Hey John,
Thanks for taking your time on going into the details here.
About our use case: We are actually not constructing UI in a background
thread, all nodes are initialized and added to the scene on the platform
thread. This is done because previously instantiating nodes on other
threads was unstable for some controls. So this issue has nothing to do
with nodes at all and is purely focused on the observable value
listeners. When initializing our application, there are some global
properties like the app language property that a lot of other listeners
and bindings depend on as a lot of stuff has to be changed when the
language changes (Not just within nodes). So in practice, we have 100+
listeners added to these important setting properties. These listeners
are added from various threads as the loading is parallelized. This
loading is to some degree done before the platform is even started.
About the listener management: Yes I'm aware that this can be tricky and
I'm doing the best to account for all of that. The GC problem is always
a bit tricky and has led to some issues in the past. But once you are
familiar with it, it's manageable. We are using a somewhat custom
implementation to create nodes, link them to properties, and control
their lifecycle properly for GC handling for that.
About the expression helper being replaced, that is unfortunate (and
also a bit weird from my unknowing perspective, at least I don't see the
reason for that). If you are already reworking the listeners in general,
then it would be nice if there was a good way to synchronize the
expressionhelper on something that is consistent.
You mentioned that there is a case that is hard to fix where a callback
occurs on another thread while the instances are manipulated on the
first thread. At least for us, that shouldn't be a problem. This issue I
reported is only about adding/removing listeners in different threads.
So I will be following the status of your PR for updates.
Best
Christopher Schnick
On 24/04/2025 03:56, John Hendrikx wrote:
Hi,
I don't think adding synchronized in ExpressionHelper is going to
really solve your problem. It will just move it elsewhere, but feel
free to let me know your exact scenario. For now I will make some
assumptions.
I'm assuming you are constructing UI's in a background thread, and
this UI requires listening to some global properties, like dark/light
mode, or any other configuration that must dynamically change your UI
that's basically global, or some global modeled state that can be
independently used, even without a UI. It's certainly not an
unreasonable scenario in larger applications that may have a lot of
configuration options -- I've been there myself. I usually call these
"global" models; they're not part of any specific piece of the user
interface. Feel free to let me know your scenario.
I'm fine with UI's being constructed on background threads; anything
that could potentially take more than a millisecond SHOULD be done on
a background thread, as otherwise animations will stutter. However,
there are several gotcha's with connecting a UI with global models
that expose properties that you must be aware of:
## Listener Management
Any UI component that listens to global properties must either:
a) unregister itself when the UI component is removed or closed (this
can be very difficult to track as FX has no #dispose method that will
be called)
b) use a weak listener (discouraged as this can lead to phantom call
backs of UI's you thought no longer existed until GC runs)
c) only register the listener when the UI is visible, and immediately
unregister when it becomes invisible (this can be largely automated
with the "when" method of ObservableValue)
Failing to do so means your UI component (including all its
children/parents as they refer to those) will never be garbage
collected as a global property is referring to it.
I highly recommend using the "when" construct here. Basically,
whenever you want to listen to a global property from a UI component
insert a "when" statement:
globalProperty.when(myComponentIsVisible).subscribe( ... ) or
addListener( ... )
Or:
uiProperty.bind(globalProperty.when(myComponentIsVisible));
This results in listeners being registered on the FX thread just
before your UI becomes visible to the user. It also removes the
listeners on the FX thread as soon as the UI becomes invisible. See
the documentation for a good condition to use with when() for this.
## Global properties may call listeners at unexpected times!
When you registered on such a property in a background thread, realize
that as soon as you do, you may get a callback from the FX thread. At
that point in time, your presumed single threaded code that you are
constructing on your isolated thread is being run by two threads. In
other words, you can get a callback from a global property halfway
during construction while your components may be in some half
constructed state. As FX controls are never safe to use concurrently
(and neither will your listener code be) this can cause intermittent
problems.
All that said, let's say we do want to proceed to make listener
management a little bit safer and prevent ExpressionHelper from going
into a bad state.
Your proposal to just synchronize the methods in ExpressionHelper will
be insufficient. ExpressionHelper replaces itself on properties all
the time, meaning that having a single invalidation listener on a
property is a different ExpressionHelper instance then when that same
property has 2 invalidation listeners or say just a single change
listener. This is done by properties like this (from ObjectPropertyBase):
@Override
publicvoidaddListener(InvalidationListener listener) {
helper= ExpressionHelper.addListener(helper, this, listener);
}
As you can see, the actual helper is getting replaced in certain cases
(it "morphs" from one internal type to another depending on what
listener types and counts are registered). That means that the first
call may be dealing with Helper#1, and the second call may also be
dealing with Helper#1 (blocking inside ExpressionHelper on a
synchronized block)... but the first call returns a new Helper,
including the new listener. When then the second call runs that was
blocked, it will replace the Helper again but without knowledge of the
listener that was added by the first call. This happens when going
from a single invalidation listener to two invalidation listeners --
it's a different helper.
There are two ways around that; you could synchronize at an earlier
level before calling ExpressionHelper, adding synchronized to the
above method and similar methods, in all property/bindings and read
only property classes (about 20 orso). Another is to synchronize on
the property itself (which is passed as "this" in the above snippet).
That still requires modifying 20 classes though as the
"removeListener" variant does not pass "this" currently, so it would
need to be explicitly passed for those as well to have something to
synchronize on.
The PR which replaces
ExpressionHelper (https://github.com/openjdk/jfx/pull/1081) faces
similar issues, but in that PR, "this" is passed already in all cases,
giving it something to synchronize on. If that PR is integrated, then
offering thread safe adding/removal of listeners for all observable
that use the new solution can be done in one central location.
Perhaps it is worth doing; as Kevin mentioned, within FX itself we've
run into problems with registering listeners that required quite some
changes in many places. A central fix may be preferable; however it
can't and won't be a full fix, as you still must deal with potential
callbacks coming in from another thread shortly after registering -- a
scenario that most developers will likely not be taking into account
while writing what they presume to be single threaded code...
--John
On 23/04/2025 18:58, 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
