Review Request: 8238358: Implementation of JEP 371: Hidden Classes
vicente.romero at oracle.com
Fri Mar 27 21:15:29 UTC 2020
The patch for nestmates  could be used as a reference. There a new
method was added to class `com.sun.tools.javac.jvm.Target`, 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.
> 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?
>> 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 has been reviewed
>>> and is in the finalized state (see specdiff and javadoc below for
>>> Hidden class is created via `Lookup::defineHiddenClass`. From JVM's
>>> 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
>>> 4. Field::setXXX method will throw IAE on a final field of a hidden
>>> regardless of the value of the accessible flag.
>>> 5. JVM_LookupDefineClass is the new JVM entry point for
>>> 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
>>> 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
>>> 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 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  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 . 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:
>>>  https://bugs.openjdk.java.net/browse/JDK-8238359
>>>  https://bugs.openjdk.java.net/browse/JDK-8240338
>>>  https://bugs.openjdk.java.net/browse/JDK-8230502
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