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Java Interoperability

This documentation shows you how to enable interoperability with Java and possible JavaScript-to-Java embedding scenarios.

Enabling Java Interoperability

As of GraalVM for JDK 21, all necessary artifacts can be downloaded directly from Maven Central. All artifacts relevant to embedders can be found in the Maven dependency group org.graalvm.polyglot. Learn more about the dependency setup in the Getting Started guide.

Polyglot Context

The preferred method of embedding JavaScript in Java is via Context. For that, a new org.graalvm.polyglot.Context is built with the hostAccess option allowing access and a hostClassLookup predicate defining the Java classes you allow access to:

Context context = Context.newBuilder("js")
    .allowHostAccess(HostAccess.ALL)
    //allows access to all Java classes
    .allowHostClassLookup(className -> true)
    .build();
context.eval("js", jsSourceCode);

See the Guide to Embedding Languages on how to interact with a guest language such as JavaScript from a Java host application.

ScriptEngine (JSR 223)

JavaScript running on a GraalVM JDK is fully compatible with JSR 223 and supports the ScriptEngine API. Internally, the GraalVM's 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);

See the ScriptEngine guide for more details on how to use it from GraalJS.

Access Java from JavaScript

GraalVM provides a set of features to allow interoperability from JavaScript to Java. While Rhino, Nashorn, and GraalJS have a mostly comparable overall feature set, they differ in exact syntax, and, partly, semantics.

Class Access

To access a Java class, GraalJS supports the Java.type(typeName) function:

var FileClass = Java.type('java.io.File');

If the host class lookup is allowed (allowHostClassLookup), the java global property is available by default. Existing code accessing, for example, java.io.File, should be rewritten to use the Java.type(name) function:

// GraalJS (and Nashorn) compliant syntax
var FileClass = Java.type("java.io.File");
// Backwards-compatible syntax
var FileClass = java.io.File;

GraalJS provides Packages, java, and similar global properties for compatibility. However, explicitly accessing the required class with Java.type is preferred whenever possible for two reasons:

It resolves the class in one step instead of trying to resolve each property as a class.
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.

The js.java-package-globals flag can be used to deactivate the global fields of Java packages (set to false to avoid creation of the fields; default is true).

Constructing Java Objects

A Java object can be constructed with JavaScript's new keyword:

var FileClass = Java.type('java.io.File');
var file = new FileClass("myFile.md");

Field and Method Access

The static fields of a Java class, or the 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('java.io.File'))("test.md");
var fileName = file.getName();

Conversion of Method Arguments

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

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

//Java
void longArg   (long arg1);
void doubleArg (double arg2);
void intArg    (int arg3);

//JavaScript
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!

Note how the argument values have to fit into the parameter types. You can override this behavior using custom target type mappings.

Method Selection

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:

//Java
void foo(int arg);
void foo(short arg);
void foo(double arg);
void foo(long arg);

//JavaScript
javaObject.foo(1);              // will call foo(short);
javaObject.foo(Math.pow(2,16)); // will call foo(int);
javaObject.foo(1.1);            // will call foo(double);
javaObject.foo(Math.pow(2,32)); // will call foo(long);

To override this behavior, an explicit method overload can be selected using the javaObject['methodName(paramTypes)'] syntax. Parameter types need to be comma-separated without spaces, and object types need to be fully qualified (for example, 'get(java.lang.String,java.lang.String[])'). Note that this is different from Nashorn which allows extra spaces and simple names. In the example above, one might always want to call, for example, foo(long), even when foo(short) can be reached with lossless conversion (foo(1)):

javaObject['foo(int)'](1);
javaObject['foo(long)'](1);
javaObject['foo(double)'](1);

Note that the argument values still have to fit into the parameter types. You can override this behavior using custom target type mappings.

An explicit method selection can also be useful when the method overloads are ambiguous and cannot be automatically resolved as well as when you want to override the default choice:

//Java
void sort(List<Object> array, Comparator<Object> callback);
void sort(List<Integer> array, IntBinaryOperator callback);
void consumeArray(List<Object> array);
void consumeArray(Object[] array);

//JavaScript
var array = [3, 13, 3, 7];
var compare = (x, y) => (x < y) ? -1 : ((x == y) ? 0 : 1);

// throws TypeError: Multiple applicable overloads found
javaObject.sort(array, compare);
// explicitly select sort(List, Comparator)
javaObject['sort(java.util.List,java.util.Comparator)'](array, compare);

// will call consumeArray(List)
javaObject.consumeArray(array);
// explicitly select consumeArray(Object[])
javaObject['consumeArray(java.lang.Object[])'](array);

Note that there is currently no way to explicitly select constructor overloads. Future versions of GraalJS might lift that restriction.

Package Access

GraalJS provides a Packages global property:

> Packages.java.io.File
JavaClass[java.io.File]

Array Access

GraalJS 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 GraalJS you can create and access Java Maps, for example, java.util.HashMap:

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

GraalJS supports iterating over such maps similar to Nashorn:

for (var key in map) {
    print(key);
    print(map.get(key));
}

List Access

In GraalJS you can create and access Java Lists, for example, java.util.ArrayList:

var ArrayList = Java.type('java.util.ArrayList');
var list = new ArrayList();
list.add(42);
list.add("23");
list.add({});

for (var idx in list) {
    print(idx);
    print(list.get(idx));
}

String Access

GraalJS 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 GraalJS uses Java strings internally to represent JavaScript strings, so the 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 for..in loop:

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

Access to JavaScript Objects from Java

JavaScript objects are exposed to Java code as instances of com.oracle.truffle.api.interop.java.TruffleMap. This class implements Java's Map interface.

JavaImporter

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

Console Output of Java Classes and Java Objects

GraalJS provides both print and console.log.

GraalJS provides a print built-in 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 GraalJS is just an alias for print, and Node's implementation is only available when running on Node.js.

Exceptions

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

try {
    Java.type('java.lang.Class')
    .forName("nonexistent");
} catch (e) {
    print(e.getMessage());
}

Promises

GraalJS provides support for interoperability between JavaScript Promise objects and Java. Java objects can be exposed to JavaScript code as thenable objects, allowing JavaScript code to await Java objects. Moreover, JavaScript Promise objects are regular JavaScript objects, and can be accessed from Java using the mechanisms described in this document. This allows Java code to be called back from JavaScript when a JavaScript promise is resolved or rejected.

Creating JavaScript `Promise` Objects That Can Be Resolved from Java

JavaScript applications can create Promise objects delegating to Java the resolution of the Promise instance. This can be achieved from JavaScript by using a Java object as the "executor" function of the JavaScript Promise. For example, Java objects implementing the following functional interface can be used to create new Promise objects:

@FunctionalInterface
public interface PromiseExecutor {
    void onPromiseCreation(Value onResolve, Value onReject);
}

Any Java object implementing PromiseExecutor can be used to create a JavaScript Promise:

// `javaExecutable` is a Java object implementing the `PromiseExecutor` interface
var myPromise = new Promise(javaExecutable).then(...);

JavaScript Promise objects can be created not only using functional interfaces, but also using any other Java object that can be executed by GraalJS (for example, any Java object implementing the Polyglot ProxyExecutable interface). More detailed example usages are available in the GraalJS unit tests.

Using `await` with Java Objects

JavaScript applications can use the await expression with Java objects. This can be useful when Java and JavaScript have to interact with asynchronous events. To expose a Java object to GraalJS as a thenable object, the Java object should implement a method called then() having the following signature:

void then(Value onResolve, Value onReject);

When await is used with a Java object implementing then(), GraalJS will treat the object as a JavaScript Promise. The onResolve and onReject arguments are executable Value objects that should be used by the Java code to resume or abort the JavaScript await expression associated with the corresponding Java object. More detailed example usages are available in the GraalJS unit tests.

Using JavaScript Promises from Java

Promise objects created in JavaScript can be exposed to Java code like any other JavaScript object. Java code can access such objects like normal Value objects, with the possibility to register new promise resolution functions using the Promise's default then() and catch() functions. As an example, the following Java code registers a Java callback to be executed when a JavaScript promise resolves:

Value jsPromise = context.eval(ID, "Promise.resolve(42);");
Consumer<Object> javaThen = (value)
    -> System.out.println("Resolved from JavaScript: " + value);
jsPromise.invokeMember("then", javaThen);

More detailed example usages are available in the GraalJS unit tests.

Multithreading

GraalJS supports multithreading when used in combination with Java. More details about the GraalJS multithreading model can be found in the Multithreading documentation.

Extending Java classes

GraalJS provides support for extending Java classes and interfaces using the Java.extend function. Note that host access has to be enabled in a polyglot context for this feature to be available.

Java.extend

Java.extend(types...) returns a generated adapter Java class object that extends the specified Java class and/or interfaces. For example:

var Ext = Java.extend(Java.type("some.AbstractClass"),
                      Java.type("some.Interface1"),
                      Java.type("some.Interface2"));
var impl = new Ext({
  superclassMethod: function() {/*...*/},
  interface1Method: function() {/*...*/},
  interface2Method: function() {/*...*/},
  toString() {return "MyClass";}
});
impl.superclassMethod();

Super methods can be called via Java.super(adapterInstance). See a combined example:

var sw = new (Java.type("java.io.StringWriter"));
var FilterWriterAdapter = Java.extend(Java.type("java.io.FilterWriter"));
var fw = new FilterWriterAdapter(sw, {
    write: function(s, off, len) {
        s = s.toUpperCase();
        if (off === undefined) {
            fw_super.write(s, 0, s.length)
        } else {
            fw_super.write(s, off, len)
        }
    }
});
var fw_super = Java.super(fw);
fw.write("abcdefg");
fw.write("h".charAt(0));
fw.write("**ijk**", 2, 3);
fw.write("***lmno**", 3, 4);
print(sw); // ABCDEFGHIJKLMNO

Note that in the nashorn-compat mode, you can also extend interfaces and abstract classes using a new operator on a type object of an interface or an abstract class:

// --experimental-options --js.nashorn-compat
var JFunction = Java.type('java.util.function.Function');
 var sqFn = new JFunction({
   apply: function(x) { return x * x; }
});
sqFn.apply(6); // 36

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