Proposal: Fully Concurrent ClassLoading (original) (raw)

David Holmes david.holmes at oracle.com
Mon Dec 10 04:08:51 UTC 2012

Thanks for the comments Mandy.

On 8/12/2012 9:46 AM, Mandy Chung wrote:

On 12/5/12 3:59 AM, David Holmes wrote:

Java 7 introduced support for parallel classloading by adding to each class loader a ConcurrentHashMap, referenced through a new field, parallelLockMap. This contains a mapping from class names to Objects to use as a classloading lock for that class name. This scheme has a number of inefficiencies. To address this we propose for Java 8 the notion of a fully concurrent classloader ...

This is a fairly simple proposal that I've written up as a blog entry: https://blogs.oracle.com/dholmes/entry/parallelclassloadingrevisitedfullyconcurrent

It's a good and simple proposal and also handles the backward compatibility nicely. I think having getClassLoadingLock() to return null for a fully concurrent loader is a good idea. In case if any code synchronizes on the returned value after migrating to fully concurrent class loader, it will catch such usage (it might be very rare)/

The only glitch with this is that a concurrent loader is-a parallel loader, so anyone trying work with an arbitrary parallel loader will encounter a problem with a concurrent loader if they don't make their code concurrent-loader aware. I am hopeful that the parallelLockMap is only being used internally by the classloaders themselves, and that there are no, or very few, external clients.

I agree with Alan's suggestion - we should consider deprecating registerAsParallelCapable.

Definitely a consideration, though as I said perhaps for 9 rather than 8.

Thanks, David

Mandy

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