Calling Python from Java through scripting engine (jython)?

I'm trying to call Jython from a Java 6 application using javax.script:

import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;

public class jythonEx
{
    public static void main (String args[]) throws ScriptException
    {
        ScriptEngineManager mgr = new ScriptEngineManager();
        ScriptEngine pyEngine = mgr.getEngineByName("python");
        try {
            pyEngine.eval("print \"Python - Hello, world!\"");
        } catch (Exception ex) {
            ex.printStackTrace();
        }       
    }
}

This is causing a NullPointerException:

java.lang.NullPointerException
        at jythonEx.main(jythonEx.java:12)

Does anyone have any idea what I'm doing wrong here?

Edit:

Thanks for the responses! I added jython.jar to the classpath and it runs properly:

java -cp "./;jython.jar" jythonEx