Hi Dan,
<snip>
It would seem to me all the instant variables used in the rollback and/or
the methods it references would have to be sent arguments to the Runnable
and the same actions rollback does and the methods it calls would have to
be done by the Runnable. Is that correct?
</snip>
Yes, but any non-final variables (like checkpoint) or mutable primitives
(like the counters) can't be used in the Runnable directly as the
reference value may change outside the constructor. They may be wrapped
in another class to get the desired behavior [1]. I think a static
class similar to Tomcat [2] might be better for this. No issues with
passing this.instanceVariable either which made me realize the real
problem was using the lambda to create the Runnable. It was passing a
reference to my test class which caused it not to be phantom reference
anymore. Fun.
<snip>
the actual StandardProcessSession as ultimately its rollback
method has to be called and this method is not static. So how is it
possible for the Runnable registered with the Cleaner not to have a handle
on the StandardProcessSession instance?
</snip>
Right you're not going to be able to use rollback as an instance method in this
case. The cleaner will not have access to the Phantom object directly. So
implementing rollback as a static method or inside the static class might
involve wrapping all the instance variables it needs into a new wrapper object
so you don't have too many parameters. It also would mean that
StandardProcessSession would need to mutate those variables through the wrapper
class rather than directly as it is now.
- Bob
[1] https://gist.github.com/bobpaulin/4cf9706c77b645dce5955bd989cac73e
[2]
https://github.com/apache/tomcat/blob/2bc4d43c9510da7e3da5d509517ad8bcd66739bd/java/org/apache/tomcat/util/net/openssl/OpenSSLContext.java#L163
On 3/19/2025 4:03 PM, Dan S wrote:
Bob,
It would seem to me all the instant variables used in the rollback and/or
the methods it references would have to be sent arguments to the Runnable
and the same actions rollback does and the methods it calls would have to
be done by the Runnable. Is that correct?
On Wed, Mar 19, 2025 at 4:57 PM Dan S <dsti...@gmail.com> wrote:
Bob thanks for your response. I am still confused how to apply the example
you have to the actual StandardProcessSession as ultimately its rollback
method has to be called and this method is not static. So how is it
possible for the Runnable registered with the Cleaner not to have a handle
on the StandardProcessSession instance?
On Tue, Mar 18, 2025 at 10:46 AM Bob Paulin <b...@apache.org> wrote:
Formatting came through poorly. Please see [1]
- Bob
[1] https://gist.github.com/bobpaulin/ce6a83e45cd6bbf34051935324f10c63
On 2025/03/18 14:40:19 Bob Paulin wrote:
Hi
In both cases it appears that finalize is being used as resource clean
up following the object being de-referenced. In both cases finalize is
not the primary means of cleanup but rather a fall-through for
exceptional cases where processing is interrupted. The consequences
are
different in each case in the case of AbstractCacheServer the
cacheServer resource is not released which is a memory concern. In the
case of StandardProcessSession it's a call to rollback which is
important for data integrity. So I'm not sure removal is without
consequences but more so for StandardProcessSession. I also agree that
implementing closeable might not be possible in all cases for
StandardProcessSession.
Cleaner looks like the simplest approach given it's wrapping some of
the
complexity in working with the ReferenceQueue[1]. There are more
sophisticated things we could do using the ReferenceQueue directly (see
[2]). There are some important caveats to implementing this that would
need to be followed to ensure it works. First we can't use references
to the object AbstractCacheServer or StandardProcessSession within the
runnable or it will not trigger (since that would create a reference
thus it would not be Phantom). It may be valuable to create a static
method on each of these classes that requires the constructor objects
to
be passed to it in order to discourage the usage references to the
object. Second we'll want to avoid passing "this.varaibles" to the
static method. Below is some example code for reference. Notice what
happens if you try to pass this.value instead of value. The cleaner is
not called because it's not a phantom ref anymore. So I'm not sure it's
required to pass the cleaning function to an additional class (as
demonstrated below) but it might help keep things cleaner and further
avoid references to the object that we're cleaning from. Hope this is
helpful.
packagecom.bobpaulin.nifi;
importjava.lang.ref.Cleaner;
importjava.lang.ref.PhantomReference;
importjava.lang.ref.ReferenceQueue;
importorg.slf4j.Logger;
importorg.slf4j.LoggerFactory;
publicclassCleanerExample {
privatestaticfinalLogger LOGGER=
LoggerFactory.getLogger(CleanerExample.class);
//Static class just for demo purposes
publicstaticclassStandardProcessorSession {
privatestaticfinalCleaner cleaner= Cleaner.create();
privatefinalCleaner.Cleanable cleanable;
privatefinalString value;
publicStandardProcessorSession(finalString value) {
this.value= value;
finalRunnable cleanUpRunner= () -> rollback(value);
this.cleanable= cleaner.register(this, cleanUpRunner);
}
publicstaticvoidrollback(finalString value) {
System.out.println("Rollback Called. Value is "+ value);
}
}
publicstaticvoidmain(finalString[] args) {
StandardProcessorSession processSession=
newStandardProcessorSession("Hello");
//Needed for testing
finalReferenceQueue<StandardProcessorSession> refQueue=
newReferenceQueue<>();
// Create a phantom reference artificially
finalPhantomReference<StandardProcessorSession> helloPhantomReference=
newPhantomReference<>(
processSession, refQueue);
newThread(() -> {
LOGGER.info("Starting ReferenceQueue consumer thread.");
booleanreferenceNotFound= true;
while(referenceNotFound) {
try{
@SuppressWarnings("unchecked")
finalPhantomReference<StandardProcessorSession> reference=
(PhantomReference<StandardProcessorSession>) refQueue
.remove();
referenceNotFound= false;
LOGGER.info("Finalized has been finalized.");
} catch(finalInterruptedException e) {
// Just for the purpose of this example.
break;
}
}
LOGGER.info("Finished ReferenceQueue consumer thread.");
}).start();
// Lose the strong reference to the object.
processSession= null;
// Attempt to trigger a GC.
System.gc();
}
}
- Bob
[1] https://inside.java/2022/05/25/clean-cleaner/
[2]
https://unitstep.net/blog/2018/03/10/java-phantomreferences-a-better-choice-than-finalize/
On 3/17/2025 4:11 PM, Dan S wrote:
I am currently working on NIFI-14340
<https://issues.apache.org/jira/browse/NIFI-14340> trying to replace
the
overridden methods finalize in both AbstractCacheServer and
StandardProcessSession as
per the javadocs
<
https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/Object.html#finalize()
finalize
is deprecated for removal.
So my first question is whether we would like a replacement for these
finalize methods or is the solution for this ticket is as simple as
just
removing them?
If we would like to replace them based on the suggestions given in
JEP 421
<https://openjdk.org/jeps/421> then I have a few other questions.
The first suggested solution is to make the class implement
java.lang.AutoCloseable and then instantiate and use it in a try with
resources statement within a single lexical scope.
It does not seem to me that is plausible for AbstractCacheServer
which is a
controller service and from what I understand is not used within a
single
lexical scope.
It also does not seem this would work for StandardProcessSession as
there
are cases where instances of ProcessSession handle transferring flow
file(s) and commits in a catch block. If used
in a try resources it would be closed before the catch block code is
called.
The second suggestion is to use Cleaners but that also seems
difficult as
it seems from what I have read the right way to implement this would
be
passing AbstractCacheServer and StandardProcessSession into a
different
class to handle cleaning. I am not sure how that would be done for
these
classes.
Can I please get guidance on how we should proceed regarding these
finalize
methods? Thank you!
