Loading custom Java classes

[DataTriny]

Hi,

I am using the GameActivity and I need to instanciate a Java class that I wrote. This class is not referenced anywhere on the Java side but it is present in the APK. When I call env.find_class inside my android_main function, I get a ClassNotFoundException. I have read that this could happen inside a thread not spawned by the JVM that would use the default ClassLoader. I have also found this comment just before the call to android_main:

// XXX: If we were in control of the Java Activity subclass then
// we could potentially run the android_main function via a Java native method
// springboard (e.g. call an Activity subclass method that calls a jni native
// method that then just calls android_main()) that would make sure there was
// a Java frame at the base of our call stack which would then be recognised
// when calling FindClass to lookup a suitable classLoader, instead of
// defaulting to the system loader. Without this then it's difficult for native
// code to look up non-standard Java classes.

I think this explain my issue. Is there a typical way around this? I've heard about the JNI_OnLoad function, but I'm afraid it would interfear.

How difficult whould it be to lift this limitation, if it is something doable?

Thanks for this amazing crate!

[rib]

Hi, sorry I missed this question when you asked it.

Yeah this is an awkward Android + JNI issue for native apps. One option is to look up the class loader for the Activity instance that you have access to and use that instead of find_class.

E.g. this is what I do in Bluey: https://github.com/rib/bluey/blob/5f351c50e6f396bc4fef9c050599f9668c63b339/bluey/src/android/session.rs#L1017

Potentially android-activity could provide some kind of utility for this but ideally I don't really want to expose jni crate types in the public API for android-activity and then effectively pin downstream crates to the same version of the jni crate.

Hope that helps though.

[DataTriny]

Thanks @rib

For now I ask the user to call a Java method in their GameActivity subclass and I'd like to avoid this if possible. I'll try your approach as it seems to be exactly what I need.
Maybe this recipe could be added somewhere in the documentation?