On 1/11/2017 10:20 PM, Peter Levart wrote:
On 11/01/17 10:04, David Holmes wrote:
On 1/11/2017 6:16 PM, Peter Levart wrote:
On 11/01/17 02:49, David Holmes wrote:
Hi Roger,
On 31/10/2017 11:58 PM, Roger Riggs wrote:
Hi Peter,
Only native resources that do not map to the heap allocation/gc
cycle need any kind
of cleanup. I would work toward a model that encapsulates the
reference to a native resource
with a corresponding allocation/release mechanism as you've
described here and in the
thread on zip.
For cleanup purposes, the independence of each resource may improve
robustness
by avoiding dependencies and opportunities for entanglements and
bugs due to exceptions
and other failures.
In the case of TPE, the native resources are Threads, which keep
running even if they are
unreferenced and are kept referenced via ThreadGroups.
I don't know the Executor code well enough to do more than
speculate, but would suggest
that a cleaner (or similar) should be registered for each thread .
Threads are not native resources to be managed by Cleaners! A live
Thread can never be cleaned. A dead thread has nothing to clean!
Right, but an idle thread, waiting for a task that will never come
since the only entry point for submitting tasks is not reachable (the
pool), may be cleaned...
cleaned? It can be interrupted if you know about it and find locate
it. But it will not be eligible for cleaning ala Cleaner as it will
always be strongly reachable.
Ah I see what you meant before. Yes, tracking Thread object with a
Cleaner does not have any sense. But tracking thread pool object with a
Cleaner and cleaning (stopping) threads as a result makes sense...
No, because live Threads will keep the thread pool strongly reachable.
If you manage to structure things such that the Threads don't keep the
pool strongly reachable then you risk having the pool cleaned while
still actively in use.
David
David
Regards, Peter
David
Regards, Peter
David
-----
For TPE, since Threads do not become unreferenced, the part of the
spec related to finalize
being called by the finalizer thread is moot.
$.02, Roger
On 10/31/2017 5:24 AM, Peter Levart wrote:
Hi,
Here are some of my thoughts...
On 10/31/17 05:37, David Holmes wrote:
Hi Roger,
On 31/10/2017 12:43 AM, Roger Riggs wrote:
Hi David,
On 10/30/2017 3:31 AM, David Holmes wrote:
Hi Andrej,
On 30/10/2017 5:02 PM, Andrej Golovnin wrote:
Hi David,
On 30. Oct 2017, at 01:40, David Holmes
<david.hol...@oracle.com> wrote:
Hi Roger,
On 30/10/2017 10:24 AM, Roger Riggs wrote:
Hi,
With the deprecation of Object.finalize its time to look at
its uses too see if they can be removed or mitigated.
So the nice thing about finalize was that it followed a
nice/clean/simple OO model where a subclass could override,
add their own cleanup and then call super.finalize(). With
finalize() deprecated, and the new mechanism being Cleaners,
how do Cleaners support such usages?
Instead of ThreadPoolExecutor.finalize you can override
ThreadPoolExecutor.terminated.
True. Though overriding shutdown() would be the semantic
equivalent of overriding finalize(). :)
In the general case though finalize() might be invoking a final
method.
Overriding shutdown() would only work when this method is
explicitly invoked by user code. Using Cleaner API instead of
finalization can not employ methods on object that is being
tracked as that object is already gone when Cleaner invokes the
cleanup function.
Migrating TPE to Cleaner API might be particularly painful since
the state needed to perform the shutdown is currently in the TPE
instance fields and would have to be moved into the cleanup
function. The easiest way to do that is to:
- rename class ThreadPoolExecutor to package-private
ThreadPoolExecutorImpl
- create new class ThreadPoolExecutor with same public API
delegating the functionality to the embedded implementation
- registering Cleaner.Cleanable to perform shutdown of embedded
executor when the public-facing executor becomes phantom reachable
This would have an added benefit of automatically shut(ing)down()
the pool when it is not reachable any more but still has idle
threads.
Anyway I'm not sure we can actually do something to try to move
away from use of finalize() in TPE. finalize() is only
deprecated - it is still expected to work as it has always
done. Existing subclasses that override finalize() must
continue to work until some point where we say finalize() is
not only deprecated but obsoleted (it no longer does anything).
So until then is there actually any point in doing anything?
Does having a Cleaner and a finalize() method make sense? Does
it aid in the transition?
As you observe, the alternatives directly using PhantomRefs or
the Cleanup do not provide
as nice a model. Unfortunately, that model has been recognized
to have a number of
issues [1]. Finalization is a poor substitute for explicit
shutdown, it is unpredictable and unreliable,
and not guaranteed to be executed.
I'm not trying to support/defend finalize(). But it does support
the very common OO pattern of "override to specialize then call
super".
I have been thinking about that and I see two approaches to enable
subclasses to contribute cleanup code:
- one simple approach is for each subclass to use it's own
independent Cleaner/Cleanable. The benefit is that each subclass
is independent of its super/sub classes, the drawback is that
cleanup functions are called in arbitrary order, even in different
threads. But for cleaning-up independent resources this should work.
- the other approach is more involving as it requires the base
class to establish an infrastructure for contributing cleanup
code. For this purpose, the internal low-level Cleaner API is most
appropriate thought it can be done with public API too. For
example (with public API):
public class BaseClass implements AutoCloseable {
protected static class BaseResource implements Runnable {
@Override
public void run() {
// cleanup
}
}
protected final BaseResource resource = createResource();
private final Cleaner.Cleanable cleanable =
CleanerFactory.cleaner().register(this, resource);
protected BaseResource createResource() {
return new BaseResource();
}
public BaseClass() {
// init BaseResource resource...
}
@Override
public final void close() {
cleanable.clean();
}
}
public class DerivedClass extends BaseClass {
protected static class DerivedResource extends BaseResource {
@Override
public void run() {
// before-super cleanup
super.run();
// after-super cleanup
}
}
@Override
protected DerivedResource createResource() {
return new DerivedResource();
}
public DerivedClass() {
super();
DerivedResource resource = (DerivedResource) this.resource;
// init DerivedResource resource
}
}
And alternative with private low-level API:
public class BaseClass implements AutoCloseable {
protected static class BaseResource extends
PhantomCleanable<BaseClass> {
protected BaseResource(BaseClass referent) {
super(referent, CleanerFactory.cleaner());
}
@Override
protected void performCleanup() {
// cleanup
}
}
protected final BaseResource resource = createResource();
protected BaseResource createResource() {
return new BaseResource(this);
}
public BaseClass() {
// init BaseResource resource...
}
@Override
public final void close() {
resource.clean();
}
}
public class DerivedClass extends BaseClass {
protected static class DerivedResource extends BaseResource {
protected DerivedResource(DerivedClass referent) {
super(referent);
}
@Override
protected void performCleanup() {
// before-super cleanup
super.performCleanup();
// after-super cleanup
}
}
@Override
protected DerivedResource createResource() {
return new DerivedResource(this);
}
public DerivedClass() {
super();
DerivedResource resource = (DerivedResource) this.resource;
// init DerivedResource resource
}
}
Regards, Peter
ThreadPoolExecutor has a responsibility to cleanup any native
resources it has allocated (threads)
and it should be free to use whatever mechanism is appropriate.
Currently, the spec for finalize
does not give it that freedom.
I suppose in hindsight we (JSR-166 EG) could have described
automatic shutdown without mentioning the word "finalize", but
that ship has sailed. We did specify it and people can expect it
to be usable and they can expect it to work. While encouraging
people to move away from finalization is a good thing you have to
give them a workable replacement.
The initiative is to identify and remediate existing uses of
finalization in the JDK.
The primary concern is about subclasses that reply on the
current spec.
If I'm using grepcode correctly[2], it does not show any
subclasses of ThreadPoolExecutor that
override finalize; so it may be a non-issue.
Pardon my skepticism if I don't consider "grepcode" as a
reasonable indicator of whether there are subclasses of TPE that
override finalize. Only a small fraction of source code is in the
open.
And I have to agree with Martin that the current documentation
for finalize() in TPE is somewhat inappropriate. If you deprecate
something you're supposed to point the user to the preferred way
of doing something other than the deprecated method - but there
is none. finalize() in TPE should not have been deprecated until
we provided that alternative.
I think the way forward here would be to:
1. Change TPE.finalize() to final to force any subclasses to
migrate to the new mechanism going forward.
2. Implement the new mechanism - presumably Cleaner - and
document how to achieve the effect of "override to specialize
then call super" (presumably by overriding shutdown() instead).
then in a few releases we remove TPE.finalize() (at the same time
as we remove it from Object).
Cheers,
David
Regards, Roger
[1] https://bugs.openjdk.java.net/browse/JDK-8165641
[2]
http://grepcode.com/search/usages?type=method&id=repository.grepcode.com%24java%24root@jdk%24openjdk@8u40-b25@java%24util%24concurrent@ThreadPoolExecutor@finalize%28%29&k=d
