It's worth pointing out that the example you linked with the cdecl is not showcasing the swift-java interoperability but using "raw" JNI patterns as if you would do it with C.

This example does showcase the interoperability: https://github.com/swiftlang/swift-android-examples/tree/mai... you don't have to annotate or write any "weird" cdecls or really touch JNI details yourself when using the swift-java interop support https://github.com/swiftlang/swift-java