#java #jni #api-bindings


j4rs stands for 'Java for Rust' and allows effortless calls to Java code, from Rust

2 releases

0.18.1 Mar 31, 2024
0.18.0 Mar 31, 2024

#51 in Programming languages


7.5K SLoC

Contains (JAR file, 2.5MB) j4rs-0.18.0-SNAPSHOT-jar-with-dependencies.jar


crates.io Maven Central Build

j4rs stands for 'Java for Rust' and allows effortless calls to Java code from Rust and vice-versa.




use j4rs::{Instance, InvocationArg, Jvm, JvmBuilder};

// Create a JVM
let jvm = JvmBuilder::new().build()?;

// Create a java.lang.String instance
let string_instance = jvm.create_instance(
    "java.lang.String",     // The Java class to create an instance for
    InvocationArg::empty(), // An array of `InvocationArg`s to use for the constructor call - empty for this example

// The instances returned from invocations and instantiations can be viewed as pointers to Java Objects.
// They can be used for further Java calls.
// For example, the following invokes the `isEmpty` method of the created java.lang.String instance
let boolean_instance = jvm.invoke(
  &string_instance,       // The String instance created above
  "isEmpty",              // The method of the String instance to invoke
  InvocationArg::empty(), // The `InvocationArg`s to use for the invocation - empty for this example

// If we need to transform an `Instance` to some Rust value, the `to_rust` should be called
let rust_boolean: bool = jvm.to_rust(boolean_instance)?;
println!("The isEmpty() method of the java.lang.String instance returned {}", rust_boolean);
// The above prints:
// The isEmpty() method of the java.lang.String instance returned true

// Static invocation
let _static_invocation_result = jvm.invoke_static(
  "java.lang.System",     // The Java class to invoke
  "currentTimeMillis",    // The static method of the Java class to invoke
  InvocationArg::empty(), // The `InvocationArg`s to use for the invocation - empty for this example

// Access a field of a class
let system_class = jvm.static_class("java.lang.System")?;
let system_out_field = jvm.field(&system_class, "out");

// Retrieve an enum constant using the field
let access_mode_enum = jvm.static_class("java.nio.file.AccessMode")?;
let access_mode_write = jvm.field(&access_mode_enum, "WRITE")?;

// Retrieve a nested class (note the use of `$` instead of `.`)
let state = jvm.static_class("java.lang.Thread$State")?;

Instancess of Java Lists and Maps can be created with the java_list and java_map functions:

let rust_vec = vec!["arg1", "arg2", "arg3", "arg33"];

// Generate a Java List. The Java List implementation is the one that is returned by java.util.Arrays#asList
let java_list_instance = jvm.java_list(

let rust_map = HashMap::from([
    ("Potatoes", 3),
    ("Tomatoes", 33),
    ("Carrotoes", 333),

// Generate a java.util.HashMap.
let java_map_instance = jvm.java_map(

Passing arguments from Rust to Java

j4rs uses the InvocationArg enum to pass arguments to the Java world.

Users can benefit of the existing TryFrom implementations for several basic types:

let i1 = InvocationArg::try_from("a str")?;      // Creates an arg of java.lang.String
let my_string = "a string".to_owned();
let i2 = InvocationArg::try_from(my_string)?;    // Creates an arg of java.lang.String
let i3 = InvocationArg::try_from(true)?;         // Creates an arg of java.lang.Boolean
let i4 = InvocationArg::try_from(1_i8)?;         // Creates an arg of java.lang.Byte
let i5 = InvocationArg::try_from('c')?;          // Creates an arg of java.lang.Character
let i6 = InvocationArg::try_from(1_i16)?;        // Creates an arg of java.lang.Short
let i7 = InvocationArg::try_from(1_i64)?;        // Creates an arg of java.lang.Long
let i8 = InvocationArg::try_from(0.1_f32)?;      // Creates an arg of java.lang.Float
let i9 = InvocationArg::try_from(0.1_f64)?;      // Creates an arg of java.lang.Double

And for Vecs:

let my_vec: Vec<String> = vec![

let i10 = InvocationArg::try_from(my_vec.as_slice())?;

The j4rs apis accept InvocationArgs either as references, or values:

let inv_args = InvocationArg::try_from("arg from Rust")?;
let _ = jvm.create_instance("java.lang.String", &[&inv_args])?; // Pass a reference
let _ = jvm.create_instance("java.lang.String", &[inv_args])?;  // Move

The Instances returned by j4rs can be transformed to InvocationArgs and be further used for invoking methods as well:

let one_more_string_instance = jvm.create_instance(
  "java.lang.String",     // The Java class to create an instance for
  InvocationArg::empty(), // The `InvocationArg`s to use for the constructor call - empty for this example

let i11 = InvocationArg::try_from(one_more_string_instance)?;

To create an InvocationArg that represents a null Java value, use the From implementation with the Null struct:

let null_string = InvocationArg::from(Null::String);                // A null String
let null_integer = InvocationArg::from(Null::Integer);              // A null Integer
let null_obj = InvocationArg::from(Null::Of("java.util.List"));     // A null object of any other class. E.g. List

Passing custom arguments from Rust to Java

Custom types, for which there is no TryFrom implementation, are also supported via serialization.

To use a custom struct MyBean as an InvocationArg it needs to be serializable:

#[derive(Serialize, Deserialize, Debug)]
struct MyBean {
    someString: String,
    someInteger: isize,

Then, an InvocationArg can be created like:

let my_bean = MyBean {
    someString: "My String In A Bean".to_string(),
    someInteger: 33,
let ia = InvocationArg::new(&my_bean, "org.astonbitecode.j4rs.tests.MyBean");

And it can be used as an argument to a Java method that accepts org.astonbitecode.j4rs.tests.MyBean instances.

Of course, there should exist a respective Java class in the classpath for the deserialization to work and the custom Java Object to be created:

package org.astonbitecode.j4rs.tests;

public class MyBean {
    private String someString;
    private Integer someInteger;

    public MyBean() {

    public String getSomeString() {
        return someString;

    public void setSomeString(String someString) {
        this.someString = someString;

    public Integer getSomeInteger() {
        return someInteger;

    public void setSomeInteger(Integer someInteger) {
        this.someInteger = someInteger;

Async support

(v0.16.0 onwards)

j4rs supports .async/.await viaJvm::invoke_async function. The function returns a Future, which is completed via the Receiver of a oneshot channel.

In Java side, the methods that can be invoked by invoke_async, must return a Java Future. When the Java Future completes, the Java side of j4rs invokes native Rust code that completes the pending Rust Future with either success or failure, using the Sender of the oneshot channel that was created when the invoke_async was called.

For example, assuming we have a Java method that returns a Future:

package org.astonbitecode.j4rs.tests;

public class MyTest {
  private static ExecutorService executor = Executors.newSingleThreadExecutor();

  // Just return the passed String in a Future
  public Future<String> getStringWithFuture(String string) {
    CompletableFuture<String> completableFuture = new CompletableFuture<>();
    executor.submit(() -> {
      return null;
    return completableFuture;

We can invoke it like following:

let s_test = "j4rs_rust";
let my_test = jvm.create_instance("org.astonbitecode.j4rs.tests.MyTest", InvocationArg::empty())?;
let instance = jvm.invoke_async(&my_test, "getStringWithFuture", &[InvocationArg::try_from(s_test)?]).await?;
let string: String = jvm.to_rust(instance)?;
assert_eq!(s_test, string);

Please note that it is better for the Java methods that are invoked by the invoke_async function to return a CompletableFuture, as this improves performance.

j4rs handles simple Java Futures that are not CompletableFutures with polling, using an internal one-threaded ScheduledExecutorService.

This has apparent performance issues.


An Instance may be casted to some other Class:

let instantiation_args = vec![InvocationArg::try_from("Hi")?];
let instance = jvm.create_instance("java.lang.String", instantiation_args.as_ref())?;
jvm.cast(&instance, "java.lang.Object")?;

Java arrays and variadics

// Create a Java array of Strings
let s1 = InvocationArg::try_from("string1")?;
let s2 = InvocationArg::try_from("string2")?;
let s3 = InvocationArg::try_from("string3")?;

let arr_instance = jvm.create_java_array("java.lang.String", &[s1, s2, s3])?;
// Invoke the Arrays.asList(...) and retrieve a java.util.List<String>
let list_instance = jvm.invoke_static("java.util.Arrays", "asList", &[InvocationArg::from(arr_instance)])?;

Java Generics

// Assuming the following map_instance is a Map<String, Integer>
// we may invoke its put method
jvm.invoke(&map_instance, "put", &[InvocationArg::try_from("one")?, InvocationArg::try_from(1)?])?;

Java primitives

Even if auto boxing and unboxing is in place, j4rs cannot invoke methods with primitive int arguments using Integer instances.

For example, the following code does not work:

let ia = InvocationArg::try_from(1_i32)?;
jvm.create_instance("java.lang.Integer", &[ia])?;

It throws an InstantiationException because the constructor of Integer takes a primitive int as an argument:

Exception in thread "main" org.astonbitecode.j4rs.errors.InstantiationException: Cannot create instance of java.lang.Integer at org.astonbitecode.j4rs.api.instantiation.NativeInstantiationImpl.instantiate(NativeInstantiationImpl.java:37) Caused by: java.lang.NoSuchMethodException: java.lang.Integer.(java.lang.Integer) at java.base/java.lang.Class.getConstructor0(Class.java:3349) at java.base/java.lang.Class.getConstructor(Class.java:2151) at org.astonbitecode.j4rs.api.instantiation.NativeInstantiationImpl.createInstance(NativeInstantiationImpl.java:69) at org.astonbitecode.j4rs.api.instantiation.NativeInstantiationImpl.instantiate(NativeInstantiationImpl.java:34)

In situations like this, the java.lang.Integer instance should be transformed to a primitive int first:

let ia = InvocationArg::try_from(1_i32)?.into_primitive()?;
jvm.create_instance("java.lang.Integer", &[ia]);

Java instances chaining

use j4rs::{Instance, InvocationArg, Jvm, JvmBuilder};

// Create a JVM
let jvm = JvmBuilder::new().build()?;

// Create an instance
let string_instance = jvm.create_instance(
  &[InvocationArg::try_from(" a string ")?],

// Perform chained operations on the instance
let string_size: isize = jvm.chain(string_instance)
    .invoke("trim", InvocationArg::empty())?
    .invoke("length", InvocationArg::empty())?

// Assert that the string was trimmed
assert!(string_size == 8);

Callback support

j4rs provides support for Java to Rust callbacks.

These callbacks come to the Rust world via Rust Channels.

In order to initialize a channel that will provide Java callback values, the Jvm::invoke_to_channel should be called. It returns a result of InstanceReceiver struct, which contains a Channel Receiver:

// Invoke of a method of a Java instance and get the returned value in a Rust Channel.

// Create an Instance of a class that supports Native Callbacks
// (the class just needs to extend the 
// `org.astonbitecode.j4rs.api.invocation.NativeCallbackToRustChannelSupport`)
let i = jvm.create_instance(

// Invoke the method
let instance_receiver_res = jvm.invoke_to_channel(
  &i,                         // The instance to invoke asynchronously
  "performCallback",          // The method to invoke asynchronoysly
  InvocationArg::empty()      // The `InvocationArg`s to use for the invocation - empty for this example

// Wait for the response to come
let instance_receiver = instance_receiver_res?;
let _ = instance_receiver.rx().recv();

In the Java world, a Class that can do Native Callbacks must extend the org.astonbitecode.j4rs.api.invocation.NativeCallbackToRustChannelSupport

For example, consider the following Java class.

The performCallback method spawns a new Thread and invokes the doCallback method in this Thread. The doCallback method is inherited by the NativeCallbackToRustChannelSupport class.

package org.astonbitecode.j4rs.tests;

import org.astonbitecode.j4rs.api.invocation.NativeCallbackToRustChannelSupport;

public class MyTest extends NativeCallbackToRustChannelSupport {

    public void performCallback() {
        new Thread(() -> {
            doCallback("THIS IS FROM CALLBACK!");


Using Maven artifacts

Since release 0.6.0 there is the possibility to download Java artifacts from the Maven repositories. While it is possible to define more repos, the maven central is by default and always available.

For example, here is how the dropbox dependency can be downloaded and get deployed to be used by the rust code:

let dbx_artifact = MavenArtifact::from("com.dropbox.core:dropbox-core-sdk:3.0.11");

Additional artifactories can be used as well:

let jvm: Jvm = JvmBuilder::new()


Maven artifacts are added automatically to the classpath and do not need to be explicitly added.

A good practice is that the deployment of maven artifacts is done by build scripts, during the crate's compilation. This ensures the classpath is properly populated during the actual Rust code execution.

Note: the deployment does not take care the transitive dependencies yet.

Adding jars to the classpath

If we have one jar that needs to be accessed using j4rs, we need to add it in the classpath during the JVM creation:

let entry = ClasspathEntry::new("/home/myuser/dev/myjar-1.0.0.jar");
let jvm: Jvm = JvmBuilder::new()

j4rs Java library

The jar for j4rs is available in the Maven Central. It may be used by adding the following dependency in a pom:


Note that the scope is provided. This is because the j4rs Java resources are always available with the j4rs crate.

Use like this in order to avoid possible classloading errors.

j4rs in android

Rust side

  1. Define your crate as cdylib in the Cargo.toml:
name = "myandroidapp"
crate-type = ["cdylib"]
  1. Implement a jni_onload function and apply the provided JavaVM to the j4rs like following:
const JNI_VERSION_1_6: jint = 0x00010006;

pub extern fn jni_onload(env: *mut JavaVM, _reserved: jobject) -> jint {

Java side

Create an Activity and define your native methods normally, as described here.

Note: If you encounter any issues when using j4rs in older Android versions, this may be caused by Java 8 compatibility problems. This is why there is a Java 7 version of j4rs:


Update: Java 7 is no more supported. j4rs 0.13.1 is the last version.

JavaFX support

(v0.13.0 onwards)

Steps to build a JavaFX UI

1. Have Rust, cargo and JDK 11 (or above) installed

2. Retrieve the JavaFX dependencies for j4rs:

A good idea is that this happens during build time, in order the dependencies to be available when the actual Rust application starts and the JVM is initialized. This can happen by adding the following in a build script:

	use j4rs::JvmBuilder;
use j4rs::jfx::JavaFxSupport;

fn main() {
    let jvm = JvmBuilder::new().build().unwrap();

3. Implement the UI:

There are two choices here; either build the UI using FXML, or, build it traditionally, using Java code. In the code snippets below, you may find comments with a short description for each line.

3.a Implement the UI with Java calls to the JavaFX API
// Create a Jvm with JavaFX support
let jvm = JvmBuilder::new().with_javafx_support().build()?;

// Start the JavaFX application.
// When the JavaFX application starts, the `InstanceReceiver` channel that is returned from the `start_javafx_app` invocation
// will receive an Instance of `javafx.stage.Stage`.
// The UI may start being built using the provided `Stage`.
let stage = jvm.start_javafx_app()?.rx().recv()?;

// Create a StackPane. Java code: StackPane root = new StackPane();
let root = jvm.create_instance("javafx.scene.layout.StackPane", InvocationArg::empty())?;

// Create the button. Java code: Button btn = new Button();
let btn = jvm.create_instance("javafx.scene.control.Button", InvocationArg::empty())?;
// Get the action channel for this button
let btn_action_channel = jvm.get_javafx_event_receiver(&btn, FxEventType::ActionEvent_Action)?;
// Set the text of the button. Java code: btn.setText("Say Hello World to Rust");
jvm.invoke(&btn, "setText", &["A button that sends events to Rust".try_into()?])?;
// Add the button to the GUI. Java code: root.getChildren().add(btn);
  .invoke("getChildren", InvocationArg::empty())?
  .invoke("add", &[btn.try_into()?])?

// Create a new Scene. Java code: Scene scene = new Scene(root, 300, 250);
let scene = jvm.create_instance("javafx.scene.Scene", &[
// Set the title for the scene. Java code: stage.setTitle("Hello Rust world!");
jvm.invoke(&stage, "setTitle", &["Hello Rust world!".try_into()?])?;
// Set the scene in the stage. Java code: stage.setScene(scene);
jvm.invoke(&stage, "setScene", &[scene.try_into()?])?;
// Show the stage. Java code: stage.show();
jvm.invoke(&stage, "show", InvocationArg::empty())?;

3.b Implement the UI with FXML

I personally prefer building the UI with FXMLs, using for example the Scene Builder.

The thing to keep in mind is that the controller class should be defined in the root FXML element and it should be fx:controller="org.astonbitecode.j4rs.api.jfx.controllers.FxController"

Here is an FXML example; it creates a window with a label and a button:

<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.scene.control.Button?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.layout.HBox?>
<?import javafx.scene.layout.VBox?>
<?import javafx.scene.text.Font?>

<VBox alignment="TOP_CENTER" maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="400.0" prefWidth="725.0" spacing="33.0" xmlns="http://javafx.com/javafx/11.0.1" xmlns:fx="http://javafx.com/fxml/1" fx:controller="org.astonbitecode.j4rs.api.jfx.controllers.FxController">
        <Label text="JavaFX in Rust">
                <Font size="65.0" />
        <Label text="This UI is loaded with a FXML file" />
        <HBox alignment="CENTER" prefHeight="100.0" prefWidth="200.0" spacing="10.0">
                <Button id="helloButton" mnemonicParsing="false" text="Say Hello" />

The id of the elements can be used to retrieve the respective Nodes in Rust and act upon them (eg. adding Event Listeners, changing the texts or effects on them etc).

// Create a Jvm with JavaFX support
let jvm = JvmBuilder::new().with_javafx_support().build()?;

// Start the JavaFX application.
// When the JavaFX application starts, the `InstanceReceiver` channel that is returned from the `start_javafx_app` invocation
// will receive an Instance of `javafx.stage.Stage`.
// The UI may start being built using the provided `Stage`.
let stage = jvm.start_javafx_app()?.rx().recv()?;

// Set the title for the scene. Java code: stage.setTitle("Hello Rust world!");
jvm.invoke(&stage, "setTitle", &["Hello JavaFX from Rust!".try_into()?])?;
// Show the stage. Java code: stage.show();
jvm.invoke(&stage, "show", InvocationArg::empty())?;

// Load a fxml. This returns an `FxController` which can be used in order to find Nodes by their id,
// add Event Listeners and more.
let controller = jvm.load_fxml(&PathBuf::from("./fxml/jfx_in_rust.fxml"), &stage)?;

// Wait for the controller to be initialized. This is not mandatory, it is here to shoe that the functionality exists.
let _ = controller.on_initialized_callback(&jvm)?.rx().recv()?;
println!("The controller is initialized!");

// Get the InstanceReceiver to retrieve callbacks from the JavaFX button with id helloButton
let hello_button_action_channel = controller.get_event_receiver_for_node("helloButton", FxEventType::ActionEvent_Action, &jvm)?;

For a complete example, please have a look here.

Java to Rust support

(v0.12.0 onwards)

  • Add the two needed dependencies (j4rs and j4rs_derive) in the Cargo.toml and mark the project as a cdylib, in order to have a shared library as output. This library will be loaded and used by the Java code to achieve JNI calls.

  • Annotate the functions that will be accessible from the Java code with the call_from_java attribute:

fn my_function_with_no_args() {
    println!("Hello from the Rust world!");
    // If you need to have a Jvm here, you need to attach the thread
    let jvm = Jvm::attach_thread().unwrap();
    // Now you may further call Java classes and methods as usual!

For a complete example, please have a look here.

Note: JNI is used behind the scenes, so, any conventions in naming that hold for JNI, should hold for j4rs too. For example, underscores (_) should be escaped and become _1 in the call_from_java definition.

Portability assumptions after Rust build (shipping a j4rs application)

During build, j4rs creates a jassets directory which contains the "java world" that is needed for the crate to work. It is always automatically populated with Java libraries and can be considered something like a default classpath container that should always be available.

By default, jassets lies in the same directory with the crate-generated artifacts (under CARGO_TARGET_DIR), so there should not be any issues during development.

But how can the application be shipped after the implementation is done?

Someone may specify a different base_path for j4rs during the Jvm initialization, issuing something like:

let jvm_res = j4rs::JvmBuilder::new()

The base_path defines the location of two directories that are needed for j4rs to work; namely jassets and deps.

  1. jassets contains the j4rs jar and other jars that may be deployed using Maven.
  2. deps should contain the j4rs dynamic library. This is needed to achieve callbacks from java to rust. The deps dir is not needed if the application does not execute Java->Rust callbacks.

So, someone may have their application binary under eg. /usr/bin, and the jassets and deps directories under /opt/myapp/, or $HOME/.myapp, or anywhere else.

An example directory tree could be:

+ --- usr
|      + --- bin
|             + --- myapp
+ --- opt
       + --- myapp 
              + --- jassets
              + --- deps

Moreover, there is also a utility function that automatically performs copying of the two directories under a specific path. The Jvm::copy_j4rs_libs_under function can be called by the build script of the crate that is being shipped:


After that, /opt/myapp will contain everything that is needed in order j4rs to work, as long as the Jvm creation is done using the with_base_path method:

let jvm_res = j4rs::JvmBuilder::new()


I get java.lang.NoSuchMethodError: java.net.URLClassLoader.<init>(Ljava/lang/String;[Ljava/net/URL;Ljava/lang/ClassLoader;)V

j4rs uses a custom ClassLoader, that needs minimum Java version 9. In order to use the default classloader that supports older Java versions, invoke the JvmBuilder::with_default_classloader when building the Jvm.

How can I enable debug logging?

j4rs uses the log crate, so, logging may be configured accordingly, depending on the chosen implementation.

However, it also supports console logging, which is configured with setting the env var J4RS_CONSOLE_LOG_LEVEL.

Accepted values are debug, info, warn, error and disabled.


At your option, under:


~80K SLoC