JEP 468 updating non-updatable fields

[Tagir Valeev]

Hello!

It's interesting that when language designers make the code easier to
write, somebody may complain that it's too easy :-)
I think it's a perfect place for static analysis tooling. One may invent an
annotation like `@NonUpdatable`
with the `RECORD_COMPONENT` target and use it on such fields, then create
an annotation processor
(ErrorProne plugin, IntelliJ IDEA inspection, CodeQL rule, etc.), that will
check the violations and fail the build if there are any.
Adding such a special case to the language specification would be an
overcomplication.

With best regards,
Tagir Valeev.

On Sun, Jan 25, 2026 at 11:48 PM Brian Goetz <brian.goetz at oracle.com> wrote:

> The important mental model here is that a reconstruction (`with`)
> expression is "just" a syntactic optimization for:
>
>  - destructure with the canonical deconstruction pattern
>  - mutate the components
>  - reconstruct with the primary constructor
>
> So the root problem here is not the reconstruction expression; if you can
> bork up your application state with a reconstruction expression, you can
> bork it up without one.
>
> Primary constructors can enforce invariants _on_ or _between_ components,
> such as:
>
>     record Rational(int num, int denom) {
>         Rational { if (denom == 0) throw ... }
>     }
>
> or
>
>     record Range(int lo, int hi) {
>         Range { if (lo > hi) throw... }
>     }
>
> What they can't do is express invariants between the record / carrier
> state and "the rest of the system", because they are supposed to be simple
> data carriers, not serialized references to some external system.  A
> class that models a database row in this way is complecting entity state
> with an external entity id.  By modeling in this way, you have explicitly
> declared that
>
>     rec with { dbId++ }
>
> *is explicitly OK* in your system; that the components of the record can
> be freely combined in any way (modulo enforced cross-component
> invariants).  And there are systems in which this is fine!  But you're
> imagining (correctly) that this modeling technique will be used in systems
> in which this is not fine.
>
> The main challenge here is that developers will be so attracted to the
> syntactic concision that they will willfully ignore the semantic
> inconsistencies they are creating.
>
>
>
>
> On 1/25/2026 1:37 PM, Andy Gegg wrote:
>
> Hello,
> I apologise for coming late to the party here - Records have been of
> limited use to me but Mr Goetz's email on carrier classes is something that
> would be very useful so I've been thinking about the consequences.
>
> Since  carrier classes and records are for data, in a database application
> somewhere or other you're going to get database ids in records:
> record MyRec(int dbId, String name,...)
>
> While everything is immutable this is fine but JEP 468 opens up the
> possibility of mutation:
>
> MyRec rec = readDatabase(...);
> rec = rec with {name="...";};
> writeDatabase(rec);
>
> which is absolutely fine and what an application wants to do.  But:
> MyRec rec = readDatabase(...);
> rec = rec with {dbId++;};
> writeDatabase(rec);
>
> is disastrous.  There's no way the canonical constructor invoked from
> 'with' can detect stupidity nor can whatever the database access layer does.
>
> In the old days, the lack of a 'setter' would usually prevent stupid code
> - the above could be achieved, obviously, but the code is devious enough to
> make people stop and think (one hopes).
>
> Here there is nothing to say "do not update this!!!" except code comments,
> JavaDoc and naming conventions.
>
> It's not always obvious which fields may or may not be changed in the
> application.
>
> record MyRec(int dbId, int fatherId,...)
> probably doesn't want
> rec = rec with { fatherId = ... }
>
> but a HR application will need to be able to do:
>
> record MyRec(int dbId, int departmentId, ...);
> ...
> rec = rec with { departmentId = newDept; };
>
> Clearly, people can always write stupid code (guilty...) and the current
> state of play obviously allows the possibility (rec = new MyRec(rec.dbId++,
> ...);) which is enough to stop people using records here but carrier
> classes will be very tempting and that brings derived creation back to the
> fore.
>
> It's not just database ids which might need restricting from update, e.g.
> timestamps (which are better done in the database layer) and no doubt
> different applications will have their own business case restrictions.
>
> Thank you for your time,
> Andy Gegg
>
>
>
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