无法使用NDK从Android Studio中的.so文件调用本机方法（库加载成功）

Question

1. I had created the sample project(HellWorldJNI) that runs with Android Ndk and prints "Hello world from Jni" message from the native C.

2. And then i extract the HelloWorldJNI.apk file of the sample from the Output/apk directory of the project.

3. So i got the all the .so files from different devices(like arm64-v8a,armeabi,x86..etc)

4. Now i created a new project (CallJNIFromSoFile) and created jniLibs folder inside main folder , and copied the all .so files of HelloWorldJNI app into this new project.

5. After that i load the library and the function name of the HelloWorldJNI App like this

    static { System.loadLibrary("hello-android-jni"); } public native static String getMsgFromJni(); 

6. Here i got strucked, i am able load the "hello-android-jni" library without any problem. But the native method getMsgFromJni() from the above is showing in RED color and not able call that method.

   Not at all receiving the message "Hello world from Jni"

   This is my Logcat Output:

    java.lang.UnsatisfiedLinkError: No implementation found for java.lang.String at com.example.hari.myopencvvsample.MainActivity.getMsgFromJni(Native Method) 

Please suggest am i missing anything other than the above steps. I tried many ways like regenerating the .so files and then put it again,but didn't worked . Sorry for my english.

Thanks in advance.

Answer 1

You need to provide a JNI implementation of the method getMsgFromJni() . The .so files are shared libraries, which you can link to and use from C or C++ code, but you need to provide an interface for any C or C++ methods you plan to call from Java, hence J ava N ative I nterface.

If you look at the src/main/jni folder in your HelloWorldJNI project, you will see a C or C++ file which implements any native methods called from Java in that project. You can also refer to the NDK samples for more help. The hello-libs example is a good place to start for your question.

Answer 2

This is completely bad idea to copy *so files between Android projects. JNI will find your function by the name which depends from the name of your Android application java-class hierarchy. Accordingly your description you compiled HelloWorldJni apk first and you have symbols there something like:

T Java_com_example_helloworldjni_MainActivity_getMsgFromJni

As you see there is your java-class hierarchy included in name of this function. And then, you are creating your own application with different class-names and your program looks for something like:

Java_com_example_hari_myopencvvsample_MainActivity_getMsgFromJni

inside your library, but as you know - it is not there. I'd suggest you to read the article https://developer.android.com/ndk/samples/sample_hellojni.html with its part about c implementation carefully, and repeat all the steps for your new CallJNIFromSoFile project.

Answer 3

Use targetSdkVersion less than 23 it will be working fine..

         defaultConfig {
             versionCode 1
             versionName "1.0.0"

             minSdkVersion 15
             targetSdkVersion 22