RFR: 8291360: Create entry points to expose low-level class file information

[Harold Seigel]

On Sun, 31 Jul 2022 22:05:15 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Please review this change to fix JDK-8291360.  This fix adds entry points getClassFileVersion() and getClassAccessFlagsRaw() to class java.lang.Class.  The new entry points return the current class's class file version and its raw access flags.
>> 
>> The fix was tested by running Mach5 tiers 1-2 on Linux, Mac OS, and Windows, and Mach5 tiers 1-3 on Linux x64.  Additionally, the JCK lang, vm, and api tests and new regression tests were run locally on Linux x64.
>> 
>> Thanks, Harold
>
> src/hotspot/share/prims/jvm.cpp line 4064:
> 
>> 4062:   assert(c->is_instance_klass(), "must be");
>> 4063:   InstanceKlass* ik = InstanceKlass::cast(c);
>> 4064:   return (ik->minor_version() << 16) | ik->major_version();
> 
> I'm curious why the format is minor:major rather than major:minor ?

This was requested by @RogerRiggs 

Major high vs low bits requires either:
  High bits:   val >>>16
  Low bits:     val & 0xffff

Except for preview, the minor version is 0, so putting major version  in the low bits allows trivial comparison (without masking); but may lead to bugs due to carelessness.

> src/java.base/share/classes/java/lang/Class.java line 4698:
> 
>> 4696:      *
>> 4697:      * If the class is an array type then the access flags of the component type is
>> 4698:      * returned.  If the class is a primitive then ACC_ABSTRACT | ACC_FINAL | ACC_PUBLIC.
> 
> The `ACC_ABSTRACT` seems odd - is that way of indicating this "class" can't be instantiated? Is there some spec document that explains this choice?

I don't know why API's such as JVM_GetClassModifiers() and JVM_GetClassAccessFlags return ACC_ABSTRACT | ACC_FINAL | ACC_PUBLIC for primitives.  Nor could I find a spec that discussed access flags for primitives.  But, I didn't want to change what's been returned for primitives in existing code.

> test/hotspot/jtreg/runtime/ClassFile/ClassFileVersionTest.java line 31:
> 
>> 29:  * @modules java.base/java.lang:open
>> 30:  * @compile classFileVersions.jcod
>> 31:  * @run main/othervm --enable-preview ClassFileVersionTest
> 
> What preview feature is being used here?

No preview feature is being used but --enable-preview is needed here for the JVM to accept a class with a minor version of 65535.

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PR: https://git.openjdk.org/jdk/pull/9688