How to import visual c++ library into android studio project?

Question

I created a shared visual c++ cross platform mobile library in visual studio 2017 for iOS, UWP, and Android. I was successful able to create a Windows RT Component wrapper for the library to use in a C# UWP. I removed the iOS library as I did not need it. All that remain is the Android project. Currently I am struggling how to write a wrapper for the shared library and import it into android studio. I looked the documentation provided on MSDN, but it goes more in depth on creating a cross platform c++ application and not enough detail on how to leverage the shared static libs.

RandString.java:

package com.myapplication;

/**
 * Created by yorel56 on 7/26/2017.
 */

public class RandString {
    static{
        System.loadLibrary("libRandString");
    }
    public native String GetString();
    public native String GetString(int index);
}

build.gradle (module):

apply plugin: 'com.android.application'

android {
    compileSdkVersion 25
    buildToolsVersion "25.0.2"
    defaultConfig {
        applicationId "com.myapplication"
        minSdkVersion 15
        targetSdkVersion 25
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
        externalNativeBuild {
            cmake {
                cppFlags ""
            }
        }
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
    externalNativeBuild {
        cmake {
            path "CMakeLists.txt"
        }
    }
    sourceSets.main {
        jni.srcDirs = [] //disable automatic ndk-build call
        jniLibs.srcDir 'src/main/libs/' //integrate your libs from libs instead of jniLibs
    }

    tasks.withType(JavaCompile) {
        compileTask -> compileTask.dependsOn(nativeLibsToJar)
    }

    task ndkBuild(type: Exec, description: 'compile native code') {
        def ndkDir = "C:\\Users\\yorel56\\AppData\\Local\\Android\\sdk\\ndk-bundle"
        workingDir "src/main/jni"
        commandLine "$ndkDir/ndk-build"
    }

    task nativeLibsToJar(type: Jar, description: 'create a jar archive of the native libs') {
        destinationDir file("$buildDir/native-libs")
        baseName 'native-libs'
        extension 'jar'
        from fileTree(dir: 'libs', include: '**/*.so')
        into 'lib/'
    }
    nativeLibsToJar.dependsOn {
        ndkBuild  // comment that, when you don't want to rerun ndk-build script
    }
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    compile fileTree(dir: "$buildDir/native-libs", include: 'native-libs.jar')
    androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
        exclude group: 'com.android.support', module: 'support-annotations'
    })
    compile 'com.android.support:appcompat-v7:25.3.1'
    compile 'com.android.support.constraint:constraint-layout:1.0.2'
    testCompile 'junit:junit:4.12'
}

The error i am getting currently is that 'native' is not allowed here, and when i choose to let Android Studio fix it by creating a function in native_lib.cpp, I continue to get errors. I tried following the documentation here.

Answer 1

You can use the libRandString.so which was built with Visual Studio; you don't need the externalNativeBuild block and not ndkBuild task and not the nativeLibsToJar task in your build.gradle. Android Studio will pick up the prebuilt files from jniLibs directory and pack them into the APK for you.