EMF Thread Safety

In my past customer project we designed an RCP application that was supposed to monitorartificial-horizon-attitude-indicator and manage a distributed flight simulation. The simulation consists of dozens of independent simulation nodes that interchange messages and signals over a local network.

Basically the RCP application manages a huge and dynamic model graph that reflects all relevant information of the underlying flight simulation. All that information is displayed simultaneously in multiple views and editors.

The domain model is based on EMF and comprises about 100 classes with hundreds of thousands of runtime instances. The application continuously receives network messages on multiple background jobs and updates parts of the domain model accordingly.

Since a flight simulation is a highly dynamic system with dozens of model updates per second, we stumbled upon two major problems:

  • An unresponsive user interface
  • Concurrent domain model modifications

To avoid unresponsive user interfaces you should:

  • Use the SWT.VIRTUAL style for all your viewers
  • Omit EMF touch events
  • Never block the UI Thread
  • Read the post about EMF.Edit performance

EMF models are not thread-safe by default and writing multithreaded applications is not that simple. The more complex our application became, the more often we got concurrent modification exceptions and had problems with filtering and sorting operations.

As you can see in the table below, the only thread-safe operation on EMF instances is reading a single-valued attribute.

Single-Valued Attribute Multi-Valued Attribute
+ Read – Iterate
– Write – Add/Remove

It’s not obvious that writing a single-valued attribute from multiple threads is forbidden. However attaching an EAdapter in one thread while propagating a set event in another thread fires concurrent modification exceptions as well.

There are two established solutions to avoid multithreading problems in EMF.

EMF Transaction

EMF transaction uses transactional editing domains to synchronize read and write operations on different threads. All write operations are triggered by commands that are executed on the domain’s transactional commandstack.

final TransactionalEditingDomain domain = TransactionalEditingDomain.Registry.INSTANCE.getEditingDomain("instanceName");


All Read operations have to run in an exclusive context and the result is passed back as follows:

try {

Double r =(Double)editingDomain.runExclusive(new RunnableWithResult.Impl<Double>() {

  public void run() {

 label.setText("Sum = " +r);

} catch (InterruptedException e) {}

In addition the framework contains several classes for EMF.edit programming.

//EMF.edit support
viever.setContentProvider(new TransactionalAdapterFactoryContentProvider(domain,af);
viever.setLabelProvider(new TransactionalAdapterFactoryLabelProvider(domain,af);

I gave up figuring out how EMF transaction actually is working behind the scenes. It’s really complex and using the framework requires a consequent usage throughout the entire application. While synchronizing write operations using commands is doable, encapsulating all read operations on a shared editing domain ends up in writing many lines of additional boilerplate code. Especially UI related code becomes quite more complex and it’s very likely that team members simply fade out synchronization when just bringing up a simple info dialog that shows parts of your domain model.

That’s why we decided to use a simpler solution.

Model synchronization on the UI Thread

According to the EMF FAQ, EMF itself does not ensure thread-safety in application model implementations. Data structures are unsynchronized, the expectation is that a complete instance of the model (including resources and resource set, if present) should only be accessed by one thread at a time.

By one thread at one time can also be realized by using always the same single thread. An excellent candidate for such a thread is the UI/Display thread.

The advantage of using this thread is that all model operations triggered from inside the UI thread ( e.g. initializing views, databinding, event handlers ) run faultless without any further synchronization. Since more than 80% of all our application code is running in the UI thread we didn’t have to think about concurrency too much.

However all applications require some long running – mostly IO intensive – background operations that affect the application’s domain model. Fortunately a solution for that is really straightforward.

Write operations in background jobs

Below is an example how to modify the domain model in a background job. By the way never use Java threads directly. Prefer higher a level abstraction like the Eclipse jobs API or executor services.

new Job("Model Update") {

   protected IStatus run(IProgressMonitor monitor) {

       //create model in long running operation
	Signal signal = fetchFromWebserviceOrWhatEver();

       //merge instance in UI thread afterwards


After the long running operation has finished, the new model instance has to be merged immediately. Model updates are pretty short running operations and it’s no problem to run them in the UI thread. Actually this thread is idling and waiting for user input most of its time. However using the display object directly is not recommended because not all OSGI bundles should necessarily have dependencies to SWT.

That’s why we used a simple EMFTRansactionHelper utility class that provides methods to modify EMF models thread-safe. In our RCP application the class is initialized on startup as shown below. For headless unit tests running on the server, this class is initialized with another synchronizer.

public Object start(IApplicationContext context) throws Exception {

 Display display = PlatformUI.createDisplay();
 EMFTransactionHelper.setSynchronizer((runnable) ->display.syncExec(runnable));

The previous example can then be written as:

new Job("Model Append") {

protected IStatus run(IProgressMonitor monitor) {

 Signal signal = ModelFactory.eINSTANCE.createSignal();
 signal.setValue((int) Math.random());

 EMFTransactionHelper.addElementExclusive(model,ModelPackage.Literals.MODEL__SIGNALS, signal);

Read operations in background jobs

Many long running operations only need read access to the domain model. For instance generating reports or PDF documents. Unlike reading single-valued attributes from multiple threads iterating over multi-valued collections in background jobs is really problematic.

One option is cloning (EcoreUtil.copy()) parts of the application model in the UI thread and use the cloned model for background processing.

If cloning is not possible, the collection must be copied synchronized in the job itself. Here is an example:

new Job("Read Only") {

protected IStatus run(IProgressMonitor monitor) {

  //clone list thread-safe
  List<Signal> cL = EMFTransactionHelper.cloneCollectionExclusive(model.getSignals);		

  //iterate over clone
  cL.forEach(s-> ...)

Demo Application

The video below shows a very simple application that demonstrates both synchronization techniques. Each viewer contains 10.000 model instances that are modified continuously. The sourcecode is available here.

As you can see, the user interface still reacts very smooth. Both solutions have their pros and cons. For our use case the second solution was the better option. For other uses cases EMF transaction might be the better choice.

Virtual containers in tree viewers using EMF.Edit

Maybe you already stumbled upon the problem to display model instances with hundreds or even thousands of child elements in a tree based viewer. Displaying all that children as direct successors of their parent (shown in the figure on the left) is confusing and slows down the UI.

An alternative approach is to hook in virtual folders, that contain a subset of the parent’s child elements instead. This approach is shown in the figure on the right and might be familiar to you since it’s also used in the Eclipse ‘Variables’ view.

If you use EMF.Edit based item providers to display your domain objects read on!

The technique shown here is a slightly modified version of the technique explained in chapter 19.2.3 Adding Non-Model Intermediary View Objects of the standard Eclipse Modeling Framework book.

The source code below is used to display the right tree viewer in the illustration above. It’s usual EMF.Edit programming. The only difference is the usage of a PartitionedContentProvider instead of an AdapterFactoryContentProvider.

You find the implementation of PartitionedContentProvider here. It’s packaged into a plugin but feel free to copy that single class into your own project. Overwrite the method getVirtualFolderSize() to limit the maximum number of container’s direct children. If the number exceeds this limit, virtual folders are hooked in instead. If the virtual folder size is -1 virtual folders are not created.

final AdapterFactory adapterFactory = new ComposedAdapterFactory(ComposedAdapterFactory.Descriptor.Registry.INSTANCE);
domain = new AdapterFactoryEditingDomain(adapterFactory, new BasicCommandStack());

viewer = new TreeViewer(parent, SWT.FLAT | SWT.MULTI);

viewer.addDragSupport(DND.DROP_MOVE, new Transfer[] { LocalTransfer.getInstance() }, new ViewerDragAdapter(viewer));
viewer.addDropSupport(DND.DROP_MOVE, new Transfer[] { LocalTransfer.getInstance() }, new EditingDomainViewerDropAdapter(domain, viewer));
viewer.setLabelProvider(new AdapterFactoryLabelProvider(adapterFactory));

// we limit the maximal child elements for containers to 100
viewer.setContentProvider(new PartitionedContentProvider(adapterFactory) {

       protected int getVirtualFolderSize(Object folder) {
          return folder instanceof Container ? 100 : DEFAULT_FOLDER_SIZE;

Embedded HTML 5 browser in Java

Today HTML 5 is one of the most sophisticated and popular UI technology available. Unfortunately writing business code using Javascript is a mess. If you are used to Java’s type safety, the modularity of OSGi and all that dozens of third party libraries, you know that plain Javascript is a huge step backwards. The Java Virtual Machine and HTML complement each other for many years in web based applications. So why not combine both technologies to create rich desktop applications? A platform independent user interface on top of a platform independent programming language – a perfect match!

I was very excited when I heard about the HTML 5 support in JavaFX. However the ‘Webview’ rendering is very bad and not comparable to most recent web browsers. Fortunately the browser support in SWT was improved with Eclipse Luna and embedding XULRunner 24.x is now possible.

XULRunner is the primary HTML rendering engine used by Firefox and is available on all major operating systems. The WebGL support in XULRunner is great, audio and video playback are implemented. Also the rendering result on different platforms is almost identical so you don’t have to fight with all that different browser quirks as usual in web development.

The demo below shows an SWT based application with embedded XULRunner that renders a simple WebGL scene using three.js.

The SWT browser API supports calling Java from Javascript and vice versa. I added the ‘Take Screenshot’ and ‘Add Image’ button just to demonstrate the interaction ability. The screen capture itself is based on the solution I posted a while ago . The source code for the demo is available here.

After you can not rely on a proper XULRunner installed on your user’s target machine, bundle your application together with the appropriate XULRunner version. For OSGi based applications this is really straightforward and an exemplary implementation can be found here (Sorry I couldn’t integrate the OSX version yet).

The org.mozilla.xulrunner bundle is the platform independent host bundle with helper classes to instantiate XULRunner based browsers. The XULRunner binaries are provided by platform specific fragment bundles, one fragment for each operating system characteristic.


XULRunner is not supported for GTK3 so under Linux you have to use a start script to run your application:

export SWT_GTK3=0

At the moment I’m playing with different approaches to combine Java and HTML. Single page apps with Java calls using AngularJS and Bootstrap, server centric solution using Vaadin and RAP. So far I have absolutely no idea what works best but I let you know in the near future on this blog.

Screen capture with Java and SWT

Creating screenshots with Java is really straightforward. The more complicated part is to implement a convenient way to select a rectangular area on the screen. The snippet below shows how to implement a rectangular clipping selector using SWT and the Tracker widget.


import org.eclipse.swt.SWT;
import org.eclipse.swt.events.*;
import org.eclipse.swt.graphics.*;
import org.eclipse.swt.widgets.*;

public class ClippingSelector {

    public Rectangle select() {

        final Display display = Display.getDefault();

        //convert desktop to image
        final Image backgroundImage = new Image(display, display.getBounds().width, display.getBounds().height);
        GC gc = new GC(display);
        gc.copyArea(backgroundImage, display.getBounds().x, display.getBounds().y);

        //invisible shell and parent for tracker
        final Shell shell = new Shell(display.getActiveShell(), SWT.NO_BACKGROUND | SWT.ON_TOP);
        shell.setCursor(new Cursor(Display.getCurrent(), SWT.CURSOR_CROSS));

        final Rectangle result = new Rectangle(0, 0, 0, 0);

        shell.addMouseListener(new MouseAdapter() {
            public void mouseDown(MouseEvent e) {

                Tracker tracker = new Tracker(shell, SWT.RESIZE);
                tracker.setRectangles(new Rectangle[] { new Rectangle(e.x, e.y, 0, 0) });

                Rectangle selection = tracker.getRectangles()[0];

                result.width = selection.width;
                result.height = selection.height;

                result.x = shell.toDisplay(selection.x, selection.y).x;
                result.y = shell.toDisplay(selection.x, selection.y).y;



        shell.addKeyListener(new KeyAdapter() {
            public void keyPressed(KeyEvent e) {
                shell.dispose(); //any key pressed close shell

        shell.addShellListener(new ShellAdapter() {
            public void shellDeactivated(ShellEvent e) {
                shell.dispose(); //close shell if another shell is activated


        shell.addPaintListener(new PaintListener() {
            public void paintControl(PaintEvent e) {
                e.gc.drawImage(backgroundImage, -1, -1); //paint background image on invisible shell


        while (!shell.isDisposed()) {
            if (!display.readAndDispatch()) {


        return result;


    public static void main(String[] args) {

        final Display display = new Display();
        final Shell shell = new Shell(display);

        final Rectangle rect = new ClippingSelector().select();

        if (rect.height == 0 || rect.width == 0) return;

        //we show the selected area in a new shell. Just for demonstration!
        final Image image = new Image(display, rect);
        final GC gc = new GC(display);
        gc.copyArea(image, rect.x, rect.y);

        shell.addPaintListener(new PaintListener() {

            public void paintControl(PaintEvent e) {
                e.gc.drawImage(image, 0, 0);

        while (!shell.isDisposed()) {
            if (!display.readAndDispatch()) display.sleep();