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

[Roger Riggs]

Hi Mandy,

On 2/22/2016 4:41 PM, Mandy Chung wrote:
On Feb 21, 2016, at 10:55 AM, Peter Levart <peter.lev...@gmail.com> wrote:

I added new method to Cleaner:

    public boolean helpClean() { }

I’m in two minds of making this a public method. An explicit way to enqueue 
pending references as well as invoke Cleanable::clean.  If it goes in, the 
method name needs to be renamed.
I would suggest a name related to cleaning a single Cleanable.

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


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.lev...@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.