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

Jan Lahoda jan.lahoda at
Fri Mar 27 11:31:33 UTC 2020

Hi Mandy,

Regarding the javac changes - should those be switched on/off depending 
the Target? Or, if one compiles with e.g. --release 14, will the newly 
generated output still work on JDK 14?


On 27. 03. 20 0:57, Mandy Chung wrote:
> 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).
> Webrev:
> 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 
> class.
> 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. 
> There
>     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 
> declared
>     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 
> protected
>     method in its supertype in a different package
> This patch also updates StringConcatFactory, LambdaMetaFactory, and 
> LambdaForms
> to use hidden classes.  The webrev includes changes in nashorn to hidden 
> class
> 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 
> intends
> 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 
> classes.
> javadoc/specdiff
> JVMS 5.4.4 change:
> CSR:
> Thanks
> Mandy
> [1]
> [2]
> [3]

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