Lambda with anonymous or local class inside unnecessarily captures 'this'

[Archie Cobbs]

Hi Chen,

You're right, I had it backwards re: old vs. new behavior. Instead, I'm
guessing the new (and better) behavior in the
"withLocalClassInsideNoInstance" case is just a side effect of recent code
refactorings. Also my speculation that the reason for the capture was due
to the newly added null check was incorrect; instead, the capture was
already there before.

So to summarize (correct me if I'm wrong) it should be possible to
eliminate the capture for local and anonymous classes when the outer
instance is not actually used, and that would fix both of the remaining
cases in Tagir's test ("withAnonymousRunnableInside"
and "withLocalClassInside").

Thanks for clarifying!

-Archie

On Fri, Nov 14, 2025 at 9:15 AM Chen Liang <chen.l.liang at oracle.com> wrote:

> The situation in which non-member nested classes capture the enclosing
> instance unconditionally can be treated as an implementation artifact of
> javac; we discussed this within Oracle a while ago. The enclosing instance
> capture being the leading parameter and the null check are for linkage, but
> local and anonymous classes do not have such linkage links, and it should
> be JLS-compliant that unused enclosing instance capture is dropped
> altogether. There are local and anonymous classes in early construction
> context and static context that don't perform enclosing instance capture,
> yet reflectively they are not distinguished from the ones that do, so I
> believe the compatibility impact is low.
>
> Re Archie: I think Tagir means that for 23 and below, the lambdas with
> local or anonymous classes are still captured. It is interesting
> withLocalClassInsideNoInstance() is suddenly deduplicated.
>
> P.S. "Deduplication" is a confusing term - in javac I believe it refers to
> identical lambda expressions reusing the same InvokeDynamic constant (which
> still generates different call sites per JVMS rules), but in this thread it
> refers to the singleton lambda instances.
>
>

-- 
Archie L. Cobbs
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