RFR [9] 8140687: Move @Contended to the jdk.internal.vm.annotation package

[Chris Hegarty]

On 11 Nov 2015, at 13:11, Doug Lea <dl at cs.oswego.edu> wrote:

> On 11/11/2015 07:06 AM, Chris Hegarty wrote:
> 
>>>>>> http://cr.openjdk.java.net/~chegar/8140687/00/
>>>>> 
>>> 
>>> While this is up for review, could someone please explain
>>> again why @Contended should not be a public annotation?
>> 
>> Sorry Doug, I don’t have the answer to your question. I was
>> not involved in the original @Contended support. Since it
>> was added in JDK 8, we have some experience with it now,
>> so  maybe this would help influence any future discussion on
>> its possible inclusion in Java SE.
>> 
> 
> Sorry to be a pest about this.

Not a problem. This is a good discussion to have now.

> But, as I should have noticed
> before, taking away sun.misc.Contended without providing any
> alternative for non-JDK users seems to be the only exception to
> the stated policy of only removing functionality that has a jdk9
> equivalent, or else making it public. If this exception was not
> deliberate, then it is better to reconsider this now vs later.

JEP 260 [1] lists a number of “critical” internal API’s that will remain
accessible in JDK 9. It specifically calls out the following:

  - sun.misc.Cleaner
  - sun.misc.{Signal,SignalHandler}
  - sun.misc.Unsafe
  - sun.reflect.Reflection::getCallerClass
  - sun.reflect.ReflectionFactory

 "Suggested additions to this list, justified by real-world use cases
   and estimates of developer and end-user impact, are welcome.”

Are you suggesting that @Contended be added to this list?

If @sun.misc.Contended remains as is, and no new standard Java
SE API is defined, then an alias or similar will be required. The
reason for this is that eventually the ‘sun.misc' package will be split
out into a new module “jdk.unsupported”, and the base module, 
which is a consumer of @Contended, cannot have a dependency
on any other module. This work is possible, just requires a different
solution.

So I think the questions we need to answer are as follows:
 a) Is it desirable to have @Contended as part of Java SE?
    i) If so, is this doable for JDK 9?
 b) If ‘No’ to a or i, do we consider @Contended “critical”,
      as per JEP 260 ? 

-Chris.

[1] https://bugs.openjdk.java.net/browse/JDK-8132928