Question

My application is an Android app with some native code.

The Java code starts a thread via new Thread(new Runnable), in the native code, I attach that thread to the JVM via AttachCurrentThread.

The native function which is called in that thread is an endless loop which will listen for simple events(boolean variables) and call another native function which by using the proper JNIEnv pointer and jobject will fill 2 java variables on the Java side and call a void method to display the data. The reason for that loop is that the overhead to calling the function normally(has a lot of allocation and destruction to do) is most likely huge.

The only problem is that while I can renew a JNIEnv pointer, I have no idea how to do it for a jobject.

And by jobject I mean the jobject which is passed to the native code via the function call like so

jint Java_com_example_example_MainActivity_NativeFunc( JNIEnv* env,
                                              jobject obj)
{
    return;
}

Answer 1

The Java code starts a thread via new Thread(new Runnable), in the native code, I attach that thread to the JVM via AttachCurrentThread.

You don’t have to attach the JVM to a thread that was created with Java code, it is already attached.

If I understood your question correctly you get a jobject once and want to access that object later in other native methods. To accomplish that, you can use NewGlobalRef:

jobject myGlobalReference = env->NewGlobalRef(obj);

You can store that jobject in a global C/C++ variable and use it later. It will stay valid until the JVM dies or you delete the reference with DeleteGlobalRef. Keep in mind that the garbage collection won’t collect the referenced object if all Java references to it are gone as long as you don’t delete your global reference!