Proposal: Fully Concurrent ClassLoading

David M. Lloyd david.lloyd at
Thu Dec 6 15:06:56 UTC 2012

On 12/06/2012 05:35 AM, Alan Bateman wrote:
> On 05/12/2012 11:59, 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:
> The jdk7 implementation is very unfortunate, it's a pity this wasn't
> caught before 7 shipped.
> I think the proposal is good, it preserves compatibility, and if there
> are loaders calling registerAsParallelCapable now (probably very few)
> then it they may be able to change to using registerAsFullyConcurrent
> without too much work.
> I am tempted to suggest that registerAsParallelCapable should be
> deprecated too.

I'm sorry I missed the original post, and I just want to add my 
wholehearted support for this idea.  Our application server's class 
loader implementation can be configured (on certain JVMs) to run in a 
lock-free mode and we have seen good performance and memory footprint as 
a result on these particular JVMs.

As far as the defineClass concurrent redefine issue goes - our current 
implementation simply does a try/catch for redefinition exceptions.  If 
such an exception occurs, we load the concurrently defined class and 
return that.  In practice, even with many threads, we would see 
relatively few such collisions.  But, a tryDefineClass or 
defineClassIfNotPresent would be a welcome addition for certain.

I'm very glad to see that Mr. Holmes has taken this initiative and found 
solutions to the various stumbling blocks.


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