otavia-projects / mill-rust-jni   0.2.4

Apache License 2.0 GitHub

A mill plugin for build rust jni code!

Scala versions: 3.x 2.13 2.12
Mill plugins: 0.11 0.10


A mill plugin for build rust jni code!

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In your project build script build.sc, import the plugin with the following code:

import $ivy.`io.github.otavia-projects::mill-rust_mill$MILL_BIN_PLATFORM:{version}`
import io.github.otavia.jni.plugin.RustJniModule

Then define a jni source code module, the module name libjni can be anything you like.

object libjni extends RustJniModule {


You can then choose to generate a rust jni project template using the following command

mill libjni.nativeInit

This command will generate the native directory in the libjni module, the native directory is A simple Cargo project with the following project directory structure.

|-- libjni
|   |-- native
|   |   |-- Cargo.toml
|   |   |-- src
|   |   |   |-- lib.rs

You can of course create the native project structure manually, but if the native directory already exists the nativeInit command will be ignored.

Customized settings

The RustJniModule contains several special definitions that you can override.

Definition Description
defaultNativeName Define the jni library name for the nativeInit command to generate the project template
release Define cargo build mode
cargoBuildEnvs Environment variables used in cargo build. Since 0.2.0

Also contains the following commands or targets.

Definition Description
nativeName jni library name, parsed from Cargo.toml [package].name
rustSourceRoot rust source code root, i.e. {module}/native
localTarget The rust target that can be run on the current computer, set using the environment variable MILL_RUST_TARGET
crossTargets The targets supported by rust cross-compilation are set using the environment variable MILL_CROSS_RUST_TARGET, with a comma separating the targets
compileNative Compiling rust code
otherNativeLibraries Other exists native library, must contains native dir in it.
resources The output of compileNative and otherNativeLibraries are appended to resources, so the resulting binary library is automatically included in the module's localClasspath

Building jar packages

Since the results of compileNative are automatically appended to the resources of the module, the jar package of the module built with mill will include all the generated binary libraries. jar package will contain the following directory structure.

|-- native
|   |-- {rust_target1}
|   |   |-- {library_name}
|   |-- {rust_target2}
|   |   |-- {library_name}


Using jni-generated libraries is very simple, for example you have the following building script:

object libjni extends RustJniModule { // You can also extends PublishModule to publish this library jar to maven central 

  override def release: Boolean = true


object jni_jvm_interface extends ScalaModule { // jni_jvm_interface is example module, it can be anything you like.
  override def scalaVersion = "3.2.1"

  // use this to dependent maven central jni module, or other dependencies.
  // if this module is include jni jvm interface, you can also add a loader helper by this project.
  override def ivyDeps: T[Loose.Agg[Dep]] = Agg(
    ivy"{organization}:{artifactId}:{version}", // some dependency
    ivy"io.github.otavia-projects::jni-loader:{version}" // loader helper

  // use this to dependent local jni module dependencies.
  override def moduleDeps: scala.Seq[JavaModule] = scala.Seq(libjni)


Then define the native methods on the jvm in the jni_jvm_interface module:

defined by scala

package io.github.example

import io.github.otavia.jni.loader.NativeLoader

object RustJNI extends NativeLoader("libjni") {
  @native def add(a: Int, b: Int): Int

// or in class

class Adder(val base: Int) extends NativeLoader("libjni") {
  @native def plus(term: Int): Int

defined by java

package io.github.example;

import io.github.otavia.jni.loader.NativeLoader;

class JavaJNI extends NativeLoader {
    private int base = 0;

    JavaJNI() {

    public static native int add(int a, int b);

    public native void plus(int term);


Correspondingly, the code implemented in the libjni module native/src/lib.rs is

use jni::JNIEnv;
use jni::objects::*;
use jni::sys::*;

pub unsafe extern "C" fn Java_io_github_example_RustJNI_00024_add<'local>(mut env: JNIEnv<'local>, this: JObject<'local>, a: jint, b: jint) -> jint {
    a + b

pub unsafe extern "C" fn Java_io_github_example_Adder_plus<'local>(mut env: JNIEnv<'local>, this: JObject<'local>, term: jint) -> jint {

pub unsafe extern "C" fn Java_io_github_example_JavaJNI_add<'local>(mut env: JNIEnv<'local>, clz: JClass<'local>, a: jint, b: jint) -> jint {

pub unsafe extern "C" fn Java_io_github_example_JavaJNI_plus<'local>(mut env: JNIEnv<'local>, this: JObject<'local>, term: jint) {

NativeLoader uses the environment variable RUN_RUST_TARGET to specify the correct runtime rust target, for example on the author's x86_64 Windows 10 machine


If not specified, NativeLoader will speculate on possible targets, which may result in not loading the correct library file.

rust jni method naming rules

This plugin does not use javah to generate C headers, because the corresponding jni method naming rules are very simple:

  1. Use the Java_ prefix for the method name.
  2. Next, use the package name of the class where the native method is located, and convert the dot to an underscore.
  3. Next, use the class name, or in the case of scala object objects, using the object name + _00024.
  4. Next is the native method name.