The custom wrapper solution to synchronize sounds like a good idea. I
will experiment with that. Would it not also be a solution to add such a
synchronized property wrapper class to JavaFX itself?
The reason for why the expression helper replaces itself makes sense.
However, is that still a design goal of modern JavaFX to implement these
optimizations just to make the memory footprint of instances a bit smaller?
On 25/04/2025 00:14, John Hendrikx wrote:
On 24/04/2025 22:07, Christopher Schnick wrote:
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.
Okay, so you're adding listeners in parallel, but not changing the
property at the same time listeners are being added? In other words,
there is never a listener being added/removed while concurrently
listeners are being notified? That should be relatively safe (if
properties were thread safe in that regard :))
For a short term solution, one thing you can do is to wrap these
global properties in a custom wrapper, delegating all methods to the
original FX class. You then add `synchronized` to all the add/remove
listener methods. The users of these properties won't know the
difference, and ExpressionHelper should no longer get in a bad state.
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.
ExpressionHelper replaces itself depending on the type and count of
listeners on the property; it's an optimization to use as little
memory as possible as most properties have 0 listeners, and the next
most common amount of listeners is 1. For those two special cases,
ExpressionHelper is null (for 0 listeners) or it is one of the
SingleXXX instances (which take less memory than the variants that
have an ArrayList).
The reworking is mostly to fix another problem, and potentially making
listener management synchronized can be done in either case.
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.
As long as you're aware that such a fix (or the wrapper I mentioned
above) only gives partial thread safety and may confuse your own
listeners if they were recently added and immediately get notified... :)
The PR mentioned is not specifically created to solve this problem; if
the team agrees that making listener management thread-safe is the way
forward, it will probably be a separate PR and ticket. The work
involved however is fairly trivial and could be done before or after
the PR mentioned is integrated.
--John
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
