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

coleen.phillimore at oracle.com coleen.phillimore at oracle.com
Mon Mar 30 14:18:58 UTC 2020



On 3/30/20 5:54 AM, David Holmes wrote:
> Sorry to jump in on this but it caught my eye though I may be missing 
> a larger context ...
>
> On 30/03/2020 7:30 pm, 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;
>
> Why does hidden imply weak here?

has_class_mirror_holder() implies weak.

Coleen
>
> David
> -----
>
>>   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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