[jarp-developer] Swing and threads

[Filip L. <fil...@ma...>]

Hi,

After browsing through the existing petri net analysis code (e.g.
PetriStatesEnumAnalysis.java) I have the following comments:

Swing is not thread-safe, so only one thread should update or call Swing
components, unless the method in question explicitely is documented as
reentrant. After initialization, the dispatcher thread is the thread that is
calling all the Listeners registered for the various components, and as a
rule of thumb, only this thread should call Swing components.

Other threads can place small "jobs" in the Swing event queue by registering
a Runnable with the javax.swing.SwingUtilities.invokeAndWait (for
synchronous updates) and .invokeLater (for asynchronous updates) methods.

To track the status of work going on in another thread, one normally
registers via invokeLater a Runnable that queries the status of the work,
updates the gui, waits a while and then register itself again if the thread
is still working.

To get the result after work is done in case the result is not updated like
above, the working thread can use invokeLater to register a runnable to
update the work just once.

The following outline should illustration what I mean (without exception
handling, though, and with one gui for both status and result):


public class MyAnalyzer {

  class Task implements Runnable {
    public Task(Runnable onCompletion) { this.onCompletion = onCompletion; }
    public void stop() { interrupted = true; }
    public boolean isRunning() { return working; }
    public boolean wasInterrupted { return interrupted; }
    public void run() {  // invoked in work thread
      while (working && !interrupted) {
        doWork();
      }
      working = false;
      if ( onCompletion != null ) SwingUtilities.invokeLater(onCompletion);
    }
    public Object getStatus() { ... }
    public Object getResult() { ... }

    private boolean working = true;
    private boolean interrupted = false;
    private Runnable onCompletion = null;
    private void doWork() {{  // invoked in work thread
      ...
      if ( done ) working = false;
      ...
    }
  }

  public void execute() {    // invoked in dispatcher thread
    makeGui();
    startTask();
    registerStatus();
  }

  private Task task = new Task();
  private Component gui;

  private void makeGui() {
    gui = new ...;
    JButton stopButton = ...
    stopButton.addActionListener( new ActionListener() {
      public void actionPerformed() {
        if ( task != null ) task.stop();
      }
    });
  }

  private void registerStatus() {
    SwingUtilities.invokeLater( new Runnable() {
      public void run() {  // invoked in dispatcher thread
        gui.setStatus( task.getStatus() );
        Thread.sleep(1000);
        if ( task.isRunning() ) SwingUtilities.invokeLater(this);
      }
    });
  }

  private void startTask() {
    Runnable onCompletion = new Runnable() {
      public void run() {  // invoked in dispatcher thread
        stopButton.setEnable(false);
        if ( task.wasInterrupted() )
          gui.setStatus(task.getStatus());
        else
          gui.setResult(task.getResult());
        task = null;
      }
    }
    task = new Task( onCompletion );
    Thread t = new Thread( task );
    t.run();
  }
}



BR,
---
Filip Larsen <fil...@ma...>