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ScriptEngine Implementation

GraalVM provides a JSR-223 compliant javax.script.ScriptEngine implementation for running JavaScript. Note that this feature is provided for legacy reasons in order to allow easier migration for implementations currently based on a ScriptEngine. We strongly encourage users to use the org.graalvm.polyglot.Context interface in order to control many of the settings directly and benefit from finer-grained security settings in GraalVM.

Prerequisite #

NOTE: As of GraalVM for JDK 21, GraalVM no longer includes ScriptEngine by default. If you relied on that, you will have to migrate your setup to explicitly depend on the script engine module and add it to the module path.

To get the js-scriptengine module, use a Maven dependency, like follows:

<dependency>
    <groupId>org.graalvm.js</groupId>
    <artifactId>js-scriptengine</artifactId>
    <version>${graalvm.version}</version>
</dependency>
<dependency>
    <groupId>org.graalvm.js</groupId>
    <artifactId>js</artifactId>
    <version>${graalvm.version}</version>
    <scope>runtime</scope>
</dependency>

If you are not using mvn, you will need to add the js-scriptengine.jar file to the module path manually, for example: --module-path=languages/js/graaljs-scriptengine.jar. In some case, you may also need to add --add-modules org.graalvm.js.scriptengine to the command line, to ensure that the ScriptEngine will be found. An explicit dependency on the org.graalvm.js.scriptengine module is only required if you want to use GraalJSScriptEngine directly (see below). Finally, it is also possible to use jlink to generate a custom Java runtime image that contains the JS ScriptEngine.

An example pom.xml can be found in the GraalJS repository on GitHub.

Recommendation: Use CompiledScript API #

To avoid unnecessary re-compilation of JS sources, it is recommended to use CompiledScript.eval instead of ScriptEngine.eval. This prevents JIT-compiled code from being garbage-collected as long as the corresponding CompiledScript object is alive.

Single-threaded example:

ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName("js");
CompiledScript script = ((Compilable) engine).compile("console.log('hello world');");
script.eval();

Multi-threaded example:

ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName("js");
CompiledScript script = ((Compilable) engine).compile("console.log('start');var start = Date.now(); while (Date.now()-start < 2000);console.log('end');");
new Thread(new Runnable() {
    @Override
    public void run() {
        try {
            // Create ScriptEngine for this thread (with a shared polyglot Engine)
            ScriptEngine engine = manager.getEngineByName("js");
            script.eval(engine.getContext());
        } catch (ScriptException scriptException) {
            scriptException.printStackTrace();
        }
    }
}).start();
script.eval();

Setting Options via Bindings #

The ScriptEngine interface does not provide a default way to set options. As a workaround, GraalJSScriptEngine supports setting some Context options through Bindings. These options are:

• polyglot.js.allowHostAccess <boolean>
• polyglot.js.allowNativeAccess <boolean>
• polyglot.js.allowCreateThread <boolean>
• polyglot.js.allowIO <boolean>
• polyglot.js.allowHostClassLookup <boolean or Predicate<String>>
• polyglot.js.allowHostClassLoading <boolean>
• polyglot.js.allowAllAccess <boolean>
• polyglot.js.nashorn-compat <boolean>
• polyglot.js.ecmascript-version <String>

These options control the sandboxing rules applied to evaluated JavaScript code and are set to false by default, unless the application was started in the Nashorn compatibility mode (--js.nashorn-compat=true).

Note that using ScriptEngine implies allowing experimental options. This is an exhaustive list of allowed options to be passed via Bindings; in case you need to pass additional options to GraalVM JavaScript, you need to manually create a Context as shown below.

To set an option via Bindings, use Bindings.put(<option name>, true) before the engine’s script context is initialized. Note that even a call to Bindings#get(String) may lead to context initialization. The following code shows how to enable polyglot.js.allowHostAccess via Bindings:

ScriptEngine engine = new ScriptEngineManager().getEngineByName("JavaScript");
Bindings bindings = engine.getBindings(ScriptContext.ENGINE_SCOPE);
bindings.put("polyglot.js.allowHostAccess", true);
bindings.put("polyglot.js.allowHostClassLookup", (Predicate<String>) s -> true);
bindings.put("javaObj", new Object());
engine.eval("(javaObj instanceof Java.type('java.lang.Object'));"); // it will not work without allowHostAccess and allowHostClassLookup

This example will not work if the user calls, e.g., engine.eval("var x = 1;"), before calling bindings.put("polyglot.js.allowHostAccess", true);, since any call to eval forces context initialization.

Setting Options via System Properties #

Options to the JavaScript engine can be set via system properties before starting the JVM by prepending polyglot.:

java -Dpolyglot.js.ecmascript-version=2022 MyApplication

Or, options to the JavaScript engine can be set programmatically from within Java before creating ScriptEngine. This, however, only works for the options passed to the JavaScript engine (like js.ecmascript), not for the six options mentioned above that can be set via the Bindings. Another caveat is that those system properties are shared by all concurrently executed ScriptEngines.

Manually Creating Context for More Flexibility #

Context options can also be passed to GraalJSScriptEngine directly, via an instance of Context.Builder:

ScriptEngine engine = GraalJSScriptEngine.create(null,
        Context.newBuilder("js")
        .allowHostAccess(HostAccess.ALL)
        .allowHostClassLookup(s -> true)
        .option("js.ecmascript-version", "2022"));
engine.put("javaObj", new Object());
engine.eval("(javaObj instanceof Java.type('java.lang.Object'));");

This allows setting all options available in GraalVM JavaScript. It does come at the cost of a hard dependency on GraalVM JavaScript, e.g., the GraalJSScriptEngine and Context classes.

Supported File Extensions #

The GraalVM JavaScript implementation of javax.script.ScriptEngine supports the js file extension for JavaScript source files, as well as the mjs extension for ES modules.