Interacting with UI threads in Java/J2ME

Question

I'm writing a J2ME application. One of the pieces is something that polls the contents of a directory periodically, and, if there are any new things, paints them on the screen. I've done this by having the UI form launch a polling thread with a pointer back to itself, and when the polling thread finds something it calls back to the form and calls a syncrhonized method to update it's display. This seems to work fine.

The question I have is this. In C#/.NET I know it is not nice to have non-UI threads updating the UI, and the correct way to handle this is to delegate it up to the UI thread.

E.g. the following:

public void DoSomeUIThing()
{
    if (this.uiComponent.InvokeRequired)
    {
        this.uiComponent.Invoke(someDelegateThatCallsBackToThis);
    }
    else
    {
        this.uiComponent.Text = "This is the update I want to happen";
    }
}

Is there a J2ME equivalent for how to manage this process? How about Java? Or does Java/J2ME just play nicer in regard to this? If not, how is this done?

[EDIT] It appears that Swing supports what I'm asking about via the SwingUtilities.invokeLater() and invokeAndWait() methods. Is there an equivalent framework for J2ME?

Answer 1

Regarding Java, what you are describing looks like a SwingWorker (worker thread).

When a Swing program needs to execute a long-running task, it usually uses one of the worker threads, also known as the background threads.

A Swing program includes the following kinds of threads:

• Initial threads, the threads that execute initial application code.
• The event dispatch thread, where all event-handling code is executed. Most code that interacts with the Swing framework must also execute on this thread.
• Worker threads, also known as background threads, where time-consuming background tasks are executed.

Single-thread rule:
Once a Swing component has been realized, all code that might affect or depend on the state of that component should be executed in the event-dispatching thread.

When used in a J2EE context, you need to be careful when you are referencing a SwingWorker from an EJB.

Regarding J2ME, it depends if you are developing your application as a standard MIDlet that will run on any MIDP-enabled device, or for instance as a RIMlet, a CLDC-based application that uses BlackBerry-specific APIs and therefore will run only on BlackBerry devices.

Because unlike MIDP's UI classes, RIM's are similar to Swing in the sense that UI operations occur on the event thread, which is not thread-safe as in MIDP. To run code on the event thread, an application must obtain a lock on the event object, or use invokeLater() or invokeAndWait() – extra work for the developer, but sophistication comes with a price tag.

But for LCDUI, you can access a form from multiple threads.

Answer 2

There are many profiles of Java ME. If you mean MIDP then Display.runSerially is what you want.

For AWT (Swing) you would use EventQueue.invokeLater (SwingUtilities.invokeLater is only necessary due to Java 1.1 not having the EventQueue method - 1.2 is about to celebrate its tenth birthday). For the Common DOM API, use DOMService.invokeLater.

No matter what claims a GUI API may make about thread-safety they are probably wrong (some of the claims of Swing are removed in JDK7 because they are not implementable). In any case, application code unlikely to be thread-safe.

Answer 3

For j2me apps you probably want to keep it simple. The main thing is to touch UI components only in the event thread. The direct way of doing this is to use invokeLater or invokeAndWait. Depending on your libraries you won't have access to anything more than that. In general if these aren't provided in your platform it probably equates to there being no thread support and not being an issue. For example the blackberry does support it.

Answer 4

If you develop under SWT this is accomplished by means of asyncExec() method of Display object. You pass an object implementing Runnable so the UI thread executes the changes done in other thread.

This is an example borrowed from here

public void itemRemoved(final ModelEvent me)
{
   final TableViewer tableViewer = this.viewer;

   if (tableViewer != null)
   {
      display.asyncExec(new Runnable()
      {
         public void run()
         {
            tableViewer.remove(me.getItem());
         }
      }
   }
}

Answer 5

I can attest that the MIDP UI toolkit is indeed thread-safe, as I have large MIDlets with complex GUI running on millions of phones made by almost every manufacturer, and I have never seen a problem in that regard.