In both cases only a partial fix can be applied that can ensure that at a minimum the listener management doesn't get into a bad state. The issue of what happens when a callback occurs on a second thread while instances are being manipulated on the first thread will remain.
The partial fix would involve synchronizing on the property, which can be done in either all properties themselves, or in the helper (as they are passed the property reference). For ExpressionHelper, removeListener currently doesn't pass the property reference, but it could be added. For the new PR, this is already the case. --John On 23/04/2025 20:54, Nir Lisker wrote: > John is replacing some of the ExpressionHelper uses (properties and > bindings) through https://github.com/openjdk/jfx/pull/1081. It's still > single threaded, but I think the new implementation there should be > the center point of this discussion. > > On Wed, Apr 23, 2025 at 9:41 PM Kevin Rushforth > <kevin.rushfo...@oracle.com> wrote: > > 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 > > >
