How to execute a Ruby file in Java?

Question

I am trying to run Ruby code using Java. This code gives me an empty result. (result: null) Could any one please help me finding the error?

Java :

    public static void callRuby(){

      ScriptEngine jruby = new ScriptEngineManager().getEngineByName("jruby");
        //process a ruby file
        try {           
             jruby.eval(new BufferedReader(new FileReader("rubyTest.rb")));
             jruby.put("a", "2");
             jruby.put("b", "3");   
             System.out.println("result: " +jruby.get("res"));

        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (ScriptException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

}

rubyTest.rb

def addition(a,b)
puts a
puts b
res = a + b
end

I'm not sure it works this way, since res is a local variable that the ScriptEngine knows nothing about... try to return the value from the function and print what eval() returns... — jamp
– jamp, Commented Jan 28, 2015 at 15:32

Solomon Slow · Accepted Answer · 2015-01-28 15:27:08Z

2

Your rubyTest.rb script defines a function, addition(a, b), but where do you ever call that function?

This defines the function in the interpreter:

jruby.eval(new BufferedReader(new FileReader("rubyTest.rb")));

Then you assign top-level variables a and b, and then you ask for the value of res, but I don't see anywhere where your program calls the addition(a,b) function.

answered Jan 28, 2015 at 15:27

Solomon Slow

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2 Comments

eriv Over a year ago

can you tell me please,how can i call the addition(a,b)?

Solomon Slow Over a year ago

Do you know Ruby? If not, time to learn. ruby-lang.org/en/documentation/quickstart Basically, you can either add the call at the end of your script file, or you can use jruby.eval(...) to evaluate a Ruby expression that calls the function. I haven't personally used JRuby from within a Java program, so I don't know the exact details. Maybe just as simple as jruby.eval("addition(a,b)");

Ewaryst Schulz · Accepted Answer · 2015-11-06 07:48:51Z

James Answer contains already the solution.

It's just as simple as jruby.eval("addition(2,3)");

If you really need to work with variables a, b you can make them global (with all the sideeffects of global variables) $a, $b and assign the values like that

 jruby.eval("$a = 2");
 jruby.eval("$b = 3");
 System.out.println("result: " + jruby.eval("addition($a,$b)"));

The Bindings mechanism of Java Scripting seem not to work as the following code FAILS:

 Bindings bindings = jruby.createBindings();
 bindings.put("a", 2);
 bindings.put("a", 3);
 // variation A
 System.out.println("result: " + jruby.eval("addition(a,b)", bindings));

 // variation B
 jruby.setBindings(bindings, ScriptContext.ENGINE_SCOPE);
 System.out.println("result: " + jruby.eval("addition(a,b)"));

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