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

Vicente Romero vicente.romero at
Fri Mar 27 21:15:29 UTC 2020

Hi Mandy,

The patch for nestmates [1] could be used as a reference. There a new 
method was added to class ``, named: 
`hasNestmateAccess` which checks if a target is ready for nestmates or 
not. I think that you can follow a similar approach here.



On 3/27/20 12:29 PM, Mandy Chung wrote:
> Hi Jan,
> Good point.  The javac change only applies to JDK 15 and later and the 
> lambda proxy class is not a nestmate when running on JDK 14 or earlier.
> I probably need the help from langtools team to fix this.  I'll give 
> it a try.
> Mandy
> On 3/27/20 4:31 AM, Jan Lahoda wrote:
>> 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?
>> Jan
>> 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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