android-jni-hello-world

This repo shows a minimal reproducible example of how to integrate Android Java code with native C++ code.

The following are notes that I took while reading through the official Android documentation.

General Overview: how to add C/C++ code to your Android project?

1. Place the C/C++ code in a cpp directory inside /src/app/main.
2. Create a CMakeLists.txt build script in the cpp directory. See the comments in CMakeLists.txt.
3. In the module's build.gradle file, provide the path to CMakeLists.txt, along with optional parameters. See the comments in app/build.gradle.
4. Java or Kotlin code can then call functions in your native library through JNI.

What happens when you build a project with C/C++?

1. Gradle calls upon your external build script, CMakeLists.txt.
2. Cmake follows the commands in the CMakeLists.txt build script to compile the C/C++ code into a native library, which is a shared object library (a .so file).
   • If a native library is named native-lib, the resulting .so file is called libnative-lib.so.
   • It prepends "lib" and uses the .so extension. See the comments in CMakeLists.txt.
3. Gradle packages this .so file in your app’s APK.

What happens when you run a project with C/C++?

1. Java/Kotlin code loads the native library using System.loadLibrary(libraryName).
   • Pass the library name as a string. For example, System.loadLibrary("native-lib").
2. Java/Kotlin can call any native function as a normal Java/Kotlin function. See JNI rules.

NOTE: Need to read again https://developer.android.com/studio/projects/configure-cmake#add-other-library