The javafx.beans package for me at least isn't necessarily bound to
JavaFX nodes. In our case, we use observable values for more than just
for JavaFX nodes. They are useful on their own. Obviously in most cases
they are used in relation to nodes, but in our case we instantiate
properties and add listeners (across multiple threads) before the
Platform is even started.
On 23/04/2025 20:59, Andy Goryachev wrote:
Even though JavaFX explicitly permits creating Nodes and Scenes in a
thread other than the Application Thread, I think it is still a bad
idea, and I would strongly suggest against doing so. The code might
work - initially - but you will soon discover that it presents a
constant source of issues, especially after the application is deployed.
I would also question the value of such a design. How many
milliseconds is being saved by trying to instantiate Nodes in a
background thread? If you create only a few objects, there is
absolutely no benefit (and a huge maintenance burden), but if there
are too many objects created then maybe one is doing something wrong,
perhaps instead one should try to create things in batches?
So my recommendation would remain the same: please don't. Always
access JavaFX objects from the Application Thread.
-andy
*From: *openjfx-dev <openjfx-dev-r...@openjdk.org> on behalf of Kevin
Rushforth <kevin.rushfo...@oracle.com>
*Date: *Wednesday, April 23, 2025 at 11:41
*To: *openjfx-dev@openjdk.org <openjfx-dev@openjdk.org>
*Subject: *Re: ExpressionHelper thread-safety
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
>
