RFR: 8353835: Implement JEP 500: Prepare to Make Final Mean Final [v2]

[David Holmes]

On Tue, 23 Sep 2025 14:39:34 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

>> src/hotspot/share/runtime/fieldDescriptor.cpp line 53:
>> 
>>> 51:   // write protected fields (JLS 17.5.4)
>>> 52:   if (is_final() && is_static() && ik == vmClasses::System_klass() &&
>>> 53:       (offset() == java_lang_System::in_offset() || offset() == java_lang_System::out_offset() || offset() == java_lang_System::err_offset())) {
>> 
>> I thought the ability to mutate these fields was restricted to the native implementations of `setIn` et al. and was not allowed via regular reflection? Otherwise how can these fields be considered as "trusted finals"??
>
>> Otherwise how can these fields be considered as "trusted finals"??
> 
> FTR `System.in` et al are not considered as trusted finals. There's a special logic in `ciField::initialize_from()` to disable corresponding optimizations, so the fields aren't treated as constants.

@iwanowww they are not special-cased here:

bool fieldDescriptor::is_trusted_final() const {
  InstanceKlass* ik = field_holder();
  return is_final() && (is_static() || ik->is_hidden() || ik->is_record());
}

and so would be reported as "trusted finals".

@AlanBateman my point is, if these can only be changed via JNI code through the setX native methods, then why do we have to special case them in this code. ??

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PR Review Comment: https://git.openjdk.org/jdk/pull/25115#discussion_r2373552612