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

[Remi Forax]

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.

So +1

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