Crate jnix

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This crate provides high-level extensions to help with the usage of JNI in Rust code. Internally, it uses the jni-rs crate for the low-level JNI operations.

Some helper traits are provided, such as:

  • AsJValue: for allowing a JNI type to be convected to a JValue wrapper type.
  • IntoJava: for allowing a Rust type to be converted to a Java type.
  • FromJava: for allowing a Rust type to be created from a Java type.

A JnixEnv helper type is also provided, which is a JNIEnv wrapper that contains an internal class cache for preloaded classes.

If compiled with the derive feature flag, the crate also exports procedural macros to derive IntoJava and to derive FromJava, which makes writing conversion code a lot easier. An example would be:

use jnix::{
    jni::{objects::JObject, JNIEnv},
    JnixEnv, FromJava, IntoJava,

// Rust type definition
#[derive(Default, FromJava, IntoJava)]
#[jnix(package = "my.package")]
pub struct MyData {
    number: i32,
    string: String,

// A JNI function called from Java that creates a `MyData` Rust type, converts it to a Java
// type and returns it.
pub extern "system" fn Java_my_package_JniClass_getData<'env>(
    env: JNIEnv<'env>,
    _this: JObject<'env>,
    data: JObject<'env>,
) -> JObject<'env> {
    // Create the `JnixEnv` wrapper
    let env = JnixEnv::from(env);

    // Convert parameter to Rust type
    let data = MyData::from_java(&env, data);

    // Create a new `MyData` object by converting from the Rust type. Since a smart pointer is
    // returned from `into_java`, the inner object must be "leaked" sothat the garbage collector
    // can own it afterwards
package my.package;

public class MyData {
    public MyData(int number, String string) {
        // This is the constructor that is called by the generated `IntoJava` code
        // Note that the fields don't actually have to exist, the only thing that's necessary
        // is for the target Java class to have a constructor with the expected type signature
        // following the field order of the Rust type.

    // These getters are called by the generated `FromJava` code
    public int getNumber() {
        return 10;

    public String getString() {
        return "string value";


  • pub extern crate jni;


  • Provides access to JNI functions.


  • Returns a value as its JValue representation.
  • Conversion from a Java type into its Rust equivalent.
  • Conversion from a type into its Java representation.