Running a Python program in Java using Jython

Question

I wrote a Python program that consists out of five .py script files. I want to execute the main of those python scripts from within a Java Application.

What are my options to do so? Using the PythonInterpreter doesn't work, as for example the datetime module can't be loaded from Jython (and I don't want the user to determine his Python path for those dependencies to work).

I compiled the whole folder to .class files using Jython's compileall. Can I embed these .class files somehow to execute the main file from within my Java Application, or how should I proceed?

Answer 1

Have a look at the ProcessBuilder class in java: https://docs.oracle.com/javase/7/docs/api/java/lang/ProcessBuilder.html.

The command used in the java constructor should be the same as what you would type in a command line. For example:

Process p = new ProcessBuilder("python", "myScript.py", "firstargument").start();

(the process builder does the same thing as the python subprocess module).

Have a look at running scripts through processbuilder

N.B. as for the Jython part of the question, if you go to the jython website (have a look at the FAQ section of their website www.jython.org). Check the entry "use jython from java".

Answer 2

I'm also interested in running Python code directly within Java, using Jython, and avoiding the need for an installed Python interpreter.

The article, 'Embedding Jython in Java Applications' explains how to reference an external *.py Python script, and pass it argument parameters, no installed Python interpreter necessary:

#pymodule.py - make this file accessible to your Java code
def square(value):
return value*value

This function can then be executed either by creating a string that executes it, or by retrieving a pointer to the function and calling its call method with the correct parameters:

//Java code implementing Jython and calling pymodule.py
import org.python.util.PythonInterpreter;
import org.python.core.*;

public class ImportExample {
   public static void main(String [] args) throws PyException
   {
       PythonInterpreter pi = new PythonInterpreter();
       pi.exec("from pymodule import square");
       pi.set("integer", new PyInteger(42));
       pi.exec("result = square(integer)");
       pi.exec("print(result)");
       PyInteger result = (PyInteger)pi.get("result");
       System.out.println("result: "+ result.asInt());
       PyFunction pf = (PyFunction)pi.get("square");
       System.out.println(pf.__call__(new PyInteger(5)));
   }
}

• Jython's Maven/Gradle/etc dependency strings can be found at http://mvnrepository.com/artifact/org.python/jython-standalone/2.7.1

• Jython JavaDoc

Answer 3

It is possible to load the other modules. You just need to specify the python path where your custom modules can be found. See the following test case and I am using the Python datatime/math modules inside my calling function (my_maths()) and I have multiple python files in the python.path which are imported by the main.py

@Test
public void testJython() {

    Properties properties = System.getProperties();
    properties.put("python.path", ".\\src\\test\\resources");
    PythonInterpreter.initialize(System.getProperties(), properties, new String[0]);

    PythonInterpreter interpreter = new PythonInterpreter();
    interpreter.execfile(".\\src\\test\\resources\\main.py");

    interpreter.set("id", 150); //set variable value
    interpreter.exec("val = my_maths(id)"); //the calling function in main.py

    Integer returnVal = (Integer) interpreter.eval("val").__tojava__(Integer.class);
    System.out.println("return from python: " + returnVal);
}