What is the use of Invocable in java?


Until now I only know that it is an interface that has the following methods:

  • invokeMethod ()

  • invokeFunction ()

  • getInterface ()

But I do not understand what each of them is for and exactly where the Invocable interface is concerned.

asked by FrEqDe 14.02.2017 в 06:17

2 answers


This interface is used in machines that interpret other programming languages like javascript , which for example is done with Nashorn.

Imagine you have javascript like:

String javascript = "function multiplica(a, b) { return a * b; }\n"
     + "function divide(a, b) { return a / b; }"

With the script machine you implement the Invocable interface to call functions from Java with invokeFunction () :

ScriptEngineManager engineManager = new ScriptEngineManager();
ScriptEngine engine = engineManager.getEngineByName("nashorn");
// cargemos el código para que la maquina lo evalua

// la maquina implementa la interfaz Invocable
Invocable js = (Invocable) engine;
// podemos llamar a funciones así:
// eso imprime 15

We can also create java interfaces with getInterface () to access the script:

interface Calculador{
    multiplica(int a, int b);
    divide(int a, int b);

// con eso podemos obtener una interfaz de Java del script
Calculador calc = js.getInterface(Calculador.class);
// imprime 6
// imprime 2

The last method invokeMethod () is similar to invokeFunction but allows you to call methods of an object:

// cargemos un objeto en javascript
Object jsObjeto = engine.eval(new FileReader("Printer.js"));
// llamamos a Printer.print( nombre )
js.invokeMethod(jsObjeto, "print", "foo" );

Nashorn Documentation in English (Oracle)

answered by 14.02.2017 / 07:18

When a scripts engine (which implements javax.script.ScriptEngine ) also implements, in turn, the interface javax.script.Invocable , means that you can execute procedures, functions or methods that have previously been compiled .

That is, instead of using the eval of javax.script.ScriptEngine method, you can invoke the procedure or function or method directly from a Java program. It should be noted that the implementation of the interface Invocable by a scripts engine is optional. Before invoking any procedure, function or method, you should check if the script engine is an instance of this interface, make a cast to Invocable , and then execute the methods of this interface.

This interface contains four methods. The two versions of the method getInterface() allow to obtain an instance of a Java interface which is implemented in a language of scripting (eg: factorial.js ). The invokeFunction() method allows you to invoke a higher level function (eg: parseInt() ) written in a scripting language. The invokeMethod() method allows invocation of object methods (eg: obj.toString() ) written in a scripting language.

If you want to make an invocation of this type, you can follow the following steps:

  • First check if the scripts engine is Invocable :

    ScriptEngineManager manager = new ScriptEngineManager();
    ScriptEngine engine = manager.getEngineByName("jruby");
    if (engine instanceof Invocable) {
        // Es invocable :-)
    } else {
        // No es invocable :'(
  • Make a cast to the ScriptEngine to Invocable reference:

    Invocable inv = (Invocable) engine;
  • Evaluate a script for the engine to compile it and save the compiled for future invocations:

    String script = "def sumar(a, b)\n  return a + b\nend";
  • Invoking the procedure, function or method:

    Object result = inv.invokeFunction("sumar", 1, 2);
  • Ademdum

    A scripts engine can also implement the interface javax.script.Compilable which allows you to compile scripts to execute it repeatedly.


    • Klauer, N. (2008). InvokingFunctionsExample.java. Retrieved from link
    • McQueeney, T. (2007). Introducing the Java scripting API. Recovered from link
    • Sharan, K. (2014). Scripting in Java: Integrating with Groovy and JavaScript. Berkeley, CA: Apress.
    answered by 15.04.2017 в 05:49