[concurrency-interest] Await...Signal - Catching IllegalMonitorStateException???

David Harrigan dharrigan at gmail.com
Tue Oct 24 14:37:27 EDT 2006


Hi David,

I appreciate your email back tremendously, it helps to clarify what I'm
trying to do :-) The thing
is, I am waiting for a state to happen. The first thread X should wait (for
a certain amount of time)
until thread Y has done it's job. If Y continues for too long, X should
continue and Y should be
abandoned, otherwise Y will return in time and X can process the results.

I believe this is a good model for Lock, newCondition and await(with
timeout) and signal and
after the helpful comments back, I'm now passing in the lock as well as the
condition into
thread Y, which then calls await on the condition, after reacquiring the
lock. Doing it this way
I now do not get any exceptions.

Does this feel right to you?

btw, and this is a shout out to all those who have contributed to Java
Concurrency in
Practice - what an excellent booK!!!!!! :-)

-=david=-

On 10/24/06, David Holmes <dcholmes at optusnet.com.au> wrote:
>
>  David,
>
> You are misusing await()/signal(). The whole point of Conditions is to
> wait for some state to be achieved. Hence you should acquire the lock and
> while the state is not what you expect, perform the await(). The thread that
> is updating the state will acquire the same lock, update the state and
> perform the signal()/signalAll(). You can abstract out the state changes if
> the "change" is actually just "event X occurred" - in which case a Semaphore
> or CountDownLatch might be suitable as a coordination mechanism.
>
> Sorry this is a rushed reply and I don't have the cycles right now to
> fully understand what you are trying to do.
>
> Cheers,
> David Holmes
>
> -----Original Message-----
> *From:* concurrency-interest-bounces at cs.oswego.edu [mailto:
> concurrency-interest-bounces at cs.oswego.edu]*On Behalf Of *Mike Quilleash
> *Sent:* Tuesday, 24 October 2006 4:21 AM
> *To:* David Harrigan; concurrency-interest at cs.oswego.edu
> *Subject:* Re: [concurrency-interest] Await...Signal - Catching
> IllegalMonitorStateException???
>
> I think the call to
>
> condition.signal();
>
> in this code will always throw this exception.  You should never be
> catching this exception explicitly, it is intended to inform you when you
> have a logic error in your use of a condition.  If you look at the docs for
> Condition.await()/signal()/signalAll(), it says that the owning lock, in
> this case "lock", must be held by the current thread before a call to any of
> the above functions.  Failure to do this results in the exception.
>
> When the X thread is in await() it will release the lock and go into a
> wait-state, in this case it will wait until signal() is called on the
> Condition or the await() times out.  When this happens it will wake up,
> reaquire the lock and continue (you later release the lock again
> correctly).  It will wait to reacquire the lock if something else holds
> it.  It is done this way to avoid race conditions where a call to await()
> could "miss" a call to signal().
>
> In short to avoid the exception you should acquire the lock in your Parse
> thread before calling signal() and release it immediatly after in a finally
> block.
>
> Also worth noting that to make a thread sleep for a period of time
> Thread.sleep() is better and safer than using synchronized + wait() as it
> won't interfere with anything else using "this" as a synchornization object.
>
> HTH.
>
>  ------------------------------
> *From:* concurrency-interest-bounces at cs.oswego.edu [mailto:
> concurrency-interest-bounces at cs.oswego.edu] *On Behalf Of *David Harrigan
> *Sent:* 23 October 2006 18:46
> *To:* concurrency-interest at cs.oswego.edu
> *Subject:* [concurrency-interest] Await...Signal - Catching
> IllegalMonitorStateException???
>
> Hi,
>
> Firstly let me say that I'm really happy to find this list. I find
> threading
> difficult, but I'm trying to get to trips with it as best I can, and I'm
> sure
> this list will help my minisule understanding...
>
> Okay, I am trying to get to trips with the concurrency features in Java
> 1.5, and I've got an issue that I'm trying to understand. I have this
> basic program structure...
>
> ....
> ....
> private Lock lock = new ReentrantLock();
> private Condition condition = lock.newCondition();
> ....
> Parser parser = new Parser(condition);
> lock.lock();
> try {
>     new Thread(parser).start();
>     if(!condition.await(2000, TimeUnit.MILLISECONDS)) {
>         if(parser.isReading()) {
>             if(!condition.await(2000, TimeUnit.MILLISECONDS)) {
>                 setTimedOutWhilstReading(true);
>             }
>         }
>         setTimedOutWhilstWaitingForResponse(true);
>     }
> } catch(InterruptedException e) {
> } finally {
>   lock.unlock();
> }
>
> then in Parser I have this...
>
> ....
> ....
> public Parser(final Condition condition) {
>     this.condition = condition;
> }
>
> public void run() {
>     try {
>        synchronized(this) {
>            wait(10000); // <--------- simulate this parser taking 10
> seconds reading, causing timeout in calling thread
>        }
>     } catch (InterruptedException e) {
>     }
>
>     try {
>         condition.signal();
>     } catch(IllegalMonitorStateException e) {
>     }
> }
>
>
>
> Okay, let me try and explain:
>
> I have a thread (X) that creates a parser (Y). X has to check for a few
> things,
> namely that Y hasn't timed out waiting for a response (the first await in
> X) then
> if it has timed out X checks to see if Y has started to parse a response.
> I've
> simulated in Y that Y takes 10 seconds to do it's business (ignorning a
> response read etc..I'm just interested in overall time at this point).
>
> Now, in X, because the two awaits have timed out (total ~4 seconds), X has
> moved on and exited out of it's run() and the calling thread of X does
> some
> extra things...etc...
>
> But, because X has moved on, when I call condition.signal() I have to
> catch
> an IllegalMonitorStateException! This seems to me a bit "strange." I can
> perhaps
> hazzard a guess that because the lock (it's condition) in X no longer
> exists, then
> calling condition.signal() in Y causes this, but my question is - is this
> correct? Do
> I have to do this? Am I not tripping out?
>
> Now, the reason I have condition.signal() is that in the version of Parser
> that
> does the parsing and returns in < 4 seconds, then X which is sitting in an
> await state, calling condition.signal() is the right thing to do - I have
> to tell X
> that Y is finished....
>
> What I'm trying to understand is what happens if Y takes a long long time,
> what
> I have to do to X (and Y) for the states to be properly managed....
>
> I hope this is clear? If not, then please do ask me for clarification...
>
> Thanks so so much, and I look forward to participating in this list!
>
> -=david=-
>
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>
>
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