RFR: JDK-8149925 We don't need jdk.internal.ref.Cleaner any more

Peter Levart peter.levart at gmail.com
Tue Feb 23 16:35:22 UTC 2016

Hi Roger, Mandy,

Here's another webrev:


I renamed the method and reworded the specification. Is this better now?

On 02/22/2016 10:56 PM, Roger Riggs wrote:
> Hi Mandy,
> On 2/22/2016 4:41 PM, Mandy Chung wrote:
>> The existing way to do that is to register phantom references in your 
>> own ReferenceQueue and then drain the queue at appropriate point.  
>> Would you consider having a method to return ReferenceQueue 
>> maintained by the cleaner instead?
> If the queue is exposed, there is no assurance that the cleanable 
> function would be called.
> Any caller could introduce a bug by not doing the proper cleaning.
> I was more concerned with the crossing of Reference.tryHandlePending 
> with the cleaning thread.
> The method description does not mention anything related to the 
> Reference processing thread
> though that is all implementation.  The @implNote might be a bit more 
> concise and less informal.
> Roger

Yes, ReferenceHandler thread is just an implementation detail. The 
specification of java.lang.Reference subclasses doesn't even mention it. 
It talks about GC enqueueing Reference objects:

"Suppose that the garbage collector determines at a certain point in time...
...At the same time or at some later time it will enqueue those 
newly-cleared weak references that are registered with reference queues."

So in this respect ReferenceHandler is just an extension of GC Reference 
discovery/enqueuing logic, delegated to a background thread on Java 
side. The Cleaner.cleanNextPending() method tries to be true to its 
specification - it tries to invoke next cleanup action for which GC has 
determined that its monitored object is phantom reachable without 
waiting for ReferenceHandler thread to transfer it from pending list to 
the queue.

Since Reference.tryHandlePending is just transfering Reference objects 
from one list to another and does that using two locks (the 
Reference.lock and the ReferenceQueue.lock) but never holds them both 
together or calls any outside code while holding any of the locks, 
there's no danger of dead-locking, if that was your concern.

Regards, Peter

>> Other than this new method, the change looks good to me.
>> Mandy
>>> I think this form of method that just does one quantum of work and 
>>> returns a boolean indicating whether there's more work waiting is a 
>>> better fit for some clients that might want to do just a limited 
>>> amount of work at once (like for example 
>>> sun.java2d.Disposer.pollRemove that you mentioned). This method also 
>>> deals with helping the ReferenceHandler thread, which is necessary 
>>> to be able to "squeeze" out all outstanding work. As Cleaner is in 
>>> the same package as Reference and helping ReferenceHandler thread is 
>>> implicitly included in Cleaner.helpClean(), the JavaLangRefAccess 
>>> interface and a field in SharedSecrets can be removed.
>>> I also simplified the API in sun.nio.fs.NativeBuffer and replaced 
>>> the accessor of Cleanable with a simple void free() method (called 
>>> free because it deallocates memory).
>>> I think this will have to be submitted to CCC for approval, right? 
>>> Can you help me with it?
>>> Regards, Peter
>>> On 02/17/2016 11:34 PM, Kim Barrett wrote:
>>>>> On Feb 17, 2016, at 4:06 AM, Peter Levart <peter.levart at gmail.com> 
>>>>> wrote:
>>>>> Hi Kim,
>>>>> Thanks for looking into this. Answers inline…
>>>> Peter,
>>>> Thanks for the explanations.
>>>>> On 02/17/2016 01:20 AM, Kim Barrett wrote:
>>>>>>> However, I recall Roger saying there were existing tests that
>>>>>> failed when certain uses of sun.misc.Cleaner were replaced with
>>>>>> java.lang.ref.Cleaner. I don't remember the details, but maybe Roger
>>>>>> does.
>>>>> If the failing test was the following:
>>>>> java/nio/Buffer/DirectBufferAllocTest.java
>>>>> ...then it has been taken care of (see Bits.java).
>>>> That looks familiar. And yes, I see what you did there, and I don't
>>>> think Roger's initial prototype testing did anything similar, so
>>>> indeed this is likely the failure he encountered.
>>>> Though I'm still inclined to object to that form of drainQueue (see
>>>> below).
>>>>>> I don't understand why CleanerImpl needs to be changed to public in
>>>>>> order to provide access to the new drainQueue. Wouldn't it be better
>>>>>> to add Cleaner.drainQueue?
>>>>> An interesting idea. But I don't know if such functionality is 
>>>>> generally useful enough to commit to it in a public API.
>>>> java.desktop:sun.java2d.Disposer.pollRemove seems to me to be
>>>> addressing an essentially similar requirement, with
>>>> java.desktop:sun.font.StrikeCache being the user of that API.
>>>> Of course, that's already got all the scaffolding for using phantom
>>>> references, and there's no need to rewrite it to use
>>>> java.lang.ref.Cleaner.  But maybe there are others like this?
>>>> In any case, I really dislike the approach of exposing the CleanerImpl
>>>> object to get at this functionality.
>>>>>> Some of the other changes here don't seem related to the problem at
>>>>>> hand. ...
>>>>> One thing that this change unfortunately requires is to get rid of 
>>>>> lambdas and method references in the implementation and 
>>>>> dependencies of java.land.ref.Cleaner API, because it gets used 
>>>>> early in the boot-up sequence when java.lang.invoke infrastructure 
>>>>> is not ready yet.
>>>> Oh, right!  Bootstrapping issues!
>>>>> The alternative to CleanerCleanable is a no-op Runnable 
>>>>> implementation passed to PhantomCleanableRef constructor. …
>>>> OK.  Now I understand.
>>>>> Making CleanerImpl implement Runnable …
>>>> I'd be fine with this if the CleanerImpl wasn't exposed as part of the
>>>> drainQueue functionality.
>>>>>> ------------------------------------------------------------------------------ 
>>>>>> src/java.base/share/classes/sun/nio/fs/NativeBuffer.java
>>>>>>    76     Cleaner.Cleanable cleanable() {
>>>>>>    77         return cleanable;
>>>>>>    78     }
>>>>>> [pre-existing, but if we're changing things anyway...]
>>>>>> I'm kind of surprised by an accessor function (both here and in the
>>>>>> original code) rather than a cleanup function.  Is there a use case
>>>>>> for anything other than buffer.cleanable().clean()?
>>>>> It can't be, since both old Cleaner and new Cleanable have only 
>>>>> got a single method - clean().
>>>> So this could be replaced with
>>>>    void clean() {
>>>>      cleanable.clean();
>>>>    }
>>>> To me, that seems better.
>>>>>> Similarly for the DirectBuffer interface.
>>>>> This one is a critical method, used by various 3rd party softwares...
>>>> I want to cover my ears when people start talking about some of the
>>>> things that have done…  OK.

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