Calling Python from Java through scripting engine (jython)?

Question

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

Canzona

Which one is line 12? pyEngine.eval("");? – Simoniac

@ilikeorangutans: Yes, that's the line 12. – Mediative

That's right. I'm presuming that the ScriptEngineManager can't find the python engine, but I don't see what I'm doing wrong that would cause that... – Canzona

Answer 1

You have to register your engine first.

From: ScriptEngineManager.getEngineByName:

[...] first searches for a ScriptEngineFactory that has been registered as a handle [...] Returns null if no such factory was found

The user guide says to use it with JSR-223 you have to:

As of Jython 2.5.1 an implementation of JSR 223 is bundled in jython.jar. Simply add jython to your CLASSPATH and ask for the python script engine.

Did you do that already?

EDIT About your comment: I think you should open a new question, you'll get better answers.

Mediative

No, I never did that, thanks! I've used JavaScript through javax.script before without doing anything, so I clearly missed that step. Let me try it now... – Canzona

@Canzona Great!, probably that will do. Let us know if it work would you? – Mediative

@Canzona I'm glad!! good luck with that python + java integration ;) – Mediative

Answer 2

You'd probably have to register a ScriptEngineFactory for'python'

Humdrum