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

[Mandy Chung]

On Mon, 7 Dec 2020 21:29:23 GMT, Lois Foltan <lfoltan 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/hotspot/share/prims/jvm.cpp line 2130:
> 
>> 2128:     JvmtiVMObjectAllocEventCollector oam;
>> 2129:     Array<u2>* subclasses = ik->permitted_subclasses();
>> 2130:     int length = subclasses == NULL ? 0 : subclasses->length();
> 
> Minor comment - you don't really need the check of subclasses == NULL here since subclasses will never be NULL.  You could just assign length to subclasses->length();

+1.   is_sealed returns true iff `_permitted_subclasses != NULL`

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