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

> From: "Caleb" <[email protected]>
> To: "dev" <dev-GSC0n/0aZo1d/SJB6HiN2Ni2O/[email protected]>
> Sent: Monday, April 13, 2026 3:11:55 PM
> Subject: Re: [JVM Question] Making Friends with Ad-Hoc Classes

> I know how to emit ad-hoc classes - I also know that hidden classes specifically
> don't have an entry in the symbol table

you mean their name is not registered inside a classloader, 

> and can be GC'd,

that depends on the ClassOptions you provide (see ClassOption.STRONG) 

> making them particularly useful for single-function classes - I just don't know
> how you'd make a class you don't know the name of a friend/innerclass.

defineHiddenClass takes a lookup as parameter and if you use ClassOption.NESTMATE then the defined class is a nestmate of the lookupClass of the Lookup. 

Caleb, if you want to know more, email me directly, because it starts to be OT for the goovy-dev mailing list. 

regards, 
Rémi 

> On Mon, Apr 13, 2026, 3:13 AM Remi Forax < [ mailto:[email protected] |
> [email protected] ] > wrote:

>>> From: "Caleb" < [ mailto:[email protected] | [email protected] ] >
>>> To: "dev" < [ mailto:dev-GSC0n/0aZo1d/SJB6HiN2Ni2O/[email protected] | 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..
>> |
>> 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