JNI

Setup in Android using NDK

1. Install NDK

Android Studio → SDK Manager → SDK Tools tab

Enable:
- ✅ NDK (Side by side)
- ✅ CMake
- ✅ LLDB

2. Create new C/C++ source files

To add new C/C++ source files to an existing project, proceed as follows:

If you don't already have a cpp/ directory in the main source set of your app, create one as follows:
1. Open the Project pane on the left side of the IDE and select the Project view from the menu.
2. Navigate to your-module > src.
3. Right-click on the main directory and select New > Directory.
4. Enter cpp as the directory name and click OK.
Right-click the cpp/ directory and select New > C/C++ Source File.
Enter a name for your source file, such as native-lib.
From the Type menu, select the file extension for your source file, such as .cpp.

After you add new C/C++ files to you project, you still need to configure CMake to include the files in your native library.

Edit c++ file:

#include <jni.h>
#include <string>

extern "C"
JNIEXPORT jstring JNICALL
Java_com_packagename_myapp_MainActivity_stringFromJNI(JNIEnv *env, jobject thiz) {
    std::string hello = "Hello from C++ 👀";
    return env->NewStringUTF(hello.c_str());
}

3. Create a CMake build script

To create a plain text file that you can use as your CMake build script, proceed as follows:

Open the Project pane from the left side of the IDE and select the Project view from the drop-down menu.
Right-click on the root directory of your-module and select New > File.
Enter "CMakeLists.txt" as the filename and click OK.

Add the below code in your CMakeLists.txt:

# Sets the minimum version of CMake required to build your native library.
# This ensures that a certain set of CMake features is available to
# your build.

cmake_minimum_required(VERSION 3.4.1)

# Specifies a library name, specifies whether the library is STATIC or
# SHARED, and provides relative paths to the source code. You can
# define multiple libraries by adding multiple add_library() commands,
# and CMake builds them for you. When you build your app, Gradle
# automatically packages shared libraries with your APK.

add_library( # Specifies the name of the library.
             native-lib

             # Sets the library as a shared library.
             SHARED

             # Provides a relative path to your source file(s).
             src/main/cpp/native-lib.cpp )

4. Configure Gradle

To manually configure Gradle to link to your native library, you need to add the externalNativeBuild block to your module-level build.gradle file and configure it with either the cmake or ndkBuild block:

android {
  ...
  defaultConfig {...}
  buildTypes {...}

  // Encapsulates your external native build configurations.
  externalNativeBuild {

    // Encapsulates your CMake build configurations.
    cmake {

      // Provides a relative path to your CMake build script.
      path = file("CMakeLists.txt")
    }
  }
}

5. Use it in Code

Refer Android Docs

companion object {
    init {
        System.loadLibrary("native-lib");
    }
}
        

static {
    System.loadLibrary("native-lib");
}
        

This function call loads the .so file upon application startup.

external fun stringFromJNI(): String

public native String stringFromJNI();

The native keyword in this method declaration tells the virtual machine that the function is in the shared library (that is, implemented on the native side).

Last modified: 25 March 2026