Review Request: 8238358: Implementation of JEP 371: Hidden Classes
mandy.chung at oracle.com
Fri Mar 27 16:29:46 UTC 2020
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
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 class.
>> 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
>> 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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