Creating a JNI Class in Kotlin

In order to invoke a JNI we need to create a Kotlin class with extrenal method and a C function with specific signature for each external method in Kotlin.

Let’s assume that we have the following class in Kotlin:

package hr.com.unix.kotlin.jni.example

class ExampleJNI {
    companion object {
        init {
            // We could also use System.loadLibrary if we package lib in jar
            System.load("/path/to/cmake-build-debug/libExampleJNI.so")
        }
    }
    external fun add(a: Int, b: Int): Int
}

For this to work, we will need to create CMake C++ project with following CMakeLists.txt config:

cmake_minimum_required(VERSION 3.25)
project(ExampleJNI)

set(CMAKE_CXX_STANDARD 17)

add_library(ExampleJNI SHARED
        main.cpp
        )
target_include_directories(ExampleJNI PUBLIC
        /usr/lib/jvm/java-17-openjdk-amd64/include
        /usr/lib/jvm/java-17-openjdk-amd64/include/linux
        )

the function with following main.cpp file:

#include <iostream>

#include "jni_md.h"
#include "jni.h"

extern "C" {
    jint Java_hr_com_unix_kotlin_jni_example_ExampleJNI_add(
            JNIEnv *env,
            jobject obj,
            jint a,
            jint b
    ) {
        return a + b;
    }
}