Proposal: Fully Concurrent ClassLoading (original) (raw)

David Holmes david.holmes at oracle.com
Mon Dec 10 03:53:21 UTC 2012

Hi Peter,

I think your middle-ground sounds like it may have been a better choice for JDK 7. But for this proposal I want to try and leave parallel loaders behind and move to the fully concurrent model.

The real crux of my proposal is how often you really do get concurrent load attempts and so waste effort doing concurrent findClass for the same class. Obviously you can easily construct the pathological microbenchmark for this, but I'd like to base this on real experiences.

The question then is whether some additional flow-control can be added to the fully concurrent model, if it proves necessary.

Thanks, David

On 6/12/2012 9:32 PM, Peter Levart wrote:

Hi David,

What about a middle-ground mode where findLoadedClass() and delegation to parent would be called without locks and only findClass() would be guarded by a per-class-name-per-classloader lock? In this mode concurrent attempts to define the same class would still be possible, but the possibility would be much lower. Usually findClass() involves non-ignorable level of resource-intensive work (ZIP, IO, NET) before it calls defineClass()... Also, in this mode, the Map of locks could be implemented so that lock objects would be Weak(ly)Reference(d), since requests for already loaded classes would be satisfied by findLoadedClass() already. We'd just need a ConcurrentWeakValuesHashMap. There must be something I'm missing here, isn't it? Regards, Peter On 12/05/2012 12:59 PM, 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

Please discuss this proposal here. Thanks, David Holmes

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