Invoke Python modules from Java

Question

I have a Python interface of a graph library written in C - igraph (the name of library). My need is to invoke the python modules pertaining to this graph library from Java

Answer 1

You can use jep.getValue() to retrieve a value from script's global dictionary.

There are caveats to that concerning scope levels in Python, so most people find it clearer to pass a Java class to python and set the return value in that instance in Python. After the script completes, the Java code will then have the result.

For example:

==> Java

class ReturnValueClass {
   public int scriptResult;
};
ReturnValueClass value = new ReturnValueClass();
jep.set("retval", value);

==> Python

# do something
pass
# write the return value
retval.scriptResult = some_python_value

==> Java

System.out.println(value.scriptResult);

Hope that helps,

Mike (I wrote Jep)

Answer 2

Never tried it. But I recently stumbled on a project named Jepp that may be of interest to you.

Jepp embeds CPython in Java. It is safe to use in a heavily threaded environment, it is quite fast and its stability is a main feature and goal.

Answer 3

If you want to call C functions from Java, JNA (Java Native Access) is probably the way to go. JNA allows you to call functions in native libraries without having to write the C glue code (as you would have to when using JNI), and automatically maps between primitive data types in Java and C. A simple example might look like this:

import com.sun.jna.Native;
import com.sun.jna.Library;

public class PrintfWrapper {
    public interface CLibrary extends Library {
        CLibrary INSTANCE = (CLibrary)Native.loadLibrary("c", CLibrary.class);
        void printf(String formatString, Object... args);
    }

    public static void main(String[] args) {
        CLibrary.INSTANCE.printf("Hello, world\n");
    }
}

However, things will get complicated with igraph because igraph uses many data structures that cannot be mapped directly into their Java counterparts. There is a project called JNAerator which should be able to generate the JNA source from igraph's header files, but I have never tried it and chances are that the results will still need some manual tweaking.

Also note that a Java interface for igraph is being developed slowly but steadily and it might become useful in a few months or so.