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javajavascriptnashorn

Java8 Javascript Nashorn exception: no current Global instance for nashorn

I want to retrieve object generated in JS store them in Java and later call methods on them. This worked with Java 7, now with Java 8 i get an exception:

Exception in thread "main" java.lang.IllegalArgumentException: no current Global instance for nashorn
    at jdk.nashorn.api.scripting.NashornScriptEngine.invokeImpl(NashornScriptEngine.java:492)
    at jdk.nashorn.api.scripting.NashornScriptEngine.invokeMethod(NashornScriptEngine.java:238)

I have modified the official example from here http://docs.oracle.com/javase/8/docs/technotes/guides/scripting/prog_guide/api.html a bit.

Now i created a minimal example to produce this exception. It seems like, if a JS object is passed to Java via return value, it is different to the case JS calls a Java object's method and passes the object.

public class InvokeScriptMethod {

    static Object o1;

    public static class Context {
        public void add( Object o ){
            InvokeScriptMethod.o1 = o;
        }
    }

    public static void main(String[] args) throws Exception {
        ScriptEngineManager manager = new ScriptEngineManager();
        ScriptEngine engine = manager.getEngineByName("nashorn");

        engine.put("ctx", new Context());

        engine.eval("function bla(){\n"
                + "var obj = new Object();\n"
                + "obj.var1 = 3;\n"
                + "obj.hello = function(name) { print('Hello, ' + this.var1 + ' ' + name); this.var1++; };\n"
                + "ctx.add(obj);\n"
                + "return obj;\n"
                + "}");

        Invocable inv = (Invocable) engine;

        Object obj = inv.invokeFunction("bla");

        System.out.printf("retrieved as return value        : %s %s\n", obj.getClass(), obj);
        System.out.printf("retrieved via call to java object: %s %s\n", o1.getClass(), o1);

        inv.invokeMethod(obj, "hello", "Script Method!");
        inv.invokeMethod(o1, "hello", "Script Method!"); // <-- exception
    }
}

program output:

retrieved as return value        : class jdk.nashorn.api.scripting.ScriptObjectMirror [object Object]
retrieved via call to java object: class jdk.nashorn.internal.scripts.JO jdk.nashorn.internal.scripts.JO@105fece7
Hello, 3 Script Method!
Exception in thread "main" java.lang.IllegalArgumentException: no current Global instance for nashorn

obj is a ScriptObjectMirror as it is expected, o1 is an internal object. http://cr.openjdk.java.net/~sundar/8023631/webrev.00/src/jdk/nashorn/api/scripting/NashornScriptEngine.java.html line 481 shows how this exception is thrown. So I think, there is something wrong by wrapping the "naked" JS object into a ScriptObjectMirror when passing as argument to Java.

Now I have 2 questions: 1. Is this a bug in my code? Or a bug in the Java8 nashorn? 2. Is there a way that i can work around this exception bug keeping the same call scheme.

Thanks Frank

Solution

  • I've found that calling inv.invokeFunction does work. So instead of calling inv.invokeMethod attempting to directly call a method on a javascript object, I proxy all invocations through an intermediate javascript global scope function (proxyMethodCall).

    var test = {
  init = function() {
    // call back into my Scala code to register a callback to invoke the method "foo"
    // for a particular condition.
    registerCallback(this, "foo", "myarg");
  },

  foo: function(arg) {
    // here I am in my foo function
  }
}

// the Scala code knows to call this proxy method as a workaround
// to invoke the method in the target (thiz) javascript object.
function proxyMethodCall(thiz, methodName, arg) {
  thiz[methodName].call(thiz, arg);
}

// Scala code...
val inv = engine.asInstanceOf[Invocable]
inv.invokeFunction("proxyMethodCall", thiz, methodName, methodArgument)