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
