Java and Javascript bind function with arrays as parameter

Question

I am trying to acheve the following. A Java application should run a javascript via the ScriptEngineManager/ScriptEngine and the javascript should be able to call functions form the Java application. This works great as long as only a simple Variable is passed to and from. But I cannot get it to pass arrays. Here is my code:

private static class test implements Function<Integer[], Integer[]> {
    @Override
    public Integer[] apply(Integer[] msg) {
        for(int i = 0; i < msg.length; i++) System.out.println(msg[i]);     
        Integer[] a = {1, 2, 3};           
        return a;
    }
}

public static void main(String[] args) throws ScriptException {
    String ps = "var e = [0x04, 0x05, 0x06]; var a = send(e); for(i in a) print(i);";
    ScriptEngineManager sm = new ScriptEngineManager();
    ScriptEngine eng = sm.getEngineByName("JavaScript");
    Bindings mbind = eng.createBindings();
    mbind.put("send", new test());
    eng.setBindings(mbind, ScriptContext.ENGINE_SCOPE);
    Object t = eng.eval(ps);
}

I get the error:

Exception in thread "main" java.lang.ClassCastException: jdk.nashorn.api.scripting.ScriptObjectMirror cannot be cast to [Ljava.lang.Integer; at scripttest$test.apply(scripttest.java:1) at jdk.nashorn.internal.scripts.Script$\^eval_.:program(:1) at jdk.nashorn.internal.runtime.ScriptFunctionData.invoke(ScriptFunctionData.java:637) at jdk.nashorn.internal.runtime.ScriptFunction.invoke(ScriptFunction.java:494) at jdk.nashorn.internal.runtime.ScriptRuntime.apply(ScriptRuntime.java:393) at jdk.nashorn.api.scripting.NashornScriptEngine.evalImpl(NashornScriptEngine.java:446) at jdk.nashorn.api.scripting.NashornScriptEngine.evalImpl(NashornScriptEngine.java:403) at jdk.nashorn.api.scripting.NashornScriptEngine.evalImpl(NashornScriptEngine.java:399) at jdk.nashorn.api.scripting.NashornScriptEngine.eval(NashornScriptEngine.java:155) at javax.script.AbstractScriptEngine.eval(Unknown Source) at scripttest.main(scripttest.java:26)

Does anyone have an idea on how to pass arrays?

Thanks! Regards

Please note that when your Javascript tries to print the result, it actually iterates over a's fields, not members - for(... in ...) is a syntax to iterate over fields of an object, so if a is indeed a 3 member array, the output would be 0\n1\n2\n regardless of the array's content. The Javascript syntax for array iteration is for(... of ...). — Guss
– Guss, Commented Mar 28, 2019 at 10:02

Guss · Accepted Answer · 2019-03-28 10:20:44Z

0

Reflection on Arrays is a bit of a problem in Java (primitive types, and all that junk), so JSR-223 script engines have trouble with them. I've tried the same code with the new GraalVM and that suffers from the same problem - the engine gives up on finding a good mapping and just tries to submit its native representation of the array value (Nashorn uses ScriptObjectMirror while GraalVM uses PolyglotMap).

If you want your host function to look like it is accepting a "real array" (implementation of a guest array), your host function will need to accept the underlying ScriptEngine's internal implementation and learn to operate on that. According to this SO answer and the ScriptObjectMirror API docs you should be able to use the obj.to(int[].class) method to extract the internal integer array from the proxy object.

The main disadvantage of this approach is that your host function must support each specific internal representation of each specific ScriptEngines you might want to support.

As the Nashorn Javascript engine is deprecated in current JDKs, you might want to support GraalVM as well, in case of which your implementation might look something like this:

private static class test implements Function<Object, Integer[]> {
    @Override
    public Integer[] apply(Object inp) {
        int[] msg;
        if (inp instanceof ScriptObjectMirror)
            msg = ((ScriptObjectMirror)inp).to(int[].class);
        else if (inp instanceof AbstractMap)
            msg = Value.asValue(inp).as(int[].class);
        else
            return null;
        for (int i = 0; i < msg.length; i++)
            System.out.println(msg[i]);
        Integer[] a = { 1, 2, 3 };
        return a;
    }
}

answered Mar 28, 2019 at 10:20

Guss

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1 Comment

Guss Over a year ago

BTW, regarding the AbstractMap stuff in there, unfortunately the GraalVM Polyglot engine internal types cannot be detected using instanceof because none of them are public classes. In an implementation that only supports GraalVM, I'm guessing you're supposed to just Value.asValue() whatever input you get and then use Value's API to take it from there, but I'm pretty sure Value won't like Nashorn's proxy objects.

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