Using JNI to call a Rust function that passes and returns a struct

[HaidaLu]

Hey,
I am new to Rust and now am trying to use JNI to call a Rust function that passes and returns a struct. A struct may define like this:

struct stud
{
	long int num;
	float score;
};

But since there is no struct in Java or JNI parameter conversion. I was thinking to create a corresponding class in Java. But don't know how to implement it.

I have googled this problem, there are very few references and just mentioned maybe I can use jlong. I am still very confused and don't know how to implement it.

Could anyone enlighten me or is there any reference or tutorial regarding this?

Thanks in advance!

[Dushistov]

You can look at what my tool generates and do the same:

For such code:

struct Strud {
    num: i64,
    score: f32,
}

fn my_func() -> Strud {
    todo!()
}

foreign_class!(class Strud {
    self_type Strud;
    private constructor = empty;
    fn num(&self) -> i64 {
        this.num
    }
    fn score(&self) -> f32 {
        this.score
    }
});

foreign_class!(class Statics {
    fn my_func() -> Strud;
});

it generates:

#[no_mangle]
pub extern "C" fn Java_com_example_rust_Statics_do_1my_1func(env: *mut JNIEnv, _: jclass) -> jlong {
    let mut ret: Strud = my_func();
    let ret: jlong = <Strud>::box_object(ret);
    ret
}

where box_object looks like this:

        let this: Box<Strud> = Box::new(this);
        let this: *mut Strud = Box::into_raw(this);
        this as jlong

and one of access functions to data:

#[no_mangle]
pub extern "C" fn Java_com_example_rust_Strud_do_1num(
    env: *mut JNIEnv,
    _: jclass,
    this: jlong,
) -> jlong {
    let this: &Strud = unsafe { jlong_to_pointer::<Strud>(this).as_mut().unwrap() };
    let mut ret: i64 = { this.num };
    let mut ret: jlong = ret;
    ret
}

generated Java code for Strud:

public final class Strud {
    /*package*/ long mNativeObj;

    private Strud() {}

    public final long num() {
        long ret = do_num(mNativeObj);

        return ret;
    }
    private static native long do_num(long self);

[HaidaLu]

Thank you!!