RFR: 8256867: Classes with empty PermittedSubclasses attribute cannot be extended

[Chris Hegarty]

On Mon, 7 Dec 2020 23:47:40 GMT, Mandy Chung <mchung at openjdk.org> wrote:

>> Please review this fix for JDK-8256867.  This change no longer throws a ClassFormatError exception when loading a class whose PermittedSubclasses attribute is empty (contains no classes).  Instead, the class is treated as a sealed class which cannot be extended nor implemented.  This new behavior conforms to the JVM Spec.
>> 
>> This change required changing Class.permittedSubclasses() to return an empty array for classes with empty PermittedSubclasses attributes, and to return null for non-sealed classes.
>> 
>> This fix was tested with Mach5 tiers 1-2 on Linux, MacOS, and Windows, and tiers 3-5 on Linux x64.
>> 
>> Thanks, Harold
>
> src/java.base/share/classes/java/lang/Class.java line 4396:
> 
>> 4394:      * is unspecified. If this {@code Class} object represents a primitive type,
>> 4395:      * {@code void}, an array type, or a class or interface that is not sealed,
>> 4396:      * then null is returned.
> 
> nit: s/null/`{@code null}`
> 
> I'd suggest to clarify if this sealed class or interface has no permitted subclass, something like this:
> Returns an array containing {@code Class} objects representing the
> direct subinterfaces or subclasses permitted to extend or
> implement this class or interface if it is sealed.  The order of such elements
> is unspecified.   The array is empty if this sealed class or interface has no
> permitted subclass. 
> 
> `@return` needs to be revised as well:
> @return an array of {@code Class} objects of the permitted subclasses of this sealed class or interface,
>      or {@null} if this class or interface is not sealed

Mandy's suggested wording is good.

I would like to add one more additional point of clarification. It would
be good to strongly connect `isSealed` and `getPermittedClasses` in a
first-class way in normative spec ( similar to isRecord and
getRecordComponents ).

For example, 

  to `isSealed` add: "getPermittedSubclasses returns a non-null but possibly
       empty value for a sealed class."

  to `getPermittedSubclasses`: "If this class is not a sealed class, that is {@link
     * #isSealed()} returns {@code false}, then this method returns {@code null}.
     * Conversely, if {@link #isSealed()} returns {@code true}, then this method
     * returns a non-null value."

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PR: https://git.openjdk.java.net/jdk/pull/1675