On Mar 25, 2025, at 12:16 PM, Andy Goryachev
<andy.goryac...@oracle.com> wrote:
Thank you, Christopher, for clarification!
Personally, I would consider this to be a problem with the
application design: the code should limit the number of alerts
shown to the user. Do you really want the user to click through
hundreds of alerts?
Nevertheless, you are right about the need for the platform to
gracefully handle the case of too many nested event loops - by
throwing an exception with a meaningful message, as Martin proposed
inhttps://github.com/openjdk/jfx/pull/1741
Cheers,
-andy
*From:*Christopher Schnick <crschn...@xpipe.io>
*Date:*Tuesday, March 25, 2025 at 11:52
*To:*Andy Goryachev <andy.goryac...@oracle.com>
*Cc:*OpenJFX <openjfx-dev@openjdk.org>
*Subject:*Re: [External] : Re: JVM crashes on macOS when entering
too many nested event loops
Hey Andy,
so I think I was able to reproduce this issue for our application.
There are two main factors how this can happen:
- We use an alert-based error reporter, meaning that we have a
default uncaught exception handler set for all threads which will
showAndWait an Alert with the exception message
- As I reported yesterday
withhttps://mail.openjdk.org/pipermail/openjfx-dev/2025-March/052963.html,
there are some rare exceptions that can occur in a normal event
loop without interference of the application, probably because of a
small bug in the bounds calculation code
If you combine these two factors, you will end up with an infinite
loop of the showAndWait entering a nested event loop, the event
loop throwing an internal exception, and the uncaught exception
handler starting the same loop with another alert. I don't think
this is a bad implementation from our side, the only thing that we
can improve is to maybe check how deep the uncaught exception loop
is in to prevent this from occurring indefinitely. But I would
argue this can happen to any application. Here is a sample code,
based on the reproducer from the OutOfBounds report from yesterday:
import javafx.application.Application;
import javafx.application.Platform;
import javafx.scene.Scene;
import javafx.scene.control.Alert;
import javafx.scene.control.Button;
import javafx.scene.layout.Region;
import javafx.scene.layout.StackPane;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;
import java.io.IOException;
import java.util.Arrays;
public class ParentBoundsBug extends Application {
@Override
public void start(Stage stage) throws IOException {
Thread./setDefaultUncaughtExceptionHandler/((thread,
throwable) -> {
throwable.printStackTrace();
if (Platform./isFxApplicationThread/()) {
var alert = new Alert(Alert.AlertType./ERROR/);
alert.setHeaderText(throwable.getMessage());
alert.setContentText(Arrays./toString/(throwable.getStackTrace()));
alert.showAndWait();
} else {
// Do some other error handling for non-platform threads
// Probably just show the alert with a runLater()
// For this example, there are no exceptions
outside the platform thread
}
});
// Run delayed as Application::reportException will only be called
for exceptions
// after the application has started
Platform./runLater/(() -> {
Scene scene = new Scene(createContent(), 640, 480);
stage.setScene(scene);
stage.show();
stage.centerOnScreen();
});
}
private Region createContent() {
var b1 = new Button("Click me!");
var b2 = new Button("Click me!");
var vbox = new VBox(b1, b2);
b1.boundsInParentProperty().addListener((observable,
oldValue, newValue) -> {
vbox.setVisible(!vbox.isVisible());
});
b2.boundsInParentProperty().addListener((observable,
oldValue, newValue) -> {
vbox.setVisible(!vbox.isVisible());
});
vbox.boundsInParentProperty().addListener((observable,
oldValue, newValue) -> {
vbox.setVisible(!vbox.isVisible());
});
var stack = new StackPane(vbox, new StackPane());
stack.boundsInParentProperty().addListener((observable,
oldValue, newValue) -> {
vbox.setVisible(!vbox.isVisible());
});
return stack;
}
public static void main(String[] args) {
/launch/();
}
}
If the same OutOfBounds exception from the reported I linked
happens in the bounds calculation, which happens approximately 1/5
runs for me, this application will enter new event loops until it
crashes. If the OutOfBounds doesn't trigger, it will just throw a
StackOverflow but won't continue the infinite loop of nested event
loops. So for the reproducer it is important to try a few times
until you get the described OutOfBounds.
I attached the stacktrace of how this fails. The initial
StackOverflow causes infinitely many following exceptions in the
nested event loop.
Best
Christopher Schnick
On 25/03/2025 18:28, Andy Goryachev wrote:
Dear Christopher:
Were you able to root cause why your application enters that
many nested event loops?
I believe a well-behaved application should never experience
that, unless there is some design flaw or a bug.
-andy
*From:*Christopher Schnick<crschn...@xpipe.io>
<mailto:crschn...@xpipe.io>
*Date:*Monday, March 10, 2025 at 19:45
*To:*Andy Goryachev<andy.goryac...@oracle.com>
<mailto:andy.goryac...@oracle.com>
*Subject:*[External] : Re: JVM crashes on macOS when entering
too many nested event loops
Our code and some libraries do enter some nested event loops at
a few places when it makes sense, but we didn't do anything to
explicitly provoke this, this occurred naturally in our
application. So it would be nice if JavaFX could somehow guard
against this, especially since crashing the JVM is probably the
worst thing that can happen.
I looked at the documentation, but it seems like the public API
at Platform::enterNestedEventLoop does not mention this.
From my understanding, the method
Platform::canStartNestedEventLoop is potentially the right
method to indicate to the caller that the limit is close by
returning false.
And even if something like an exception is thrown when a nested
event loop is started while it is close to the limit, that
would still be much better than a direct crash.
Best
Christopher Schnick
On 10/03/2025 18:51, Andy Goryachev wrote:
This looks to me like it might be hitting the (native)
thread stack size limit.
c.s.glass.ui.Application::enterNestedEventLoop() even warns
about it:
* An application may enter several nested loops
recursively. There's no
* limit of recursion other than that imposed by the native
stack size.
-andy
*From:*openjfx-dev<openjfx-dev-r...@openjdk.org>
<mailto:openjfx-dev-r...@openjdk.org>on behalf of Martin
Fox<martinfox...@gmail.com> <mailto:martinfox...@gmail.com>
*Date:*Monday, March 10, 2025 at 10:10
*To:*Christopher Schnick<crschn...@xpipe.io>
<mailto:crschn...@xpipe.io>
*Cc:*OpenJFX<openjfx-dev@openjdk.org>
<mailto:openjfx-dev@openjdk.org>
*Subject:*Re: JVM crashes on macOS when entering too many
nested event loops
Hi Christopher,
I was able to reproduce this crash. I wrote a small routine
that recursively calls itself in a runLater block and then
enters a nested event loop. The program crashes when
creating loop 254. I’m not sure where that limit comes from
so it’s possible that consuming some other system resource
could lower it. I couldn’t see any good way to determine
how many loops are active by looking at the crash report
since it doesn’t show the entire call stack.
I did a quick trial on Linux and was able to create a lot
more loops (over 600) but then started seeing erratic
behavior and errors coming from the Java VM. The behavior
was variable unlike on the Mac which always crashes when
creating loop 254.
Martin
> On Mar 7, 2025, at 6:24 AM, Christopher
Schnick<crschn...@xpipe.io> <mailto:crschn...@xpipe.io>wrote:
>
> Hello,
>
> I have attached a JVM fatal error log that seemingly was
caused by our JavaFX application entering too many nested
event loops, which macOS apparently doesn't like.
>
> As far as I know, there is no upper limit defined on how
often an event loop can be nested, so I think this is a bug
that can occur in rare situations.
>
> Best
> Christopher Schnick<hs_err_pid.txt>
