Re: Synchronization when collecting data from the EDT?
On 06/05/2011 08:47 PM, John B. Matthews wrote:
I don't know that it does, I guess I was hoping that when Sun wrote
invokeLater and AndWait that they assumed that you were getting there
from another thread and put some sort of synchronization in. And
maybe I'm looking for some insight as to how you would do this any
other way. I still think that invokeAndWait has problems if the
current thread is interrupted before execution reaches the
synchronized block in the Runnable.
Focusing on invokeAndWait() [1], I understood it to be the standard way
to wait for data to arrive on the EDT. In particular, it promises to
block until the supplied Runnable's run method has been called on the
EDT. I don't understand why the Runnable needs a synchronized block.
The problem with using invokeAndWait() is that it won't block if the
thread is interrupted and it won't start the Runnable() either.
The synchronized block is to make sure that the data retrieved from the
GUI elements on the EDT were visible to the current thread. I've come
up with what I think is a better way for the project I'm working on and
that is to use a Semaphore.
Semaphore sem = new Semaphore(1);
..
..
while (true) {
try {
sem.acquire();
EventQueue.invokeLater(new Runnable() {
public void run() {
// read data from GUI elements
sem.release();
}
});
sem.acquire(); // blocks here until done above
// do stuff with the GUI element data
} catch (InterruptedException ie) {
// skip the do stuff if interrupted
} finally {
sem.release();
}
}
A widely used implementation queues the Runnable and waits [2], using a
private AWTInvocationLock. The Runnable, wrapped in an InvocationEvent,
is subsequently dispatched [3]. When finished, the same
AWTInvocationLock is used to notifyAll().
Of the ways a thread can exit wait() [4], an InterruptedException
appears to be the only one to worry about. It looks like
InvocationEvent is always set to catch Throwable, so you'll have to
catch InvocationTargetException, too. Finally, invokeAndWait() doesn't
call wait() in a loop, as recommended [4]; but I don't think the
calling thread cares if it's not scheduled immediately on waking [5].
The InvocationTargetException can't be thrown unless something in the
Runnable throws it, so I don't think that is as big a problem as the
interrupted thread issue.
Having said that, I see that the Swing tutorial no longer uses
invokeAndWait() in this context. Instead, it focuses on SwingWorker
[6], which can be launched from any thread.
I didn't use a SwingWorker because it's really set up to be a self
terminating thing and I need to continually update data from my GUI
elements to current thread and the SwingWorker goes the other way, do
something on the current thread and publish it on the EDT.
So John, back to your question about the synchronized block, how would
you get data from the EDT to your current thread without some sort of
synchronization?
--
Knute Johnson
s/knute/nospam/