Calling @JSFunction from JavaScript, TypeError: Cannot find default value for object

Question

I'm calling a @JSFunction annotated method of a ScriptableObject

The JavaScript file

Target = Packages.com.acme.rhino.Target;

function evaluate() {
    var t = Target();
    t.addModifier("foobar", 1);
    return t;
}

The Java File

public class Target extends ScriptableObject {
    private static final long serialVersionUID = 1L;
    public List<Modifier> modifiers = new LinkedList<>();

    @JSConstructor
    public Target() {
    }

    @JSFunction
    public void addModifier(final String message, final int value) {
        modifiers.add(new Modifier(message, value));
    }

    public int getValue() {
        int sum = 0;
        for (final Modifier modifier : modifiers) {
            sum += modifier.getValue();
        }
        return sum;
    }

    @Override
    public String getClassName() {
        return "Target";
    }
}

But I get

org.mozilla.javascript.EcmaError: TypeError: Cannot find default value for object.
    at org.mozilla.javascript.ScriptRuntime.constructError(ScriptRuntime.java:3687)
    at org.mozilla.javascript.ScriptRuntime.constructError(ScriptRuntime.java:3665)
    at org.mozilla.javascript.ScriptRuntime.typeError(ScriptRuntime.java:3693)
    at org.mozilla.javascript.ScriptRuntime.typeError1(ScriptRuntime.java:3705)
    at org.mozilla.javascript.ScriptableObject.getDefaultValue(ScriptableObject.java:976    )
    at org.mozilla.javascript.ScriptableObject.getDefaultValue(ScriptableObject.java:895    )
    at org.mozilla.javascript.ScriptRuntime.toString(ScriptRuntime.java:761)
    at org.mozilla.javascript.ScriptRuntime.notFunctionError(ScriptRuntime.java:3774)
    at org.mozilla.javascript.ScriptRuntime.getPropFunctionAndThisHelper(ScriptRuntime.    java:2269)
    at org.mozilla.javascript.ScriptRuntime.getPropFunctionAndThis(ScriptRuntime.    java:2251)
    at org.mozilla.javascript.optimizer.OptRuntime.callProp0(OptRuntime.java:83)
    at org.mozilla.javascript.gen.script_5._c_evaluate_1(script:6)
    at org.mozilla.javascript.gen.script_5.call(script)
    at org.mozilla.javascript.ContextFactory.doTopCall(ContextFactory.java:394)
    at org.mozilla.javascript.ScriptRuntime.doTopCall(ScriptRuntime.java:3091)
    at org.mozilla.javascript.gen.script_5.call(script)

and don't know where to go from there. When I don't call addModifier method the given code works, and given the error notFunctionError down in the stack trace I think that Rhino doesn't interpret the given method as a JavaScript Function.

• OSX 10.8.2
• Java 7
• Rhino 1.7R4

Complete Maven project that reproduces the error can be found here

Answer 1

tl;dr see these two alternatives.

The problem with the approach above is that Target.prototype is not properly set up in the script scope. See the static ScriptableObject.defineClass() method for details on how to properly define prototypes in a script scope.

You have a couple alternatives for providing the Target constructor to your scripts. The first alternative would be to always define the Target constructor for all scripts. This works well if you know ahead of time that you want Target to be globally available. This basically comes down to the following:

final Context context = Context.enter();
try {
  final ScriptableObject scope = context.initStandardObjects();
  ScriptableObject.defineClass(scope, Target.class, false, true);
  context.evaluateString(scope, script, "script", 1, null);
  // etc.
} finally {
  Context.exit();
}

If instead you want the script author to decide which constructors are necessary, the second alternative is to provide the defineClass function to scripts. With this function, script authors can "import" any scriptable objects on their class path (which may be more than you want to allow). To provide the defineClass functions to scripts, do the following after entering the context:

final Context context = Context.enter();
try {
  final ScriptableObject scope = context.initStandardObjects();
  scope.defineFunctionProperties(
          new String[] {"defineClass"},
          Global.class,
          ScriptableObject.DONTENUM);

  context.evaluateString(scope, script, "script", 1, null);
  // etc.
} finally {
  Context.exit();
}

And then, the JavaScript author makes use of the Target constructor with the following:

defineClass("com.acme.rhino.Target");
// whatever `getClassName()` returns is now available
var target = new Target();

In both of the above branches, I've made a couple other changes that set you up better if you add more to the Target constructor. The zero argument constructor doesn't need the @JSConstructor annotation. If you later want to have a constructor that accepts arguments, this zero argument constructor will be used as the prototype constructor, and you can use the @JSConstructor annotation on a method that will be used to initialize your object. Depending on how you author this constructor method, it will become important to use the new keyword in your JavaScript.

In short, the Packages.com.acme... syntax is not useful for getting access to ScriptableObject constructors from scripts.