Preload attribute
Frederic Parain
frederic.parain at oracle.com
Thu Jun 1 18:34:57 UTC 2023
The current support of the PreLoad attribute in HotSpot is very lenient:
- the VM tries to load all classes listed in the attribute, there's
no attempt to check if a listed class is used in the declaration of a
field or as the type of a method argument or return value
- the VM tries to load those classes at link time (after linking of
super-interfaces, before verification), but this timing is constrained
by some HotSpot internal designs, timing could be different on another VM
- all error/exceptions thrown during an attempt to load one of those
classes is caught and discarded (silent failure)
- if the loading is successful, there's no check that the class is a
value class. It could be an identity class or an interface
AFAICT, this implementation matches Dan Smith's answers to your questions.
Fred
On 6/1/23 1:53 PM, Dan Heidinga wrote:
> A couple of questions about the spec for the Preload attribute[0].
> The current spec says it indicates "certain classes contain
> information that may be of interest during linkage."
>
> The Preload attribute removes one need for Q modifiers while allowing
> calling convention optimizations and layout decisions to be made early.
>
> The current spec is quite vague on what classes should be included in
> the attribute and on when / what the VM will do with those classes (or
> even if it does anything). I think it's time to tighten up the spec
> for Preload attribute and specify:
> * what the VM will do with classes listed in the attribute
> * when those classes will be loaded (ie: somewhere in JVMS 5.3)
> * how invalid cases are handled, including circularities (Class A's
> Preload mentions B <: A)
> * what types of classes can be listed (any? only values?)
>
> And there's probably other issues to clarify. Otherwise, the current
> spec isn't clear enough for users to know when to add a class, how it
> will be treated, and any potential edge cases to avoid.
>
> It probably makes sense to start from the current Hotspot handling of
> the attribute and fine tune that into the spec?
>
> --Dan
>
> [0]
> https://cr.openjdk.org/~dlsmith/jep401/jep401-20230404/specs/value-objects-jvms.html#jvms-4.7.31
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