lambda method naming changes in jdk 24

[Maurizio Cimadamore]

Hi Egor,
I confirm that this change is deliberate. There were several changes 
required to make JEP 513 (Flexible Constructor Bodies) work. Some of 
these changes resulted in significant rewrites of the compiler backend 
(to move lambda translation ahead of inner class translation). The bulk 
of the work (and some history) is here:

https://github.com/openjdk/jdk/pull/19836

This change created some compatibility issues for serializable lambdas, 
which were later addressed here:

https://github.com/openjdk/jdk/pull/20349

This latter change dropped a lot of accidental complexity from the javac 
LambdaToInner pass, and that meant making the logic for computing the 
names serializable and non-serializable lambdas more robust.

This meant that, unfortunately, the (unspecified) name of some 
non-serializable lambda changed. This compatibility issue is described 
in the associated CSR:

https://bugs.openjdk.org/browse/JDK-8337558

The difference you see is caused by when the name of the translated 
lambda method is generated -- it used to be computed _after_ the end of 
the lambda body, now it is computed _before_. This means that as soon as 
we see the outermost lambda for `r3` we give it the first number -- then 
the nested lambda for `r4` gets the second number.

This allows us to avoid having to "patch" the names of the translated 
symbols, which minimizes the amount of mutation in our code, making it 
less brittle. So, unless there's a big reason as to why these names 
should go back the way they were, we'd like to keep the code the way it 
is :-)

Cheers
Maurizio

On 11/07/2025 16:08, Egor Ushakov wrote:
> Hi everyone!
>
> it looks like in jdk 24 compiler started naming lambda methods 
> slightly differently,
> consider the code:
> public class Lambdas1 {
>      public static void main(String[]args) {
>          Runnable r3 = () -> {
>              Runnable r4 = () ->System.out.println(new Exception().getStackTrace()[0]);
>              r4.run();
>              System.out.println(new Exception().getStackTrace()[0]);
>          };
>          r3.run();
>      }
> }
>
> with jdk 23 the output is:
> Lambdas1.lambda$main$*0*(Lambdas1.java:7)
> Lambdas1.lambda$main$*1*(Lambdas1.java:9)
>
> with jdk 24:
> Lambdas1.lambda$main$*1*(Lambdas1.java:4)
> Lambdas1.lambda$main$*0*(Lambdas1.java:6)
>
> I know that this is not specified anywhere, but Intellij IDEA relied 
> on this and it caused issues with debugger features:
> https://youtrack.jetbrains.com/issue/IDEA-375811
>
> Was this change really intended? Could anyone please point me to the 
> fix that caused this?
>
> Thanks!
> Egor
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