[nestmates] Name of a hidden class and stack trace

[Brian Goetz]

This trick has worked well for us with anon classes, and there’s already plenty of code out there that does `name.contains(‘/‘)` to defend against it, so I see no reason to break from this convention.

> On Aug 29, 2019, at 2:34 PM, Mandy Chung <mandy.chung at oracle.com> wrote:
> 
> I'm writing a JEP proposal of hidden/nestmate/weak classes prototyped in the nestmates branch (JDK-8171335).  Attached describes about hidden classes (thanks to Alex Buckley for the help) and description about nestmates/weak will come next.
> 
> A hidden class cannot be named by other class.   Ideally a hidden class should  be nameless.  For troubleshooting and stack trace, a hidden class needs a name.   So the proposal has been:
> 
> Class::getName returns a name for a hidden class and the name is unique in the runtime package namespace.  That is, there is no two Class objects with the same name in the same runtime package.
> 
> Below shows the stack trace where a hidden class throws an exception.
> 
> $ java -XX:+UnlockDiagnosticVMOptions -XX:+ShowHiddenFrames DefineClass foo/Foo.class
> java.lang.Error: error
>     at foo.Foo_/0x0000000800b79258.run(Foo.java:9)
>     at DefineClass.main(DefineClass.java:18)
> 
> The current impl includes `/` in the hidden class's name to disjoint from the ordinary class names
> (same trick as VM anonymous class).
> 
> Another class calls Class::forName with the hidden class's name which would fail since that's not a valid binary name.   If someone attempts to spin a class referencing this hidden class's name with replace('.', '/'), it may attempt to load a class named 'foo/Foo_/0x0000000800b79258'  and may succeed if such a class file exists and can be located by class loader.  This is no difference than today as one can spin a class file to any class names.
> 
> Class::getName may return an invalid binary name if it's a hidden class.  It will impact existing code that expects the returned name is a valid binary name or use the name to perform class lookup.  I think the compatibility concern should not be high.
> 
> Thought?
> 
> Mandy
>