Recipe 8.11 Multithreading SWT Applications

8.11.1 Problem

You're running code in a worker thread, and you need to interact with the user interface in an SWT application.

8.11.2 Solution

Use the Display.getDefault method to get access to the main UI thread. Then use the asyncExec or syncExec methods, passing them a Runnable object that will interact with user interface elements.

8.11.3 Discussion

In an SWT application, the main thread, called the UI thread, is the thread responsible for handling events and dispatching them to widgets. In other GUI frameworks such as AWT or Swing, you don't have to interact with the UI thread directly, but the SWT architecture is different (SWT uses a message-pump architecture to support plug-ins in Eclipse).

If you've got a lot of intensive work to do, it isn't a good idea to burden the UI thread, and you might want to launch a worker thread. On the other hand, the UI thread is the only thread that can interact with SWT user interface elements without throwing an exception.

So, how do you interact with user interface elements from worker threads? You can use the UI thread's asyncExec and syncExec methods, passing a Runnable object that works with the user interface element you want. Here's an example. In this worker thread code, we want to display some text in a text widget. To do that, you get the current Display object using the Display.getDefault method and set the text in the text widget like so:

Display.getDefault( ).asyncExec(new Runnable( )
{
    public void run( )
    {
        textWidget.setText("Worker thread task has finished.");
    }
});