How do you build a custom native module in Android (Kotlin) and connect it to React Native?

Short answer

Create a class that extends ReactContextBaseJavaModule, annotate exported methods with @ReactMethod, and register the class in a custom ReactPackage added to getPackages() in MainApplication.

Explanation

Set the module name with getName(). Methods annotated with @ReactMethod can receive basic arguments (Strings, Numbers, ReadableMap) and use a Promise to return results or errors asynchronously to JavaScript. This ReactContextBaseJavaModule approach belongs to the old architecture and works through the compatibility layer; new projects should prefer Turbo Native Modules with Codegen.

Code example

// Kotlin code under android/
class BatteryModule(reactContext: ReactApplicationContext) : ReactContextBaseJavaModule(reactContext) {
    override fun getName() = "BatteryModule"

    @ReactMethod
    fun getBatteryLevel(promise: Promise) {
        try {
            val level = 85 // read from the Android system (BatteryManager)
            promise.resolve(level)
        } catch (e: Exception) {
            promise.reject("ERROR_BATTERY", e.message)
        }
    }
}

Common mistakes

• Trying to return complex custom objects directly without converting them to WritableMap or types supported by the Bridge.

Follow-up questions

• What is Codegen in the new React Native architecture, and why is it required?
• How do you build a custom native module in iOS (Swift/Objective-C)?

Sources

• React Native docs — Turbo Native Modules: Android