synchronization understanding

[sandipan chakrabarti]

import java.util.concurrent.atomic.AtomicBoolean;
class SynchronizedCounter {
private int c = 0;
public synchronized void increment() {
try {
System.out.println("11111111");
this.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}

public synchronized void decrement() {
c--;
}

public synchronized int value() {
return c;
}
}

public class test2 extends Thread{
static SynchronizedCounter c = new SynchronizedCounter(); ;
public void run(){
c.increment();
}
public static void main(String[] args) {
test2 t = new test2();
test2 t1 = new test2();
t.start();
t1.start();
}
}

The result-----

11111111
11111111

Q if the SynchronizedCounter methods are synchronized then I can assume that when one thread enters the critical section in increment() then the other thread is blocked. then why 11111 is printed twice instead of once??

[Henry Wong]

Q if the SynchronizedCounter methods are synchronized then I can assume that when one thread enters the critical section in increment() then the other thread is blocked. then why 11111 is printed twice instead of once?

The other thread *is* blocked -- because it is waiting to acquire the lock. However, once the wait() method is called, it will release the lock, allowing the other thread to acquire the lock.


Henry

[amitt jainn]

I am totally agreed with Henry..
explaning it further syncing with the question.

this.wait() ;

actually when we call wait() on an object, it relinqushes the monitor it holds and goes to the waiting thread pool of that object ('this' in this case). Now this thread will not gain its ready state until a notify is called on this object (this in our example).

Now since the thread has already released the monitor another thread can very smoothly obtain monitor on this object.

That is why your program returned

11111111
11111111

Regards
Amit

[Raghavan Muthu]

Howdy sandipan chakrabarti ,

Kindly make use of the Code Tags when posting the java code