graal-bindgen

graal-bindgen generates safe bindings between Rust and Graal Polyglot so that you can use Java types and methods as if they were native to Rust.

• Overview
• Building
• TODO
• ArrayList example
• Constructor stubs
• Function stubs
• Pass and Receive
  • Pass and Passable
  • Receive
• Generics
• Arrays

Overview

The class macro is the primary way to generate bindings to Java types; it will generate a struct (with generics if specified) that implements Pass and Receive and has all the methods you give stubs for. The methods generated can be used like normal rust methods, however mutability is not enforced. The fully-qualified type name should precede a block containing method and constructor stubs. Java primitives like char, int, and byte are aliased to corresponding Rust types.

Building

First, make sure you have cargo-make installed, the GRAAL_HOME environment variable points to the root directory of your GraalVM installation, and the GraalVM LLVM toolchain is installed:

export GRAAL_HOME=[PATH_TO_GRAAL]
cargo install cargo-make
${GRAAL_HOME}/bin/gu install llvm-toolchain

You can then run

cargo make run

to run main.rs, or

cargo make build

to just compile it. Currently, graal-bindgen isn't published on crates.io since its build process is reliant on GraalVM being installed.

TODO

• Automated generation from Javadoc
• Generics in generics
• Generic bounds (CitrusJuice<? extends CitrusFruit>
• Static field access
• Function and type declaration from Rust
• Tests

ArrayList example

The following example uses nested Java java.util.ArrayLists, and then uses the ArrayList#toArray method to convert it to a Polyglot Array.

use crate::polyglot::*;
use std::marker::PhantomData;
use crate::types::jtypes::*;

graal_bindgen_macros::class! [java.util.ArrayList<E> {
    new();
    E get(int index);
    boolean add(E e);
    E[] toArray();
}];

let list = ArrayList::new();
let list_in_list = ArrayList::new();
for i in 0..100 {
    list_in_list.add(i);
}
list.add(list_in_list);
let array_from_list = list.get(0).toArray();
for i in 0..100 {
    println!("{}", array_from_list.get(i).unwrap());
}

Here's what the preceding code would look like in normal Rust using Vec and slices:

let mut vec = Vec::new();
let mut vec_in_vec = Vec::new();
for i in 0..100 {
    vec_in_vec.push(i);
}
vec.push(vec_in_vec);
let slice_from_vec = vec.get(0).unwrap().as_slice();
for i in 0..100 {
    println!("{}", slice_from_vec.get(i).unwrap());
}

A full implementation of java.util.ArrayList can be seen in src/builtins/mod.rs.

Constructor stubs

A stub is inferred to be a constructor if it doesn't have a return type. The Rust name of the constructor must be explicitly declared, and doesn't have to be the name of the type.

class! [java.lang.String {
    new();
    new_from(String original);
}];

will expand to a struct String with the methods String::new and String::new_from:

pub fn new() -> String {
    let polyglot_type = crate::java_type("java.lang.String");
    String::from_polyglot_value({
        unsafe { crate::polyglot::internal::polyglot_new_instance(polyglot_type) }
    })
}
pub fn new_from(original: String) -> String {
    let polyglot_type = crate::java_type("java.lang.String");
    String::from_polyglot_value({
        unsafe {
            crate::polyglot::internal::polyglot_new_instance(
                polyglot_type,
                crate::polyglot::internal::expect_variadic(original),
            )
        }
    })
}

String::new() will be equivalent to calling new String() in Java, and String::new_from(...) will be equivalent to new String(...).

Function stubs

Function stubs are composed of a return value, an optional alias, a name, and arguments.
<return type> [alias] <function_name> ( [<type> <arg_name>]* ); Aliases make the Rust function name different to the function name in Java; this is useful when you have an overloaded method since Rust does not support overloading and will not compile if two functions have the same name. If left empty, the Rust binding name is presumed to be the same as the Java function name.

class! [java.util.ArrayList<E> {
    int add_at add(int index, E element);
    int add(E element);
}];

will expand to a struct ArrayList with the methods ArrayList::add_at and ArrayList::add:

pub fn add_at(&self, index: int, element: E) -> int {
    return int::from_polyglot_value({
        unsafe {
            crate::polygot::internal::polyglot_invoke(
                self.ptr,
                crate::polygot::internal::make_cstring("add").as_ptr(),
                crate::polygot::internal::expect_variadic(index),
                crate::polygot::internal::expect_variadic(element),
            )
        }
    });
}
pub fn add(&self, element: E) -> int {
    return int::from_polyglot_value({
        unsafe {
            crate::polygot::internal::polyglot_invoke(
                self.ptr,
                crate::polygot::internal::make_cstring("add").as_ptr(),
                crate::polygot::internal::expect_variadic(element),
            )
        }
    });
}

Pass and Receive

The Pass and Receive traits indicate that describe how a type can safely be passed to and received from Graal Polyglot.

Pass and Passable

The Pass<T: Passable> trait requires fn pass(&self) -> T to be implemented. Passable is a marker trait for whether a type is safe to directly pass to polyglot, and because unboxed primitives are passed directly but all other types are passed using a pointer. For all objects, pass should be implemented so that it returns its inner pointer (a *Value) like this:

fn pass(&self) -> *mut Value {
    self.ptr
}

Receive

The Receive trait defines how a Polyglot value can be used to construct a type. Usually, it should just instantiate a struct Self with its backing pointer set to the given *mut Value like this:

fn from_polyglot_value(value: *mut Value) -> Self {
    Self { ptr: value }
}

Generics

class! supports generics; the generic type must be Pass + Receive. Due to the poor design decision of treating *mut Values and primitives differently (even though they can be passed to polyglot directly), Pass makes it so that there needs to be an extra parameter for each desired generic. The first generic parameters are the ones you specify, followed by a Passable bound for each one you specified after. Type inference should sort this out, but if you need to specify explicitly, you can tell Rust to still infer the Passable bounds like this:

let my_arraylist: ArrayList<i32, _> = ArrayList::new();

If something goes really wrong, you can explicitly specify the Passable. For primitives, this will be the same as the main type. For other Objects, this will be *mut Value.

Arrays

Arrays are represented by JavaArray. Currently, creating and updating elements in them has not been implemented and Index cannot be implemented, since the trait requires a reference to be returned. The return value of .get() is an Option; if the index is out of bounds, it will be None, otherwise it will be Some(value_at_index).