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

Harold Seigel harold.seigel at oracle.com
Mon Mar 30 12:28:10 UTC 2020


Hi Serguei,

The bottom class is always the first class.  The bottom class of 
[Integer and [[[Integer is Integer in both cases.

Thanks, Harold

On 3/30/2020 5:30 AM, serguei.spitsyn at oracle.com wrote:
> Hi Mandy,
>
> I have just one comment so far.
>
> http://cr.openjdk.java.net/~mchung/valhalla/webrevs/hidden-classes/webrev.03/src/hotspot/share/classfile/classLoaderHierarchyDCmd.cpp.frames.html 
>
>
>  356   void add_classes(LoadedClassInfo* first_class, int num_classes, 
> bool has_class_mirror_holder) {
>  357     LoadedClassInfo** p_list_to_add_to;
>  358     bool is_hidden = first_class->_klass->is_hidden();
>  359     if (has_class_mirror_holder) {
>  360       p_list_to_add_to = is_hidden ? &_hidden_weak_classes : 
> &_anon_classes;
>  361     } else {
>  362       p_list_to_add_to = &_classes;
>  363     }
>  364     // Search tail.
>  365     while ((*p_list_to_add_to) != NULL) {
>  366       p_list_to_add_to = &(*p_list_to_add_to)->_next;
>  367     }
>  368     *p_list_to_add_to = first_class;
>  369     if (has_class_mirror_holder) {
>  370       if (is_hidden) {
>  371         _num_hidden_weak_classes += num_classes;
>  372       } else {
>  373         _num_anon_classes += num_classes;
>  374       }
>  375     } else {
>  376       _num_classes += num_classes;
>  377     }
>  378   }
>
>  Q1: I'm just curious, what happens if a cld has arrays of hidden 
> classes?
>      Is the bottom_klass always expected to be the first?
>
>
> Thanks,
> Serguei
>
>
> On 3/26/20 16:57, 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:
>> 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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