Re: Synchronization when collecting data from the EDT?

From	"John B. Matthews" <nospam@nospam.invalid>
Newsgroups	comp.lang.java.gui
Subject	Re: Synchronization when collecting data from the EDT?
Date	2011-06-07 23:51 -0400
Organization	The Wasteland
Message-ID	<nospam-7E1CB5.23515607062011@news.aioe.org> (permalink)
References	(4 earlier) <TjZGp.40183$Vp.29218@newsfe14.iad> <nospam-7A4754.23033306062011@news.aioe.org> <islktt$kt1$1@dont-email.me> <nospam-1E20CE.13382907062011@news.aioe.org> <ism7lg$ku1$1@dont-email.me>

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In article <ism7lg$ku1$1@dont-email.me>,
 Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> wrote:

> On 07/06/2011 19:38, John B. Matthews allegedly wrote:
> > In article <islktt$kt1$1@dont-email.me>,
> >  Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> wrote:
> > 
> >> On 07/06/2011 05:03, John B. Matthews allegedly wrote:
> >>> Here's my (contrived) sscce. A javax.swing.Timer increments  an 
> >>> int at 100 Hz, while a java.util.TimerTask samples the value at 1 
> >>> Hz. I'd like the API to be a little more explicit, but it does 
> >>> say that the calling thread blocks and the Runnable's dispatch 
> >>> "will happen after all pending events are processed."
> >>>
> > [code elided]
> >>
> >> John, is it intended that you didn't use a thread-safe 
> >> producer/consumer queue (a mere LinkedList)? I would have thought 
> >> one would be necessary, but since it touches upon the subject 
> >> Knute brought to our attention, I'm wondering whether there's some 
> >> kind of point involved.
> > 
> > I appreciate your insight on this. I'm afraid my example just 
> > recapitulates Knute's question without answering it: Does the 
> > invokeAndWait() API ensure correct synchronization when used as 
> > shown? I'm confident that the _implementation_ I examined is 
> > correct, as the relevant methods are synchronized. Sadly, I have a 
> > tendency to over- read the API.
> 
> Aha! So it was the point! :)

Yes, I'm arguing that invokeAndWait() alone is sufficient, but I'd 
welcome an alternative using java.util.concurrent.

> Question then, for I am not an expert when it comes to the memory 
> model: You say the implementation you examined guarantees the code's 
> correctness. This seems to me to imply that the non-EDT thread, upon 
> calling invokeAndWait(), because it locks and waits on some monitor, 
> is guaranteed to fetch a fresh copy of the LinkedList instance before 
> it proceeds to dereference it. Is that actually so? Makes sense I 
> guess, but I had never thought of it like that.

IIUC, because samples is final, it gets slightly different treatment:

<http://java.sun.com/docs/books/jls/third_edition/html/memory.html#17.5>

The reference itself is immutable, and changes to the content are 
synchronized by invokeAndWait(). Here's a variation that retains just 
the last five samples:

public void startUtilTimer() {
    final Queue<Integer> samples = new LinkedList<Integer>();
    java.util.Timer sampler = new java.util.Timer();
    sampler.scheduleAtFixedRate(new TimerTask() {

        @Override
        public void run() {
            try {
                EventQueue.invokeAndWait(new Runnable() {

                    @Override
                    public void run() {
                        samples.add(getValue());
                    }
                });
                if (samples.size() > 5) {
                    samples.remove();
                }
                System.out.println(samples);
            } catch (InterruptedException ex) {
                ex.printStackTrace(System.err);
            } catch (InvocationTargetException ex) {
                ex.printStackTrace(System.err);
            }
        }
    }, 1000, 1000);
}

> To be more precise, I believe my question boils down to this: when a 
> thread's memory is flushed, is that always a full flush, or can it be 
> a selective flush, and if so, how does the selection happen?

IIUC, the memory model uses the notion of a happens-before relationship:

<http://java.sun.com/docs/books/jls/third_edition/html/memory.html#17.4.5
>

I usually rely on this summary of Memory Consistency Properties:

<http://download.oracle.com/javase/6/docs/api/java/util/concurrent/packag
e-summary.html>

> > I can't find a Sun/Oracle example using invokeAndWait() dated later 
> > than 2000, well after the advent of java.util.concurrent. Would it 
> > be correct to infer that using, say BlockingQueue, would obviate 
> > the need for invokeAndWait()?
> 
> You mean "well before", don't you?

Quite right; thanks.

> A BlockingQueue would only be a good solution if the non-EDT thread 
> is waiting for a value, rather than for a process to finish. But it 
> would seem to me that Exchanger + CountdownLatch(1) would indeed make 
> the "AndWait()" part obsolete. Or at least provide a complete 
> alternative.

Sadly, I lack experience in this area. The critical thing would be not 
to block the EDT. I'm thinking a BlockingQueue passed via invokeLater(), 
which is _not_ synchronized.

-- 
John B. Matthews
trashgod at gmail dot com
<http://sites.google.com/site/drjohnbmatthews>

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Thread

Synchronization when collecting data from the EDT? Knute Johnson <nospam@knutejohnson.com> - 2011-06-04 17:38 -0700
  Re: Synchronization when collecting data from the EDT? Knute Johnson <nospam@knutejohnson.com> - 2011-06-04 17:50 -0700
  Re: Synchronization when collecting data from the EDT? Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> - 2011-06-05 17:52 +0200
    Re: Synchronization when collecting data from the EDT? Knute Johnson <nospam@knutejohnson.com> - 2011-06-05 10:47 -0700
      Re: Synchronization when collecting data from the EDT? "John B. Matthews" <nospam@nospam.invalid> - 2011-06-05 23:47 -0400
        Re: Synchronization when collecting data from the EDT? Knute Johnson <nospam@knutejohnson.com> - 2011-06-05 22:11 -0700
          Re: Synchronization when collecting data from the EDT? Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> - 2011-06-06 14:31 +0200
          Re: Synchronization when collecting data from the EDT? "John B. Matthews" <nospam@nospam.invalid> - 2011-06-06 23:03 -0400
            Re: Synchronization when collecting data from the EDT? Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> - 2011-06-07 18:51 +0200
              Re: Synchronization when collecting data from the EDT? "John B. Matthews" <nospam@nospam.invalid> - 2011-06-07 13:38 -0400
                Re: Synchronization when collecting data from the EDT? Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> - 2011-06-08 00:10 +0200
                Re: Synchronization when collecting data from the EDT? "John B. Matthews" <nospam@nospam.invalid> - 2011-06-07 23:51 -0400
                Re: Synchronization when collecting data from the EDT? Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> - 2011-06-10 01:17 +0200
                Re: Synchronization when collecting data from the EDT? Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> - 2011-06-10 01:25 +0200
                Re: Synchronization when collecting data from the EDT? "John B. Matthews" <nospam@nospam.invalid> - 2011-06-09 22:34 -0400
                Re: Synchronization when collecting data from the EDT? Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> - 2011-06-10 05:22 +0200
                Re: Synchronization when collecting data from the EDT? markspace <-@.> - 2011-06-10 08:04 -0700
                Re: Synchronization when collecting data from the EDT? "John B. Matthews" <nospam@nospam.invalid> - 2011-06-10 21:13 -0400
                Re: Synchronization when collecting data from the EDT? Knute Johnson <nospam@knutejohnson.com> - 2011-06-12 12:51 -0700
                Re: Synchronization when collecting data from the EDT? markspace <-@.> - 2011-06-07 23:31 -0700
                Re: Synchronization when collecting data from the EDT? Knute Johnson <nospam@knutejohnson.com> - 2011-06-08 23:07 -0700
                Re: Synchronization when collecting data from the EDT? "John B. Matthews" <nospam@nospam.invalid> - 2011-06-09 18:35 -0400
                Re: Synchronization when collecting data from the EDT? Knute Johnson <nospam@knutejohnson.com> - 2011-06-12 12:45 -0700
            Re: Synchronization when collecting data from the EDT? Knute Johnson <nospam@knutejohnson.com> - 2011-06-08 23:05 -0700
              Re: Synchronization when collecting data from the EDT? "John B. Matthews" <nospam@nospam.invalid> - 2011-06-09 14:35 -0400
  Re: Synchronization when collecting data from the EDT? Roedy Green <see_website@mindprod.com.invalid> - 2011-06-05 13:39 -0700
    Re: Synchronization when collecting data from the EDT? Knute Johnson <nospam@knutejohnson.com> - 2011-06-05 22:17 -0700
      Re: Synchronization when collecting data from the EDT? markspace <-@.> - 2011-06-07 14:51 -0700
  Re: Synchronization when collecting data from the EDT? markspace <-@.> - 2011-06-05 17:46 -0700
    Re: Synchronization when collecting data from the EDT? Knute Johnson <nospam@knutejohnson.com> - 2011-06-05 22:41 -0700
      Re: Synchronization when collecting data from the EDT? Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> - 2011-06-06 14:35 +0200

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