[lworld] RFR: JDK-8251986: [lworld] implement Class::valueType and Class::referenceType in Java

[Roger Riggs]

On Tue, 18 Aug 2020 19:47:36 GMT, Mandy Chung <mchung at openjdk.org> wrote:

> Now that javac generates the reference projection class as a sealed type,
> this patch reimplements  `Class::valueType` and `Class::referenceType` in Java.
> VM no longer has any knowledge of the value projection type and reference projection type.
> 
> This patch includes a temporary workaround to disable the preview feature check
> for sealed types because inline types are not (yet) a preview feature in valhalla repo.
> This workaround should be removed when it's time to make inline class a preview feature.

src/java.base/share/classes/java/lang/Class.java line 631:

> 630:         } else if (isReferenceProjectionType()) {
> 631:             projectionTypes = new Class<?>[] { valueProjectionType(), this };
> 632:         } else {

It seems odd that isReferenceProjectType() uses projectionTypes before it is initialized (or is it re-initialized).
(Though there is a check for null).
The logic is not very straightforward; though I haven't looked at how to make it easier to follow.

src/java.base/share/classes/java/lang/Class.java line 666:

> 665:                 try {
> 666:                     Class<?> valType = Class.forName(cn, false, getClassLoader());
> 667:                     if (valType.isInlineClass()) {

The lookup by name implies that hidden classes can not be value classes (because they cannot be resolved by name).

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PR: https://git.openjdk.java.net/valhalla/pull/152