Who is getting locked?

[Adolfo Eloy]

Hi all.

I have a conceptual doubt about threads. If I have an object that has the method that follows:

	
private synchronized void makeWithDrawal(int amt) {
	if (acct.getBalance() >= amt) {
		System.out.println(Thread.currentThread().getName()
			+ " is going to withdraw");
		try {
			Thread.sleep(500);
		} catch (InterruptedException ex) { }
		acct.withdraw(amt);
		System.out.println(Thread.currentThread().getName()
			+ "completes the withdrawal");
	} else {
		System.out.println("Not enough in account for "
			+ Thread.currentThread().getName()
			+ " to withdraw " + acct.getBalance());
	}
}

I'd like to confirm that the object whose method is synchronized is the object that has been locked.
So the instance variables such as acct who is a reference to an object isn't locked.

Am I right?

Thanks

[Jeff Verdegan]

We don't really refer to anything as "being locked." Rather, we say that we obtain a lock. But if by "locking X" you mean "obtaining X's lock", then, yes, you're correct. The object whose lock we obtain is the one whose synchronized method we are executing.

public synchronized void foo() {
  // body
}

public void foo() {
  synchronized (this) {
    // body
  }
}

The above methods are equivalent. Both obtain the lock for the "this" object and hold it for the entire body of the method.