Proposal: Fully Concurrent ClassLoading (original) (raw)

Zhong Yu zhong.j.yu at gmail.com
Wed Dec 12 16:24:05 UTC 2012

On Wed, Dec 12, 2012 at 10:05 AM, Peter Levart <peter.levart at gmail.com> wrote:

On 12/12/2012 04:51 PM, Zhong Yu wrote:

If a class loader is declared fully concurrent, yet getClassLoadingLock() is invoked, what's the harm of returning a dedicated lock anyway, exactly like what's done before? To encourage people to not use locking in the first place ;-)

In that case throwing an exception is probably better than returning a null.

No, seriously, to be able to remove the deprecated feature in the future, perhaps.

Peter

On Tue, Dec 11, 2012 at 7:40 PM, David Holmes <david.holmes at oracle.com> wrote:

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. Thanks, David 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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