ReflectionData space optimization in java.lang.Class (JEP-149)

Peter Levart peter.levart at
Thu Dec 13 10:13:26 UTC 2012

Hi Mandy, David and others,

Here's the updated version of the patch for ReflectionData:

I split long lines as proposed by Mandy. I also split reflectionData() 
method in two methods, so the fast-path method is smaller and might get 
better chances of being in-lined.

I also found code-paths that evaluated reflectionData() method more than 
once for each external call. It's the methods:

private Field[] privateGetDeclaredFields(boolean publicOnly)


private Method[] privateGetDeclaredMethods(boolean publicOnly)

which are called from:

private Field[] privateGetPublicFields(Set<Class<?>> traversedInterfaces)


private Method[] privateGetPublicMethods()

respectively. I therefore introduced overloaded variants of the former 
methods taking a ReflectionData parameter like the following:

     private Field[] privateGetDeclaredFields(boolean publicOnly) {
         return privateGetDeclaredFields(publicOnly, reflectionData());
     // A variant called from methods that already obtained 
ReflectionData instance
     private Field[] privateGetDeclaredFields(boolean publicOnly, 
ReflectionData<T> rd) {

the same for privateGetDeclaredMethods. This is not for performance 
reasons (though it might help) but for correctness. Each external call 
should be a separate "transaction" and should work on the same 
ReflectionData instance. The "transaction" is only guaranteed on the 
level of a particular java.lang.Class instance though. Some methods also 
invoke other Class instances (to gather inherited public methods / 
fields) and those invocations might be separate transactions in the face 
of concurrent class re-definitions. But we are not going to implement a 
database here, are we?

I prepared some micro benchmarks for individual public methods. Here're 
the results:

they indicate no noticeable performance decreases. Some methods are in 
fact faster (more in-linable?):

- getFields - 4x
- getDeclaredConstructor - 4x ... 10x
- getDeclaredMethod - 3x

Here's the code for micro-benchmarks:

Regards, Peter

On 12/12/2012 11:59 PM, Mandy Chung wrote:
> Hi Peter,
> On 12/12/12 4:31 AM, Peter Levart wrote:
>> Hi all,
>> Ok, no problem. So here's a patch that summarizes what I did in the 
>> previous patch, but only for reflective fields (Field[], Method[], 
>> Constructor[]) not including annotations:
> Finally able to make time to review this patch.  It's good work. While 
> it's good to see the synchronization issue with annotations be fixed, 
> separating the cache for reflection and annotation helps.  As David 
> replied, he will take your patch and run with it for JEP-149.
> The change looks good.  Nit: there are several long lines 
> L2235,2244,2245,2249,2269 etc that should be broken into separate 
> lines.  The remaining open question is the performance difference in 
> the reflectionData() method and how well it will be jit'ed.  In the 
> common case, there is no class redefinition where 
> classCachesOnClassRedefinition() is essentially like an nop.  I 
> believe David will look at the footprint and performance numbers as he 
> has initiated the performance testing (need to do it with this new 
> patch).
> Thanks
> Mandy
>> The annotations part is unchanged semantically, but I had to:
>> - modify private method clearCachesOnClassRedefinition to only 
>> include invalidation of annotations and declaredAnnotations fields. I 
>> also renamed it to clearAnnotationCachesOnClassRedefinition
>> - renamed lastRedefinedCount to lastAnnotationsRedefinedCount and, 
>> since this field is now only accessed while holding a lock (from 
>> synchronized initAnnotationsIfNecessary), I removed the volatile 
>> keyword.
>> That's it.
>> While looking at this unchanged part of code some more, I see other 
>> races as well. For example: two concurrent accesses to annotations 
>> combined with redefinition of a class can result in NPE. Here's a 
>> serial execution:
>> Thread 1:
>>     getAnnotation(annotationType);
>>         initAnnotationsIfNecessary();
>> VM:
>>     classRedefinedCount++;
>> Thread 2:
>>     getAnnotation(annotationType);
>>         initAnnotationsIfNecessary();
>>             clearAnnotationCachesOnClassRedefinition();
>>                 annotations = null;
>> Thread 1:
>>         return AnnotationSupport.getOneAnnotation(annotations, 
>> annotationClass);
>>         // 'annotations' field can be null
>> So this needs to be fixed sooner or later.
>> Joel!
>> Are your JSR 308 canges involving java.lang.Class too?
>> Regards, Peter
>> On 12/12/2012 11:59 AM, Joel Borggrén-Franck wrote:
>>> Hi all,
>>> First, thanks all of you that are involved in this!
>>> I agree with David here, lets split this up (for now) and do 
>>> reflective objects in the context of jep-149 and annotations 
>>> separately.
>>> As you may know there are even more annotation coming in with JSR 
>>> 308 annotations on type (use), so I want to complete that work first 
>>> and then do the effort of reducing contention and overhead for both 
>>> type and regular annotations and also fixing up the behaviour for 
>>> redefine/retransform class.
>>> One other point to consider is that some of the fields in 
>>> java/lang/reflect/ classes are known by the VM so not all changes in 
>>> Java-land are actually doable. Glancing over your patches very 
>>> quickly I don't think you have done anything to upset the VM, but 
>>> then I am not an expert in this area.
>>> Also, with the VM permgen changes we might have to rethink our 
>>> assumptions in order to reduce total overhead. For example as I 
>>> understand it previously we could just ship the same pointer into 
>>> permgen for the annotations arrays, now that isn't possible so we 
>>> create a new copy of the array for every Field/Method/Constructor 
>>> instance. Perhaps there is some clever way of eliminating those copies.
>>> So while I think your patches generally makes sense, I think it is 
>>> prudent to delay this for annotations until all our new annotation 
>>> features are in.
>>> cheers
>>> /Joel
>>> On Dec 10, 2012, at 7:18 AM, David Holmes <david.holmes at> 
>>> wrote:
>>>> Hi Peter,
>>>> Sorry for the delay on this.
>>>> Generally your VolatileData and my ReflectionHelper are doing a 
>>>> similar job. But I agree with your reasoning that all of the cached 
>>>> SoftReferences are likely to be cleared at once, and so a 
>>>> SoftReference to a helper object with direct references, is more 
>>>> effective than a direct reference to a helper object with 
>>>> SoftReferences. My initial stance with this was very conservative 
>>>> as the more change that is introduced the more uncertainty there is 
>>>> about the impact.
>>>> I say the above primarily because I think, if I am to proceed with 
>>>> this, I will need to separate out the general reflection caching 
>>>> changes from the annotation changes. There are a number of reasons 
>>>> for this:
>>>> First, I'm not at all familiar with the implementation of 
>>>> annotations at the VM or Java level, and the recent changes in this 
>>>> area just exacerbate my ignorance of the mechanics. So I don't feel 
>>>> qualified to evaluate that aspect.
>>>> Second, the bulk of the reflection caching code is simplified by 
>>>> the fact that due to current constraints on class redefinition the 
>>>> caching is effectively idempotent for fields/methods/constructors. 
>>>> But that is not the case for annotations.
>>>> Finally, the use of synchronization with the annotations method is 
>>>> perplexing me. I sent Joe a private email on this but I may as well 
>>>> raise it here - and I think you have alluded to this in your 
>>>> earlier emails as well: initAnnotationsIfNecessary() is a 
>>>> synchronized instance method but I can not find any other code in 
>>>> the VM that synchronizes on the Class object's monitor. So if this 
>>>> synchronization is trying to establish consistency in the face of 
>>>> class redefinition, I do not see where class redefinition is 
>>>> participating in the synchronization!
>>>> So what I would like to do is take your basic VolatileData part of 
>>>> the patch and run with it for JEP-149 purposes, while separating 
>>>> the annotations issue so they can be dealt with by the experts in 
>>>> that particular area.
>>>> I'm sorry it has taken so long to arrive at a fairly negative 
>>>> position, but I need someone else to take up the annotations 
>>>> gauntlet and run with it.
>>>> Thanks,
>>>> David

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