repaint method and design question

Discussion in 'Java' started by conrad, Jun 29, 2008.

  1. conrad

    conrad Guest

    I have two classes:
    one for the GUI the other for various scanning methods
    on a file system.

    I'm experiencing an issue where, while scanning,
    the main pane, if minimized and brought back up,
    is blank. So I'm thinking I need to constantly
    repaint while scanning?

    In terms of design: should I be passing around
    GUI objects(instance variables) from my GUI class
    to my file system scanning class?

    Or should I handle it differently?

    --
    conrad
     
    conrad, Jun 29, 2008
    #1
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  2. conrad

    Stefan Ram Guest

    conrad <> writes:
    >In terms of design: should I be passing around
    >GUI objects(instance variables) from my GUI class
    >to my file system scanning class?


    If you use standard GUI components they should handle
    it all for you.

    You supply a model, and according to the observer pattern
    your UI delegate will be notified of changes to the model
    and will update the view.

    When you design custom components you implement (overwrite)
    »paintComponent«. But also in this case, it will be called
    by Java, whenever necessary (for example, when an obscured part
    of your component becomes visible). Maybe you want to study

    http://google.to/search?q=java custom-painting tutorial

    or

    http://google.to/search?q=java observer ui-delegate model
     
    Stefan Ram, Jun 29, 2008
    #2
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  3. conrad

    Roedy Green Guest

    On Sat, 28 Jun 2008 17:58:37 -0700 (PDT), conrad <>
    wrote, quoted or indirectly quoted someone who said :

    >I'm experiencing an issue where, while scanning,
    >the main pane, if minimized and brought back up,
    >is blank. So I'm thinking I need to constantly
    >repaint while scanning?


    I don't know what you meant by "scanning" but if it is as all CPU
    intensive or long lasting, you should do it on a separate thread, not
    the EDT. See http://mindprod.com/jgloss/swingthreads.html
    for details.
    --

    Roedy Green Canadian Mind Products
    The Java Glossary
    http://mindprod.com
     
    Roedy Green, Jun 29, 2008
    #3
  4. In article <>,
    Roedy Green <> wrote:

    > On Sat, 28 Jun 2008 17:58:37 -0700 (PDT), conrad <>
    > wrote, quoted or indirectly quoted someone who said :
    >
    > >I'm experiencing an issue where, while scanning,
    > >the main pane, if minimized and brought back up,
    > >is blank. So I'm thinking I need to constantly
    > >repaint while scanning?

    >
    > I don't know what you meant by "scanning" but if it is as all CPU
    > intensive or long lasting, you should do it on a separate thread, not
    > the EDT. See http://mindprod.com/jgloss/swingthreads.html
    > for details.


    Roedy's right. Here's the outline of a simple model that may guide your
    re-factoring. It uses a javax.swing.Timer to notify the GUI of interim
    results (not compiled):

    class Scanner extends Observable implements ActionListener, Runnable {

    private int interval = 1000 / 4; // 4 Hz
    private Timer timer;
    private Results results;

    public Scanner() {
    super();
    timer = new Timer(interval, this);
    }

    public void run() {
    timer.start()
    // lengthy code that generates results
    timer.stop();
    setChanged();
    notifyObservers(results);
    }

    public void actionPerformed(ActionEvent e) {
    // interim progress
    setChanged();
    notifyObservers(results);
    }
    }

    public class ScanView extends JPanel implements Observer {
    ...
    public void update(Observable o, Object arg) {
    Results results = (Results) arg;
    // fill in GUI components with results
    this.repaint();
    }
    }
    ....
    Scanner scanner = new Scanner();
    new Thread((Runnable) scanner).start();

    --
    John B. Matthews
    trashgod at gmail dot com
    home dot woh dot rr dot com slash jbmatthews
     
    John B. Matthews, Jun 29, 2008
    #4
  5. In article <>,
    "Peter Duniho" <> wrote:

    > On Sun, 29 Jun 2008 10:02:10 -0700, John B. Matthews <>
    > wrote:
    >
    > >> I don't know what you meant by "scanning" but if it is as all CPU
    > >> intensive or long lasting, you should do it on a separate thread, not
    > >> the EDT. See http://mindprod.com/jgloss/swingthreads.html
    > >> for details.

    > >
    > > Roedy's right. Here's the outline of a simple model that may guide your
    > > re-factoring. It uses a javax.swing.Timer to notify the GUI of interim
    > > results (not compiled):

    >
    > Please correct me if I'm wrong, but I see nothing in the code you posted
    > that ensures that the observer notification is done on the EDT. The
    > documentation specifically says that there are basically no guarantees as
    > to how the observer's update() method is called, never mind that it's
    > always called on the EDT (in fact, the docs also say that subclasses of
    > the Observable class may "deliver notifications on separate threads").


    I appreciate your critical analysis! This is an outline of an approach
    I've used (successfully) several times for simulations in which a view
    displays the interim results calculated by an asynchronously running
    model. I'm sure I've overlooked a critical caveat or two:)

    My proposed Scanner implements Runnable, so it's a separate thread. I
    recall reading the "separate threads" warning, but I understood the
    warning to refer to notification order not being preserved across
    threads. Timer(int, ActionListener) ensures that notification is done on
    the EDT, but I'm not sure that is required. More importantly, the
    Observer is running on the EDT, as it extends an AWT Component.

    > Since the notification happens in response to the timer event, and since
    > that event happens on the EDT, as long as the base Observable class always
    > executes notifications on the same thread on which notifyObservers() is
    > called, you're safe. But the docs don't restrict the Observable class in
    > this way. I personally wouldn't depend on the current implementation to
    > never change, unless there was some specific Java documentation that
    > promises that the Observable class will never change in this respect.


    The javax.swing.Timer I proposed runs on a separate thread; only it's
    action handler executes on the EDT. One _could_ put drawing code in the
    handler, but sending a notification instead keeps the model and view
    separate. I thinka java.util.Timer would do as well.

    > As long as I'm replying, I'll also point out that your sample code
    > conveniently skips demonstrating any of the synchronization code that
    > would be required in order to successfully respond to the timer while the
    > other thread is busily working on the time-consuming operation. This code
    > could in fact be included, yes...but it would not be an insignificant
    > change. In fact, it's arguably "the hard part" of the whole sample.


    Yes, a caveat is warranted: The data structure that the model is
    updating must be consistent for a given instant of simulated time, or
    the view must receive a consistent copy/snapshot. In the OP's case, it
    wouldn't do to have the scanner updating the same TableModel used by the
    view!

    > Personally, I'd skip the timer altogether, and simply update the UI at
    > convenient intervals throughout processing based simply on progress (say,
    > every N files, where N is selected based on how long each file would take
    > to process). Then just use invokeAndWait() or invokeLater() directly from
    > the processing thread to call code that would update the GUI. This allows
    > the processing thread to package up the interesting data synchronously
    > without any actual explicit synchronization overhead, and send it off to
    > the GUI via the EDT, taking advantage of the EDT's own built-in
    > synchronization mechanisms.


    This approach makes better sense than mine for the OP's file scanning
    operation. Of course, the scanner thread still has to decide when it has
    useful, self-consistent, interim results. My approach may be more suited
    to watching a numerical simulation evolve, where there's no _a_priori_
    way to know a preferred update point, other than "periodically".

    > Just my two cents. :)


    Worth much more! What, with compound interest and the price of copper,
    etc. :)

    --
    John B. Matthews
    trashgod at gmail dot com
    home dot woh dot rr dot com slash jbmatthews
     
    John B. Matthews, Jun 30, 2008
    #5
  6. In article <>,
    "Peter Duniho" <> wrote:

    > On Mon, 30 Jun 2008 15:32:07 -0700, John B. Matthews <>
    > wrote:
    >
    > > [...]
    > > My proposed Scanner implements Runnable, so it's a separate thread. I
    > > recall reading the "separate threads" warning, but I understood the
    > > warning to refer to notification order not being preserved across
    > > threads.

    >
    > Well, IMHO the docs are ambiguous. To me, the important part is that they
    > go out of their way to say that one should not rely on the notification
    > being done on a particular thread. The warning is in the context of
    > dealing with sub-classes of Observable, and so there _could_ be an
    > implication that as long as the sub-class doesn't change the default
    > implementation, the notification will be done on the same thread on which
    > notifyObservers() is called.


    Yes, the subclass that implements Observable should behave correctly,
    but I read the paragraph about notification order as a warning to
    Observers that an Observable may change the order and thread.

    > But that's an assumption, and as we all know, assumptions are not nearly
    > as valuable as things printed in black and white. :)


    Agreed!

    > > Timer(int, ActionListener) ensures that notification is done on
    > > the EDT,

    >
    > Now this is exactly what I'm talking about. The Swing Timer class
    > does ensure that the Action event itself happens on the EDT.
    > However, there's nothing about Timer that guarantees that when you
    > call notifyObservers(), the notification itself happens on the same
    > thread on which that method was called.
    >
    > For example, one implementation could create a copy of the observer
    > list and then ship that off to yet another thread to process the
    > list asynchronously.
    >
    > I don't even know for sure that the _current_ implementation keeps
    > the notification on the same thread as which notifyObservers() was
    > called (though I do agree it's likely, I haven't bothered to check
    > it myself, and the fact is it could be implementation-dependent),
    > but it does seem to me that the docs don't promise that even if it's
    > true now, it always will be true.


    I don't think it matters as long as the Observer respects the
    order/thread warning above. In particular, my proposed Observer's
    update(Observable, Object) method ends with repaint(). The hidden
    assumption is that repaint() must be invoked from a JComponent, which
    uses the RepaintManager to run from the EDT:

    <http://java.sun.com/products/jfc/tsc/articles/painting/index.html#paint_
    process>

    > > but I'm not sure that is required. More importantly, the
    > > Observer is running on the EDT, as it extends an AWT Component.

    >
    > It depends on what you mean by "is running on the EDT". It's true
    > that your Observer-implementing class is a Swing (not just AWT)
    > component. But that doesn't guarantee that code in that class is
    > run on the EDT. In fact, that's the whole issue: it _is_ a Swing
    > component and so when you execute Swing-dependent code in the class,
    > you have to go out of your way to ensure that happens on the EDT.


    Good point! I should have said JComponent.

    > If that's what you meant, then yes...I agree. But if your statement was
    > intended to mean that because you've sub-classed JFrame, you're assured
    > that calls to your sub-class are always on the EDT, that would be wrong.


    Yes, for JFrame. I proposed JPanel, which is a JComponent, but your
    point is well taken.

    > > The javax.swing.Timer I proposed runs on a separate thread; only
    > > it's action handler executes on the EDT. One _could_ put drawing
    > > code in the handler, but sending a notification instead keeps the
    > > model and view separate. I thinka java.util.Timer would do as well.

    >
    > See above. I agree that the Timer Action event is raised on the EDT,
    > but that doesn't necessarily guarantee that the Observer's update()
    > method is called on the EDT. There's a missing link between your
    > actionPerformed() method being called, and the Observer's update()
    > method being called that involves ambiguously defined behavior on
    > the part of the Java run-time.


    As long as the Observer's update() repaints a lightweight container, I
    don't think it matters if the notification doesn't come from the EDT.

    > >> As long as I'm replying, I'll also point out that your sample code
    > >> conveniently skips demonstrating any of the synchronization code
    > >> that would be required in order to successfully respond to the
    > >> timer while the other thread is busily working on the
    > >> time-consuming operation. This code could in fact be included,
    > >> yes...but it would not be an insignificant change. In fact, it's
    > >> arguably "the hard part" of the whole sample.

    > >
    > > Yes, a caveat is warranted: The data structure that the model is
    > > updating must be consistent for a given instant of simulated time,
    > > or the view must receive a consistent copy/snapshot. In the OP's
    > > case, it wouldn't do to have the scanner updating the same
    > > TableModel used by the view!

    >
    > Well, more specifically, since you know that the Timer Action event
    > will be raised on the EDT, and you also know that the scanning work
    > is done on a thread other than the EDT, you are assured that you
    > need some kind of synchronization mechanism. Even if this is as
    > simple as having the scanning thread making immutable copies of its
    > progress at regular intervals and assigned references to the data to
    > a volatile variable, _some_ sort of effort needs to be made to keep
    > things synchronized.


    Agreed, although there are stochastic models where no synchronization at
    all gives satisfactory results.

    > >> Personally, I'd skip the timer altogether, and simply update the
    > >> UI at convenient intervals throughout processing based simply on
    > >> progress (say, every N files, where N is selected based on how
    > >> long each file would take to process). Then just use
    > >> invokeAndWait() or invokeLater() directly from the processing
    > >> thread to call code that would update the GUI. This allows the
    > >> processing thread to package up the interesting data synchronously
    > >> without any actual explicit synchronization overhead, and send it
    > >> off to the GUI via the EDT, taking advantage of the EDT's own
    > >> built-in synchronization mechanisms.

    > >
    > > This approach makes better sense than mine for the OP's file
    > > scanning operation. Of course, the scanner thread still has to
    > > decide when it has useful, self-consistent, interim results. My
    > > approach may be more suited to watching a numerical simulation
    > > evolve, where there's no _a_priori_ way to know a preferred update
    > > point, other than "periodically".

    >
    > Agreed...to the extent that it might be difficult to determine what a
    > "preferred update point" is, a timer-based approach might work
    > better. I'm not sure that that applies to a numerical simulation,
    > but that's only because of the broadness of that genre of
    > algorithms. Some numerical simulations lend themselves just fine to
    > predictable update points, others might not.
    >
    > Note, however, that the more likely that an algorithm is specifically
    > in need of a timer-based approach, the more complex the
    > synchronization mechanism will have to be (i.e. just copying
    > immutable results to a volatile variable won't do, since if you
    > could do that, it'd imply that you do in fact have predictable
    > update points :) ).


    Don't similar synchronization problems attend to using the invoke*
    utilities?

    > >> Just my two cents. :)

    > >
    > > Worth much more! What, with compound interest and the price of copper,
    > > etc. :)

    >
    > Thanks. :)


    --
    John B. Matthews
    trashgod at gmail dot com
    home dot woh dot rr dot com slash jbmatthews
     
    John B. Matthews, Jul 1, 2008
    #6
  7. In article <>,
    Lew <> wrote:

    > John B. Matthews wrote:
    > > I don't think it matters as long as the Observer respects the
    > > order/thread warning above. In particular, my proposed Observer's
    > > update(Observable, Object) method ends with repaint(). The hidden
    > > assumption is that repaint() must be invoked from a JComponent, which
    > > uses the RepaintManager to run from the EDT:

    >
    > I see nothing in the Javadocs for JComponent#repaint() that indicates
    > that the method is thread safe.


    Yes, the SDN article is referenced in Component, and perhaps elsewhere.

    > But you showed us
    > > <http://java.sun.com/products/jfc/tsc/articles/painting/index.html#
    > > paint_process>

    >
    > which explains that repaint() is thread safe.


    Only for JComponents: "JComponent.repaint() registers an asynchronous
    repaint request to the component's RepaintManager, which uses
    invokeLater() to queue a Runnable to later process the request on the
    event dispatching thread."

    > This worries me, as I had thought the Javadocs to be normative.


    A linked article is less dispositive, I suppose, but not entirely
    unreliable:)

    > > ... there are stochastic models where no synchronization at
    > > all gives satisfactory results.

    >
    > How do variable values communicate across threads in that case?
    >
    > Or by "stochastic" do you mean that it doesn't matter, because the
    > results are intended to be random?


    Yes, the results evolve in apparent random order: many kinds of
    fractals; diffusion limited aggregation, which has a fractal character;
    any iteration over a set of lattice points in which you don't care that
    some points are from iteration n-1 and some from iteration n.

    > I really do wonder how one handles the memory-model concerns in that
    > "stochastic models" scenario. Would you be so gracious as to explain?


    My point was only that sometimes it doesn't matter if the model is
    changing under you. When it matters, you have to synchronize access, of
    course. Sorry if I seemed to be arguing to the contrary.

    Oh, and I hope I am always gracious!

    --
    John B. Matthews
    trashgod at gmail dot com
    home dot woh dot rr dot com slash jbmatthews
     
    John B. Matthews, Jul 2, 2008
    #7
  8. In article <>,
    "Peter Duniho" <> wrote:

    > On Tue, 01 Jul 2008 12:57:37 -0700, John B. Matthews <>
    > wrote:
    >
    > > Yes, the subclass that implements Observable should behave correctly,

    >
    > Define "correctly". The point of the statements in the documentation
    > is that the Observable class is not under any restriction regarding
    > how it deals with notification. That's my point. There's a whole
    > range of what can be considered "correct", including any variety of
    > implementations that don't allow you assume that the Observer's
    > update() method is called on the EDT.


    I think we agree on this for subclasses.

    > > but I read the paragraph about notification order as a warning to
    > > Observers that an Observable may change the order and thread.

    >
    > Yes, it may. And IMHO this can include the base Observable class.


    I must demur: "The default implementation provided in the Observable
    class will notify Observers in the order in which they registered
    interest..." tells me that order is preserved if my subclass doesn't
    change it. If I do change it, Observers must accept that.

    The phrases "...this notification mechanism is has nothing to do with
    threads..." and "Each observer has its update method called..." tell me
    that my subclass will execute the update() method of a registered
    Observer even if they're in different threads, and Observers have to
    accept what that implies.

    > > [...]
    > >> I don't even know for sure that the _current_ implementation keeps
    > >> the notification on the same thread as which notifyObservers()
    > >> was called (though I do agree it's likely, I haven't bothered to
    > >> check it myself, and the fact is it could be
    > >> implementation-dependent), but it does seem to me that the docs
    > >> don't promise that even if it's true now, it always will be true.

    > >
    > > I don't think it matters as long as the Observer respects the
    > > order/thread warning above. In particular, my proposed Observer's
    > > update(Observable, Object) method ends with repaint(). The hidden
    > > assumption is that repaint() must be invoked from a JComponent,
    > > which uses the RepaintManager to run from the EDT:
    > >
    > > <http://java.sun.com/products/jfc/tsc/articles/painting/index.html#p
    > > aint_ process>

    >
    > Unfortunately, the documentation is not clear as to whether repaint()
    > can be called from a thread other than the EDT. Just because it
    > queues a Runnable to the EDT, that doesn't necessarily mean that it
    > doesn't itself need to be also called from the EDT.


    Well, the article claims "JComponent.repaint() registers an asynchronous
    repaint request to the component's RepaintManager, which uses
    invokeLater() to queue a Runnable," which is precisely the approach you
    proposed earlier.

    > More significantly, even if repaint() is legal to be called from any
    > thread, not just the EDT, in presenting a code example we should be
    > VERY explicit that we are relying on that specific behavior. Other
    > interactions with a component (AWT or Swing) could easily still be
    > required to be done on the EDT, even if the repaint() method need not
    > be (and to be clear: I remain unconvinced that the documentation
    > promises that to be true).
    >
    > A naïve reader of the code example might incorrectly extrapolate from
    > the lack of special handling for repaint() that no special handling
    > is needed generally.


    I agree. The scheme relies on repainting JComponents, and then only if
    the SDN article is dispositive. It's a hidden assumption that I should
    document.

    > >> > but I'm not sure that is required. More importantly, the
    > >> > Observer is running on the EDT, as it extends an AWT Component.
    > >>
    > >> It depends on what you mean by "is running on the EDT". It's true
    > >> that your Observer-implementing class is a Swing (not just AWT)
    > >> component. But that doesn't guarantee that code in that class is
    > >> run on the EDT. In fact, that's the whole issue: it _is_ a Swing
    > >> component and so when you execute Swing-dependent code in the
    > >> class, you have to go out of your way to ensure that happens on
    > >> the EDT.

    > >
    > > Good point! I should have said JComponent.

    >
    > That's not what I am talking about. Whether you're talking Swing or
    > EDT, you need to make sure you are making calls to the component on
    > the EDT.


    I agree. The Observer's update() method should defer drawing, but it can
    get interim results and update component data models, e.g. a
    BufferedImage or a TableModel. It then calls repaint() to schedule the
    screen update on the EDT.

    > >> If that's what you meant, then yes...I agree. But if your statement was
    > >> intended to mean that because you've sub-classed JFrame, you're assured
    > >> that calls to your sub-class are always on the EDT, that would be wrong.

    > >
    > > Yes, for JFrame. I proposed JPanel, which is a JComponent, but your
    > > point is well taken.

    >
    > Again, for what I'm talking about, whether you are using JFrame or JPanel
    > doesn't matter. You still have the same "must be on EDT" requirement.
    >
    > >> > The javax.swing.Timer I proposed runs on a separate thread; only
    > >> > it's action handler executes on the EDT. One _could_ put drawing
    > >> > code in the handler, but sending a notification instead keeps the
    > >> > model and view separate. I thinka java.util.Timer would do as well.
    > >>
    > >> See above. I agree that the Timer Action event is raised on the EDT,
    > >> but that doesn't necessarily guarantee that the Observer's update()
    > >> method is called on the EDT. There's a missing link between your
    > >> actionPerformed() method being called, and the Observer's update()
    > >> method being called that involves ambiguously defined behavior on
    > >> the part of the Java run-time.

    > >
    > > As long as the Observer's update() repaints a lightweight container, I
    > > don't think it matters if the notification doesn't come from the EDT.

    >
    > Well, I haven't seen any documentation that promises that. It could be
    > true, but I have no reason at the moment to believe that. But again, the
    > repaint() method is a very specific method; there is no general promise
    > that when interacting with an AWT or Swing component from the Observer's
    > update() method, it doesn't matter if the notification doesn't come from
    > the EDT. In many other scenarios, it definitely would matter.
    >
    > Even if we grant that repaint() has special rules (and again, I'm not
    > convinced we should), the more general rule still needs to be followed for
    > other kinds of calls to the component. Presumably, if you're updating the
    > GUI, you will in fact find such calls, and they will need to be executed
    > on the EDT.


    I welcome your skepticism, but the documentation for Component.repaint()
    makes repeated reference to the SDN article, and the article mentions
    calling paint() from the EDT as step one for both AWT and Swing
    components.

    > > [...]
    > >> Note, however, that the more likely that an algorithm is
    > >> specifically in need of a timer-based approach, the more complex
    > >> the synchronization mechanism will have to be (i.e. just copying
    > >> immutable results to a volatile variable won't do, since if you
    > >> could do that, it'd imply that you do in fact have predictable
    > >> update points :) ).

    > >
    > > Don't similar synchronization problems attend to using the invoke*
    > > utilities?

    >
    > I don't really know what you mean here. By design, invokeLater() and
    > invokeAndWait() address synchronization issues related specifically
    > to those methods.


    I see the run() method getting called from the EDT, but with no special
    synchronization, other than being added to the end of the queue.

    > The code being invoked does need to deal with synchronization,
    > possibly. But when I write code like this, typically the only data
    > being used in the invoked code is the bare minimum data required to
    > update the GUI. In particular, the extraction of the relevant
    > information from my model has already been done, on the same thread
    > that's processing the model, and copied to some specific place used
    > by the invoked code (sometimes this is as simple as a "final" local
    > variable used in an anonymous class).
    >
    > In this paradigm, because only one thread is ever actually using the
    > model itself (the processing thread for the model), no
    > synchronization issues exist with respect to the model, and the
    > invoke*() methods automatically deal with synchronization issues
    > that may exist with respect to the component itself.


    Is the model also running on the EDT?

    > As I said, I'm not sure I understand your question. But my first
    > impression of what you're asking leads me to answer "no, similar
    > problems do not exist".


    I appreciate your patience helping me vet the approach. It's immensely
    helpful in understanding it's limitations.

    --
    John B. Matthews
    trashgod at gmail dot com
    home dot woh dot rr dot com slash jbmatthews
     
    John B. Matthews, Jul 2, 2008
    #8
  9. In article <>,
    Lew <> wrote:

    > John B. Matthews wrote:
    > > Yes, the SDN article is referenced in Component, and perhaps elsewhere.

    >
    > I finally figured that out, after spending some quality time with the
    > Javadocs.
    >
    > It doesn't help that some of that outrigger documentation has been
    > superseded,
    > but the docs don't reflect the new knowledge.
    >
    > >>> ... there are stochastic models where no synchronization at
    > >>> all gives satisfactory results.

    >
    > Lew wrote:
    > >> How do variable values communicate across threads in that case?
    > >>
    > >> Or by "stochastic" do you mean that it doesn't matter, because the
    > >> results are intended to be random?

    >
    > John:
    > > Yes, the results evolve in apparent random order: many kinds of
    > > fractals; diffusion limited aggregation, which has a fractal character;
    > > any iteration over a set of lattice points in which you don't care that
    > > some points are from iteration n-1 and some from iteration n.

    >
    > But without synchronization, the display thread might only see
    > iteration zero forever.


    In practice I haven't seen this. The model, an Observable, executes the
    view's update(Observable Object) method, and the view either asks the
    model for data or receives it as a parameter. For some models, the data
    may be no more than a reference to an array of primitive types, which
    the model continues to update even as the view accesses it to compose,
    for example, a BufferedImage.

    > >> I really do wonder how one handles the memory-model concerns in
    > >> that "stochastic models" scenario. Would you be so gracious as to
    > >> explain?

    >
    > > My point was only that sometimes it doesn't matter if the model is
    > > changing under you. When it matters, you have to synchronize
    > > access, of

    >
    > But don't model changes by very necessity have to be communicated
    > somewhere, eventually? Absent synchronization, either via the
    > 'synchronized' keyword or via any of the several other mechanisms
    > that Java provides, there is no guarantee in the memory model that
    > those changes would ever escape to another thread that needs them.


    I can see this for more complex, non-thread-safe data structures. I
    imagine a Vector, which is synchronized, or one of the
    Collections.synchronized[Collection|List|Map|etc.] would do as well.

    > And if another thread doesn't need them, then it's not a
    > multithreaded scenario.
    >
    > I'm not sure failing to communicate model state actually contributes
    > to the stochasticism.


    Yes, the model is intended to be stochastic, not factitious:)

    > > Sorry if I seemed to be arguing to the contrary.

    >
    > You are not.
    >
    > > Oh, and I hope I am always gracious!

    >
    > You are so.


    --
    John B. Matthews
    trashgod at gmail dot com
    home dot woh dot rr dot com slash jbmatthews
     
    John B. Matthews, Jul 2, 2008
    #9
  10. In article <>,
    "Peter Duniho" <> wrote:

    > On Tue, 01 Jul 2008 22:07:16 -0700, John B. Matthews <>
    > wrote:
    >
    > > [...]
    > >> > but I read the paragraph about notification order as a warning
    > >> > to Observers that an Observable may change the order and thread.
    > >>
    > >> Yes, it may. And IMHO this can include the base Observable class.

    > >
    > > I must demur: "The default implementation provided in the
    > > Observable class will notify Observers in the order in which they
    > > registered interest..." tells me that order is preserved if my
    > > subclass doesn't change it. If I do change it, Observers must
    > > accept that.

    >
    > Nothing I wrote has anything to do with the _order_ in which
    > observers are notified.


    Ah, I misunderstood. I thought you were saying that the base Observable
    class might change the order, when the documentation says the opposite.
    I understand that subclasses may do so.

    > > The phrases "...this notification mechanism is has nothing to do
    > > with threads..." and "Each observer has its update method
    > > called..." tell me that my subclass will execute the update()
    > > method of a registered Observer even if they're in different
    > > threads, and Observers have to accept what that implies.

    >
    > What you wrote supports my point. Yet, you seem to be including it
    > with the intent to refute my point.
    >
    > I can only conclude that either you misunderstand my point, or you
    > misunderstand the implications of "will execute the update() method
    > of a registered Observer even if they're in different threads".


    Yes, I misunderstood your point.

    > On a slight tangent: it's not really clear from the above quote that
    > you understand an important point about threading. Specifically, an
    > _object_ is not on a specific thread. The _execution_ of some code
    > is on a specific thread. A given class can have code that is
    > executed on arbitrarily many threads. There is no connection
    > between a given object and a given thread, except whatever
    > connection is part of the actual explicit implementation.
    >
    > If you're confused about this, that could explain some of the
    > disconnect here.


    Of course, I don't know what I don't know, but I think I understand:)
    Lew helpfully pointed out my ambiguous antecedent, so let me restate:

    Starting a new thread for a Runnable [e.g. new Thread((Runnable)
    scanner).start()] causes the Runnable's run() method to begin executing
    on a new thread. Invoking notifyObservers() causes the registered
    Observer's update() method to execute on that same thread, even though
    other methods in the Observable may be executing on another thread. In
    particular, an Observable that extends JPanel (or some other top-level
    container) may have (at most) one method executing on the EDT.

    I think this is the genesis of your welcomed criticism of my approach.

    > > [...]
    > >> Unfortunately, the documentation is not clear as to whether
    > >> repaint() can be called from a thread other than the EDT. Just
    > >> because it queues a Runnable to the EDT, that doesn't necessarily
    > >> mean that it doesn't itself need to be also called from the EDT.

    > >
    > > Well, the article claims "JComponent.repaint() registers an
    > > asynchronous repaint request to the component's RepaintManager,
    > > which uses invokeLater() to queue a Runnable," which is precisely
    > > the approach you proposed earlier.

    >
    > No, it's not.
    >
    > To be clear: Lew has helped by pointing out _some_ documentation
    > (such as it is) that explains that the repaint() method can be
    > successfully called from a thread other than the EDT thread. So
    > we've got that point out of the way.
    >
    > But, the approach I proposed has nothing to do with whether the
    > _painting_ of the control is done on the EDT or not (and in fact, I
    > take as granted that it is, whatever else the implementation may be,
    > since that's a requirement of AWT and Swing).
    >
    > So, the fact that when you call repaint(), that ultimately queues a
    > Runnable to the event queue that will eventually do the actual
    > repainting of the controls has absolutely nothing to do with how
    > data is synchronized between the processing thread and the GUI (EDT)
    > thread.


    I agree.

    > >> More significantly, even if repaint() is legal to be called from any
    > >> thread, not just the EDT, in presenting a code example we should be
    > >> VERY explicit that we are relying on that specific behavior. Other
    > >> interactions with a component (AWT or Swing) could easily still be
    > >> required to be done on the EDT, even if the repaint() method need not
    > >> be (and to be clear: I remain unconvinced that the documentation
    > >> promises that to be true).
    > >>
    > >> A naïve reader of the code example might incorrectly extrapolate from
    > >> the lack of special handling for repaint() that no special handling
    > >> is needed generally.

    > >
    > > I agree. The scheme relies on repainting JComponents, and then only if
    > > the SDN article is dispositive. It's a hidden assumption that I should
    > > document.

    >
    > Well, more importantly, you gloss over the real work: actually taking the
    > model data and using it to update the GUI. The repaint() method itself
    > doesn't do anything except actually redraw the GUI. For that to be a
    > useful operation, you have to first _change_ the GUI so that when it
    > redraws, the state of the GUI is actually different, reflecting the
    > current state of processing.
    >
    > So yes, while the lack of a guarantee that the update() method is called
    > on the EDT doesn't hurt us with respect to calling repaint() (sort
    > of...see * below), it certainly would if you actually meant to _do_
    > anything with the GUI that presents the current state of processing.
    >
    > (*) Note my comments in my reply to Lew regarding the thread safety of
    > calling repaint(). The docs do explain that you can call repaint() from a
    > thread other than the EDT. But they also say that any method that's
    > thread-safe will specifically say so, and repaint() doesn't say so. This
    > means that while it's legal to call repaint() from a non-EDT thread, you
    > are still required to address synchronization yourself, since repaint()
    > isn't a thread-safe method.
    >
    > In other words, you're really only safe calling repaint() in one respect.
    > There's another, arguably more important respect, in which calling it from
    > another thread without synchronization is _not_ safe.


    Permit me to defer comment to that part of the thread.

    > > [...]
    > >> That's not what I am talking about. Whether you're talking Swing or
    > >> EDT, you need to make sure you are making calls to the component on
    > >> the EDT.

    > >
    > > I agree. The Observer's update() method should defer drawing,

    >
    > It does, by virtue of the repaint() method deferring actual drawing.
    >
    > > but it can
    > > get interim results and update component data models, e.g. a
    > > BufferedImage or a TableModel. It then calls repaint() to schedule the
    > > screen update on the EDT.

    >
    > But those "component data models" need to be synchronized. If you are not
    > executing your code on the EDT, then they may be being used by code
    > running on the EDT at the same time you are modifying them. And that's on
    > top of the fact that Swing does not automatically handle cross-thread
    > updates to data models for GUI components.
    >
    > For example (from
    > http://java.sun.com/javase/6/docs/api/javax/swing/package-summary.html#threadi
    > ng):
    >
    > ...if a TableModel is attached to a JTable, the TableModel
    > should only be modified on the event dispatching thread.
    > If you modify the model on a separate thread you run the
    > risk of exceptions and possible display corruption
    >
    > Conversely, if you _are_ executing your code on the EDT, then the data
    > model from which you're getting your actual information (that is, the
    > processing thread's model data structure) is what needs to be synchronized.
    >
    > The bottom line is that you have a processing thread that's generating
    > data, but you can only display data on the EDT, which is by definition
    > going to be a different thread from the processing thread. You _must_
    > address synchronization of those threads any time you want data to flow
    > from one thread to the other. And the only way to present the results of
    > the processing (interim results or otherwise) to the user is to have data
    > flow from one thread to the other.
    >
    > On top of all this, the Observable class doesn't give us any assurance
    > that Observer implementers are going to be called on the same thread on
    > which Observable.notifyObservers() is called. So your Observer
    > implementer cannot make the assumption that it's update() method is called
    > on the EDT, even if you call notifyObservers() on the EDT.
    >
    > And finally (to conclude this particular logical progression), since your
    > Observer implementation cannot make the assumption that its update()
    > method is called on the EDT, it must itself explicitly ensure that any
    > interaction it has with Swing is done on the EDT, except possibly for
    > things that are explicitly documented as not needing that (in this case,
    > that would be the repaint() method, but not any of the other things you
    > might do in order to update the GUI).


    These point are well taken, and thank you for pointing out the
    TableModel caveat. I've been modifying a BufferedImage's
    WriteableRaster, so any problems may have been more in-apparent than
    impossible.

    > > [...]
    > >> Even if we grant that repaint() has special rules (and again, I'm not
    > >> convinced we should), the more general rule still needs to be followed
    > >> for
    > >> other kinds of calls to the component. Presumably, if you're updating
    > >> the
    > >> GUI, you will in fact find such calls, and they will need to be executed
    > >> on the EDT.

    > >
    > > I welcome your skepticism, but the documentation for Component.repaint()
    > > makes repeated reference to the SDN article, and the article mentions
    > > calling paint() from the EDT as step one for both AWT and Swing
    > > components.

    >
    > As I noted above, I concede the point that repaint() itself may be called
    > from a thread other than the EDT. However, please note:
    >
    > -- that doesn't help with any of the other methods that you'll need to
    > call in order to update the GUI, and
    > -- since repaint() isn't documented as being thread-safe, you still
    > have a synchronization issue even with that method
    >
    > >> > [...]
    > >> >> Note, however, that the more likely that an algorithm is
    > >> >> specifically in need of a timer-based approach, the more complex
    > >> >> the synchronization mechanism will have to be (i.e. just copying
    > >> >> immutable results to a volatile variable won't do, since if you
    > >> >> could do that, it'd imply that you do in fact have predictable
    > >> >> update points :) ).
    > >> >
    > >> > Don't similar synchronization problems attend to using the invoke*
    > >> > utilities?
    > >>
    > >> I don't really know what you mean here. By design, invokeLater() and
    > >> invokeAndWait() address synchronization issues related specifically
    > >> to those methods.

    > >
    > > I see the run() method getting called from the EDT, but with no special
    > > synchronization, other than being added to the end of the queue.

    >
    > You see which "run() method" getting called from the EDT? How _isn't_
    > making sure some code is executed on the same thread as some other code
    > not obviously synchronizing those two pieces of code?
    >
    > >> The code being invoked does need to deal with synchronization,
    > >> possibly. But when I write code like this, typically the only data
    > >> being used in the invoked code is the bare minimum data required to
    > >> update the GUI. In particular, the extraction of the relevant
    > >> information from my model has already been done, on the same thread
    > >> that's processing the model, and copied to some specific place used
    > >> by the invoked code (sometimes this is as simple as a "final" local
    > >> variable used in an anonymous class).
    > >>
    > >> In this paradigm, because only one thread is ever actually using the
    > >> model itself (the processing thread for the model), no
    > >> synchronization issues exist with respect to the model, and the
    > >> invoke*() methods automatically deal with synchronization issues
    > >> that may exist with respect to the component itself.

    > >
    > > Is the model also running on the EDT?

    >
    > No, not the model I'm referring to. As I said, "only one thread is ever
    > actually using the model itself (the processing thread for the model)".
    > Since by definition, the processing thread is not the EDT, obviously the
    > model is not "running on the EDT" (inasmuch as the model could be
    > "running" on any thread...I wouldn't describe it that way, but rather as
    > simply the _processing thread_ is what's "running", and it modifies the
    > model as it runs; that's a bit of a semantic nit-pick though).


    Yes, I confounded the model as a class with the model's run() method.

    On reflection, I see the clear contract the invokeLater() and
    invokeAndWait() methods offer with respect to the EDT, and my approach
    may have too many limitations.

    --
    John B. Matthews
    trashgod at gmail dot com
    home dot woh dot rr dot com slash jbmatthews
     
    John B. Matthews, Jul 2, 2008
    #10
  11. In article <>,
    Lew <> wrote:

    > John B. Matthews wrote:
    > > The phrases "...this notification mechanism is has nothing to do with
    > > threads..." and "Each observer has its update method called..." tell me
    > > that my subclass will execute the update() method of a registered
    > > Observer even if they're in different threads, and Observers have to
    > > accept what that implies.

    >
    > The determination of the thread in which an action occurs depends on the
    > caller, not the called. It really doesn't make sense to say that an object
    > is
    > "in" a thread - only actions occur in a thread.
    >
    > Between that and the dangling antecedent for "they", I'm not certain I
    > understand the sentence, but I think you are saying that the thread in which
    > an Observer's update() will occur is the thread in which the observed object
    > makes the call. That would be the case.


    Yes. Thank you for pointing out the ambiguity. I tried to make myself
    clearer in my response to Pete.

    --
    John B. Matthews
    trashgod at gmail dot com
    home dot woh dot rr dot com slash jbmatthews
     
    John B. Matthews, Jul 3, 2008
    #11
  12. In article <>,
    "Peter Duniho" <> wrote:

    > On Wed, 02 Jul 2008 14:07:47 -0700, John B. Matthews <>
    > wrote:
    >
    > > [...]
    > >> On a slight tangent: it's not really clear from the above quote
    > >> that you understand an important point about threading.
    > >> Specifically, an _object_ is not on a specific thread. The
    > >> _execution_ of some code is on a specific thread. A given class
    > >> can have code that is executed on arbitrarily many threads.
    > >> There is no connection between a given object and a given thread,
    > >> except whatever connection is part of the actual explicit
    > >> implementation.
    > >>
    > >> If you're confused about this, that could explain some of the
    > >> disconnect here.

    > >
    > > Of course, I don't know what I don't know, but I think I
    > > understand:) Lew helpfully pointed out my ambiguous antecedent, so
    > > let me restate:
    > >
    > > Starting a new thread for a Runnable [e.g. new Thread((Runnable)
    > > scanner).start()] causes the Runnable's run() method to begin
    > > executing on a new thread. Invoking notifyObservers() causes the
    > > registered Observer's update() method to execute on that same
    > > thread, even though other methods in the Observable may be
    > > executing on another thread.

    >
    > Well, that depends on how you call notifyObservers(). The code you
    > posted does _not_ actually do this though. It calls
    > notifyObservers() from the actionPerformed() method, which is called
    > by the Timer class on the EDT, not the thread on which the
    > Observable's run() method was called.


    Worse! A serious flaw in the outline I proposed is that update() runs on
    the EDT when sent by the Timer's actionPerformed() method, but the final
    notification executes update() on the thread on which the
    Observable's run() method was called.

    > At _best_, this will cause the registered Observer's update() method
    > to be executed on the EDT, but in any case certainly not on the
    > thread on which the Observable's run() method was called. At worst,
    > the notifyObservers() method itself might actually use yet another
    > thread for the purpose of calling the Observer's update() method
    > (I'd be surprised in the Sun implementation of the base Observable
    > class did that, but you never really know and the docs go to some
    > length to warn that you cannot count on that happening).


    I am somewhat reassured that "... the action event handlers for Timers
    execute on ... the event-dispatching thread," and that the Observable's
    "... notification mechanism has nothing to do with threads..."

    > > In particular, an Observable that extends JPanel (or some other top-
    > > level container) may have (at most) one method executing on the EDT.

    >
    > I suppose that depends on your definition of "executing". Personally, I'd
    > say that any method still in the call stack is "executing", and in that
    > sense, any class at all can have any number of methods executing on the
    > EDT.


    Well, "executing" in the sense that drawing methods should execute only
    on a single thread, the EDT.

    > > I think this is the genesis of your welcomed criticism of my approach.

    >
    > If by that you mean that, since your restatement of your previous
    > statement is flawed, then so too is your proposed approach to the original
    > question, then yes...I suppose this could be that "genesis". :)


    Permit me to amend my proposed outline, clarifying the need to
    synchronize shared data and to execute update() on the EDT:

    /**
    * A model capable of running on a separate thread.
    * A javax.swing.Timer is used for interim updates so that the
    * actionPerformed() method executes on the event-dispatching thread.
    */
    class Model extends Observable implements ActionListener, Runnable {

    private int interval = 1000 / 4; // 4 Hz
    private Timer timer; // javax.swing.Timer required
    private Results results; // a synchronized Collection
    private boolean done;

    public Model() {
    super();
    timer = new Timer(interval, this);
    }

    public void run() {
    done = false;
    timer.start()
    // generate results
    done = true;
    }

    public void actionPerformed(ActionEvent e) {
    if (done) timer.stop;
    // notify Observer(s) of progress
    setChanged();
    notifyObservers(results);
    }
    }

    /**
    * A view capable of processing interim results from a model.
    */
    public class View extends JPanel implements Observer {
    View(Model model) {
    ...
    model.addObserver(this);
    }

    /**
    * Act on notifications from the model.
    * This method relies on notifications being sent from the
    * actionPerformed() method of a javax.swing.Timer in order
    * to execute on the event-dispatching thread.
    */
    public void update(Observable o, Object arg) {
    Results results = (Results) arg;
    // process results for display
    this.repaint();
    }
    }
    ....
    Model model = new Model();
    new Thread((Runnable) model).start();

    > > [...] These point are well taken, and thank you for pointing out
    > > the TableModel caveat. I've been modifying a BufferedImage's
    > > WriteableRaster, so any problems may have been more in-apparent
    > > than impossible.

    >
    > I suspect that you were just lucky to have chosen an implementation
    > that is unlikely to have run into problems with WriteableRaster.
    > BufferedImage/WriteableRaster don't have the same EDT-related
    > limitations that JTable, etc. would have. But they aren't
    > thread-safe either.
    >
    > In your case however, my impression is that you are simply updating
    > the BufferedImage periodically, and my guess is that your period is
    > much longer than it would take to display the image on-screen. In
    > that particular implementation, you pretty much guarantee that the
    > EDT is done with the BufferedImage by the time you get around to
    > trying to change it again, and you wouldn't normally cause the EDT
    > to try to access it again until after you've finished changing it.
    >
    > You probably can, if you try, cause a subtle display bug by forcing a
    > redraw of the image at the exact moment that the processing thread
    > is updating the BufferedImage. But you'd have to have just the
    > right timing (this is typical of threading bugs...the vast majority
    > of the time, things work fine, but once in a blue moon you hit the
    > timing just right, and things fail...sometimes quite dramatically :)
    > ). And even doing that, in this particular case the error would
    > probably be very subtle, depending on how exactly you're updating
    > the BufferedImage (if you're just adding new things to the image,
    > then at worst the image displayed would have a partial update...if
    > you clear the image and redraw it from scratch, then you might have
    > a more easily-noticed problem).


    Yes. In previous tests, the shared data was simply a byte[][], so the
    display was correct even in the worst case: a final update _not_
    executing on the EDT, while paintComponent() was still executing _on_
    the EDT.

    > > [...]
    > > On reflection, I see the clear contract the invokeLater() and
    > > invokeAndWait() methods offer with respect to the EDT, and my approach
    > > may have too many limitations.

    >
    > I guess that depends on your definition of "limitations". But personally,
    > I'd say the contrary is true: your approach has too few limitations. :)
    > In particular, using invokeLater/AndWait() is somewhat more limited. But
    > with that limitation comes a clearer, simpler way to move data from one
    > thread to another.
    >
    > Your approach is actually more flexible, but it carries with it a heavy
    > burden to fully address all of the threading issues, which include both
    > the requirement to ensure that all methods (with few exceptions, like
    > repaint()) for a AWT/Swing component are called on the EDT, and the
    > requirement that you synchronize access to shared data structures between
    > any code running on the EDT and on your processing thread.


    Yes, I am reluctant to abandon the approach completely; I like the
    Observer/Observable pattern too well:) Thank you, too for guiding me to
    the (latent) bug described above.

    > I would not expect there to typically be a need for the flexibility that
    > would justify the burden. Thus my suggestion to use the simpler, albeit
    > less flexible approach. :)


    --
    John B. Matthews
    trashgod at gmail dot com
    home dot woh dot rr dot com slash jbmatthews
     
    John B. Matthews, Jul 3, 2008
    #12
  13. In article <>,
    "Peter Duniho" <> wrote:

    > On Thu, 03 Jul 2008 04:25:36 -0700, John B. Matthews <>
    > wrote:
    >
    > > [...]
    > >> > Starting a new thread for a Runnable [e.g. new Thread((Runnable)
    > >> > scanner).start()] causes the Runnable's run() method to begin
    > >> > executing on a new thread. Invoking notifyObservers() causes the
    > >> > registered Observer's update() method to execute on that same
    > >> > thread, even though other methods in the Observable may be
    > >> > executing on another thread.
    > >>
    > >> Well, that depends on how you call notifyObservers(). The code you
    > >> posted does _not_ actually do this though. It calls
    > >> notifyObservers() from the actionPerformed() method, which is called
    > >> by the Timer class on the EDT, not the thread on which the
    > >> Observable's run() method was called.

    >
    > Please note, from the above: "...not the thread on which the Observable's
    > run() method was called"
    >
    > > Worse! A serious flaw in the outline I proposed is that update() runs on
    > > the EDT when sent by the Timer's actionPerformed() method, but the final
    > > notification executes update() on the thread on which the
    > > Observable's run() method was called.

    >
    > Ugh.
    >
    > No, you are incorrect. Again, this seems to be a flaw in your
    > understanding about how threads work. It is _definitely_ not true that
    > "the final notificatione executes update() on the thread on which the
    > Observable's run() method was called". There is no way for Java to do
    > that, without you implementing it explicitly yourself.
    >
    > I even reiterated the point here:
    >
    > >> At _best_, this will cause the registered Observer's update() method
    > >> to be executed on the EDT, but in any case certainly not on the
    > >> thread on which the Observable's run() method was called.

    >
    > Note in particular the last phrase in the sentence, starting with "but in
    > any case...".


    You've been extraordinarily patient, so I am reluctant to be
    contradictory. In my original post, notifyObservers() is called each
    time actionPerformed() is called by the Timer; and one final time in
    run(), after the timer stops:

    public void run() {
    timer.start()
    // lengthy code that generates results
    timer.stop();
    setChanged();
    notifyObservers(results);
    }

    public void actionPerformed(ActionEvent e) {
    // interim progress
    setChanged();
    notifyObservers(results);
    }

    The call stack shows a series of calls to update() via the EDT and a
    final one in an anonymous thread on which the Observable's run() method
    was called. It is definitely true. It was a bug. I fixed it in the
    amended code below.

    > >> At worst, the notifyObservers() method itself might actually use
    > >> yet another thread for the purpose of calling the Observer's
    > >> update() method (I'd be surprised in the Sun implementation of
    > >> the base Observable class did that, but you never really know and
    > >> the docs go to some length to warn that you cannot count on that
    > >> happening).

    > >
    > > I am somewhat reassured that "... the action event handlers for
    > > Timers execute on ... the event-dispatching thread," and that the
    > > Observable's "... notification mechanism has nothing to do with
    > > threads..."

    >
    > I don't see how that reassures you.


    Observable states "that this notification mechanism has nothing to do
    with threads..." The method notifyObservers() says, "Each observer has
    its update method called..." Where does "yet another thread" come from?
    Moreover, the warning specifically refers to subclasses of Observable
    and the Observer's reliance on order. As long as my subclass of
    Observable doesn't start another thread to issue notifications,
    notifyObservers() will execute update() on the EDT, as arranged by Timer.

    > >> > In particular, an Observable that extends JPanel (or some other
    > >> > top-level container) may have (at most) one method executing on
    > >> > the EDT.
    > >>
    > >> I suppose that depends on your definition of "executing".
    > >> Personally, I'd say that any method still in the call stack is
    > >> "executing", and in that sense, any class at all can have any
    > >> number of methods executing on the EDT.

    > >
    > > Well, "executing" in the sense that drawing methods should execute only
    > > on a single thread, the EDT.

    >
    > Again, you're not really being clear. Drawing methods may call other
    > drawing methods.
    >
    > Either you are saying that the current instruction being executed can only
    > be in one method at a time (which is trivially true and uninteresting) or
    > you are saying that the EDT executes exactly one method per event queue
    > item (which is obviously false and could lead to some unfortunate mistakes
    > if it's believed).


    I see your point that "executing" includes methods still in the call
    stack. I understand that AWTEvents are dispatched from the EventQueue
    sequentially and in the same order as they are put on the queue.

    > It's not clear why the statement you made is relevant to the question at
    > hand anyway, but inasmuch as you believe it to be, that could be at least
    > partially responsible for your errors.
    >
    > >> > I think this is the genesis of your welcomed criticism of my approach.
    > >>
    > >> If by that you mean that, since your restatement of your previous
    > >> statement is flawed, then so too is your proposed approach to the
    > >> original
    > >> question, then yes...I suppose this could be that "genesis". :)

    > >
    > > Permit me to amend my proposed outline, clarifying the need to
    > > synchronize shared data and to execute update() on the EDT:

    >
    > The clarification is fine as far as it goes. But you still have a
    > synchronization bug and you are still relying on undocumented behavior.


    I understand the need for synchronization; I specified a synchronized
    Collection, but the comment was elided. I believe the behavior is
    documented, as described above.

    > In particular:
    >
    > > public void run() {
    > > done = false;
    > > timer.start()
    > > // generate results
    > > done = true;
    > > }

    >
    > The above method runs on thread A and modifies the data structure that
    > keeps the results of the processing.
    >
    > > public void actionPerformed(ActionEvent e) {
    > > if (done) timer.stop;
    > > // notify Observer(s) of progress
    > > setChanged();
    > > notifyObservers(results);
    > > }

    >
    > The above method runs on the EDT. In the current implementation of the
    > Java run-time I'm testing on, it winds up calling this method...
    >
    > > /**
    > > * Act on notifications from the model.
    > > * This method relies on notifications being sent from the
    > > * actionPerformed() method of a javax.swing.Timer in order
    > > * to execute on the event-dispatching thread.
    > > */
    > > public void update(Observable o, Object arg) {
    > > Results results = (Results) arg;
    > > // process results for display
    > > this.repaint();
    > > }

    >
    > ...on the same thread (i.e. the EDT).


    I see the same result using the amended code.

    > On the one hand, this is good for the purposes of applying whatever
    > changes have happened to whatever components are used to display the
    > results. On the other hand, you are relying on undocumented behavior, and
    > in addition you fail to synchronize access to the data structure that is
    > being modified on thread A.


    I specified a synchronized Collection above, but this might be more
    clear:

    Results results = (Results) arg;
    synchronized(results) {
    // process results for display
    }

    > The one positive aspect of the example is dependent on one of the negative
    > aspects, and fails to address the other negative aspects. I don't think
    > this is a good thing.
    >
    > > [...]
    > >> Your approach is actually more flexible, but it carries with it a
    > >> heavy burden to fully address all of the threading issues, which
    > >> include both the requirement to ensure that all methods (with few
    > >> exceptions, like repaint()) for a AWT/Swing component are called
    > >> on the EDT, and the requirement that you synchronize access to
    > >> shared data structures between any code running on the EDT and on
    > >> your processing thread.

    > >
    > > Yes, I am reluctant to abandon the approach completely; I like the
    > > Observer/Observable pattern too well:) Thank you, too for guiding me to
    > > the (latent) bug described above.

    >
    > I'm afraid I can't take any credit yet, since so far you seem to be
    > believing there's a bug where there isn't, and failing to understand the
    > bugs where they are. :(


    No, the spurious, final notification was calling update() on a thread
    other than the EDT. That was a bug waiting to happen. You helped me look
    at this critically. Thank you.

    I think we agree that the contract for Timer specifies that action event
    handlers for Timers execute on the EDT.

    > There's nothing fundamentally unfixable by your approach, but at the very
    > least, I think the difficulty in even _explaining_ the bugs is strongly
    > illustrative of the challenges involved in getting that approach to work
    > correctly. It's definitely not an implementation that someone
    > inexperienced with multi-threaded code should be trying to attempt.


    I'm no expert on multi-threading, but I sometimes get to track down bugs
    in code written by people who are. :)

    > Note also that it's not that there's anything wrong with using the
    > Observer/Observable pattern per se. You could almost as easily apply that
    > pattern to the approach I've suggested. The biggest problem is the lack
    > of any assurance that the Observer's update() method will be called on the
    > same thread that the Observable used to call notifyObservers(), but that's
    > addressable with a (potentially redundant) call to invokeLater/AndWait().


    I believe my interpretation is reasonable, but I remain open.

    --
    John B. Matthews
    trashgod at gmail dot com
    home dot woh dot rr dot com slash jbmatthews
     
    John B. Matthews, Jul 4, 2008
    #13
  14. In article <>,
    "Peter Duniho" <> wrote:

    > On Thu, 03 Jul 2008 20:43:56 -0700, John B. Matthews <>
    > wrote:
    > > [...]
    > > In my original post, notifyObservers() is called each time
    > > actionPerformed() is called by the Timer; and one final time in
    > > run(), after the timer stops:

    >
    > Ah, I see. I failed to process the crucial word "final" (as in, "but the
    > final notification executes...") in your previous post. I thought you
    > were making the statement about _all_ notifications. (I'll blame it on
    > the fact that "final" wound up at the end of a line, and it's easy to miss
    > words at the end of a line :) ).
    >
    > At least I was correct about there being "no way for Java to do that,
    > without implementing it explicitly yourself". You _did_ implement it
    > explicitly yourself.
    >
    > Sorry for the confusion.


    Not at all; I'd overlooked it for much longer! Plus, I always see
    something new in the debugger and profiler:)

    > > [...]
    > > Observable states "that this notification mechanism has nothing to do
    > > with threads..." The method notifyObservers() says, "Each observer has
    > > its update method called..." Where does "yet another thread" come from?

    >
    > It comes from either a change in the Java run-time implementation, or from
    > a sub-class that modifies the default behavior. The latter you already
    > understand. The former seems unlikely to me, but given that the
    > documentation doesn't provide a promise against the possibility, I'm
    > loathe to actually _depend_ on that.


    Ah, I see the value in this distinction: the former may be less likely,
    but it still deserves notice; the latter definitely requires a clear
    prohibition against monkeying with the notification thread, as
    established by Timer.

    > But even the possibility of a different sub-class of Observable is an
    > important possibility. Sample code almost never gets run as-is. People
    > start there and then modify it. There's no way to guarantee that the
    > Observable class is the default one, or a sub-class that doesn't modify
    > the default threading behavior.
    >
    > This is an issue that documentation, perhaps even to a degree that seems
    > excessive, can address successfully. But it does need to be
    > well-documented in the sample.


    I agree emphatically.

    > > [...]
    > > I understand the need for synchronization; I specified a
    > > synchronized Collection [in] the comment[...]

    >
    > You did have a comment, but since your code doesn't actually demonstrate
    > the initialization and use of the Results, I'm afraid that a naïve reader
    > may gloss over or completely ignore the advice. IMHO it should be much
    > more explicit. Also consider that many scenarios don't even lend
    > themselves to having results of processing being stored in a collection.
    >
    > It's nice to have collection classes that internalize synchronization, but
    > a person without that option may find themselves lost without more
    > explicit guidance as to how to synchronize across the threads (or may not
    > even realize the importance of the comment describing the "Results" class
    > as a synchronized collection).


    Yes, a single, easily-overlooked comment is insufficient. The shared
    data may range from a single primitive type to an arbitrarily complex
    data structure, and synchronization may be required for none, some or
    all of the data. I've had excellent results where "none" was required,
    but I want to experiment more with "some" and "all" for a time.

    > > [...]


    --
    John B. Matthews
    trashgod at gmail dot com
    home dot woh dot rr dot com slash jbmatthews
     
    John B. Matthews, Jul 4, 2008
    #14
  15. In article <>,
    Lew <> wrote:

    > John B. Matthews wrote:
    > > Yes, a single, easily-overlooked comment is insufficient. The shared
    > > data may range from a single primitive type to an arbitrarily complex
    > > data structure, and synchronization may be required for none, some or
    > > all of the data. I've had excellent results where "none" was required,
    > > but I want to experiment more with "some" and "all" for a time.

    >
    > Any time you share data between threads, it is never the case that
    > the amount of synchronization required is "none". There are
    > circumstances where it might appear that the code is not broken for a
    > while, but it's about as safe as smoking cigarettes thinking you
    > don't risk cancer just because it hasn't happened yet.


    No smoking! :)

    > Without some form of synchronization, there is absolutely no guarantee that
    > writes made by one thread will ever be visible for reads in another.
    >
    > None.


    Certainly a program must be correctly synchronized, but I'd naturally
    like to use explicit synchronization no more than is required.

    For example, elements of an int array are modified atomically and do not
    need synchronization to ensure sequential consistency. While iterating,
    a single writer and a single reader can race on at most one element; the
    reader simply sees the previous value. Each full iteration is perceived
    as atomic, although one element may be from iteration n-1. The Timer's
    final notification ensures that the last complete writer iteration
    happens before the start of the last reader iteration. The result is
    correct with no explicit synchronization. In the absence of word
    tearing, a similar case may be made for byte or short array. This is
    what I meant by "none"; please correct me if I'm wrong.

    In contrast, consider a model that reports interim progress as a
    percentage of completion. If an Observer receives a new Number as the
    actual parameter in update(), there is no shared data and no need for
    synchronization at all.

    I see the classes of java.util.concurrent.atomic extend the notion of
    volatile, but they are new to me.

    Of course, I already love the names. :)

    --
    John B. Matthews
    trashgod at gmail dot com
    home dot woh dot rr dot com slash jbmatthews
     
    John B. Matthews, Jul 5, 2008
    #15
  16. In article <>,
    Lew <> wrote:

    > John B. Matthews wrote:
    > > Certainly a program must be correctly synchronized, but I'd naturally
    > > like to use explicit synchronization no more than is required.

    >
    > It is required *every* time threads share data.

    [...]
    > Atomicity is not the problem. Visibility is.

    [...]
    > Changes written by one thread may never be seen by another.

    [...]
    > The happens-before relationship will not exist without synchronization.

    [...]

    Ah, I think I'm back on the reservation. The memory model doesn't
    _preclude_ visibility across threads, it just can't _guarantee_ it
    without synchronization.

    > > In contrast, consider a model that reports interim progress as a
    > > percentage of completion. If an Observer receives a new Number as the
    > > actual parameter in update(), there is no shared data and no need for
    > > synchronization at all.

    >
    > As long as the thread is the same.
    >
    > > I see the classes of java.util.concurrent.atomic extend the notion of
    > > volatile, but they are new to me.

    >
    > Yes, they are a mere nearly four years old and introduced with a version now
    > in its End-of-Life processing.


    Yes, I'm late to this corner of the party. :)

    > The important thing is that they implement explicit synchronization
    > internally.
    >
    > Without synchronization, there is no happens-before established. Without
    > happens-before, changes may *never* propagate to a different thread.


    Thanks!

    --
    John B. Matthews
    trashgod at gmail dot com
    home dot woh dot rr dot com slash jbmatthews
     
    John B. Matthews, Jul 6, 2008
    #16
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