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

[serguei.spitsyn at oracle.com]

Hi Mandy,

This looks good from the Serviceability point of view.

 > 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.

This can potentially impact the JDWP agent as it uses the JNI to set 
fields values.
Please, see ObjectReference#setValue:
https://docs.oracle.com/en/java/javase/14/docs/api/jdk.jdi/com/sun/jdi/ObjectReference.html#setValue(com.sun.jdi.Field,com.sun.jdi.Value)

Thanks,
Serguei


On 6/15/20 14:58, Mandy Chung wrote:
> 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