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Creating Javascript Function for Java Code

Description of the problem

A somewhat contrived example illustrating my question. Imagine that we have a library of javascript functions, which is already supported and updated daily by the army of frontend developers. To be specific, imagine that one of these functions looks like this:

function employeesForStore(store) { var dictionary = { "downtown": ["Joe", "Mary", "Steve"], "uptown": ["Jules", "Vincent", "Matt"], // and so on for hundreds of locations }; return dictionary[store]; }

NOTE. Please ignore the implementation details of this function. The actual function can be much more complicated than a simple search in the JSON dictionary, and suppose we do not know any details about the implementation of the js function. We only know that it takes a String argument and returns an array of arrays.

Now we would like to use this feature in our Java code. That is, in our Java code, we would like to “load” this function, and then be able to call it several times, passing it String args and getting the results String[] or ArrayList<String> .

From searching SO and google so far, I understand that this will be related to using:

  • javax.script.ScriptEngineManager
  • javax.script.ScriptEngine
  • and possibly scriptEngine.getContext() to pass values ​​to a function and get the results.

I'm a little hazy about the details of the above, especially since most of the examples I found include running javascript code at a time, instead of using the javascript function for Java.

Sample code that I would like to see

  • Assuming the js function is in the file "my_functions.js", load this file in Java so that all its functions are available for use.
  • Call employeesForStore("downtown") and save its results in native java String[] or List<String> in a variable named downtownResults .
  • Same as 2, but call employeesForStore("uptown") and save uptownResults in the variable
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2 answers

Create an interface to act like a facade in your JavaScript code.

Here is an example using the Rhino implementation implemented in the Oracle Java 1.7 implementation:

 package demo; import java.io.*; import java.util.*; import java.util.concurrent.atomic.AtomicReference; import javax.script.*; public class StoreData { public static interface Stores { public String[] employees(String store); } public static Stores stores() throws IOException, ScriptException { ScriptEngineManager sem = new ScriptEngineManager(); ScriptEngine engine = sem.getEngineByName("JavaScript"); AtomicReference<Stores> ref = new AtomicReference<>(); engine.put("ref", ref); String adapt = "ref.set(" + "new Packages.demo.StoreData.Stores({employees:employeesForStore})" + ");"; try (Reader myFns = new FileReader("my_functions.js")) { // TODO encoding engine.eval(myFns); engine.eval(adapt); return ref.get(); } } public static void main(String[] args) throws IOException, ScriptException { List<String> employees = Arrays.asList(stores().employees("uptown")); System.out.println(employees); } }

By specifying an interface, we let Rhino bind JavaScript types to Java types (String, String [], etc.)

The JRE specification does not provide any guarantees about which scripting mechanisms should be provided, so it may be wise to rely on an external engine. I don't know if that will change Nashorn .

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You can use Rhino API to run JS code in java

This tutorial covers the requested examples.

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