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

Mike Quilleash mike.quilleash at subexazure.com
Mon Oct 23 14:20:47 EDT 2006


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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