Using Python from within Java

> Possible Duplicate:
> Java Python Integration

I have a large existing codebase written in 100% Java, but I would like to use Python for some new sections of it. I need to do some text and language processing, and I'd much rather use Python and a library like [NLTK][1] to do this.

I'm aware of the Jython project, but it looks like this represents a way to use Java and its libraries from within Python, rather than the other way round - am I wrong about this?

If not, what would be the best method to interface between Java and Python, such that (ideally) I can call a method in Python and have the result returned to Java?

[1]: http://www.nltk.org "NLTK - The Natural Language Toolkit"

> I'm aware of the Jython project, but > it looks like this represents a way to > use Java and its libraries from within > Python, rather than the other way > round - am I wrong about this?

Yes, you are wrong. You can either call a command line interpreter to run python code using Jyton or use python code from Java. In the past there was also a python-to-Java compiler, but it got discontinued with Jython 2.2

I would write a Python module to handle the text and language processing, and then build a small bridge in jython that your java program can interact with. The jython bridge will be a very simple one, that's really only responsible for forwarding calls to the python module, and return the answer from the python module to the java module. Jython is really easy to use, and setup shouldn't take you more than 15 minutes.

Best of luck!

I don't think you could use NLTK from Jython, since it depends on Numpy which isn't ported to the JVM. If you need NLTK or any other native CPython extension, you might consider using some IPC mechanism to communicate between CPython and the JVM. That being said, there is a project to allow calling CPython from Java, called Jepp:

http://jepp.sourceforge.net/

The reverse (calling Java code from CPython) is the goal of JPype and javaclass:

sourceforge.net/projects/jpype/

pypi.python.org/pypi/javaclass/0.1

I've never used any of these project, so I cant't vow for their quality.

Jython is a Python implementation running on the JVM. You can find information about embedding Python in an existing Java app in the user guide.

I don't know the environment that you're working in, but be aware that mixing languages in the same app can quickly lead to a mess. I recommend creating Java interfaces to represent the operations that you plan to use, along with separately-packaged implementation classes that wrap the Python code.

IN my opinion, Jython is exactly what you are looking at.
It is an implementation of Python within the JVM; as such, you can freely exchange objects and, for instance, inherit from a Java class (with some limitations).

Note that, its major strength point (being on top of of JVM) is also its major drawback, because it cannot use all (C)Python extension written in C (or in any other compiled language); this may have an impact on what you are willing to do with your text processing.

For more information about what is Jython, its potential and its limitations, I suggest you reading the Jython FAQ.

Simply run the Python interpreter as a subprocess from within Java.

Write your Python functionality as a proper script, which reads from stdin and writes to stdout.

Use the Java Runtime class to spawn a subprocess that runs your Python script. This is very simple to do and provides a very clean interface.

---

Edit

import simplejson
import sys
for request in sys.stdin.readlines():
    args = simplejson.loads( request )
    result = myFunction( args['this'], args['that'] )
    sys.stdout.writeline( simplejson.dumps( result ) + "\n" )

The interface is simple, structured and very low overhead.

Remember to first check from those paying for the development that they're OK with the codebase needing a developer who knows both Python and Java from now on, and other cost and maintainability effects you've undoubtedly already accounted for.

See: http://www.acm.org/about/se-code 1.06, 2.03, 2.09, 4.03, 4.05, 6.07