First Export

This guide walks through exporting a simple Swift class and calling it from Kotlin.

1. Annotate Swift code

Create a Swift file in your target and annotate the types you want to expose with @jvm:

import Swift4j

@jvm
class GreetingService {
    func greet(name: String, _ response: (Greeting) -> Void) {
        response(Greeting(name: name))
    }
}

@jvm
class Greeting {
    let name: String

    init(name: String) {
        self.name = name
    }

    func getMessage() -> String {
        "Swift says hello to \(name)"
    }
}

Both types appear in the Kotlin signature of greet, so both must be annotated with @jvm.

2. Generate Kotlin bridging classes

Run the generate-java-bridging SPM plugin from your package directory:

swift package plugin generate-java-bridging --product MyLibrary

The generated files are written to:

.build/plugins/generate-java-bridging/outputs/MyLibrary/main/java/

Copy or symlink this directory into your Android or Java project’s source tree.

3. Build the native library

swift build -c release

This produces libMyLibrary.dylib (macOS) or libMyLibrary.so (Linux/Android). Copy this file to wherever your JVM project expects native libraries.

4. Call from Kotlin

import MyLibrary.GreetingService

fun main() {
    System.loadLibrary("MyLibrary")

    val service = GreetingService()
    service.greet("Kotlin") { greeting ->
        println(greeting.getMessage())
    }
}

System.loadLibrary("MyLibrary") must be called before any Swift type is used. On Android, call it in Application.onCreate() or Activity.onCreate().

Next steps

• Classes — properties, static methods, object lifecycle
• Structs — value types and inout parameters
• Closures — callbacks and lambda passing
• Code Generation — SPM plugin options and output structure