How wait() and notify() works?

[K Raj Kumar]

Hi Ranchers!
This is the code below;

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(); //1
        }
        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();
    }
   }
}

I could understand that at label 1, we have used wait(); to prevent ThreadB from finishing or completing the calculation, its stack. And, then we used notify to release the waiting thread and complete the ThreadA.
Please explain the inner details esp synchronisation with object b to call wait() on the object and then how notify() acts.
Source:SCJP 6 study guide, Kathy sierra and Bert bates , Threads, page 748.
Thanks in advance!
[ December 23, 2008: Message edited by: Raj kumar ]

[Punit Singh]

 
public class NotifyDoesNotReleaseLock {
final static Object obj=new Object();
static class Thread1 extends Thread{
@Override
public void run() {
synchronized (obj) {
try {
System.out.println("I am going to wait....");
obj.wait(); 
 
    quote: Here thread1 will call obj.wait() and release the lock on obj and wait until any other thread call notify on this obj, obj.notity(). 
 

System.out.println("Now wait is over... phew");
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}
static class Thread2 extends Thread{
@Override
public void run() {
 
 
    quote: Here thread2 will wait to take lock on obj, that is already taken by thread1. When thread1 will call obj.wait(), then it will release the lock on obj, then only this thread2 will enter in this synchronized block. 
 

 synchronized (obj) {
 System.out.println("I am going to notify my partner....");
obj.notify();
System.out.println("But I have not released the lock still...");
System.out.println("First I will loop 10 times, then release the lock...");
for(int i=0;i<10;i++){
System.out.println("I have lock. Count is: "+i);
}

}
}
}
 
Output:
I am going to wait....
I am going to notify my partner....
But I have not released the lock still...
First I will loop 10 times, then release the lock...
I have lock. Count is: 0
I have lock. Count is: 1
I have lock. Count is: 2
I have lock. Count is: 3
I have lock. Count is: 4
I have lock. Count is: 5
I have lock. Count is: 6
I have lock. Count is: 7
I have lock. Count is: 8
I have lock. Count is: 9
Now wait is over... phew
 

Try to run and understand this code.

[Punit Singh]

I could understand that at label 1, we have used wait(); to prevent ThreadB from finishing or completing the calculation, its stack. And, then we used notify to release the waiting thread and complete the ThreadA.
Please explain the inner details esp synchronisation with object b to call wait() on the object and then how notify() acts.

We have used wait(); to prevent main thread from finishing.

What is happening here:

b.start();

Main thread called this b.start() to start the threadb.

b.wait(); //1

Main thread called wait() on object b.It will release lock from the b and will wait here until any other thread call notify on the same object. So it will wait here for infinite time.

In between threadb's run method is working its task. After completing
its for loop it will call notify, that is actually this.notify() and here this object is the same object on which Main thread is waiting.

public void run()
{
synchronized(this){
for(int i=0; i<100; i++){
total+=i;
}
notify();//this.notify
}
}

When threadb will call this.notify() and Main thread will wake up and wait for lock on the same object, notify() does not release lock, but when
synchronized(this){} block will complete lock will be released by threadb.

Main thread will get the lock on object b again and it will become runnable again, now thread schedular will select it and Main thread will start executing.

[Brendan Healey]

forget it....