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

Vicente Romero vicente.romero at oracle.com
Fri Mar 27 22:48:52 UTC 2020


Hi Mandy,

On 3/27/20 6:29 PM, Mandy Chung wrote:
> Hi Vicente,
>
> hasNestmateAccess is about VM supports static nestmates on JDK release 
> >= 11.

I was not suggesting the use of `hasNestmateAccess` but to follow the 
same approach which is adding a new method at class `Target` to check if 
the new goodies were in the given target
>
> However this is about javac --release 14 and the compiled classes may 
> run on JDK 14 that lambda and string concat spin classes that are not 
> nestmates. I have a patch with Jan's help:
>
> http://cr.openjdk.java.net/~mchung/valhalla/webrevs/8171335/webrev-javac-target-release-14/index.html

which is what the patch above is doing

>
> (you can apply the above patch on valhalla repo "nestmates" branch)
>
> About testing, I wanted to run BridgeMethodsForLambdaTest and 
> TestLambdaBytecode test with --release 14 but it turns out not 
> straight-forward.  Any help would be appreciated.
>
> thanks
> Mandy

Vicente
>
> On 3/27/20 2:15 PM, Vicente Romero wrote:
>> Hi Mandy,
>>
>> The patch for nestmates [1] 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.
>>
>> Thanks,
>> Vicente
>>
>> [1] http://hg.openjdk.java.net/jdk/jdk/rev/2f2af62dfac7
>>
>> 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.
>>>
>>> Mandy
>>>
>>> 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?
>>>>
>>>> Jan
>>>>
>>>> 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 [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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