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Graal JavaScript to Java interoperability

Graal JavaScript is a JavaScript (ECMAScript) language execution runtime. It allows interoperability with Java code. This document describes the features and usage of this JavaScript to Java interoperability feature.

For a reference of Graal JavaScript's public API, see Migration guides for Rhino and Nashorn are available.

Launching Graal JavaScript

Depending on how you build Graal JavaScript, it is started in different ways. GraalVM CE or EE by default ships with a js and node native launcher. The following examples assume this setup is used.

Enabling Java interoperability

In GraalVM CE or EE images, the js and node binaries are started in an ahead-of-time compiled native mode by default. In that mode, Java interoperability is not available.

To enable Java interoperability, the --jvm option has to be provided to the native launcher. This way, Graal JavaScript is executed on a traditional JVM and allows full Java interoperability.


To load Java classes you need to have them on the Java classpath. You can specify the classpath with the --jvm.classpath=<classpath> option (or short: --jvm.cp=<classpath>).

node --jvm --jvm.cp=/my/class/path
js --jvm --jvm.cp=/my/class/path

Polyglot Context

The preferred method of launching Graal JavaScript with Java interop support instance is via polyglot Context. For that, a new org.graalvm.polyglot.Context is built with the hostAccess option set:

Context context = Context.newBuilder("js").allowHostAccess(true).build();
context.eval("js", jsSourceCode);

See for more details.

ScriptEngine (JSR 223)

The org.graalvm.polyglot.Context is the preferred execution method for interoperability with languages and tool of the GraalVM. In addition, Graal JavaScript is fully compatible with JSR 223 and supports the ScriptEngine API. Internally, the Graal JavaScript ScriptEngine wraps a polyglot context instance.

ScriptEngine eng = new ScriptEngineManager().getEngineByName("graal.js");
Object fn = eng.eval("(function() { return this; })");
Invocable inv = (Invocable) eng;
Object result = inv.invokeMethod(fn, "call", fn);

Java interoperability features

Rhino, Nashorn and Graal JavaScript provide a set of features to allow interoperability from JavaScript to Java. While the overall feature set is mostly comparable, the engines differ in exact syntax, and partly, semantics.

Class access

To access a Java class, Graal JavaScript supports the Java.type(typeName) function.

var FileClass = Java.type('');

By default, Java classes are not automatically mapped to global variables, e.g., there is no java global property in Graal JavaScript. Existing code accessing e.g. should be rewritten to use the Java.type(name) function.

var FileClass = Java.type(""); //Graal JavaScript compliant syntax
var FileClass =;              //FAILS in Graal JavaScript

Graal JavaScript provides a Packages global property (and java etc. if the js.nashorn-compat option is set) for compatibility. However, explicitly accessing the required class with Java.type should be preferred whenever possible for two reasons:

  1. It allows resolving the class in one step rather than trying to resolve each property as a class.
  2. Java.type immediately throws a TypeError if the class cannot be found or is not accessible rather than silently treating an unresolved name as a package.

Constructing Java objects

Java objects can be constructed with JavaScript's new keyword.

var FileClass = Java.type('');
var file = new FileClass("");

Field and method access

Static fields of a Java class or fields of a Java object can be accessed like JavaScript properties.

var JavaPI = Java.type('java.lang.Math').PI;

Java methods can be called like JavaScript functions.

var file = new (Java.type(''))("");
var fileName = file.getName();

Conversion of method arguments

JavaScript is defined to operate on the double number type. Graal JavaScript might internally use additional Java data types for performance reasons (e.g., the int type).

When calling Java methods, a value conversion might be required. This happens when the Java method expects a long parameter, and a int is provided from Graal JavaScript (type widening). If this conversion caused a lossy conversion, a TypeError is thrown.

void longArg   (long arg1);
void doubleArg (double arg2);
void intArg    (int arg3);
javaObject.longArg(1);     //widening, OK
javaObject.doubleArg(1);   //widening, OK
javaObject.intArg(1);      //match, OK

javaObject.longArg(1.1);   //lossy conversion, TypeError!
javaObject.doubleArg(1.1); //match, OK
javaObject.intArg(1.1);    //lossy conversion, TypeError!

Selection of method

Java allows overloading of methods by argument types. When calling from JavaScript to Java, the method with the narrowest available type that the actual argument can be converted to without loss is selected.

void foo(int arg);
void foo(short arg);
void foo(double arg);
void foo(long arg);
//JavaScript;              //will call foo(short arg);,16)); //will call foo(int arg);;            //will call foo(double arg);,32)); //will call foo(long arg);

Note that there currently is no way of overriding this behavior from Graal JavaScript. In the example above, one might want to always call foo(int arg), even when foo(short arg) can be reached with lossless conversion (foo(1)). Future versions of Graal JavaScript might lift that restriction by providing an explicit way to select the method to be called.

Package access

Graal JavaScript provides a Packages global property.


Array access

Graal JavaScript supports the creation of Java arrays from JavaScript code. Both the patterns suggested by Rhino and Nashorn are supported:

//Rhino pattern
var JArray = Java.type('java.lang.reflect.Array');
var JString = Java.type('java.lang.String');
var sarr = JArray.newInstance(JString, 5);

//Nashorn pattern
var IntArray = Java.type("int[]");
var iarr = new IntArray(5);

The arrays created are Java types, but can be used in JavaScript code:

iarr[0] = iarr[iarr.length] * 2;

Map access

In Graal JavaScript you can create and access Java Maps, e.g. java.util.HashMap.

var HashMap = Java.type('java.util.HashMap');
var map = new HashMap();
map.put(1, "a");

Graal JavaScript supports iterating over such map similar to Nashorn:

for (var key in map) {

List access

In Graal JavaScript you can create and access Java Lists, e.g. java.util.ArrayList.

var ArrayList = Java.type('java.util.ArrayList');
var list = new ArrayList();

for (var idx in list) {

String access

Graal JavaScript can create Java strings with Java interoperability. The length of the string can be queried with the length property. Note that length is a value property and cannot be called as a function.

var javaString = new (Java.type('java.lang.String'))("Java");
javaString.length === 4;

Note that Graal JavaScript uses Java strings internally to represent JavaScript strings, so above code and the JavaScript string literal "Java" are actually not distinguishable.

Iterating properties

Properties (fields and methods) of Java classes and Java objects can be iterated with a JavaScript loop.

var m = Java.type('java.lang.Math')
for (var i in m) { print(i); }
> E
> PI
> abs
> sin
> ...

This is working as in existing Rhino code.

Access to JavaScript objects from Java

JavaScript objects are exposed to Java code as instances of This class implements Java's Map interface.


The JavaImporter feature is available only in Nashorn compatibility mode (js.nashorn-compat option).

Console output of Java classes and Java objects

Graal JavaScript provides both print and console.log.

Graal JavaScript provides a print builtin function compatible with Nashorn.

The console.log is provided by Node.js directly. It does not provide special treatment of interop objects. Note that the default implementation of console.log on Graal JavaScript is just an alias for print, and Node's implementation is only available when running on Node.js.


Exceptions thrown in Java code can be caught in JavaScript code. They are represented as Java objects.

try {
} catch (e) {


Graal JavaScript supports multithreading by creating several Context objects from Java code. Multiple JavaScript engines can be created from a Java application, and can be safely executed in parallel on multiple threads.

On the other hand, Graal JavaScript interoperability prevents the creation of Java threads from JavaScript applications. This could lead to unmanagable synchronization problems like data races in a language that is not prepared for multithreading.