ExpressionHelper thread-safety

[Kevin Rushforth]

This came up most recently in the discussion of 
https://github.com/openjdk/jfx/pull/1697

As noted by you and in that PR, properties are not thread-safe. If two 
threads add a listener concurrently, or if one thread adds a listener 
while and another thread notifies the listeners, it is likely to fail.

So the question is: Is it worth doing something about this? And if so, 
how far do we go?

Making the add/remove listeners operations on ExpressionHelper (and 
related classes?) thread-safe so that listeners could be added or 
removed on any thread concurrently with each other and with the 
operation off firing a listener probably wouldn't be too hard or have 
much downside (the performance impact should be negligible and it is 
unlikely to cause a deadlock).

You still wouldn't be able to modify a property on more than one thread, 
nor control the thread on which listeners are notified (they are 
notified on the thread that mutates the property), so it won't magically 
solve all your threading issues; and you still would need to deal with 
the fact that your listener can be called on a different thread than the 
one which added it.

I'd like to hear from Andy, John, and others as to whether they think 
there is value in providing partial thread-safety for the add/remove 
listener methods of properties.

-- Kevin


On 4/23/2025 9:58 AM, Christopher Schnick wrote:
> Hello,
>
> I encountered a rare exception where adding listeners to an observable 
> value might break when they are added concurrently. This is due to 
> ExpressionHelper not being synchronized. I thought about how to fix 
> this on my side, but it is very difficult to do. As this is not a 
> typical platform thread issue, in my opinion it should be possible to 
> add listeners to one observable value from any thread without having 
> to think about any potential synchronization issues (which I can't 
> solve other than just running everything on one thread).
>
> Even worse, due to the size and array being two different variables 
> and being incremented unsafely, once such a concurrent modification 
> occurs, this invalid state will persist permanently and will cause 
> exceptions on any further method call as well. The only solution is to 
> restart the application.
>
> This is how a stack trace looks like when this occurs:
>
> 21:25:38:840 - error: Index 2 out of bounds for length 2
> java.lang.ArrayIndexOutOfBoundsException: Index 2 out of bounds for 
> length 2
>     at 
> com.sun.javafx.binding.ExpressionHelper$Generic.addListener(ExpressionHelper.java:248)
>     at 
> com.sun.javafx.binding.ExpressionHelper$Generic.addListener(ExpressionHelper.java:200)
>     at 
> com.sun.javafx.binding.ExpressionHelper.addListener(ExpressionHelper.java:65)
>     at 
> javafx.beans.binding.ObjectBinding.addListener(ObjectBinding.java:86)
>     at javafx.beans.binding.StringBinding.bind(StringBinding.java:114)
>     at javafx.beans.binding.Bindings$7.<init>(Bindings.java:428)
>     at 
> javafx.beans.binding.Bindings.createStringBinding(Bindings.java:426)
>     at 
> io.xpipe.app.util.StoreStateFormat.shellEnvironment(StoreStateFormat.java:24)
>     at 
> io.xpipe.ext.proc.env.ShellEnvironmentStoreProvider.informationString(ShellEnvironmentStoreProvider.java:155)
>     at 
> io.xpipe.app.comp.store.StoreEntryWrapper.update(StoreEntryWrapper.java:228)
>     at 
> io.xpipe.app.comp.store.StoreViewState.lambda$updateContent$1(StoreViewState.java:147)
>     at java.lang.Iterable.forEach(Iterable.java:75)
>     at 
> io.xpipe.app.comp.store.StoreViewState.updateContent(StoreViewState.java:147)
>     at 
> io.xpipe.app.comp.store.StoreViewState.init(StoreViewState.java:93)
>     at 
> io.xpipe.app.core.mode.BaseMode.lambda$onSwitchTo$1(BaseMode.java:109)
>     at io.xpipe.app.util.ThreadHelper.lambda$load$0(ThreadHelper.java:78)
>     at java.lang.Thread.run(Thread.java:1447)
>
> 21:25:38:847 - error: Index 3 out of bounds for length 2
> java.lang.ArrayIndexOutOfBoundsException: Index 3 out of bounds for 
> length 2
>     at 
> com.sun.javafx.binding.ExpressionHelper$Generic.addListener(ExpressionHelper.java:248)
>     at 
> com.sun.javafx.binding.ExpressionHelper$Generic.addListener(ExpressionHelper.java:200)
>     at 
> com.sun.javafx.binding.ExpressionHelper.addListener(ExpressionHelper.java:65)
>     at 
> javafx.beans.binding.ObjectBinding.addListener(ObjectBinding.java:86)
>     at javafx.beans.binding.StringBinding.bind(StringBinding.java:114)
>     at javafx.beans.binding.Bindings$7.<init>(Bindings.java:428)
>     at 
> javafx.beans.binding.Bindings.createStringBinding(Bindings.java:426)
>     at 
> io.xpipe.app.util.StoreStateFormat.shellEnvironment(StoreStateFormat.java:24)
>     at 
> io.xpipe.ext.proc.env.ShellEnvironmentStoreProvider.informationString(ShellEnvironmentStoreProvider.java:155)
>     at 
> io.xpipe.app.comp.store.StoreEntryWrapper.update(StoreEntryWrapper.java:228)
>     at 
> io.xpipe.app.comp.store.StoreEntryWrapper.lambda$setupListeners$3(StoreEntryWrapper.java:143)
>     at 
> io.xpipe.app.util.PlatformThread.lambda$runLaterIfNeeded$0(PlatformThread.java:318)
>     at 
> com.sun.javafx.application.PlatformImpl.lambda$runLater$4(PlatformImpl.java:424)
>     at 
> com.sun.glass.ui.InvokeLaterDispatcher$Future.run$$$capture(InvokeLaterDispatcher.java:95)
>     at 
> com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java)
>
> This full log goes up to index 50 out of bounds due to the recurring 
> nature of this exception.
>
> Looking at the implementation of ExpressionHelper, I don't see any 
> harm in just synchronizing the methods, at least from my perspective. 
> But I guess that is up to the developers to decide. The only real 
> solution I have as an application developer is to perform all 
> initialization on one thread or just hope that this error is rare 
> enough, both of which aren't great options. So I hope that a potential 
> synchronization of the ExpressionHelper methods can be considered.
>
> Best
> Christopher Schnick
>