RFR: 8246778: Compiler implementation for Sealed Classes (Second Preview)

[Jan Lahoda]

On Wed, 2 Dec 2020 19:31:59 GMT, Dan Smith <dlsmith at openjdk.org> wrote:

>>> Additional changes may be needed to Class.permittedSubclasses() and/or Class.isSealed() as part of fixing bug JDK-8256867. The JVM is being changed to treat classes with empty PermittedSubclasses attributes as sealed classes that cannot be extended (or implemented).
>>> 
>>> Current thinking is that Class.permittedSubclasses() will return an empty array for both non-sealed classes and for sealed classes with empty PermittedSubclasses attributes. And, Class.isSealed() will return False in the former case and True in the latter. This will require changing the implementation of Class.isSealed() to call the JVM directly instead of calling Class.permittedSubclasses().
>>> 
>>> Does this seem like a reasonable way to handle this corner case?
>> 
>> I suggest `Class::getPermittedSubclasses` to return a `non-null` array if this `Class` is sealed, i.e. this class is derived from a `class` file with the presence of `PermittedSubclasses` attribute regardless of its content (the attribute could be empty or contains zero or more entries which is a properly loaded permitted subtype.
>> 
>> If this `Class` is not sealed, `Class::getPermittedSubclasses` returns null (see `Class::getRecordComponents` and some other reflection APIs as precedence).
>
>> I suggest `Class::getPermittedSubclasses` to return a `non-null` array if this `Class` is sealed, i.e. this class is derived from a `class` file with the presence of `PermittedSubclasses` attribute regardless of its content (the attribute could be empty or contains zero or more entries which is a properly loaded permitted subtype.
>> 
>> If this `Class` is not sealed, `Class::getPermittedSubclasses` returns null (see `Class::getRecordComponents` and some other reflection APIs as precedence).
> 
> Agree, that seems reasonable. Often, methods in `Class` with an array return type default to an empty array, but `getRecordComponents` is a good example of returning `null` when an empty array is meaningful.

I've changed Class.getPermittedSubclasses to return null for classes that are not sealed here:
https://github.com/openjdk/jdk/pull/1483/commits/7056143822ff62dfbb1d294c67352ed3892317c2
with follow-up changes to tests here:
https://github.com/openjdk/jdk/pull/1483/commits/537e267fb6802ab5cf38bf26978039383cc6309a

How does this look?

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PR: https://git.openjdk.java.net/jdk/pull/1483