[gclist] A Java question.

Johny johny@medan.wasantara.net.id
Wed, 26 Mar 1997 21:12:49 +0700

> From: John D. Mitchell <johnm@non.net>
> To: gclist@iecc.com
> Subject: Re: [gclist] A Java question.
> Date: Wednesday, March 26, 1997 3:41 AM
> >>>>> "Charles" == Charles Fiterman <cef@geodesic.com> writes:
> >>>>>> Mimir Reynisson <skelmir@qualia.com> writes:
> [...]
> >> All Java VM's must call finalizers and all as far as I know do.
> >> finalize() is used to free up bitmaps, file references, and other
> >> non-Java-related "garbage". I think you may be talking about the fact
> >> that once a finalizer has been called once, it will not be called
> >> again. So if you create a reference to the object being finalized in
> >> finalizer, you essentially create a zombie object and finalize() will
> >> not be called the next time it tries to gc that object. Most of the
> >> this is not a problem.
> Hmm...  The JLS (Java Language Specification) says "The Java language
> not specify how soon a finalizer will be invoked, except to say that it
> will be happen before the storage for the object is reused."
> It's also interesting to note that upon exit of the Java run-time, it is
> *not* obliged to run finalize() on everything.  The Java v1.1 stuff
> supposedly makes this configurable by using the
> java.lang.System.runFinalizersOnExit() method but I haven't tried it.
> It is Bad Practice(tm) to rely on finalization for reclaiming resources
> which are heavily contended for (file handles) or otherwise where
> timeliness is important (e.g., sockets).
> > I have nothing against zombies but am worried about the zombie getting
> > freed or any other pointer to a free object as those would allow me to
> > create a virus. The point of Java is that the type system prevents the
> > creation of viruses. There is also the question of finalizer order.
> There was, IIRC, a lot of discussion about this in the Java newsgroups so
> you probably want to check DejaNews.  Also, check the archives of the
> advanced-java@xcf.berkeley.edu mailing list (as it has a higher
> signal-to-noise ratio :-).
> Note that there is the java.lang.System.runFinalization() and
> java.lang.System.gc() methods (which are pretty much just a simple way to
> invoke java.lang.Runtime.{runFinalization,gc}()) so you can "force" gc
> finalization.  Heck, you can even call finalizer methods directly.
> The Java specification gives *no* guarantees about the relative ordering
> the calls to the various objects' finalize() method.
> In terms of having access to freed resources via references that you are
> holding which are accessible when a finalizer is invoked, check out the
> Section 12.6.1, page 232, "Implementing Finalization" for all sorts of
> state-based discussion of Java finalization.  Basically, finalization and
> freeing are separate "passes".
> Hope this helps,
> 		John