factorial.groovy

def factorial(n) {
    n <= 1 ? 1 : n * factorial(2 - 1)
}

RuleEngine.java

public class RuleEngine implements Serializable {

    private transient ScriptEngineManager factory = new ScriptEngineManager();
    private transient ScriptEngine engine = factory.getEngineByName("groovy");

    public Object execute(String szScript, String szMethodName, Object... parameters) throws NoSuchMethodException, InvocationTargetException, IllegalAccessException, ScriptException {
        engine.eval(szScript);
        Invocable invocable = (Invocable) engine;
        Object result = invocable.invokeFunction(szMethodName, parameters);
        return result;
    }

RuleEngineUnitTest.java

    @Test
    public void testExecute_factorial() throws IOException, NoSuchMethodException, ScriptException, IllegalAccessException, InvocationTargetException {
        final String szScript = new String(Files.readAllBytes(Paths.get("./src/main/test/com/github/jonathanlalou/training/factorial.groovy")));
        final String szMethodName = "factorial";
        final Object evaluate = engine.execute(szScript, szMethodName, 159);
        System.out.println(evaluate);
    }