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

[Roger Riggs]

On Thu, 7 Jul 2022 22:41:55 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 515:
> 
>> 513:      *         {@code null} if there was no mapping for {@code key}.
>> 514:      *         (A {@code null} return can also indicate that the map
>> 515:      *         previously associated {@code null} with {@code key}.)
> 
> Need to specify: A null return can also indicate the entry is discarded if the value policy is DISCARD and the key is a value object.

A subtle point...  The WeakHashmap *may* contain an identity class instance that compares as `equals()` with the value object. In that case, the instance would be returned from `getEntry`.
Similarly, `put()` would return the previous entry (that was `equals`) to a value entry that is discarded.
This will need some more spec detail.

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