jni-toolbox

This is a simple crate built around jni-rs to automatically generate JNI-compatible extern functions.

It also wraps functions returning Result<>, making short-circuiting easy.

Usage

Just specify package and class on your function, and done!

#[jni_toolbox::jni(package = "your.package.path", class = "ContainerClass")]
fn your_function_name(arg: String) -> Result<Vec<String>, String> {
  Ok(arg.split('/').map(|x| x.to_string()).collect())
}

Conversions

Every type that is meant to be sent to Java must implement IntoJavaObject (or, unlikely, IntoJavaPrimitive); every type that is meant to be received from Java must implement FromJava. Most primitives and a few common types should already be implemented.

impl<'j> IntoJavaObject for MyClass {
  type T = jni::objects::JObject<'j>
  fn into_java(self, env: &mut jni::JNIEnv<'j>) -> Result<Self::T, jni::errors::Error> {
    let hello = env.new_string("world")?;
    // TODO!!
  }
}

Pointers

Note that, while it is possible to pass raw pointers to the JVM, it is not safe by default and must be done with extreme care.

Exceptions

Errors are thrown automatically when a Result is an error. For your errors to work, you must implement the JniToolboxError trait for your errors, (which just returns the path to your Java error class) and then make a Java error wrapper which can be constructed with a single string argument.

Functions returning Results will automatically have their return value unwrapped and, if is an err, throw an exception and return early.

impl JniToolboxError for MyError {
  fn jclass(&self) -> String {
    "my/package/some/MyError".to_string()
  }
}

package my.package.some;
public class MyError extends Throwable {
  public MyError(String x) {
    // TODO
  }
}

To throw simple exceptions, it's possible to use the exception attribute. Pass the exception's fully qualified name (must have a constructor that takes in a single String argument).

Examples

The following function:

#[jni(package = "mp.code", class = "Client", ptr)]
fn connect(config: Config) -> Result<Client, ConnectionError> {
  super::tokio().block_on(Client::connect(config))
}

generates a matching expanded function invoking it:

fn connect(config: Config) -> Result<Client, ConnectionError> {
  super::tokio().block_on(Client::connect(config))
}

#[no_mangle]
#[allow(unused_unit)]
pub extern "system" fn Java_mp_code_Client_connect<'local>(
  mut env: jni::JNIEnv<'local>,
  _class: jni::objects::JClass<'local>,
  config: <Config as jni_toolbox::FromJava<'local>>::From,
) -> <Client as jni_toolbox::IntoJava<'local>>::Ret {
  use jni_toolbox::{FromJava, IntoJava, JniToolboxError};
  let config_new = match jni_toolbox::from_java_static::<Config>(&mut env, config) {
    Ok(x) => x,
    Err(e) => {
      let _ = env.throw_new(e.jclass(), format!("{e:?}"));
      return std::ptr::null_mut();
    }
  };
  let result = connect(config_new);
  let ret = match result {
    Ok(x) => x,
    Err(e) => match env.find_class(e.jclass()) {
      Err(e) => panic!("error throwing Java exception -- failed resolving error class: {e}"),
      Ok(class) => match env.new_string(format!("{e:?}")) {
        Err(e) => panic!("error throwing Java exception --  failed creating error string: {e}"),
        Ok(msg) => match env.new_object(class, "(Ljava/lang/String;)V", &[jni::objects::JValueGen::Object(&msg)]) {
          Err(e) => panic!("error throwing Java exception -- failed creating object: {e}"));
          Ok(obj) => match env.throw(jni::objects::JThrowable::from(obj)) {
            Err(e) => panic!("error throwing Java exception -- failed throwing: {e}"),
            Ok(_) => return std::ptr::null_mut(),
          },
        },
      },
    },
  };
  match ret.into_java(&mut env) {
    Ok(fin) => fin,
    Err(e) => {
      let _ = env.throw_new(e.jclass(), format!("{e:?}"));
      std::ptr::null_mut()
    }
  }
}

Status

This crate is early and intended mostly to maintain codemp's Java bindings, so things not used there may be missing or slightly broken. However, the crate is also quite small and only runs at compile time, so trying it out in your own project should not be a problem.