<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
</head>
<body>
<p>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?<br>
<br>
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? <br>
</p>
<div class="moz-cite-prefix">On 25/04/2025 00:14, John Hendrikx
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:ea0ec13f-4345-4842-99b5-c3b057a1fcbe@gmail.com">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<br>
<div class="moz-cite-prefix">On 24/04/2025 22:07, Christopher
Schnick wrote:<br>
</div>
<blockquote type="cite"
cite="mid:1baaf4de-8329-472a-b9c2-4bcbc8f760d5@xpipe.io">
<meta http-equiv="Content-Type"
content="text/html; charset=UTF-8">
<p>Hey John,<br>
<br>
Thanks for taking your time on going into the details here.<br>
<br>
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.<br>
</p>
</blockquote>
<br>
<p>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
:))<br>
</p>
<p>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.</p>
<blockquote type="cite"
cite="mid:1baaf4de-8329-472a-b9c2-4bcbc8f760d5@xpipe.io">
<p> <br>
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.<br>
<br>
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.<br>
</p>
</blockquote>
<p>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).</p>
<p>The reworking is mostly to fix another problem, and potentially
making listener management synchronized can be done in either
case.<br>
</p>
<blockquote type="cite"
cite="mid:1baaf4de-8329-472a-b9c2-4bcbc8f760d5@xpipe.io">
<p> <br>
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.<br>
</p>
</blockquote>
<p>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... :)</p>
<p>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.<br>
</p>
<p>--John<br>
</p>
<blockquote type="cite"
cite="mid:1baaf4de-8329-472a-b9c2-4bcbc8f760d5@xpipe.io">
<p> <br>
Best<br>
Christopher Schnick<br>
</p>
<div class="moz-cite-prefix">On 24/04/2025 03:56, John Hendrikx
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:f0c074e8-176b-4465-a239-2dfb781e361e@gmail.com">
<meta http-equiv="Content-Type"
content="text/html; charset=UTF-8">
<p>Hi,</p>
<p>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.<br>
</p>
<p>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.</p>
<p>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:</p>
<p>## Listener Management</p>
<p>Any UI component that listens to global properties must
either:<br>
<br>
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)<br>
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)<br>
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)</p>
<p>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.</p>
<p>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:<br>
<br>
globalProperty.when(myComponentIsVisible).subscribe(
... ) or addListener( ... )<br>
<br>
Or: <br>
<br>
uiProperty.bind(globalProperty.when(myComponentIsVisible));<br>
<br>
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.<br>
</p>
<p>## Global properties may call listeners at unexpected
times!<br>
<br>
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.</p>
<p>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. <br>
</p>
<p>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):<br>
</p>
<div style="background-color:#ffffff;padding:0px 0px 0px 2px;">
<div
style="color:#000000;background-color:#ffffff;font-family:"Consolas";font-size:11pt;white-space:pre;"><p
style="margin:0;"><span style="color:#000000;"> </span><span
style="color:#646464;">@Override</span></p><p
style="margin:0;"><span style="color:#000000;"> </span><span
style="color:#0000a0;font-weight:bold;">public</span><span
style="color:#000000;"> </span><span
style="color:#0000a0;font-weight:bold;">void</span><span
style="color:#000000;"> addListener(InvalidationListener listener) {</span></p><p
style="margin:0;"><span style="color:#000000;"> </span><span
style="color:#0000c0;background-color:#f0d8a8;">helper</span><span
style="color:#000000;"> = ExpressionHelper.</span><span
style="color:#000000;font-style:italic;">addListener</span><span
style="color:#000000;">(</span><span
style="color:#0000c0;background-color:#d4d4d4;">helper</span><span
style="color:#000000;">, </span><span
style="color:#0000a0;font-weight:bold;">this</span><span
style="color:#000000;">, listener);</span></p><p
style="margin:0;"><span style="color:#000000;"> }</span></p></div>
</div>
<p>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.<br>
</p>
<p>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.<br>
</p>
<p>The PR which replaces ExpressionHelper (<a
class="moz-txt-link-freetext"
href="https://github.com/openjdk/jfx/pull/1081"
moz-do-not-send="true">https://github.com/openjdk/jfx/pull/1081</a>)
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.</p>
<p>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...<br>
<br>
--John</p>
<br>
<div class="moz-cite-prefix">On 23/04/2025 18:58, Christopher
Schnick wrote:<br>
</div>
<blockquote type="cite"
cite="mid:f69784b1-6614-47e0-94f1-b5d94f0e46e8@xpipe.io">Hello,
<br>
<br>
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). <br>
<br>
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. <br>
<br>
This is how a stack trace looks like when this occurs: <br>
<br>
21:25:38:840 - error: Index 2 out of bounds for length 2 <br>
java.lang.ArrayIndexOutOfBoundsException: Index 2 out of
bounds for length 2 <br>
at
com.sun.javafx.binding.ExpressionHelper$Generic.addListener(ExpressionHelper.java:248)<br>
at
com.sun.javafx.binding.ExpressionHelper$Generic.addListener(ExpressionHelper.java:200)<br>
at
com.sun.javafx.binding.ExpressionHelper.addListener(ExpressionHelper.java:65)<br>
at
javafx.beans.binding.ObjectBinding.addListener(ObjectBinding.java:86)
<br>
at
javafx.beans.binding.StringBinding.bind(StringBinding.java:114)
<br>
at
javafx.beans.binding.Bindings$7.<init>(Bindings.java:428)
<br>
at
javafx.beans.binding.Bindings.createStringBinding(Bindings.java:426)
<br>
at
io.xpipe.app.util.StoreStateFormat.shellEnvironment(StoreStateFormat.java:24)<br>
at
io.xpipe.ext.proc.env.ShellEnvironmentStoreProvider.informationString(ShellEnvironmentStoreProvider.java:155)<br>
at
io.xpipe.app.comp.store.StoreEntryWrapper.update(StoreEntryWrapper.java:228)<br>
at
io.xpipe.app.comp.store.StoreViewState.lambda$updateContent$1(StoreViewState.java:147)<br>
at java.lang.Iterable.forEach(Iterable.java:75) <br>
at
io.xpipe.app.comp.store.StoreViewState.updateContent(StoreViewState.java:147)<br>
at
io.xpipe.app.comp.store.StoreViewState.init(StoreViewState.java:93)
<br>
at
io.xpipe.app.core.mode.BaseMode.lambda$onSwitchTo$1(BaseMode.java:109)
<br>
at
io.xpipe.app.util.ThreadHelper.lambda$load$0(ThreadHelper.java:78)
<br>
at java.lang.Thread.run(Thread.java:1447) <br>
<br>
21:25:38:847 - error: Index 3 out of bounds for length 2 <br>
java.lang.ArrayIndexOutOfBoundsException: Index 3 out of
bounds for length 2 <br>
at
com.sun.javafx.binding.ExpressionHelper$Generic.addListener(ExpressionHelper.java:248)<br>
at
com.sun.javafx.binding.ExpressionHelper$Generic.addListener(ExpressionHelper.java:200)<br>
at
com.sun.javafx.binding.ExpressionHelper.addListener(ExpressionHelper.java:65)<br>
at
javafx.beans.binding.ObjectBinding.addListener(ObjectBinding.java:86)
<br>
at
javafx.beans.binding.StringBinding.bind(StringBinding.java:114)
<br>
at
javafx.beans.binding.Bindings$7.<init>(Bindings.java:428)
<br>
at
javafx.beans.binding.Bindings.createStringBinding(Bindings.java:426)
<br>
at
io.xpipe.app.util.StoreStateFormat.shellEnvironment(StoreStateFormat.java:24)<br>
at
io.xpipe.ext.proc.env.ShellEnvironmentStoreProvider.informationString(ShellEnvironmentStoreProvider.java:155)<br>
at
io.xpipe.app.comp.store.StoreEntryWrapper.update(StoreEntryWrapper.java:228)<br>
at
io.xpipe.app.comp.store.StoreEntryWrapper.lambda$setupListeners$3(StoreEntryWrapper.java:143)<br>
at
io.xpipe.app.util.PlatformThread.lambda$runLaterIfNeeded$0(PlatformThread.java:318)<br>
at
com.sun.javafx.application.PlatformImpl.lambda$runLater$4(PlatformImpl.java:424)<br>
at
com.sun.glass.ui.InvokeLaterDispatcher$Future.run$$$capture(InvokeLaterDispatcher.java:95)<br>
at
com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java)<br>
<br>
This full log goes up to index 50 out of bounds due to the
recurring nature of this exception. <br>
<br>
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. <br>
<br>
Best <br>
Christopher Schnick <br>
<br>
</blockquote>
</blockquote>
</blockquote>
</blockquote>
</body>
</html>