Hi Jaroslav,
FWIW, I have experimented with adapting sun.misc.Cleaner for external
use. For example, it could be used as a NetBeans provided API. The
thread management is based on tracking of Cleaner (PhantomReference)
objects. When 1st Cleaner object is added to a doubly-linked list, the
thread is started, when the last Cleaner object is removed from the
list, it is stopped, so this is a kind of reference tracking (not
counting). Here's the attempt:
http://cr.openjdk.java.net/~plevart/misc/nb.Cleaner/Cleaner.java
Would this be an appropriate replacement for ActiveQueue API in NetBeans?
Regards, Peter
On 08/15/2014 11:34 AM, Jaroslav Tulach wrote:
Hi.
I'd like to thank everyone in participating in our discussion about lightweight
finalization and shaping and sharping my understanding of the problem. I
believe the
discussion has now slowed down and it is time for another summary. I have added
one
into the issue (while also changing its subject):
https://bugs.openjdk.java.net/browse/JDK-8051843?focusedCommentId=13539596&p
age=com.atlassian.jira.plugin.system.issuetabpanels:comment-
tabpanel#comment-13539596[1]
I believe our understanding of the problem has been deepen enough to focus on
creation of the actual public API for lightweight finalization. It will be my
task for up-
coming week to start necessary steps to propose such API.
-jt
PS: I'd like to especially thank to Andrew for pointing out the "this"
reference problem
and Doug for commenting it. I've reported
https://bugs.openjdk.java.net/browse/JDK-8055183[2] to resolve it somehow. I'll
be
watching from distance eager to know the result.
Dne St 13. srpna 2014 13:12:43, Jaroslav Tulach napsal(a):
Dne So 9. srpna 2014 07:18:54, Doug Lea napsal(a):
Dne Pá 8. srpna 2014 15:02:41, Stanimir Simeonoff napsal(a):
According to articles provided by Andrew[1], toString() can be on stack,
even if obj
(e.g. this for toString()) is garbage collected. This is a bit
surprising
(an OOP fan
would expect that this is GC root while its instance method is running),
but such
behavior is said to be in accordance to the 1.8 spec.
Question for Doug (as a member of jmm-dev list) then is: would not it be
wise to
change the spec to require this to be GC root while its instance method
is
being
executed?
Possible changes are being discussed. It is not clear yet
exactly what rules would better match people's expectations
while remaining implementable by JVMs, and/or whether they
would force performance degradations even
for the vast majority of code not using finalizers. It is
very possible the end result will just be to clarify rules and
provide mechanisms allowing better programmer control.
Any changes would go into effect for JDK9 at the earliest,
so if you can find a way to avoid use of finalizers
here, it would be a better option.
I'll do my best to come up with a lightweight alternative to finalization.
However:
As far as I understand Andrew's inquiry, the problem is not (that much)
related to finalizers, rather to question whether following method can ever
finish or not:
private void gcThis() {
Reference<?> ref = new WeakReference<>(this);
while (ref.get() != null) {
System.gc();
}
System.err.println("ref is gone: " + ref);
}
On Oracle's JDK8 the method has never finished for me. However when I tried
IBM's JDK (identification can be found below) the method finished every
time! A bit of consistency would be beneficial in this area, imho.
-jt
PS: While I understand why the two JDKs behave the way they behave, I don't
think this is explainable to regular Java developers. But maybe they don't
care that much and one day it will be easy to google out solution on
stackoverflow to prevent "this" being garbage collected by changing the last
method line to
System.err.println("ref is gone: " + ref + " for " + this);
The version of IBM's Java was:
~/bin/ibmjava7$ ./bin/java -version
java version "1.7.0"
Java(TM) SE Runtime Environment (build pxa6470_27sr1fp1-20140708_01(SR1
FP1)) IBM J9 VM (build 2.7, JRE 1.7.0 Linux amd64-64 Compressed References
20140707_205525 (JIT enabled, AOT enabled)
J9VM - R27_Java727_SR1_20140707_1408_B205525
JIT - tr.r13.java_20140410_61421.07
GC - R27_Java727_SR1_20140707_1408_B205525_CMPRSS
J9CL - 20140707_205525)
JCL - 20140707_01 based on Oracle 7u65-b16
--------
[1]
https://bugs.openjdk.java.net/browse/JDK-8051843?focusedCommentId=13539596&p
age=com.atlassian.jira.plugin.system.issuetabpanels:comment-
tabpanel#comment-13539596
[2] https://bugs.openjdk.java.net/browse/JDK-8055183
