Accessing JavaScript array through Java with graal.js

Question

I am migrating a project from Nashorn to graal.js. The project utilizes a large amount of scripts (over 3,400) and there's a function on the Java side which invokes a method; the method returns a JavaScript list of objects.

function filterList(ob)
{
    var list = [];
    var arr = ob.toArray();

    for(var i = 0; i < arr.length; i++)
    {
        if(global.isValid(arr[i]))
        {
            list.push(arr[i]);
        }
    }

    return list;
}

This worked fine on Nashorn previously with the use of ScriptUtils. This code was written by the developer who worked on the project before I picked it up:

try {
    Object p = iv.invokeFunction("filterList", this.getList());

    if(p != null) {
        List<MyObj> lObj = new ArrayList<>(((Map<String, MyObj>)(ScriptUtils.convert(p, Map.class))).values());
        return lObj;
    }
} catch (ScriptException | NoSuchMethodException ex) {
    ex.printStackTrace();
}

How can I access the array through Java with graal.js? I have tried using Value.asValue(p)as(MyObj[]) to no avail. I have also tried following the Nashorn migration guide where they suggest to cast the object to List or Map, to no avail either.

I'm aware of a solution where I would have to rewrite the script to just use new Java.type('java.util.ArrayList'); and return a List rather than an array - however there are thousands of scripts and rewriting all of them will be incredibly tedious.

Christian Wirth · Accepted Answer · 2019-12-20 13:29:16Z

1

you can use Value.getArraySize() and Value.getArrayElement(index):

Context context = Context.newBuilder("js").build();
Value result = context.eval("js", "var list=[1,2,'foo',true]; list;");
if (result.hasArrayElements()) {
    for (int i=0;i<result.getArraySize();i++) {
        System.out.println(result.getArrayElement(i));
    }
}

You find the full JavaDoc of the Value class in https://www.graalvm.org/sdk/javadoc/org/graalvm/polyglot/Value.html

Best, Christian

answered Dec 20, 2019 at 13:29

Christian Wirth

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