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

Mandy Chung mandy.chung at
Fri Mar 27 16:29:46 UTC 2020

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.


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]

More information about the valhalla-dev mailing list