[OJB-developers] Double checked locking & OJB

[John F. <jfr...@so...>]

Forgive me - I don't want to seem critical here - I'm pretty excited
about using OJB and the potential it has!

However, I've read in a number of places that double checked locking
(DCL) isn't really safe in Java:

http://www.javaworld.com/javaworld/jw-02-2001/jw-0209-double.html
http://www.javaworld.com/javaworld/jw-05-2001/jw-0525-double.html
http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html

I've noticed that a few spots in OJB sort of use DCL, but don't really
do full DCL as there is no second check after the synchronized()
statement (see comment):

  public static LockManager getLockManager()
  {
        if (LOCKMAN =3D=3D null)
        {
            synchronized (LockManager.class)
            {
                // This would be DCL, but the code doesn't have this
second check
                //  if (LOCKMAN =3D=3D null)
                LOCKMAN =3D createNewLockManager();
            }
        }
        return LOCKMAN;
  }

LockMapFactory, SequenceManagerFactory, PersistentFieldFactory,
ObjectCacheFactory seem to use this same style of singleton creation.

If folks believe otherwise on DCL, I'd like to know as I try to follow
that sort of stuff.  Otherwise it is probably safer to simply make these
methods synchronized.

I worry about this as I'm planning on using OJB inside a servlet
container where there are a lot of PersistenceBroker objects that may be
concurrently calling these methods (and running on a dual processor
box).

Thanks,=20
  - John Freeborg