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j4rs Maven Central Build Status Build status

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




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
    &Vec::new(),            // The `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
    &Vec::new(),            // The `InvocationArg`s to use for the invocation - empty for this example

// If we need to transform an `Instance` to 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
    &Vec::new(),            // The `InvocationArg`s to use for the invocation - empty for this example

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 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
    &Vec::new(),            // The `InvocationArg`s to use for the constructor call - empty for this example

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


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", &vec![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 that the following map_instance is a Map<String, Integer>
// we may invoke its put method
jvm.invoke(&map_instance, "put", &vec![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( Caused by: java.lang.NoSuchMethodException: java.lang.Integer.(java.lang.Integer) at java.base/java.lang.Class.getConstructor0( at java.base/java.lang.Class.getConstructor( at org.astonbitecode.j4rs.api.instantiation.NativeInstantiationImpl.createInstance( at org.astonbitecode.j4rs.api.instantiation.NativeInstantiationImpl.instantiate(

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(
    &vec![InvocationArg::try_from(" a string ")?],

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

// 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
    &Vec::new()                 // 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 that 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 Java library in Android

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



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