Java language bindings for payjoin-ffi

[ram0verflow]

Which crate is this feature request for?

payjoin-ffi

Please describe the feature you'd like to see added.

I'd like to add Java bindings for payjoin-ffi.

There is no JVM target today. I'd follow the C# layout (payjoin-ffi/java/), generate with uniffi-bindgen-java, and keep the generator in that directory rather than adding a cargo feature on payjoin-ffi.

First PR: generate, tests including v2, CI on linux/mac/windows. Not in that PR: Maven or a wallet.

Does that match what you'd want? I can open a draft after generate works locally.

Is your feature related to a problem, if so please describe it.

No response

Describe the solution you'd like

No response

Describe any alternatives you've considered

No response

Please leave any additional context

No response

[ram0verflow]

Following up on this, I did some work and found a few things.

uniffi-bindgen-java 0.5.0 requires uniffi 0.32. payjoin-ffi is on 0.31.2, and 0.5.0 rejects the metadata with Unexpected metadata type code: 116. The previous release, 0.4.2, reads 0.31 metadata but generates Java that does not compile for this crate.

From there,

I backported the 0.5.0 template fixes onto the 0.31 line. I opened IronCoreLabs/uniffi-bindgen-java#68 asking about a 0.4.3 and offered to do the work. Fixes are on a fork meanwhile and I generated bindings against that fork here

A cleaner approach would be to move payjoin-ffi to uniffi 0.32, which 0.5.0 supports unmodified. That is not limited to payjoin-ffi. uniffi-bindgen-cs is pinned at v0.11.0+v0.31.0 and uniffi-dart at v0.2.1+v0.31.2, so both would need 0.32 tags first.

A third option avoids the bindgen: a hand-written JVM layer over the official Kotlin bindings, CompletableFuture rather than coroutines. I did some work here. Works, but hand-maintained, and JNA rather than FFM.

I can open a draft PR for whichever direction you prefer, or leave this if Java bindings are not wanted at present.

[chavic]

@ram0verflow Thanks for taking this on. I see you added Kotlin and are wrapping it. I'd have the Kotlin in its own draft; it seems like that could ship faster...

Also, do you have or know of a project demanding Java? I'd like to keep an eye on that

[ram0verflow]

@chavic Thanks! I’ll separate the Kotlin work out and get a V2 integration test working first, then put up a draft PR.

For Java, my original motivation was to experiment with integrating Payjoin V2 into Sparrow Wallet. I’m not working on Sparrow officially, It’s just a personal project/interest. I needed JVM/Java support for that, and the Kotlin work came out of exploring the cross-platform JVM side, rather than Kotlin being the original goal.