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Java Forum / General / July 2008repaint method and design question
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 >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 > I have two classes: > one for the GUI the other for various scanning methods [quoted text clipped - 4 lines] > is blank. So I'm thinking I need to constantly > repaint while scanning? Your code that is scanning is preventing the event dispatcher thread (EDT) from dispatching events, because (I presume) you are scanning in response to some user input and not returning from the user input handler (a listener of some sort, probably an ActionListener, I also presume) until the scanning is done. If you don't want the GUI to freeze up while the scanning is being done, then you'll need to run the scanning on a different thread, using EventQueue.invokeLater() or EventQueue.invokeAndWait() to perform updates to the components of your GUI (so that that code is executed on the EDT where it belongs). > In terms of design: should I be passing around > GUI objects(instance variables) from my GUI class > to my file system scanning class? That's a somewhat different question. Ideally, I think your file scanning class would not know about the GUI. It'd expose some sort of listener interface that the GUI class can add a listener to in order to receive updates on the progress of the scanning (for example, you might call the listener method for each file, or each directory you scan, or you might just have a "done" event that's called when the scan is complete). Then the GUI class's handler method that's called for the event would handle invoking the actual code on the EDT. But you certainly could have the scanning class know about the GUI and just update the GUI directly. Not the best model, since it ties the classes together closely, but for quick and dirty stuff, sometimes that's just fine. Either way, you need to address the "blocking the EDT" issue if you want the GUI to update while the scanning is going on. And neither approach specifically addresses that issue. Pete > [...] >> In terms of design: should I be passing around [quoted text clipped - 14 lines] > classes together closely, but for quick and dirty stuff, sometimes > that's just fine. Sorry to follow up my own post. I should point out that there's a third option, of course: the scanning class can still not know anything about the GUI, but also not provide an asynchronous interface. In that case, the GUI class would have its own thread implementation that calls the scanning class and deals with invoking the results back to the EDT. Pete >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.  SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com > >I'm experiencing an issue where, while scanning, > >the main pane, if minimized and brought back up, [quoted text clipped - 5 lines] > 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(); SignatureJohn B. Matthews trashgod at gmail dot com home dot woh dot rr dot com slash jbmatthews >> 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 [quoted text clipped - 4 lines] > 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"). 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. 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. 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. Just my two cents. :) Pete > >> 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 [quoted text clipped - 11 lines] > 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 [quoted text clipped - 3 lines] > 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, [quoted text clipped - 5 lines] > 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. :-) SignatureJohn B. Matthews trashgod at gmail dot com home dot woh dot rr dot com slash jbmatthews > [...] > 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. But that's an assumption, and as we all know, assumptions are not nearly as valuable as things printed in black and white. :) > 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. > 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. 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. > 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 I'm replying, I'll also point out that your sample code >> conveniently skips demonstrating any of the synchronization code that [quoted text clipped - 10 lines] > 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. >> Personally, I'd skip the timer altogether, and simply update the UI at >> convenient intervals throughout processing based simply on progress [quoted text clipped - 15 lines] > 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 :) ). >> Just my two cents. :) > > Worth much more! What, with compound interest and the price of copper, > etc. :-) Thanks. :) Pete > > [...] > > My proposed Scanner implements Runnable, so it's a separate thread. I [quoted text clipped - 9 lines] > 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, [quoted text clipped - 15 lines] > 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. [quoted text clipped - 5 lines] > 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 [quoted text clipped - 7 lines] > 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 [quoted text clipped - 18 lines] > 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 [quoted text clipped - 27 lines] > 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. :) SignatureJohn B. Matthews trashgod at gmail dot com home dot woh dot rr dot com slash jbmatthews > 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. > 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 don't even know for sure that the _current_ implementation keeps [quoted text clipped - 12 lines] > <http://java.sun.com/products/jfc/tsc/articles/painting/index.html#paint_ > 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. 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. >> > but I'm not sure that is required. More importantly, the >> > Observer is running on the EDT, as it extends an AWT Component. [quoted text clipped - 7 lines] > > 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. >> 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 [quoted text clipped - 10 lines] > 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. > [...] >> Note, however, that the more likely that an algorithm is specifically [quoted text clipped - 6 lines] > 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. 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. 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". Pete > > Yes, the subclass that implements Observable should behave correctly, > [quoted text clipped - 4 lines] > 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 [quoted text clipped - 17 lines] > 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 [quoted text clipped - 7 lines] > 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. [quoted text clipped - 12 lines] > 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 [quoted text clipped - 33 lines] > 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 [quoted text clipped - 10 lines] > 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 [quoted text clipped - 10 lines] > 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. SignatureJohn B. Matthews trashgod at gmail dot com home dot woh dot rr dot com slash jbmatthews > [...] >> > but I read the paragraph about notification order as a warning to [quoted text clipped - 6 lines] > 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. > 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". 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. > [...] >> Unfortunately, the documentation is not clear as to whether repaint() [quoted text clipped - 6 lines] > 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. >> 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 [quoted text clipped - 11 lines] > 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. > [...] >> 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#threading): ...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). > [...] >> Even if we grant that repaint() has special rules (and again, I'm not [quoted text clipped - 9 lines] > 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 [quoted text clipped - 13 lines] > 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 [quoted text clipped - 12 lines] > > 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). Pete > > [...] > >> > but I read the paragraph about notification order as a warning [quoted text clipped - 10 lines] > 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 [quoted text clipped - 8 lines] > 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 [quoted text clipped - 6 lines] > 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 [quoted text clipped - 24 lines] > 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 [quoted text clipped - 35 lines] > 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 [quoted text clipped - 50 lines] > 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 [quoted text clipped - 64 lines] > 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. SignatureJohn B. Matthews trashgod at gmail dot com home dot woh dot rr dot com slash jbmatthews > [...] >> On a slight tangent: it's not really clear from the above quote that [quoted text clipped - 16 lines] > 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. 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). > 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. > 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". :) > [...] > 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). > [...] > 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. 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. :) Pete > > [...] > >> On a slight tangent: it's not really clear from the above quote [quoted text clipped - 25 lines] > 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 [quoted text clipped - 4 lines] > 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. [quoted text clipped - 3 lines] > 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 [quoted text clipped - 27 lines] > 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 [quoted text clipped - 13 lines] > 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. :) SignatureJohn B. Matthews trashgod at gmail dot com home dot woh dot rr dot com slash jbmatthews > [...] >> > Starting a new thread for a Runnable [e.g. new Thread((Runnable) [quoted text clipped - 9 lines] >> 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...". >> At worst, >> the notifyObservers() method itself might actually use yet another [quoted text clipped - 6 lines] > 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. >> > In particular, an Observable that extends JPanel (or some other top- >> > level container) may have (at most) one method executing on the EDT. [quoted text clipped - 7 lines] > 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). 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. >> [quoted text clipped - 5 lines] > 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. 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. [quoted text clipped - 7 lines] > this.repaint(); > } ...on the same thread (i.e. the EDT). 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. 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 [quoted text clipped - 8 lines] > 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. :( 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. 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(). Pete > > [...] > >> > Starting a new thread for a Runnable [e.g. new Thread((Runnable) [quoted text clipped - 34 lines] > 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 [quoted text clipped - 9 lines] > > 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 [quoted text clipped - 16 lines] > 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 [quoted text clipped - 12 lines] > 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: > [quoted text clipped - 31 lines] > > ...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 [quoted text clipped - 16 lines] > 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. SignatureJohn B. Matthews trashgod at gmail dot com home dot woh dot rr dot com slash jbmatthews >> > Worse! A serious flaw in the outline I proposed is that update() runs >> on [quoted text clipped - 16 lines] > 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. > [...] > 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. 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. > [...] >> > Permit me to amend my proposed outline, clarifying the need to [quoted text clipped - 6 lines] > Collection, but the comment was elided. I believe the behavior is > documented, as described above. 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). > [...] > I think we agree that the contract for Timer specifies that action event > handlers for Timers execute on the EDT. Yes, absolutely. For the Swing Timer class, this is essential. Pete > > [...] > > In my original post, notifyObservers() is called each time [quoted text clipped - 12 lines] > > 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 [quoted text clipped - 6 lines] > 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 [quoted text clipped - 5 lines] > 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 [quoted text clipped - 11 lines] > 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. > > [...] SignatureJohn B. Matthews trashgod at gmail dot com home dot woh dot rr dot com slash jbmatthews > 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. 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. SignatureLew > > Yes, a single, easily-overlooked comment is insufficient. The shared > > data may range from a single primitive type to an arbitrarily complex [quoted text clipped - 7 lines] > 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. :-) SignatureJohn B. Matthews trashgod at gmail dot com home dot woh dot rr dot com slash jbmatthews > 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. > For example, elements of an int array are modified atomically and do not Atomicity is not the problem. Visibility is. > 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 Wrong. Changes written by one thread may never be seen by another. The reading thread might not see anything other than initial values, those from just prior to iteration zero. > 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 Wrong. The happens-before relationship will not exist without synchronization. > 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. You are wrong. Only with synchronization is happens-before guaranteed. > 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. 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. SignatureLew > > 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 [quoted text clipped - 8 lines] > 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! SignatureJohn B. Matthews trashgod at gmail dot com home dot woh dot rr dot com slash jbmatthews Lew wrote: >> 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. :-) Do not misunderstand. I have yet to really use NIO, and that was out with 1.4! But the chance to be sarcastic was just too tempting. You demonstrated for everyone a beautiful way to respond to such, without getting all whiny or upset. There are many who could benefit from your example. SignatureLew Not humor, so no smileys. :-) > 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. SignatureLew > > The phrases "...this notification mechanism is has nothing to do with > > threads..." and "Each observer has its update method called..." tell me [quoted text clipped - 11 lines] > 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. SignatureJohn B. Matthews trashgod at gmail dot com home dot woh dot rr dot com slash jbmatthews > 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. But you showed us > <http://java.sun.com/products/jfc/tsc/articles/painting/index.html#paint_process> which explains that repaint() is thread safe. This worries me, as I had thought the Javadocs to be normative. > ... 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? I really do wonder how one handles the memory-model concerns in that "stochastic models" scenario. Would you be so gracious as to explain? SignatureLew > I see nothing in the Javadocs for JComponent#repaint() that indicates > that the method is thread safe. [quoted text clipped - 3 lines] > > which explains that repaint() is thread safe. [...] Where does it do that? And please distinguish between "thread safe" as in "need not be called on the EDT" and "thread safe" as in "can be called simultaneously from multiple threads". I read through that page and was unable to find a clear statement to the effect of either type of "thread safe". I'd appreciate it if you could direct me to the portion of the page that you believes does so. Thanks, Pete >> I see nothing in the Javadocs for JComponent#repaint() that indicates >> that the method is thread safe. [quoted text clipped - 11 lines] > effect of either type of "thread safe". I'd appreciate it if you could > direct me to the portion of the page that you believes does so. You're absolutely right. It says that the repaint() schedules the painting to happen on the EDT, not that the action is thread safe in general terms. In the context of this conversation, that means that it should be thread-safe enough for the EDT, especially under the assumptions stated by - actually, by you yourself: > 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 [quoted text clipped - 3 lines] > 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). So the promise of the cited article that the repaint() will happen on the EDT makes it, most of the time, thread-safe enough. SignatureLew >> [...] >> I read through that page and was unable to find a clear statement to [quoted text clipped - 4 lines] > painting to happen on the EDT, not that the action is thread safe in > general terms. But the latter is the question at hand. We're not concerned with whether the actual painting happens on the EDT. That can be taken as granted. The question is whether calling repaint() itself must happen on the EDT. I've seen nothing to suggest that it's not required to be, and the general rule described to me is that _all_ calls to AWT or Swing methods _must_ be done on the EDT. The repaint() method is an AWT method (being part of the java.awt.Component class), and absent clear documentation to the contrary, that means that it _must_ itself be done on the EDT. The fact that its results also happen on the EDT is inconsequential to the issue. > In the context of this conversation, that means that it should be > thread-safe enough for the EDT, especially under the assumptions stated [quoted text clipped - 8 lines] >> the invoked code (sometimes this is as simple as a "final" local >> variable used in an anonymous class). I don't know what you mean. I agree that if you design the code in the way I suggest, it's fine. But this part of the thread isn't about whether the design I propose is safe. I never would have proposed that design had I not felt it's safe. This part of the thread is about whether the other design that was proposed is safe. > So the promise of the cited article that the repaint() will happen on > the EDT makes it, most of the time, thread-safe enough. You statement that "the repaint() will happen on the EDT" is ambiguous. To me, saying "the <method name here>() will happen on..." means that the call to the method itself will happen as described (in this case, "...on the EDT"). But that is definitely not true. If you mean that the consequential painting that occurs _after_ the call to repaint() happens will happen on the EDT, then yes...I agree with that. But IMHO your statement is an awkward way of saying that, and in any case such a statement doesn't provide any insight as to the safety of calling repaint() itself on an arbitrary thread. Pete >>> [...] >>> I read through that page and was unable to find a clear statement to [quoted text clipped - 11 lines] > and the general rule described to me is that _all_ calls to AWT or Swing > methods _must_ be done on the EDT. According to the document cited, > 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. That seems pretty clear to me, that all GUI actions occur on the EDT. I don't see how whether repaint() being called off the EDT will hurt that. The ambiguity that you dislike doesn't bother me, although I see what you mean. The important things that must be on the EDT are, according to the article. The question of whether anything of repaint() other than that happens on the EDT is immaterial. Again, I see the distinction that you describe, but I don't think it matters from a practical standpoint. It's pretty clear that the document at least intends to show that JComponent#repaint() can safely be called from off the EDT, at least for the implementation described. To me, the problem isn't whether the document is clear - it seems sufficiently clear from here. The problem is whether it's reliable, the fact that it's on java.sun.com notwithstanding. My distrust of the article stems from the fact that it is not presented as a normative standard - I don't get from it that Java promises that repaint() is implemented in an EDT-safe manner, only that the particular implementation actually does so. I would like to know of any documentation that commits to JComponent#repaint() being an EDT-safe method, by which I mean it can be called from off the EDT without harm. SignatureLew > I would like to know of any documentation that commits to > JComponent#repaint() being an EDT-safe method, by which I mean it can be > called from off the EDT without harm. Maybe I should trust <http://java.sun.com/products/jfc/tsc/articles/threads/threads1.html> although its claim that one can construct the GUI off the EDT turns out not to be true. It does claim that: > There are a few exceptions to the rule that all code that might affect a > realized Swing component must run in the event-dispatching thread: [quoted text clipped - 4 lines] > respectively. The invalidate() method just marks a component and all of its > direct ancestors as requiring validation. I think that handles both our concerns, does it not? That same article seems to draw the distinction between "thread-safe" and "safe to call from any thread" that you brought to our attention. SignatureLew >> I would like to know of any documentation that commits to >> JComponent#repaint() being an EDT-safe method, by which I mean it can [quoted text clipped - 15 lines] > > I think that handles both our concerns, does it not? Sort of. First, I think your observation that it seems to claim something "known" to be false ("that one can construct the GUI off the EDT") is pertinent. That is, if one accepts that that's incorrect advice, then that calls into question the rest of the article. Second, a specification is where one would find implementation-independent descriptions of how the run-time should behave. These articles are nice but, as you might have noticed from previous discussions, I'm loathe to accept them as the last word on what I can and cannot depend on the Java run-time doing. Third, in the very specific example, the repaint() method is the only _explicit_ AWT/Swing method being called. However, you'll note the comment reading "// fill in GUI components with results", as well as my later comments pointing out that one is generally going to be actually changing the state of the GUI components. In other words, while the code sample posted has only the repaint() method, in truth there are always going to be other methods that don't share repaint()'s flexibility with respect to the EDT. This third point is the most important. In particular, the primary point in my original response was that the Observable class makes no guarantees that the Observer.update() method will be called on the same thread on which Observable.notifyObservers() is called. So the fact that the Timer's Action event is raised on the EDT doesn't imply that the subsequent Observer.update() calls are also done on the EDT thread. So even though there is apparently no need for repaint() to be called from the EDT, the other method calls that would likely be found in the Observer.update() method _do_ need to be called from the EDT and there's not any guarantee in the posted code that they would be. > That same article seems to draw the distinction between "thread-safe" > and "safe to call from any thread" that you brought to our attention. |
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