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

Chris Plummer chris.plummer at
Sat Mar 28 03:51:37 UTC 2020

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)) {
  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.

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.

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.

I still need to look over the JVMTI tests.



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:
> 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
> JVMS 5.4.4 change:
> CSR:
> Thanks
> Mandy
> [1]
> [2]
> [3]

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