Proposal: Fully Concurrent ClassLoading

David Holmes david.holmes at
Wed Dec 12 01:40:22 UTC 2012

On 11/12/2012 9:58 PM, Peter Levart wrote:
> On 12/11/2012 12:27 PM, David Holmes wrote:
>> Peter,
>> You are convincing me that all superclasses must be fully concurrent
>> too. Otherwise we are just trying to second-guess a whole bunch of
>> what-ifs. :)
> If you think some more, yes. The superclass might not use
> getClassLoadingLock() but rely on the fact that findClass() is allways
> called under a guard of per-class-name lock, for example. It's a matter
> of how far to go to prevent such miss-behaving fully-concurrent
> subclasses. So far to also prevent fully-concurrent subclasses that
> would otherwise be perfectly correct?
> Maybe not. Creating custom ClassLoaders is not an average programmer's
> job. Those that do this things will of course study the implementations
> of superclasses they extend and do the right thing. And it's reasonable
> to expect that they more or less will only extend JDK's ClassLoaders -
> but on the other hand if they only extend JDK's class loaders, they are
> not prevented to be fully-concurrent either way. Hm...

Again I think it is just too hard to try and second-guess how a 
parallel-loader might rely on the per-class locks (I actually don't see 
any reasonable use for them beyond flow-control), and then how a 
concurrent loader subclass might need to modify things.

If we simply disallow this then we can relax that constraint in the 
future if valid use-cases turn up for that capability. Of course if 
someone has a valid use-case during this discussion phase then of course 
that will influence the decision.


> Peter
>> Thanks,
>> David
>> On 11/12/2012 7:44 PM, Peter Levart wrote:
>>> On 12/11/2012 10:29 AM, David Holmes wrote:
>>>> 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?
>>> This is more or less "what if". There might be a subclass Y of say
>>> java.lang.ClassLoader that overrides loadClass or findClass, declares
>>> that it is parallel-capable and in the implementation of it's loadClass
>>> or findClass, uses getClassLoadingLock() to synchronize access to it's
>>> internal state. Now there comes class X extends Y that declares that it
>>> is fully-concurrent. Of course this will not work, X has to declare that
>>> it is parallel-capable, because Y uses getClassLoadingLock().
>>> What I suggested in the next message is to not change the registration
>>> API but rather provide getClassLoadingLock() that returns non-null locks
>>> when any of the superclasses declare that they are only
>>> parallel-capable, not fully-concurrent.
>>> Regards, Peter
>>>> 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

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