Thread wait()

[jibs parap]

Why does it say that wait() and notify() are to be called from a synchronized context? Whats the implication of it?

Can anyone explain please.

[Srinivasan thoyyeti]

wait,notify came into existance due to old Thread methods
suspend, stop,resume.

These methods have failed to manage locks obtained.
causing deadlocks.

To work with or manage locks wait and notify are introduced.

wait() -> releases the obtained lock and waits to get notified.
notify() -> notifies another thread(s) waiting on that object.


We can't handle the locks out side the synchronized() block. we can't creep into monitor state.

Thats wait,notify placed in Object sothat we can effectively manage the locks obtained on any Object.
[ August 11, 2007: Message edited by: Srinivasan thoyyeti ]

[jibs parap]

Just to make it clearer:

    
class ThreadA {
 public static void main(String [] args) {
 ThreadB b = new ThreadB();
 b.start();
 
 //synchronized(b) {
 try {
 System.out.println("Waiting for b to complete...");
 b.wait();
 } catch (InterruptedException e) {}
 System.out.println("Total is: " + b.total);
 //}
 }
 }
 
 class ThreadB extends Thread {
 int total;
 
 public void run() {
 //synchronized(this) {
 for(int i=0;i<100;i++) {
 total += i;
 }
 notify();
 //}
 }
 }

In the above programme I've commented out the synchronised block around wait() and notify()[I know it will give a run time exception] just for a hypotethic scenario; I don't seem to see the importance of lock in calling wait and notify; i.e. even without owning or releasing the lock, the waiting / notifying behaviour should happen.
I know there is a flawin my thought process, please explain.

[Manfred Klug]

Hi jibs,

I will try it with a more complex example, to show, why it is useful to remind the programmer to use synchronisation. The following code contains a problem (Besides that it doesn't use synchronisation for wait and notify). Can you find it?

class A {
    Queue<Job> jobQueue;
 
    public A(Queue<Job> jobQueue) {
        this.jobQueue = jobQueue;
    }
 
    public void enqueueJob(Job job) {
        synchronized(jobQueue) {
            jobQueue.offer(job);
        }
        jobQueue.notify();
    }
}
 
class Worker extends Runnable {
    Queue<Job> jobQueue;
 
    public Worker(Queue<Job> jobQueue) {
        this.jobQueue = jobQueue;
    }
 
    public void run() {
        while(true) {
            Job job;
            synchronized(jobQueue) {
                job = jobQueue.remove();
            }
            if (job != null) {
                // Do something, and then wait for the next job.
            } else {
                try {
                    jobQueue.wait();
                } catch (InterruptedException e) {}
            }
        }
    }
}

[ August 11, 2007: Message edited by: Manfred Klug ]

[Srinivasan thoyyeti]

Manfred Klug ,

Do you think wait,notify can be called out-side synchronized context ???

[Manfred Klug]

Originally posted by Srinivasan thoyyeti:
Do you think wait,notify can be called out-side synchronized context

If you read the text, you will see, that I know that this will not work. It is only for demonstration purposes.

[Srinivasan thoyyeti]

I am sorry for that