Re: Why "lock" functionality is introduced for all the objects?

From:

=?ISO-8859-1?Q?Arne_Vajh=F8j?= <arne@vajhoej.dk>

Newsgroups:

comp.lang.java.programmer

Date:

Thu, 21 Jul 2011 20:33:14 -0400

Message-ID:

<4e28c54c$0$308$14726298@news.sunsite.dk>

On 6/28/2011 5:29 AM, Alex J wrote:

I'm curious why Java designers once decided to allow every object to
be lockable (i.e. allow using lock on those).
I know, that out of such a design decision every Java object contain
lock index, i.e. new Object() results in allocation of at least 8
bytes where 4 bytes is object index and 4 bytes is lock index on 32-
bit JVM.
I think that it just inefficient waste of space, because not all the
objects requires to be lockable/waitable.

The better decision, IMHO, would be to introduce lock/wait mechanics
for only, say, the Lockable descendants.
The current approach seems to be very simple, but is the performance
penalty so small for not be taken into an account?
Eclipse uses tons of small objects and I guess that is why it consumes
so much memory while a significant part of it is never used.

What do you think of it?

For all the simple cases:

public class Foobar {
    ...
    private Object lock = new Object();
    ...
    public void test() {
        ...
        synchronized(lock) {
            ...
        }
        ...
     }
     ...
}

having to use LockingObject instead of Object would have worked fine.

But in more complex scenarios where you have multiple methods modifying
multiple objects, then the only safe way is to lock on the actual
objects (obviously in a fixed order to avoid deadlocks).

Arne