Re: [JVM Question] Making Friends with Ad-Hoc Classes

> From: "Caleb" <[email protected]>
> To: "dev" <dev-GSC0n/0aZo1d/SJB6HiN2Ni2O/[email protected]>
> Sent: Sunday, April 12, 2026 10:09:35 PM
> Subject: [JVM Question] Making Friends with Ad-Hoc Classes

> Hi! This isn't really related to Groovy, but I'm doing weird things with the JVM
> for a compiler plugin and I've hit a knowledge barrier I can't pass. Since you
> guys are the experts on "doing weird stuff with the JVM", I was hoping you
> might be willing to help?

> My goal is to copy existing methods, transform them, and link to those
> transformed versions with InvokeDynamic. Basically the template system from
> C++, but done at runtime.
> I've got all of the groundwork laid to do this, but I'm having trouble with
> access violations in my clones.

> Since I'm *cloning* methods, they'll be generated into a class other than the
> one being cloned. But then since I'm cloning *methods*, they need to be able to
> access PRIVATE MEMBERS of the original instance.

> I tried generating public bridge methods, but private types still break that. If
> possible, I'd like to just make my clone a "friend" of the original class like
> in C++, but I don't know how.

> I fully control the classes that clones can be made from, so I can mark anything
> else as an "inner class" or "nestmate" as needed. I just can't figure out what
> each of those ALLOW on the JVM from the specification docs alone. And you know
> how helpful google is nowadays; I couldn't find anything no matter how I
> phrased it.

> Does anyone know how to do this?

The class that encapsulates your clone should be an hidden class (this is the mechanism used to load lambda bodies) 

https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/lang/invoke/MethodHandles.Lookup.html#defineHiddenClass(byte[],boolean,java.lang.invoke.MethodHandles.Lookup.ClassOption...) 

> Thank you so much for your help!

regards, 
Rémi