I have an Android app in which I have implemented the connection with a WebSocket in the C code.

Now I would like to invoke a method of an object initialized in the Java class, via C code with JNI.

It's possible to do it?

This is my Activity:

public class MainActivity extends AppCompatActivity{

private MyCustomObject object; //This object is initialized in the life cycle of the activty

}

What I want to do is call object.myCustomMethod() from JNI.

CodePudding user response：

I tried to put the part of code for your use case.

1. Pass the custom object during onCreate to JNI

    //MainActivity.java
    public class MainActivity extends AppCompatActivity {
   
       // Used to load the 'native-lib' library on application startup.
   
       static {
           System.loadLibrary("native-lib");
       }
   
       private MyCustomObject object;
   
       protected void onCreate(Bundle savedInstanceState) {
   
           object = new MyCustomObject();
   
           //object is passed tthrough JNI call
           intJNI(object);
       }
   
       public class MyCustomObject{
   
           public void myCustomMethod(){
   
           }
   
       }
   
   
       /**
        * A native method that is implemented by the 'native-lib' native library,
        * which is packaged with this application.
        */
       public native void intJNI(MyCustomObject obj);
   }
   

2. At native side you keep the reference of the object and call it at appropriate time

   //JNI 
   static jobject globlaRefMyCustomObject;
   static JavaVM *jvm;
   
   extern  "C" JNIEXPORT void JNICALL
   Java_test_com_myapplication_MainActivity_intJNI(
           JNIEnv* env,
           jobject callingObject,
           jobject myCustomObject) {
   
           jint rs = env->GetJavaVM(&jvm);
           assert (rs == JNI_OK);
   
           //take the global reference of the object
           globlaRefMyCustomObject =     env->NewGlobalRef(myCustomObject);
   
   }
   
   //this is done in any background thread in JNI 
   void callJavaCallbackFucntion(){
   
        JNIEnv *env;
        jint rs = jvm->AttachCurrentThread(&env, NULL);
        assert (rs == JNI_OK);
   
       jclass MyCustomObjectClass = env->GetObjectClass(globlaRefMyCustomObject);
       jmethodID midMyCustomMethod = env->GetMethodID(MyCustomObjectClass, "myCustomMethod", "()V");
       env->CallVoidMethod(globlaRefMyCustomObject,midMyCustomMethod);
   
       /* end useful code */
       jvm->DetachCurrentThread();
   }
   
   
   //Release the Global refence at appropriate time
   void JNI_OnUnload(JavaVM *vm, void *reserved){
       JNIEnv* env;
       if (vm->GetEnv(reinterpret_cast<void**>(&env), JNI_VERSION_1_6) != JNI_OK) {
           return JNI_ERR;
       }
       env->DeleteGlobalRef(globlaRefMyCustomObject);
   }