How can I add methods from a Java class as global functions in Javascript using Rhino?

Question

I have a simple Java class that has some methods:

public class Utils {
    public void deal(String price, int amount) {
        // ....
    }
    public void bid(String price, int amount) {
        // ....
    }
    public void offer(String price, int amount) {
        // ....
    }
}

I would like to create an instance of this class and allow the Javascript code to call the methods directly, like so:

deal("1.3736", 100000);
bid("1.3735", 500000);

The only way I could figure out for now was to use

ScriptEngine engine = new ScriptEngineManager().getEngineByName("js");
engine.put("utils", new Utils());

and then use utils.deal(...) in the Javascript code. I can also write wrapper functions in Javascript for each method, but there should be a simpler way to do this automatically for all the public methods of a class.

Allyce

You do realise Java and JavaScript are two complete different languages. They may have similar C-like syntax, but they are not the same. – Blossomblot

Rhino is an embedded js engine for Java. This sort of thing should be achievable. I managed it in Perl with SpiderMonkey: blog.dorward.me.uk/2006/02/02/spambots_that_drink_coffee.html – Friendly

@Blossomblot This is an absolutely valid question. You can run Javascript in Java and you may need to use Java goodies in JS then. – Catlee

Answer 1

I'm not real familiar with Rhino, but something like this should work:

for(var fn in utils) {
  if(typeof utils[fn] === 'function') {
    this[fn] = (function() {
      var method = utils[fn];
      return function() {
         return method.apply(utils,arguments);
      };
    })();
  }
}

Just loop over the properties of utils,and for each one that is a function, create a global function that calls it.

EDIT: I got this working in a Groovy script, but I had to set utils in the bindings, not on the engine like in your code:

import javax.script.*

class Utils {
   void foo(String bar) {
      println bar
   }   
}

ScriptEngine engine = new ScriptEngineManager().getEngineByName("js");

engine.eval("""
for(var fn in utils) {
  if(typeof utils[fn] === 'function') {
    this[fn] = (function() {
      var method = utils[fn];
      return function() {
         return method.apply(utils,arguments);
      };
    })();
  }
}

foo('foo'); // prints foo, sure enough
""",new SimpleBindings("utils":new Utils()))

Stickle

+1 for nice use of closures. I think I'm going to go with your solution. – Allyce

How would you get this to work in Java? Would you use the following: ScriptEngine engine = new ScriptEngineManager().getEngineByName("js"); engine.put("utils", new Utils()); or something else? – Mapp

Answer 2

I'm not sure how this would work using the JSR-223 API, but with the Rhino API, you can create a FunctionObject with the method you want to add like this.

Class[] parameters = new Class[] { String.class, Integer.class };
Method dealMethod = Utils.class.getMethod("deal", parameters);
engine.put("deal", new FunctionObject("deal", dealMethod, scope));

The documentation is available at https://www.mozilla.org/rhino/apidocs/org/mozilla/javascript/FunctionObject.html.

You might need to reference the Rhino library to access the FunctionObject class, and I'm not sure how you would get the scope object with the JSR-223 API (although, it's possible that null would work).

Anthill

I don't think this will work because the deal function object has no reference to the instance. – Strongwilled

@Geoff: I was assuming they were static methods, since they would be called without a (JavaScript) this object. Looking at it again, I guess they aren't, but probably should be. – Anthill

Thanks for this. Actually, when using the Rhino API, the Context class has a javaToJS method which does exactly what I need. It looks like it should work with non-static methods too. I was hoping for a JSR-223 solution, but I don't think there is one. – Allyce

Thanks for this, the Rhino documentation is excessively unclear about this part – Epiphora

Answer 3

This is possible if you use the rhino API rather than the ScriptEngine API as explained in this answer: https://mcmap.net/q/907574/-how-do-i-call-a-method-of-a-java-instance-from-javascript.

I prefer this approach over Noah's answer as it means you don't need to execute random javascript code before each execution.

I have a working example here:

https://github.com/plasma147/rhino-play

Towrey

Answer 4

With Java Lambdas (so since 1.8) it is actually possible to just do this:

ScriptEngine engine = new ScriptEngineManager().getEngineByName("js");
BiConsumer<String, Integer> deal = Utils::deal
engine.put("deal", deal);

Granular