Does Android Wear support C++ (JNI) in Android Studio?

Question

I cannot get an Android Wear project to use C++. I am able to get a "Phone and Tablet" project to use C++. Here is what I have done.

1. Here is an image of the SDK Tools I have installed. I also have SDK API Levels 24-27 installed.

   

2. I create a new project. I check "Include C++ support". I check "Wear" (API 26: Android 8.0 (Oreo)). I select "Next" a bunch.

3. For C++ Standard I have tried all three (Toolchain Default, C++11, c++14).

   • I do not check -fexceptions or -frtti.

In the project that is created under the "mobile" Module, I have a "cpp" folder, but I do not have one under the "wear" Module.

If I create a project without "Phone and Tablet" support ("mobile" Module), then I still do not get a "cpp" folder under the "wear" Module.

I tried forcing the project to use C++ under the "wear" Module. Here is what I did.

1. I modified build.gradle (Module: wear) to look like this

   apply plugin: 'com.android.application'
   
   android {
       compileSdkVersion 26
       defaultConfig {
           applicationId "com.example.xorgaming.watchtestcpp"
           minSdkVersion 25
           targetSdkVersion 26
           versionCode 1
           versionName "1.0"
           testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
           externalNativeBuild {
               cmake {
                   cppFlags "-std=c++11"
               }
           }
       }
       buildTypes {
           release {
               minifyEnabled false
               proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
           }
       }
       externalNativeBuild {
           cmake {
               path "CMakeLists.txt"
           }
       }
   }
   
   dependencies {
       implementation fileTree(dir: 'libs', include: ['*.jar'])
       implementation 'com.google.android.support:wearable:2.2.0'
       implementation 'com.google.android.gms:play-services-wearable:11.8.0'
       implementation 'com.android.support:percent:26.1.0'
       implementation 'com.android.support:support-v4:26.1.0'
       implementation 'com.android.support:recyclerview-v7:26.1.0'
       implementation 'com.android.support:wear:26.1.0'
       compileOnly 'com.google.android.wearable:wearable:2.2.0'
   }
   

2. I restart the project which makes a "cpp" folder under the "wear" module.

3. I create a native-lib.cpp file in the "cpp" folder. It looks like this:

   extern "C"
   JNIEXPORT jstring JNICALL Java_com_example_xorgaming_watchtestcpp_MainActivity_stringFromJNIWatch(
       JNIEnv *env, jobject /* this */)
   {
       std::string hello = "Hello from C++ WATCH!";
       return env->NewStringUTF(hello.c_str());
   }
   

4. In my java onCreate() function, I call: stringFromJNIWatch().

5. Everything builds without error (green hammer)

6. When I run the project (as a wearable device) I get this error:

   No implementation found for java.lang.String com.example.xorgaming.watchtestcpp.MainActivity.stringFromJNIWatch()

Any idea what I'm doing wrong? Does Android Wear support C++?

Answer 1

Things have changed quite a bit since this question was posted. But half of the answer is: Yes, you can use C++ and Java together with Wear OS. You can follow the same instructions for Android applications.

As for why this failed, my suggestion would be double-check the paths. If you follow the instructions here you will note that the cpp directory goes in the same directory with the java directory. Typically something like MyApplication/app/src/main will have a java directory and a cpp directory. That helps everything get wrapped up into the eventual .apk