RFR: 6983726: Reimplement MethodHandleProxies.asInterfaceInstance [v20]

[Mandy Chung]

On Tue, 4 Jul 2023 14:14:03 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> Chen Liang has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 44 commits:
>> 
>>  - Merge branch 'master' into explore/mhp-iface
>>  - stage
>>    
>>    Signed-off-by: liach <liach at users.noreply.github.com>
>>  - Review comments
>>  - Code cleanup, thanks mandy!
>>  - Merge branch 'master' of https://github.com/openjdk/jdk into explore/mhp-iface
>>  - 1. Change WRAPPER_TYPES to WeakHashMap to accurately determine if the given class is
>>       the proxy class
>>    
>>    Discussion:
>>    2. I dropped ProxyClassInfo and use Lookup just to see the simplication.
>>       If wrapperInstanceTarget and wrapperInstanceType are frequently called, it makes
>>       sense to cache the method handles.
>>    
>>    3. Should it use SoftReference or WeakReference?  It depends if asInterfaceInstance
>>       will be used heavily.
>>    
>>    3. I also dropped SamInfo and getStats as it can be inlined in the caller, which
>>       I think it's clearer to see what it does in place.
>>  - SecurityManager fixed, minimize changes
>>  - Merge branch 'master' into explore/mhp-iface
>>  - Some changes per Mandy's review. SecurityManager test fails in this patch
>>  - Merge branch 'master' into explore/mhp-iface
>>  - ... and 34 more: https://git.openjdk.org/jdk/compare/0e3d91dc...44e62271
>
> src/java.base/share/classes/java/lang/invoke/MethodHandleProxies.java line 324:
> 
>> 322:             r = PROXY_LOOKUPS.get(intfc);
>> 323:         }
>> 324:         return r.get();
> 
> This doesn't look right to me. AFAICT the Lookup can be GC'd before we call `get`.
> 
>> No, it wouldn't. PROXY_LOOKUPS.get(intfc) returns the WeakReference of the lookup class which is strongly reachable in the stack frame.
> 
> I'm not sure how the lookup class (`intfc` ?) is keeping the `Lookup` alive here?

Sorry I re-read Chen's question and missing the last half of the sentence.   My reply was about:  If the referent is not GC'ed, `r.get()` returns non-null which will be returned to the caller.  If the referent becomes weakly reachable, `r.get()` will return null.

OK.  In any case, I was thinking to use a local variable to keep the lookup object strongly reachable.   I adjusted the patch to keep a class value of a holder of `WeakReference` ([ab3d8b2](https://github.com/mlchung/jdk/commit/ab3d8b2f086dfac4f8fda1ddce415270c0c53d2b)):


    private static Lookup getProxyClassLookup(Class<?> intfc) {
        WeakReferenceHolder<Lookup> r = PROXY_LOOKUPS.get(intfc);
        Lookup lookup = r.get();
        if (lookup == null) {
             // If the referent is cleared, create a new value and update cached weak reference.
            lookup = newProxyLookup(intfc);
            r.set(lookup);
        }
        return lookup;
    }

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PR Review Comment: https://git.openjdk.org/jdk/pull/13197#discussion_r1252356427