> On May 3, 2021, at 4:21 PM, Brian Goetz <brian.go...@oracle.com> wrote: > > >>> 2. Whether abstract classes are primitive superclass candidates. The >>> static compiler will check this at compilation time when it sees a >>> superclass of a primitive class, but the JVM will want to recheck anyway. >>> There are two sensible ways to handle this in the classfile: >>> >>> - An attribute that says "I am a primitive superclass candidate." The >>> static compiler puts it there, and the JVM checks it. >>> - Infer and tag. If an abstract class is loaded that is not a primitive >>> superclass candidate, the JVM injects IdentityObject as a superinterface of >>> the newly loaded class; when we go to load a primitive subclass, this will >>> fail because primitive classes cannot implement both IdentityObject and >>> PrimitiveObject. >>> >>> Reflection probably doesn't have to reflect whether a class is primitive >>> superclass candidate; it already reflects the things needed to make this >>> determination. >> This one, on the other hand, conveys a core property of a JVM class. > > John's notes in the SotV suggests that the JVM is comfortable just "figuring > it out" and not requiring an attribute. So this is the "infer and tag" > option; the VM infers this at runtime. Not clear if there is a value to > having the static compiler capture something that wasn't explicit in the > source and that has to be validated at runtime anyway.
Ha, I was just looking over this! (See email.) SotV still has an opt-in. It just describes it as a ACC_ABSTRACT flag on an <init> method, rather than a class attribute or something else. (It also describes some additional requirements, like no instance fields, but I argued in the other email that those requirements are better handled as consistency checks, not separate components to the opt-in.)
