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

[Lois Foltan]

On Mon, 7 Dec 2020 19:51:38 GMT, Harold Seigel <hseigel 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

Changes looks good Harold.
Lois

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();

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Marked as reviewed by lfoltan (Reviewer).

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