Calling Rust from Java

Question

Apart from the problem that *mut Any/*const Any are fat pointers, there is also a fact that native JNI functions use double indirection when accessing JNINativeInterface structure:

struct JNINativeInterface_;
typedef const struct JNINativeInterface_ *JNIEnv;
jint (JNICALL *GetVersion)(JNIEnv *env);

Here, you can see that JNIEnv is a pointer to JNINativeInterface_ structure which actually contains the fields you presented, and GetVersion accepts a pointer to JNIEnv – that is, it requires a pointer to a pointer to JNINativeInterface_. This Rust program works on my machine (Rust nightly is used but the same code would work in beta with an external libc crate):

#![crate_type="dylib"]
#![feature(libc)]
extern crate libc;

use libc::c_void;

#[repr(C)]
pub struct JNINativeInterface {
    reserved0: *mut c_void,
    reserved1: *mut c_void,
    reserved2: *mut c_void,
    reserved3: *mut c_void,

    GetVersion: extern fn(env: *mut JNIEnv) -> i32,

    _opaque_data: [u8; 1824]
}

pub type JNIEnv = *const JNINativeInterface;

#[no_mangle]
pub extern fn Java_tests_Test_helloJre(jre: *mut JNIEnv, class: *const c_void) {
    println!("Invoked native method, jre: {:p}, class: {:p}", jre, class);
    unsafe {
        let v = ((**jre).GetVersion)(jre);
        println!("version: {:?}", v);
    }
}

Java counterpart:

package tests;

import java.nio.file.Path;
import java.nio.file.Paths;

public class Test {
    public static native void helloJre();

    public static void main(String[] args) {
        Path p = Paths.get("libtest.dylib");
        System.load(p.toAbsolutePath().toString());
        Test.helloJre();
    }
}

Invocation:

% javac tests/Test.java
% java tests.Test
Invoked native method, jre: 0x7f81240011e0, class: 0x10d9808d8
version: 65544

65544 is 0x10008, and indeed, I’m running this under Oracle JVM 1.8.

I guess you can omit _opaque_data field as JNINativeInterface structure is always passed by pointer, so if you only need several first fields from the structure, you can declare only them and ignore the rest.

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