Proposal: Fully Concurrent ClassLoading

[David Holmes]

On 11/12/2012 7:20 PM, Peter Levart wrote:
> On 12/11/2012 03:55 AM, David Holmes wrote:
>>> Question on the source code: registerAsFullyConcurrent has confusing
>>> comment -
>>> do the super classes all need to be parallel capable? Or do the super
>>> classes all need
>>> to be FullyConcurrent? I assume the latter, so just fix the comments.
>>
>> Actually it is the former. There's no reason to require that all
>> superclasses be fully-concurrent. Of course a given loaders degree of
>> concurrency may be constrained by what it's supertype allows, but
>> there's no reason to actually force all the supertypes to be
>> fully-concurrent: it is enough that they are at least all parallel
>> capable.
>
> Hi David,
>
> There is one caveat: if ClassLoader X declares that it is
> fully-concurrent and it's superclass Y is only parallel-capable, then X
> will act as fully-concurrent (returning null from
> getClassLoadingLock()). superclass Y might or might not be coded to use
> the getClassLoadingLock(). X therefore has to know how Y is coded. To be
> defensive, X could ask for Y's registration and declare itself as only
> parallel-capable if Y declares the same so that when Y is upgraded to be
> fully-concurrent, X would become fully-concurrent automatically. To
> support situations where the same version of X would work in two
> environments where in one Y is only parallel-capable and in the other Y
> is fully-concurrent, there could be a static API to retrieve the
> registrations of superclasses.

I don't quite follow this. What code in the superclass are you 
anticipating that the subclass will use which relies on the lock? Or is 
this just an abstract "what if" scenario?

Thanks,
David
-----

> Or, to have less impact on future deprecation of old parallel-capable
> registration API, the fully-concurrent registration API:
>
> protected static boolean registerAsFullyConcurrent()
>
> might take a boolean parameter:
>
> protected static boolean registerAsFullyConcurrent(boolean
> downgradeToPrallelCapableIfAnySuperclassIsNotFullyConcurrent)
>
>
> and provide no accessible API to find out what the registration actually
> did (register as parallel-capable or fully-concurrent - return true in
> any case).
>
> Since all JDK provided ClassLoaders will be made fully concurrent, this
> might only be relevant if there is vendor A that currently provides only
> parallel-capable ClassLoader implementation and there is vendor B that
> subclasses A's loader and wants to upgrade and be backward compatible at
> the same time.
>
> Does this complicate things to much for no real benefit?
>
> Regards, Peter
>