[nestmates] Name of a hidden class and stack trace

Mandy Chung mandy.chung at oracle.com
Thu Aug 29 18:34:25 UTC 2019

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 
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.



More information about the valhalla-dev mailing list