RFR: 8078641: MethodHandle.asTypeCache can retain classes from unloading [v3]

[Mandy Chung]

On Thu, 2 Sep 2021 11:35:52 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

>> `MethodHandle.asTypeCache` keeps a strong reference to adapted `MethodHandle` and it can introduce a class loader leak through its `MethodType`.
>> 
>> Proposed fix introduces a 2-level cache (1 element each) where 1st level can only contain `MethodHandle`s which are guaranteed to not introduce any dependencies on new class loaders compared to the original `MethodHandle`. 2nd level is backed by a `SoftReference` and is used as a backup when the result of `MethodHandle.asType()` conversion can't populate the higher level cache.  
>> 
>> The fix is based on [the work](http://cr.openjdk.java.net/~plevart/jdk9-dev/MethodHandle.asTypeCacheLeak/) made by Peter Levart @plevart back in 2015.
>> 
>> Testing: tier1 - tier6
>
> Vladimir Ivanov has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Address review comments

This looks good to me.  

For the change of `MethodHandle::asType` to a final method, this needs a CSR.   Is this spec change intentional?  I wonder if `MethodHandle` should be a sealed class instead.   In any case, maybe you can consider the spec change as a separate issue.

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PR: https://git.openjdk.java.net/jdk/pull/5246