Since Groovy Dice modify some Groovy classes at runtime, we need to initialize it before use:

import net.sf.groovydice.*;

public class MyDiceRollerApp {
    static {
        new GroovyDice().initialize();

    public static void main(String[] args) {
        // now we can roll our dice!

We just need to invoke the initialize() method once for a given application, so it's fine to put that call inside a static block.

For more information on GroovyDice class, please read the Extension Points documentation.