JNI Modules

JNI Modules

The CLI calls this backend Native JNI Module. Use it when implementation belongs in C or C++ and a Promise-based JavaScript call is appropriate. The generator owns the Kotlin/Java Native Interface (JNI) bridge and registration.

Generate a module

supernote-module add local-math-jni --type jni --yes

The derived JavaScript name is MathJni and the Android namespace is com.example.math_jni.

Check the C/C++ toolchain first when needed:

supernote-module doctor --type jni

Write the C++ export

Edit the user-owned C/C++ tree:

local_modules/local-math-jni/android/src/main/cpp/

For example, edit math.cpp:

// @SupernoteExport
double add(double left, double right) {
  return left + right;
}

The marker must be immediately before an ordinary top-level C++ definition. Do not write a JNI function, JNI_OnLoad, native registration, a loader, or generated CMake entries.

Call it

JNI value returns are asynchronous at the JavaScript boundary:

import MathJni from 'local-math-jni';

const total = await MathJni.add(20, 22);

Generated JNI transports strings as UTF-8 byte arrays instead of JNI modified UTF-8.

C and C++ roles

• .cc, .cpp, and .cxx files may contain marked exports.
• .c files compile as C23 and may provide internal helpers, but cannot export.
• C++ and generated bindings compile as C++23.
• Use extern "C" guards in headers when C++ calls C helpers.

Supported boundary values are bool, double, and UTF-8 std::string by value, plus void returns. See Exports and JavaScript API for restrictions.

Build and verify

supernote-module validate local-math-jni --build --verbose
./buildPlugin.sh

Then reinstall the resulting .snplg and inspect both bridge and JavaScript logs if a call fails:

adb logcat -d -s SupernoteNativeMathJni:V ReactNativeJS:V '*:S'

Ownership

Update preserves the complete android/src/main/cpp/ tree, including your choice to delete starter files. It replaces metadata, wrappers, declarations, Gradle/CMake, generated Kotlin/C++, loading, registration, and the generated README. Remove deletes everything in the module, including the C/C++ tree.