Re: Speaking of thread safety?
On 3/10/2010 3:07 PM, John B. Matthews wrote:
In article<S9Sln.90254$Ye4.78776@newsfe11.iad>,
Knute Johnson<nospam@rabbitbrush.frazmtn.com> wrote:
On 3/10/2010 10:34 AM, markspace wrote:
Knute Johnson wrote:
Does anybody know if the Observable/Observer calls to update() are on
the EDT?
Considering that Observer is just an interface and it would depend
on the implementation, and that no Java Swing classes that I know
of actually implement Observer, I'd have to guess the answer is in
general "no."
The Observable class method, notifyObservers() is called and that
then calls all Observer.update() methods. I'm curious to know if the
Observable puts those calls onto the EDT.
As markspace has elaborated, the answer is no. Not long ago, I proposed
that on Observable can invoke notifyObservers() on the EDT, as via
javax.swing.Timer, but access to any data shared between threads must
still be correctly synchronized. I found this discussion very helpful
<http://groups.google.com/group/comp.lang.java.programmer/browse_frm/thread/4c4a6b46ffd2d31f>
and I have tried to express my understanding in this example:
<http://sites.google.com/site/drjohnbmatthews/threadwatch>
I looked through the source code and all of the methods of Observable
are synchronized on the Observable instance. So threads that read any
argument that is passed in and stored needs to be synchronized as well.
I'm not sure how to do that. The whole point of using
Observer/Observable is to disconnect the two classes.
Calling notifyObservers() from a known thread could be used to
synchronize via thread containment but the docs imply that the calling
thread isn't necessarily guaranteed to be the writing thread in the future.
In the application that I am writing, I am passing an Integer). In the
Observer.update() method I store that Integer. I'm storing it in a
volatile int so that solves the visibility problems for the reading
thread but what if you had a mutable Object to pass?
Thanks,
--
Knute Johnson
email s/nospam/knute2010/