[lworld] RFR: 8286824: [valhalla] WeakHashMap for value classes

[Roger Riggs]

On Thu, 7 Jul 2022 22:37:51 GMT, Mandy Chung <mchung at openjdk.org> wrote:

>> WeakHashMap uses WeakReference for keys, allowing it to retain only keys and values that are strongly referenced elsewhere.
>> 
>> The behavior of WeakReference (or any Reference class) is not defined for value objects.
>> 
>> Four (4) behaviors are proposed for WeakHashMap to give the developer a choice:
>>  - SOFT - Keys that are value objects are retained until memory pressure clears soft references; then the keys that are value objects are removed automatically
>>  - STRONG - Keys are value objects are retained until they are explicitly removed
>>  - THROW - Keys that are value objects can not be put into the map; UnsupportedOperationException is thrown
>>  - DISCARD - Keys that are value objects are silently discarded; as if they were removed immediately without being visible
>> 
>> The default is SOFT - key/values are removed when there is memory pressure.
>> Entries will be removed automatically at some point, though less deterministically than with WeakReferences.
>
> src/java.base/share/classes/java/util/WeakHashMap.java line 46:
> 
>> 44:  * mapping for a given key will not prevent the key from being discarded by the
>> 45:  * garbage collector, that is, made finalizable, finalized, and then reclaimed.
>> 46:  * For keys that are {@linkplain Class#isValue() Value objects}, the retention of the
> 
> should this be "value objects" rather than "Value objects"?  This comment applies to all occurrences.

I was thinking there needs to be a link target that explains value objects. Class.isValue doesn't see quite the right one. But verbiage will be needed somewhere.

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PR: https://git.openjdk.org/valhalla/pull/718