Locks and synchronization

[raja singh kumar]

We know that locks are associated with objects. In the below example program there are 3 synchronized methods. Lock is acquired for which object? Also when a thread is executing the increment method can a second thread invoke the decrement method?

public class SynchronizedCounter {
    private int c = 0;

    public synchronized void increment() {
        c++;
    }

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

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

[Campbell Ritchie]

raja singh kumar wrote:. . . . Lock is acquired for which object?

Look in the Java® Language Specification. It tells you which object is used as the monitor.
If I remember correctly, a synchronised method is equivalent to wrapping the whole content of the method in synchronized (this) { ... }

Also when a thread is executing the increment method can a second thread invoke the decrement method? . . .

That is the whole point of the synchronisation: stop a second thread accessing decrement() until increment() has completed and the lock has been released. I presume the get method was only there to show how the examplle works. Please work out whether you really need that get method to be synchronised.

[raja singh kumar]

I dont get it. Is the lock acquired on object of SynchronizedCounter when accessing synchronized method of  SynchronizedCounter?

[Paul Clapham]

Yes, that's it exactly. The "synchronized" keyword, when it doesn't specify an object to synchronize on, synchronizes on "this". Which in your example means the SynchronizedCounter which is about to execute the synchronized method.