Android NDK making library available all through the app

Issue

I am diving into the ndk stuff and I've managed to make a simple library and call it from my activity.

In order to test it, in my Activity I have used:

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

    public native static String callNativeFunction(Context context);

This means that calling MyActivity.callNativeFunction(context) does return the String value from my Cpp function.

I have 2 questions:

1. The loadLibrary is made in my main activity, however I want to be able for instance, to call the callNativeFunction function from an IntentService when the activity may be closed or from other places of my app. How can I properly load the library and have it available from all places of the app?

2. Since this is a simple function that I'll use in my project, is there anything else specific to do on release? From https://developer.android.com/studio/projects/add-native-code.html it seems that Gradle builds for all supported abis and adds them into the apk.

Solution

1. You need to load the library once. This can also be done in your Application class. Once the library is loaded, you can call the defined native methods from the appropriate classes defined in your cpp class

Application class :

public class SRApplication extends Application {
    static {
        System.loadLibrary("my-lib");
    }

    @Override
    public void onCreate() {
       super.onCreate();
    }
}

CPP :

extern "C"
jstring
Java_com_example_service_ExampleService_getNativeString (
    JNIEnv* env,
    jobject /* this */) {

    return env->NewStringUTF("yo");
}

Your Service :

package com.example.service.ExampleService;

public class ExampleService extends Service {

    @Nullable
    @Override
    public IBinder onBind(Intent intent) {
        String nativeString = getNativeString();
        return null;
    }

    public native String getNativeString();

}

2. If you don't have specific code for different cpu variants, you don't need to explicitly handle anything, the default

   externalNativeBuild { cmake { cppFlags "" } }

   is sufficient.

Answered By - Alim Parkar