Working toward making a vulkan + OpenXR app on Oculus Quest: How can I get a pointer to the JVM?

[Phil Dougherty]

Slowly figuring out how to get all of these things correctly hooked together-

on the oculus website ( https://developer.oculus.com/downloads/package/oculus-openxr-mobile-sdk/) it has the following snippet:

loaderInitializeInfoAndroid.applicationVM = data->applicationVM; 

This is part of a process to hook up the openXR loader. However, this documentation shows nothing of how to get such a pointer. Googling "NDK get JVM" isn't coming up with any straightforward documentation either...

So, how can I get access to the JVM from within C++ using the NDK?

What do I need to link against? What do I need to #include? What initialization do I need to do?

I'm using android_native_app_glue.h, which gets me access to the ANativeActivity (via struct android_app), which I also needed in the snippet from the oculus website. But nothing is there regarding the JVM.

[mic _]

ANativeActivity has a member JavaVM *vm

https://developer.android.com/ndk/reference/struct/a-native-activity 

 

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[Florian Limberger]

I don't know how the Oculus SDK integrates with Android, but typically, an Android app has a Java entrypoint to the native code in form of a native method, like this:

public class MyActivity extends Activity {

    static {

        System.loadLibrary("mylib");

    }

    public void onCreate() {

        super.onCreate();

        if (!myInit()) {

            // handle the error

        }

    }

    private static native boolean myInit();

}

This method provided by a shared library (in this example mylib.so) like described here: https://developer.android.com/training/articles/perf-jni#native-libraries

All the native functions take a pointer to a JNIEnv as first parameter, which can then used to retrieve a pointer to the JVM:

#include <jni.h>

// Do the JNI_OnLoad() dance here, described in the link above

jboolean myInit(JNIEnv *env, jclass clazz) {

    (void)clazz;

    JavaVM *jvm = NULL;

    jint result = GetJavaVM(env, &jvm);

    if (result < 0) {

        return JNI_FALSE;

    }

    return JNI_TRUE;

}

JNI Documentation is available here: https://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/jniTOC.html

Disclaimer: I wrote all code above from top of my head and tested nothing of this. I hope this helps anyway.

[Phil Dougherty]

ah awesome. that's... really strange they would have me separately pass in the ANativeActivity AND the JavaVM when one is accessible from the other...

[Florian Limberger]

As far as I understand it, ANativeActivity is for convenience and not necessary.

At work we use native code from a Service with the approach I described above.

If you use ANativeActivity, you do not need to retrieve the pointer to the JVM manually.