JavaFX Application Thread is recursively re-entrant into Eventhandler handle() method under some circumstances

Kevin Rushforth kevin.rushforth at
Sat Sep 8 12:02:42 UTC 2018

I am not aware of such a bug. If you have a test program, then you can 
file a bug here:

-- Kevin

On 9/7/2018 5:37 PM, javafx at wrote:
> Hi,
> I have a couple of very small apps (3 small classes in one case and 5 
> in another)  which demonstrate that, under some circumstances, the 
> JavaFX Application Thread will recursively re-enter 
> EventHandler#handle().
> So this means that it is already in handle (and calls therefrom) and 
> will, in some situations not complete that  processing (thus exiting 
> handle) before it reappears in the same instance of EventHandler's 
> handle method again. So this is true recursion.
> I actually don't know if this is expected behavior or not. No one I've 
> talked to expected it; the general understanding is the JavaFX 
> Application Thread (processing) is specifically single-threaded and 
> also that it will defintily complete one invocation of handle() before 
> beginning another one.
> I have to say that there is NO other Thread  in play here, at least no 
> other Thread my applications create (what's going on QuantumToolKit 
> may be a different story.)
> The material upshot of this is it can lead to apparent  program 
> incorrectness if the dev believes that it's not the case, and 100% of 
> devs I've talked to think it's not possible.
> I am happy to post or attach the classes or modules as requested but 
> first I wanted to check to see if in fact this is already known to be 
> true and is in fact  expected behavior, in which case it's a non-issue 
> and just a subtlety people are not aware of.
> Thank you so much !

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