invalidMonitorState

[mohit joshi]

Can some one give a simple example when InvalidMonitorStateException is thrown? I got these exceptions long time back, but cant recall and didnt understand at that time why exactly i got these.

[deekasha gunwant]

Hi Mohit ,
this exception is thrown when you try to call wait/notify/notifyAll any of these methods for an Object from a point in your program where u are not having a lock on that object.(i.e. u r not executing any synch. block/method of that object and still trying to call wait/notify/notifyAll)
here is a code first run it as it is. and the uncomment the lines
which are bold. again run it and u will get an exception.

class IllegalMonitorStateExample
{
public static void main(String a[])
{
AnObject o = new AnObject();
Thread inc = new Thread(new IncrUser(o));
Thread dec = new Thread(new DecrUser(o));
inc.start();
dec.start();
}
}
class IncrUser implements Runnable
{
AnObject a ;
IncrUser()
{
}
IncrUser(AnObject a)
{
this.a = a;
}
public void run()
{
while(true)
{
try
{
[B]// a.wait();[/B]
a.incr();
Thread.sleep(1000);
[B] // a.notify(); //[/B]
}
catch(InterruptedException ie)
{
ie.printStackTrace();
}
}
}
}
class DecrUser implements Runnable
{
AnObject a ;
DecrUser()
{
}
DecrUser(AnObject a)
{
this.a = a;
}
public void run()
{
while(true)
{
try
{
a.decr();
Thread.sleep(1000);
}
catch(InterruptedException ie)
{
ie.printStackTrace();
}
}
}
}
class AnObject{
int myNum = 0;
synchronized void incr() throws InterruptedException
{
if(myNum!=0)
{
System.out.println("ready to incr but muNum not zero +++++++++++++++");
wait();
}
myNum++;
print("produced : ");
notify();
}
synchronized void decr() throws InterruptedException
{
if(myNum==0)
{
System.out.println("ready to decr but myNum not 1 ----------");
wait();
}
myNum--;
print("consumed : ");
notify();
}
void print(String s)
{
System.out.println(s + " "+myNum);
}
}

regards
Deekasha

[mohit joshi]

thanks deekasha, that was great example.
Another doubt which got clarified in the process, is that the compiler doesnt ensures that the wait() notify() methods are called from within a synchronized block.

[Vladimir Kositsky]

I am not sure that notify and notifyAll throws any exception

[deekasha gunwant]

Hi Vladimir,
notify and notifyAll both throw IllegalMonitorStateException. since This exception is a subclass of Runtime exception so u r not bound to catch it (although u may if u want to).
and being a runtimeexception this exception is not mentioned in the signature of notify,notifyAll methods.
regards
Deekasha