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

NB 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);
}