Proposal: Fully Concurrent ClassLoading

Peter Levart peter.levart at
Tue Dec 11 11:58:24 UTC 2012

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...


> 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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