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
