Re: wait and spurious wakeups

From:
Patricia Shanahan <pats@acm.org>
Newsgroups:
comp.lang.java.help
Date:
Tue, 27 Nov 2007 14:08:55 -0800
Message-ID:
<fii4hm$iot$1@ihnp4.ucsd.edu>
Daniel Pitts wrote:

Patricia Shanahan wrote:

apm35@student.open.ac.uk wrote:
....

Here is my question: what is wrong with calling wait just as wait()?
The developer I am working with claims that wait suffers from the
potential to experience interrupts and spuirous wakeups. Is that
right? He suggested that the following code be used:

synchronized (listOwner) {
    while (special event not received)
    {
        listOwner.wait();
    }
}

Can someone explain this to me please? I have tried talking to the
developer but there is a communication problem (different timezones,
different languages).

....

The non-looping code would be:

synchronized (listOwner) {
    if(special event not received)
    {
        listOwner.wait();
    }
}

You need to do the test inside the synchronized block to avoid race
conditions where the notifying thread does its notify after the waiting
thread has done its test but before it enters wait.

The difference in cost between doing the "if" and a "while" is so small,
compared to the cost of a wait and inter-thread communication, that you
might just as well use "while", even if you are sure "if" would be
sufficient. It is more robust in the face of future changes, such as
adding use of interrupts.

Patricia

but if is *not* sufficient.
There can be spurious signals that don't mean the special flag has been
set.


The Object wait() documentation says "Causes current thread to wait
until another thread invokes the notify() method or the notifyAll()
method for this object."

The structure of the program appears to exclude a spurious notify, it is
not using notifyAll, and there are no Java interrupts. Under those
conditions, wouldn't a spurious return from wait represent a bug in the
wait implementation, a failure to conform to its API documentation?

Patricia

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