Handling Threads & Concurrency in JavaFX

Recently, while converting the PageFlow mode from Java3D to JavaFX in our Java PDF Viewer, I came across some ConcurrentModificationException issues, so thought it worthwhile sharing what I have learnt from fixing them.

To explain what I was doing, I was putting ImageViews onto the scene and using MouseEvents to move the Images around the scene. One of my MouseEvents made use of a Thread in order to continue moving images even when the images were no longer being clicked on.

Despite making sure that I would never concurrently ask the scene to do multiple things at once, I was still randomly receiving ConcurrentModificationExceptions as below:

Exception in runnable
java.util.ConcurrentModificationException
	at java.util.LinkedHashMap$LinkedHashIterator.nextEntry(LinkedHashMap.java:394)
	at java.util.LinkedHashMap$KeyIterator.next(LinkedHashMap.java:405)
	at javafx.scene.Parent.recomputeBounds(Parent.java:1526)
	at javafx.scene.Parent.impl_computeGeomBounds(Parent.java:1275)
	at javafx.scene.layout.Region.impl_computeGeomBounds(Region.java:2517)
	at javafx.scene.Node.updateGeomBounds(Node.java:3142)
	at javafx.scene.Node.getGeomBounds(Node.java:3102)
	at javafx.scene.Node.getLocalBounds(Node.java:3084)
	at javafx.scene.Node.containsBounds(Node.java:3430)
	at javafx.scene.layout.Region.impl_pickNodeLocal(Region.java:2378)
	at javafx.scene.Node.impl_pickNode(Node.java:3931)
	at javafx.scene.Scene$MouseHandler.pickNode(Scene.java:3454)
	at javafx.scene.Scene$MouseHandler.access$2000(Scene.java:3119)
	at javafx.scene.Scene.pick(Scene.java:1736)
	at javafx.scene.Scene.access$6900(Scene.java:169)
	at javafx.scene.Scene$MouseHandler.process(Scene.java:3284)
	at javafx.scene.Scene$MouseHandler.access$8100(Scene.java:3119)
	at javafx.scene.Scene$MouseHandler$1.run(Scene.java:3152)
	at com.sun.javafx.application.PlatformImpl$4.run(PlatformImpl.java:173)
	at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
	at com.sun.glass.ui.win.WinApplication.access$100(WinApplication.java:29)
	at com.sun.glass.ui.win.WinApplication$3$1.run(WinApplication.java:73)
	at java.lang.Thread.run(Thread.java:722)

The stack trace is not terribly helpful as the exception does not occur in my code, but after some research, it was quite clear that the issue is that the scene is NOT thread safe.

There are a couple of good resources worth reading if you are thinking of using threads in JavaFX, for example a Concurrency in JavaFX article from Oracle, or Richard Bair’s blog article on Worker Threads in JavaFX that is a little out of date, but which led me to the best resource that is in fact the API documentation for the Task class.

There is a long list of examples, but the most important one for me was one very near the end: A Task Which Modifies The Scene Graph.

 final Group group = new Group();
 Task<Void> task = new Task<Void>() {
     @Override protected Void call() throws Exception {
         for (int i=0; i<100; i++) {
             if (isCancelled()) break;
             final Rectangle r = new Rectangle(10, 10);
             r.setX(10 * i);
             Platform.runLater(new Runnable() {
                 @Override public void run() {
                     group.getChildren().add(r);
                 }
             });
         }
         return null;
     }
 };

So, if you are modifying the Scene Graph from within a Thread, you need to use Platform.runLater() in order to avoid any unwanted Exceptions.

It is also worth a reminder that if you are using a boolean declared and used outside a thread that also gets used in a thread, make sure to use the volatile keyword to disable any unwanted caching or other optimisations.

I have written several articles on converting our Java3D usage into JavaFX and you can read the other articles here.

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Leon Atherton

Developer at IDRsolutions
Leon is a Developer at IDRsolutions, focusing mainly on development of the PDF to HTML5/SVG converter. He was a speaker at JavaOne 2012, co-presenting a session titled 'Lessons Learned in Writing a PDF-to-JavaFX Converter for NetBeans'.

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