[jmm-dev] ECOOP, JVMLS

[Ludwig, Mark]

Thanks.  I didn’t mean it as a criticism, just an observation.  (I didn’t get to JavaOne in 2005.)  I’m still digesting the presentation, because the reachability problem is a lot more subtle that any of us realized  (and a few of us in my group like to think of ourselves as “systems” guys, not just “application programmers”).

When you wrote about synchronized(this) {} I had no idea you meant that literally; I thought you just omitted the implicit “…” between the braces.  When I first read the presentation and saw you did mean it literally, it blew my mind … but now it makes sense as coordinating with the synchronized method keepAlive().

Once upon a time (this code dates from Java 1.1, ca. 1997), the finalize method in this class was marked as synchronized.  It’s not now (through maintenance over the years) because we thought there was a performance problem with that (i.e., unnecessarily slowing down the Finalization thread with the lock overhead for something that could never be locked).  The performance impact of marking the finalize method synchronized should be the same as synchronized(this) {} at the top of the method since one thread is involved.  (That is, it’s not as though any other finalize method can run while this class’s finalize method is running, so it’s not as though there is any throughput increase by not marking the whole method as synchronized, is there?  I have the impression, though, that there must be a reason not to just mark the finalize method synchronized.  What is it?

I’m also surprised there is no attempt to invoke super.finalize() in your example.  Isn’t that a good idea (even required?), pro forma?

Cheers,
Mark

From: Hans Boehm, Monday, August 04, 2014 2:42 PM
That's probably a valid criticism.  I did give a 2005(?) JavaOne talk on the subject, which was attended by about 1000.

There are currently several possible variants of the pattern, all ugly.  One variant is at slide 35 in http://hboehm.info/misc_slides/java_finalizers.pdf (essentially the slides from that ancient JavaOne talk, which no longer seem to be available from a more official site, but were for a number of years).

Hans



On Mon, Aug 4, 2014 at 12:21 PM, Ludwig, Mark <ludwig.mark at siemens.com<mailto:ludwig.mark at siemens.com>> wrote:
> From: Hans Boehm, Monday, August 04, 2014 2:04 PM
>
> We've been telling people since 2005 that
> you need synchronized(this){} all over the place in code with finalizers,
> and I have yet to see a single piece of code "in the wild" that actually
> does this.
How has this message been distributed?  (The message certainly
has not been effectively conveyed....)

What is the pattern, exactly?  I assume I don't need a synchronized
block inside a synchronized method....

Thanks,
Mark Ludwig