Monitors wait and notifyAll

[Jay Brass]

Hey there,
I've been fighting with these for a couple of days and I hope one of ya'll can help me. I understand that the monitors wait and notifyAll have to be called from synchronized code and probably not the same piece of synchronized code either. But when I use 2 different methods and try to call notifyAll from the second synchronized method, I get illegalMonitorStateException error. Is this because the thread executing notifyAll doesn't have the lock on the waiting thread?
I'm callling the start method so that the 2 threads will run concurrently but for the life of me I can't seem to be able to notify either one. They seem to hang on the wait()
Can anyone help me? I'd introduce some code but it is really simple, try catch blocks with wait and notifyAll inside.
Thx
J

[Marilyn de Queiroz]

It seems that you should post the code because from your description you are doing everything correctly.

[Jay Brass]

I hope this helps clarify. Thanks again to anyone that can help.

package scjpThreads.src;
public class SimpleThreads extends Thread {
public static boolean bool;
    public SimpleThreads(String str) {
        super(str);
    }
  public void run() {
        for (int i = 0; i < 5; i++) {
            System.out.println(getName() + " " + i);
            int x = 0;
                if (x == 0){
                this.methodA();
                x++;
                }
                this.methodB();
        }
        System.out.println(getName() +" DONE!" );
      }
    public synchronized void methodA() {
        while(bool == false) {
            bool = true;
            try {
                System.out.println(getName() + " waiting from methodA");
                wait();
                } catch (InterruptedException e) {}
        }
        notifyAll();
    }
    public synchronized void methodB() {
            while(bool == true) {
                bool = false;
                try {
                    System.out.println(getName() + " waiting from methodB");
                    wait();
                    } catch (InterruptedException e) {}
            }
            notifyAll();
    }
    public static void main (String args[])
    {
        new SimpleThreads("Thread 1").start();
        new SimpleThreads("Thread 2").start();
    }
}

[William Brogden]

As far as I know, the only way you can get a
IllegalMonitorStateException error
is like you say, by calling one of the monitor related methods when the Thread is not synchronized on the object.
As far as why your threads never get notified, I can't see where you expect that to happen.
Bill
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[Jay Brass]

OK so where should I put the notifyAll() ? I tried it in the run method before the call to methodB and got IllegalMonitorStateException. I even created a new synchronized method called Notify and called notifyAll from there with yet a third thread. Same problem. My basic question is how to notify a waiting thread? Maybe I'm missing something but I just can't seem to get it to work at all. Any suggestions??
Thx
J

[Jennifer Warren]

Jay Brass, Hi there,
Thing is that theres got to be a reason for synchronizing the two methods. Usually when two methods are dependent on each other. In your code both threads calls the mehtodA and are passed in to wait state. Try putting notifyAll() before the wait(), the code will work. I hope this helps.
Regards.
Jennifer.

[Jay Brass]

Hey Jennifer,
The only way I could get the results I wanted was to do this:

public void run() {
        for (int i = 0; i < 5; i++) {
            System.out.println(getName() + " " + i);
                methodA();
        }
        System.out.println(getName() +" DONE!" );
      }
 
public synchronized void methodA() {
    try {
         System.out.println(getName() + " waiting from methodA");
         wait(1000);
         } catch (InterruptedException e) {}
public static void main (String args[])
    {
        new SimpleThreads("Thread 1").start();

Which is little different than calling sleep(1000) except that sleep is static. I peppered the code with notifyAll()'s and still couldn't get it out of the wait state. I think because when notifyAll is called the notified thread can't aquire the lock because the other thread has it. So it is forced to wait again. Then the second thread waits and no one ever notifies it. Can someone verify this? And can someone tell me where I can apply a notifyAll command to give me the same results as above?
Thx
J

[Jay Brass]

sorry there should be a
new SimpleThreads("Thread 2").start();
included in the above code. Darn that cut and paste!
J