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

[mark.reinhold at oracle.com]

2015/10/8 1:41 -0700, roger.riggs at oracle.com:
> Webrev:
> http://cr.openjdk.java.net/~rriggs/webrev-cleaner-extensible-8138696/
> 
> javadoc:
>     http://cr.openjdk.java.net/~rriggs/cleaner-doc2/

Roger -- thanks for taking this on.  The more we can do to get people
to stop using sun.misc APIs, the better.

A couple of comments/questions:

First, I think the approach in your first version is, well, cleaner.
The additional abstract classes in the second version look like a
classic case of implementation inheritance that's not subtype
inheritance, what with the overrides of the original enqueue and
isEnqueued methods to throw UnsupportedOperationException.

I understand the desire to avoid allocating too many objects, but
do we have actual use cases where that's critical?  The original
sun.misc.Cleaner has been used by both internal and external code
to do relatively heavy-weight things like unmap direct buffers and
release other kinds of native resources, which suggests that
allocating three small objects per cleaner is not a problem.

Second, the original sun.misc.Cleaner only handles phantom references.
What are the use cases for weak and soft cleaners?

Finally, how important is it to be able to unregister a cleaner?  In
all the years we've had sun.misc.Cleaner that capability has never
been needed, and leaving it out would simplify the API.

- Mark