RFR 9: 8138696 : java.lang.ref.Cleaner - an easy to use alternative to finalization

[Roger Riggs]

Hi Mandy,

On 11/2/2015 2:53 PM, Mandy Chung wrote:
>> On Oct 20, 2015, at 11:28 AM, Roger Riggs <roger.riggs at oracle.com> wrote:
>>
>> Updated Javadoc:
>>   http://cr.openjdk.java.net/~rriggs/cleaner-doc/
> I’m happy with this API to provide an easy way to migrate away from finalizers.  Some thoughts:
> 1. For an existing finalizer (say cleanup function), it will be replaced with
>       Cleaner.create().phantomCleanable(this, this::cleanup);
Not quite,  It can't reference this;  the cleanup method must not have 
any reference to the object
being cleaned up because the Cleanable holds a strong reference to the 
cleanup state and would keep
it from being xxx unreachable.
The cleaning function must only refer to the state (and behavior) that 
does the cleaning.
It could be a static method  with all the state in the arguments.
Or a instance method on a object holding only the cleanup state.
>
> It may be useful to provide a sharable Cleaner instance that doesn’t go away.
yes, for the use cases in java.base, there will be a common cleaner in 
the jdk.internal.misc package.
>
> The primary use for this Cleaner API is to replace finalizers.  I wonder it worths having a convenient method easier to relate to finalizer (say Cleaner.finalizable?)
Not just finalizers; I would stay away from the term finalizer to avoid 
overloading the understanding.
Finalization is specific specification defined semantics.
>
> 2. Do you know any existing use of soft / weak references in the JDK can benefit from this automatic cleanup mechanism? The typical cache implementation using soft/weak references is to purge the entries when the cache is accessed and manages the reference queue without a background thread.
Peter Levart offered an example of a self cleaning Weak key'ed collection.
It would be nice to have a ConcurrentHashMap with Weak keys.

>
> I’m in two minds that it sounds sensible for this Cleaner API to extend for soft/weak references whereas I am not certain of the use cases.   Anyone can share the known use cases that would be helpful.

Besides the cases already mentioned on the thread...  Anyone?

>
>> Updated Webrev:
>>    http://cr.openjdk.java.net/~rriggs/webrev-cleaner-8138696/
>
>    57     final PhantomCleanable<?> phantomCleanableList;
>    59     final WeakCleanable<?> weakCleanableList;
>    61     final SoftCleanable<?> softCleanableList;
>
> - is there any benefit of keeping separate list for each reference type?  Each cleaner has one single ReferenceRueue and it seems that keeping one single Cleanable list could achieve similar performance in most cases while the implementation might be simplified.
Since each type has to extend the corresponding Reference subtype; the 
code was easier to understand
and maintain with separate queues.  There were several iterations using 
different approaches suggested
during the earlier reviews.
>
>
>   111         thread.setPriority(Thread.MAX_PRIORITY);
>
> VM threads are near max priority.  The reference handler thread is MAX_PRORITY whereas the finalizer thread is MAX_PRIORITY-2.
I'll change it to be equal to the finalizer priority;  MAX_PRIORITY - 2 
for the default ThreadFactory
> I think the cleaner thread should not be of MAX_PRIORITY and also perhaps the caller may need a way to get the thread associated with a Cleaner so that users can set the priority if needed?
The priority should be set by the ThreadFactory;  if an application 
needs to control the thread
it can/should provide a custom ThreadFactory; that will keep the API clean.

The setting of the default thread priority should be in the default 
thread factory.

Thanks, Roger

ps.  Webrev: http://cr.openjdk.java.net/~rriggs/webrev-cleaner-8138696/

>
> Mandy
>
>