从 JNI 访问 Java class 对象方法

Question

我有一个 Android 应用程序，我在其中使用 C++ 代码实现了与 WebSocket 的连接。

现在我想通过 C++ 代码和 JNI 调用在 Java class 中初始化的对象的方法。

可以吗？

这是我的 Activity:

public class MainActivity extends AppCompatActivity{

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

}

我想做的是从 JNI 调用 object.myCustomMethod()。

Answer 1

我试着把你的用例的部分代码。

在 onCreate 期间将自定义对象传递给 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);
}

在本机端，您保留对象的引用并在适当的时候调用它

//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);
}

从 JNI 访问 Java class 对象方法

Access Java class object methos from JNI

c++

java

java-native-interface

android

android-ndk