(15) RFR: JDK-8247444: Trust final fields in records

[coleen.phillimore at oracle.com]

On 6/16/20 12:42 PM, Mandy Chung wrote:
>
>
> On 6/16/20 2:49 AM, Remi Forax wrote:
>> Hi Mndy,
>> Looks good,
>> I don't like too much the fact that there is a new boolean field in 
>> j.l.r.Field instead of using the modifiers like in the VM counterpart,
>> but it will require masking and unmasking the modifiers which is a 
>> far bigger change, too big to worth it in my opinion.
>
> Adding a new field instead of using the modifiers is a deliberate 
> choice from my side.  It's higher risk to overload the modifiers to 
> include VM-specific flags.

Thank you for this.  As far as I can see the trusted final field doesn't 
add any footprint to the java.lang.reflect.Field instance.

The VM runtime changes look okay to me, excluding the methodHandles.* 
changes which I don't know that much about.

Coleen

> I am all for a better clean up (valhalla lworld does use the modifiers 
> to cover a flag indicating if it's flattened which I am not satisfied 
> either).
>
>> So +1
>
> Thanks Remi.
>
> Mandy
>
>> Rémi
>>
>> ----- Mail original -----
>>> De: "mandy chung" <mandy.chung at oracle.com>
>>> À: "hotspot-dev" <hotspot-dev at openjdk.java.net>, "core-libs-dev" 
>>> <core-libs-dev at openjdk.java.net>, "amber-dev"
>>> <amber-dev at openjdk.java.net>
>>> Envoyé: Lundi 15 Juin 2020 23:58:19
>>> Objet: (15) RFR: JDK-8247444: Trust final fields in records
>>> This patch is joint contribution from Christoph Dreis [1] and me.
>>>
>>> Webrev: 
>>> http://cr.openjdk.java.net/~mchung/jdk15/webrevs/8247444/webrev.00/
>>> CSR: https://bugs.openjdk.java.net/browse/JDK-8247517
>>>
>>> This proposes to make final fields in records notmodifiable via
>>> reflection.  Field::set throws IAE if a Field is not modifiable.
>>> Thecurrent specification specifies the following Fields not modifiable:
>>> - static final fields in any class
>>> - final fields in a hidden class
>>>
>>> The spec of Field::set is changed to specify that records are not
>>> modifiable via reflection.
>>>   Noe that no change in Field::setAccessible(true), i.e. it will 
>>> succeed
>>> to allow existing frameworks to have read access to final fields in
>>> records.  Just no write access.
>>>
>>> VarHandle does not support write access if it's a static final field or
>>> an instance final field.
>>>
>>> This patch also proposes `sun.misc.Unsafe::objectFieldOffset` and
>>> `sun.misc.Unsafe::staticField{Offset/Base}` not to support records.
>>>
>>> No change is made in JNI.  JNI could be considered to disallow
>>> modification of final fields in records and hidden classes (static and
>>> instance fields).  But JNI has superpower and the current spec already
>>> allows to modify the value of a final static field even after it's
>>> constant folded (via JNI Set<type>Field and SetStatic<type>Field), then
>>> all bets are off.  This should be re-visited when we consider "final is
>>> truly final" for all classes.
>>>
>>> Make final fields in records not modifiable via reflection enables JIT
>>> optimization as these final fields are effectively truly final.
>>>
>>> This change impacts 3rd-party frameworks including 3rd-party
>>> serialization framework that rely on core reflection `setAccessible` or
>>> `sun.misc.Unsafe::allocateInstance` and `objectFieldOffset` etc to
>>> construct records but not using the canonical constructor.
>>> These frameworks would need to be updated to construct records via its
>>> canonical constructor as done by the Java serialization.
>>>
>>> I see this change gives a good opportunity to engage the maintainers of
>>> the serialization frameworks and work together to support new features
>>> including records, inline classes and the new serialization mechanism
>>> and which I think it is worth the investment.
>>>
>>> This is a low risk enhancement.  I'd like to request approval for a 
>>> late
>>> enhancement in JDK 15.  It extends the pre-existing code path with
>>> refactoring the hidden classes to prepare for new kinds of classes with
>>> trusted final fields.  The change is straight-forward.
>>>
>>> Can this wait to integrate in JDK 16?
>>>
>>>    It's important to get this enhancement in when record is a preview
>>> feature that we can get feedback and give time to 3rd-party
>>> serialization frameworks to look into adding the support for records.
>>> If we delayed this change in 16 and records exit preview, it would be
>>> bad for frameworks if they verify that they support records in 15 but
>>> fail in 16.  OTOH the risk of this patch is low.
>>>
>>> Performance Impact
>>>
>>> I addressed the performance concern I raised earlier myself. For
>>> reflection, VM creates the reflective Field objects and fills in
>>> MemberName when resolving a member.  VM will tag if this
>>> Field/MemberName is trusted final field.  I think this is a cleaner
>>> approach rather than in each place to check for final static and final
>>> fields in hidden or record class to determine if it has write access or
>>> not.
>>>
>>> `sun.misc.Unsafe` does not use Field::isTrustedFinalField because (1)
>>> Unsafe has been allowing access of static final fields of any classes
>>> but isTrustedFinalField is not limited to instance fields (2) Unsafe
>>> disallows access to all fields in a hidden class (not limited to 
>>> trusted
>>> final fields).  So it follows the precedence and simply checks if the
>>> declaring class is a record. `Class::isRecord` calls
>>> `Class::getSuperclass` to check if it's a subtype of `Record`.  As
>>> `Class::getSuperclass` is intrinsified, the call on isRecord on a 
>>> normal
>>> class is fast. Christoph has contributed the microbenchmarks that
>>> confirm that no performance regression.
>>>
>>> Thanks
>>> Mandy
>>> [1]
>>> https://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2020-June/040096.html 
>>>
>