Preview features and class file formats

[Alex Buckley]

On 4/21/2025 1:53 PM, Chen Liang wrote:
> Per JEP 12, reflective preview APIs do not allow preview features to be 
> used; rather, they allow programs compiled without preview to run 
> correctly in a Java Runtime Environment running with preview features 
> enabled.

Yes, that's one way of looking at it. The program compiled without 
preview is typically a framework that wants to run everywhere. If a user 
chooses to exploit preview features in their code, by passing 
--enable-preview at compile time and run time, then the framework when 
running on this user's machine can (via reflective preview APIs) 
recognize and adapt to the user's preview-using classes.

For example, when records were previewing in Java 14, a framework that 
recognized a user class as a record (via Class::isRecord, which today 
we'd call a reflective preview API) could deserialize it more 
efficiently than normal classes, since records can't self-reference.

> I just reviewed the preview APIs introduced for Value Objects JEP: it 
> has a few, such as IdentityException, Objects.requireIdentity, that 
> perform inspection on object instances but are not marked as reflective. 
> I believe they should be marked reflective, as they are critical for 
> non-preview programs to run correctly when preview is enabled (when they 
> can receive value objects, and JVM can throw IdentityException for 
> certain bytecode instructions).

Yes. There's little downside in marking a preview API concerned with 
reflection as a reflective (rather than normal) preview API.

> Meanwhile, the constructor for IdentityException are all essential
> preview APIs - they are necessary for the Value Objects feature, but
> not for non-preview programs to run correctly when preview features
> are enabled.

This sounds right.

> In particular, can inspection on object instances via
> getClass() be considered reflection as well?

It is reflection, but I don't see how it connects to the Value Objects 
API you mentioned earlier.

Alex