lshanmug]

The javadoc of Device.dispose() states that "After this method has been invoked, the receiver will answer true when sent the message isDisposed()". Will this remain true after this change to Device.isDisposed() or can it return false in some cases?

[jukzi]

Will this remain true

will remain true.
see
https://jenkov.com/tutorials/java-concurrency/java-happens-before-guarantee.html#the-java-volatile-happens-before-guarantee

[vogella]

Maybe we should add to the Javadoc that we expect isDisposed to be called from the main thread?

[jukzi]

Maybe we should add to the Javadoc that we expect isDisposed to be called from the main thread?

no. there are a lot of usages where isDisposed() is called from other threads to avoid queuing up messages if widget is already disposed. Still needs a double isDisposed() check within SWT thread though.