Access variable of ScriptContext using Nashorn JavaScript Engine (Java 8)

Question

I used the following code with the Rhino JavaScript engine in Java:

@Test
public void testRhino() throws ScriptException {
    final ScriptEngineManager factory = new ScriptEngineManager();
    final ScriptEngine engine = factory.getEngineByName("rhino");
    final String raw = "I am the raw value injected";
    final ScriptContext ctx = new SimpleScriptContext();
    ctx.setAttribute("raw", raw, ScriptContext.ENGINE_SCOPE);

    String script = "var result = 'I am a result';";
    script += "java.lang.System.out.println(raw);";
    script += "'I am a returned value';";

    final Object res = engine.eval(script, ctx);
    System.out.println(ctx.getAttribute("result"));
    System.out.println(res);
}

The output of the script (using Rhino) is:

I am the raw value injected
I am a result
I am a returned value

Within the Nashorn JavaScript engine, I get no value for the result:

@Test
public void testNashorn() throws ScriptException {
    final ScriptEngineManager factory = new ScriptEngineManager();
    final ScriptEngine engine = factory.getEngineByName("nashorn");
    final String raw = "I am the raw value injected";
    final ScriptContext ctx = new SimpleScriptContext();
    ctx.setAttribute("raw", raw, ScriptContext.ENGINE_SCOPE);

    String script = "var result = 'I am a result';";
    script += "java.lang.System.out.println(raw);";
    script += "'I am a returned value';";

    final Object res = engine.eval(script, ctx);
    System.out.println(ctx.getAttribute("result"));
    System.out.println(res);
}

returns

I am the raw value injected
null
I am a returned value

How can I access the value of the result variable of the ScriptContext using the nashorn engine?

A. Sundararajan · Accepted Answer · 2017-07-20 02:53:10Z

5

If you use ScriptEngine.createEngine API to create ENGINE_SCOPE Bindings, it'll work as expected:

import javax.script.*;

public class Main {
  public static void main(String[] args) throws Exception {

    final ScriptEngineManager factory = new ScriptEngineManager();
    final ScriptEngine engine = factory.getEngineByName("nashorn");
    final String raw = "I am the raw value injected";
    final ScriptContext ctx = new SimpleScriptContext();

    // **This is the inserted line**
    ctx.setBindings(engine.createBindings(), ScriptContext.ENGINE_SCOPE);

    ctx.setAttribute("raw", raw, ScriptContext.ENGINE_SCOPE);

    String script = "var result = 'I am a result';";
    script += "java.lang.System.out.println(raw);";
    script += "'I am a returned value';";

    final Object res = engine.eval(script, ctx);
    System.out.println(ctx.getAttribute("result"));
    System.out.println(res);
 }
}

answered Jul 20, 2017 at 2:53

A. Sundararajan

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1 Comment

AJNeufeld Over a year ago

NashornScriptEngine#createBindings() returns either a new SimpleBindings(), if global-per-engine is set, or createGlobalMirror() if it is not set. Running with either -Dnashorn.args=--global-per-engine, or adding the line System.setProperty("nashorn.args", "--global-per-engine"); before the engine is created, will both cause ctx.getAttribute("result") to again return null. Working around Nashorn's global is one of the biggest pains in embedding Nashorn in a Java application.

AJNeufeld · Accepted Answer · 2017-07-19 21:06:03Z

3

Nashorn treats the Bindings stored in ScriptContext as "read-only". Any attempt to set a variable stored in a Bindings object (or to create a new variable) will result in a new variable being created in nashorn.global which shadows the Bindings parameter by that name.

You can use the engine to "evaluate" the variable, using this code:

System.out.println( engine.eval("result", ctx) );

This is, however, quite ugly. "result" is first compiled into a script, and then that script is evaluated, to return the variable's value. Fine for testing, but perhaps a little too inefficient for a general solution.

A better, but perhaps more fragile method, is to extract the "nashorn.global" variable, and query it for the desired value.

Bindings nashorn_global = (Bindings) ctx.getAttribute("nashorn.global");
System.out.println( nashorn_global.get("result") );

See also my hack/answer in Capturing Nashorn's Global Variables for automated way of moving nashorn.global values back to a Map<String,Object> after evaluating a script.

answered Jul 19, 2017 at 21:06

AJNeufeld

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3 Comments

A. Sundararajan Over a year ago

It is possible create a new Bindings object using ScriptEngine.createBindings API. This creates a Binding that is directly backed by Nashorn's Global instance - which reflects read/writes into JS global scope always.

AJNeufeld Over a year ago

@A.Sundararajan If -Dnashorn.args=--global-per-engine is set, then the problem remains even if you use ScriptEngine.createBindings().

A. Sundararajan Over a year ago

With --globals-per-engine, there is only one underlying JS/Nashorn global object per engine. All ENGINE_SCOPE bindings will share the same underlying Nashorn global instance! As you've observed above, engine.createBindings() in --global-per-engine case would just return a SimpleBindings (not a Bindings backed by a fresh Nashorn Global instance).

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