RFR JDK-8011940 : java.lang.Class.getAnnotations() always enters synchronized method

[Peter Levart]

Hi Joel and others,

Here's a 3rd revision of this proposed patch:

http://cr.openjdk.java.net/~plevart/jdk8-tl/AnnotationData/webrev.03/

I used LinkedHashMap for annotations in this one. It means that now even 
.getAnnotations() are reported in "declaration order": 1st inherited 
(includes overridden) then declared (that are not overriding). For 
example, using @Inherited annotations A1, A2, A3:

@A1("C")
@A2("C")
class C {}

@A1("D")
@A3("D")
class D extends C {}

D.class.getAnnotations() now returns: { @A1("D"), @A2("C"), @A3("D") } ...

The LHM constructor uses the following expression to estimate the 
initial capacity of the LHM:

3326                         annotations = new LinkedHashMap<>((Math.max(
3327                                 declaredAnnotations.size(),
3328                                 Math.min(12, declaredAnnotations.size() + superAnnotations.size())
3329                             ) * 4 + 2) / 3
3330                         );


I think this strikes some balance between effectivity and accuracy of 
estimation. I could pre-scan the superclass annotations and calculate 
the exact final size of the annotations Map before constructing it 
though. Tell me if this is worth the effort.

I also created a test that tests 3 things:
- class annotation inheritance
- order of class annotations reported by .getAnnotations() and 
.getDeclaredAnnotations() methods (although not specified, the order is 
now stable and resembles declaration order)
- behaviour of class annotation caching when class is redefined


Regards, Peter

On 08/13/2013 09:52 AM, Joel Borggrén-Franck wrote:
> Hi,
>
> Comments inline,
>
> On 12 aug 2013, at 14:40, Peter Levart <peter.levart at gmail.com> wrote:
>> On 08/12/2013 12:54 PM, Joel Borggren-Franck wrote:
>> - annotation (@interface)  declarations can themselves be redefined (for example, defaults changed). Such redefinitions don't affect already initialized annotations. Default values are cached in AnnotationType which is not invalidated as a result of class redefinition. Maybe it should be. And even if AnnotationType was invalidated as a result of class redefinition, the defaults are looked-up when particular annotations are initialized and then combined with parsed values in a single values map inside each annotation instance (proxy), so invalidating AnnotationType would have no effect on those annotations.
>>
>>> 3326                 if (annotations == null) { // lazy construction
>>> 3327                     annotations = new HashMap<>();
>>> 3328                 }
>>>
>>> I think this should be a LinkedHashMap, so that iteration is predictable
>>> (I know it isn't in the current code). Also the size of the map is known
>>> so you can use a constructor with an explicit initial capacity.
>> Right, AnnotationParser does return LinkedHashMap, so at least decalredAnnotations are in parse-order. I will change the code to use LinkedHashMap for inherited/combined case too.
> Thanks.
>
>> The number of annotations that end-up in inherited/combined Map is not known in advance. I could make a separate pre-scan for superclass annotations that are @Inheritable and don't have the same key as any of declared annotations and then sum this count with declared annotations count, but I don't think this will be very effective. I could allocate a large-enough Map to always fit (the count of superclass annotations + the count of declared annotations), but that could sometimes lead to much over-allocated Maps. I could take the min(DEFAULT_CAPACITY, superclass annotations count + declared annotations count) as the initial capacity for the Map. What do you think which of those 3 alternatives is the best?
> My bad. I was thinking of the case where every inherited annotation was overridden, in that case annotations.size() == declaredAnnotations.size(). That is of course not generally true. I'm fine with handling this as a follow up since the situation is no worse than today and the surrounding code is better. However,
>
> 1) We should really measure this.
> 2) My gut feeling is that the ratio of not overridden inherited annotations to declared annotations is small IE the expected nr of annotations is much closer to declare annotations than to declared + superclass annotations.
> 3) The default initial capacity is 16 and load factor is 0.75. I do think the mean nr of annotations is closer to 3 than to 12. We are probably wasting some space here.
>
> Perhaps use min(default cap, (total annotations/0.75 (+ 1?))) for now as that will make the situation no worse than today and often better?
>
>>> Since this is a fairly significant rewrite I think it might be good to
>>> make sure our tests exercise the new code. Can you run some kind of
>>> coverage report on this?
>> I successfully ran the jdk_lang jtreg tests which contain several tests for annotations.
> I'm a bit worried these tests doesn't cover annotations + class redefine. But I might be persuaded to have more extensive testing as a follow up as well.
>
> cheers
> /Joel