Review Request: 8238358: Implementation of JEP 371: Hidden Classes
Mandy Chung
mandy.chung at oracle.com
Mon Mar 30 02:17:27 UTC 2020
On 3/27/20 8:51 PM, Chris Plummer wrote:
> Hi Mandy,
>
> A couple of very minor nits in the jvmtiRedefineClasses.cpp comments:
>
> 153 // classes for primitives, arrays, hidden and vm unsafe
> anonymous classes
> 154 // cannot be redefined. Check here so following code can
> assume these classes
> 155 // are InstanceKlass.
> 156 if (!is_modifiable_class(mirror)) {
> 157 _res = JVMTI_ERROR_UNMODIFIABLE_CLASS;
> 158 return false;
> 159 }
>
> I think this code and comment predate anonymous classes. Probably
> before anonymous classes the check was not for !is_modifiable_class()
> but instead was just a check for primitive or array class types since
> they are not an InstanceKlass, and would cause issues when cast to one
> in the code that lies below this section. When anonymous classes were
> added, the code got changed to use !is_modifiable_class() and the
> comment was not correctly updated (anonymous classes are an
> InstanceKlass). Then with this webrev the mention of hidden classes
> was added, also incorrectly implying they are not an InstanceKlass. I
> think you should just leave off the last sentence of the comment.
>
I agree with you that this comment needs update. Perhaps it should say
"primitive, array types and hidden classes are non-modifiable. A
modifiable class must be an InstanceKlass."
I leave it to Serguei who may have other opinion.
> There's some ambiguity in the application of adjectives in the following:
>
> 297 // Cannot redefine or retransform a hidden or an unsafe
> anonymous class.
>
> I'd suggest:
>
> 297 // Cannot redefine or retransform a hidden class or an unsafe
> anonymous class.
>
+1
> There are some places in libjdwp that need to be fixed. I spoke to
> Serguei about those this afternoon. Basically the
> convertSignatureToClassname() function needs to be fixed to handle
> hidden classes. Without the fix classname filtering will have problems
> if the filter contains a pattern with a '/' to filter on hidden
> classes. Also CLASS_UNLOAD events will not properly convert hidden
> class names. We also need tests for these cases. I think these are all
> things that can be addressed later.
>
Good catch. I have created a subtask under JDK-8230502:
https://bugs.openjdk.java.net/browse/JDK-8230502
> I still need to look over the JVMTI tests.
>
Thanks
Mandy
> thanks,
>
> Chris
>
> On 3/26/20 4:57 PM, 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:
>> http://cr.openjdk.java.net/~mchung/valhalla/webrevs/hidden-classes/webrev.03
>>
>>
>> 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
>> http://cr.openjdk.java.net/~mchung/valhalla/webrevs/hidden-classes/api/
>> http://cr.openjdk.java.net/~mchung/valhalla/webrevs/hidden-classes/specdiff/
>>
>>
>> JVMS 5.4.4 change:
>> http://cr.openjdk.java.net/~mchung/valhalla/webrevs/hidden-classes/Draft-JVMS-HiddenClasses.pdf
>>
>>
>> CSR:
>> https://bugs.openjdk.java.net/browse/JDK-8238359
>>
>> Thanks
>> Mandy
>> [1] https://bugs.openjdk.java.net/browse/JDK-8238359
>> [2] https://bugs.openjdk.java.net/browse/JDK-8240338
>> [3] https://bugs.openjdk.java.net/browse/JDK-8230502
>
>
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