Review request JDK-8004729: Parameter Reflection API

[Peter Levart]

On 01/10/2013 11:29 PM, Eric McCorkle wrote:
> Good catch there.  I made the field volatile, and I also did the same
> with the cache fields in Parameter.
>
> It is possible with what exists that you could wind up with multiple
> copies of identical parameter objects in existence.  It goes something
> like this
>
> thread A sees Executable.parameters is null, goes into the VM to get them
> thread B sees Executable.parameters is null, goes into the VM to get them
> thread A stores to Executable.parameters
> thread B stores to Executable.parameters
>
> Since Parameters is immutable (except for its caches, which will always
> end up containing the same things), this *should* have no visible
> effects, unless someone does == instead of .equals.
>
> This can be avoided by doing a CAS, which is more expensive execution-wise.
>
> My vote is to *not* do a CAS, and accept that (in extremely rare cases,
> even as far as concurrency-related anomalies go), you may end up with
> duplicates, and document that very well.
>
> Thoughts?
We can not guarantee the singularity of a Parameter instance (like for 
example Class instance) because of various reasons, among them:
- the Method/Constructor instances that are available via public API are 
allways copied and caching of Parameter instances is performed on each 
individual copied Method/Constructor instance. I already had a patch 
which moved caching of annotations, for example, to the "root" instance, 
but it has been postponed because of big anticipated annotations changes 
comming in. Maybe we should revisit the same also for Parameter 
instances when time comes...
- even if caching of Parameters/annotations is performed on "root" 
instances, the root instances can change over time because they are 
cached in j.l.Class via a SoftReference which can be cleared and new 
"root" instances can be created for the same methods/constructors.

So as currently stands, the effort to do CAS for Parameters is useless.

Regards, Peter

>
> On 01/10/13 16:10, Peter Levart wrote:
>> Hello Eric,
>>
>> I have another one. Although not very likely, the reference to the same
>> Method/Constructor can be shared among multiple threads. The publication
>> of a parameters array should therefore be performed via a volatile write
>> / volatile read, otherwise it can happen that some thread sees
>> half-initialized array content. The 'parameters' field in Executable
>> should be declared as volatile and there should be a single read from it
>> and a single write to it in the privateGetParameters() method (you need
>> a local variable to hold intermediate states)...
>>
>> Regards, Peter
>>
>> On 01/10/2013 09:42 PM, Eric McCorkle wrote:
>>> Thanks to all for initial reviews; however, it appears that the version
>>> you saw was somewhat stale.  I've applied your comments (and some
>>> changes that I'd made since the version that was posted).
>>>
>>> Please take a second look.
>>>
>>> Thanks,
>>> Eric
>>>
>>>
>>> On 01/10/13 04:19, Peter Levart wrote:
>>>> Hello Eric,
>>>>
>>>> You must have missed my comment from the previous webrev:
>>>>
>>>>   292     private Parameter[] privateGetParameters() {
>>>>   293         if (null != parameters)
>>>>   294             return parameters.get();
>>>>
>>>> If/when the 'parameters' SoftReference is cleared, the method will be
>>>> returning null forever after...
>>>>
>>>> You should also retrieve the referent and check for it's presence before
>>>> returning it:
>>>>
>>>> Parameter[] res;
>>>> if (parameters != null && (res = parameters.get()) != null)
>>>>      return res;
>>>> ...
>>>> ...
>>>>
>>>> Regards, Peter
>>>>
>>>> On 01/09/2013 10:55 PM, Eric McCorkle wrote:
>>>>> Hello,
>>>>>
>>>>> Please review the core reflection API implementation of parameter
>>>>> reflection.  This is the final component of method parameter reflection.
>>>>>    This was posted for review before, then delayed until the check-in for
>>>>> JDK-8004728 (hotspot support for parameter reflection), which occurred
>>>>> yesterday.
>>>>>
>>>>> Note: The check-in of JDK-8004728 was into hsx/hotspot-rt, *not*
>>>>> jdk8/tl; therefore, it may be a while before the changeset makes its way
>>>>> into jdk8/tl.
>>>>>
>>>>> Also note: since the check-in of JDK-8004727 (javac support for
>>>>> parameter reflection), there has been a failure in the tests for
>>>>> Pack200.  This is being addressed in a fix contributed by Kumar, which I
>>>>> believe has also been posted for review.
>>>>>
>>>>> The open webrev is here:
>>>>> http://cr.openjdk.java.net/~coleenp/JDK-8004729
>>>>>
>>>>> The feature request is here:
>>>>> http://bugs.sun.com/view_bug.do?bug_id=8004729
>>>>>
>>>>> The latest version of the spec can be found here:
>>>>> http://cr.openjdk.java.net/~abuckley/8misc.pdf
>>>>>
>>>>>
>>>>> Thanks,
>>>>> Eric