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

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... — griffin
– griffin, Commented Apr 20, 2010 at 0:29

OscarRyz · Accepted Answer · 2010-04-20 16:51:55Z

15

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.

edited Apr 20, 2010 at 16:51

answered Apr 20, 2010 at 0:31

OscarRyz

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3 Comments

griffin Over a year ago

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...

OscarRyz Over a year ago

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

OscarRyz Over a year ago

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

Petre Maierean · Accepted Answer · 2010-04-20 00:34:19Z

2

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

answered Apr 20, 2010 at 0:34

Petre Maierean

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