Review Request: 8238358: Implementation of JEP 371: Hidden Classes

Mandy Chung mandy.chung at
Thu Mar 26 23:57:39 UTC 2020

Please review the implementation of JEP 371: Hidden Classes. The main 
changes are in core-libs and hotspot runtime area.  Small changes are 
made in javac, VM compiler (intrinsification of Class::isHiddenClass), 
JFR, JDI, and jcmd.  CSR [1]has been reviewed and is in the finalized 
state (see specdiff and javadoc below for reference).


Hidden class is created via `Lookup::defineHiddenClass`. From JVM's point
of view, a hidden class is a normal class except the following:

- A hidden class has no initiating class loader and is not registered in 
any dictionary.
- A hidden class has a name containing an illegal character 
`Class::getName` returns `p.Foo/0x1234` whereas `GetClassSignature` 
returns "Lp/Foo.0x1234;".
- A hidden class is not modifiable, i.e. cannot be redefined or 
retransformed. JVM TI IsModifableClass returns false on a hidden.
- Final fields in a hidden class is "final".  The value of final fields 
cannot be overriden via reflection.  setAccessible(true) can still be 
called on reflected objects representing final fields in a hidden class 
and its access check will be suppressed but only have read-access (i.e. 
can do Field::getXXX but not setXXX).

Brief summary of this patch:

1. A new Lookup::defineHiddenClass method is the API to create a hidden 
2. A new Lookup.ClassOption enum class defines NESTMATE and STRONG 
option that
    can be specified when creating a hidden class.
3. A new Class::isHiddenClass method tests if a class is a hidden class.
4. Field::setXXX method will throw IAE on a final field of a hidden class
    regardless of the value of the accessible flag.
5. JVM_LookupDefineClass is the new JVM entry point for Lookup::defineClass
    and defineHiddenClass to create a class from the given bytes.
6. ClassLoaderData implementation is not changed.  There is one primary CLD
    that holds the classes strongly referenced by its defining loader.  
    can be zero or more additional CLDs - one per weak class.
7. Nest host determination is updated per revised JVMS 5.4.4. Access control
    check no longer throws LinkageError but instead it will throw IAE with
    a clear message if a class fails to resolve/validate the nest host 
    in NestHost/NestMembers attribute.
8. JFR, jcmd, JDI are updated to support hidden classes.
9. update javac LambdaToMethod as lambda proxy starts using nestmates
    and generate a bridge method to desuger a method reference to a 
    method in its supertype in a different package

This patch also updates StringConcatFactory, LambdaMetaFactory, and 
to use hidden classes.  The webrev includes changes in nashorn to hidden 
and I will update the webrev if JEP 372 removes it any time soon.

We uncovered a bug in Lookup::defineClass spec throws LinkageError and 
to have the newly created class linked.  However, the implementation in 14
does not link the class.  A separate CSR [2] proposes to update the
implementation to match the spec.  This patch fixes the implementation.

The spec update on JVM TI, JDI and Instrumentation will be done as
a separate RFE [3].  This patch includes new tests for JVM TI and
java.instrument that validates how the existing APIs work for hidden 


JVMS 5.4.4 change:



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