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Calling Java app with “subprocess” from Python and reading the Java app output

Basically what I am looking for is, I want to interact with java program while its running using python so I can access its output and pass input to it. I have managed to run a Java program using python. I want to know can i access the outputs of java program in my python program. For example. In java program: System.out.println("Enter no."); In python i should be able to get "Enter no" as string and also pass value to java program from python.a

What I managed to do till no : Python program : 

import sys
import os.path,subprocess

def compile_java(java_file):
    subprocess.check_call(['javac', java_file])

def execute_java(java_file):
    java_class,ext = os.path.splitext(java_file)
    cmd = ['java', java_class]
    subprocess.call(cmd, shell=False)            

def run_java(java_file):
    compile_java(java_file)
    execute_java(java_file)

Java Program :

import java.io.*;
import java.util.*;
class Hi
{
        public static void main(String args[])throws IOException
        {
        Scanner t=new Scanner(System.in);

        System.out.println("Enter any integer");
        int str1=t.nextInt();           
        System.out.println("You entered"+str1);     

        }
}

Thanx :)

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  • If you want to pass input to the program, you're probably going to need to use subprocess.Popen. Commented Feb 19, 2012 at 19:38
  • 1
    See: stackoverflow.com/questions/2388423/… Commented Feb 19, 2012 at 19:42

2 Answers 2

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1

If all you need is to get the output from a non-interactive execution of your Java program, use subprocess.check_output instead of subprocess.call.

http://docs.python.org/library/subprocess.html

You need Python 2.7 or newer for check_output to be available.

If you need to interact with the Java program, you can do so using Popen.communicate, where you can read the process's output and send stuff to its input using file descriptors.

You can also use the pexpect python library to automate this kind of interaction, pexpect abstracts a lot of the legwork involved in using Popen.communicate.

Note that these techniques apply for any kind of executable you need your Python program to interact with, not just Java; as long as it uses stdin and stdout, using these calls should work for you.

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Thanx for the quick reply :) I gave a try to pexpect and really liked it. Just trying to find some good tutorials on it.
1

The easiest way would be to use Jython, which is a complete Python implementation that runs in the JVM, and can interact with native Java code. But if you want to use CPython, and generally continue down the path you've sketched out above, you'll want to create a live Python Popen object that you can interact with. For example:

import sys
import os.path,subprocess

def compile_java(java_file):
    subprocess.check_call(['javac', java_file])

def execute_java(java_file):
    java_class,ext = os.path.splitext(java_file)
    cmd = ['java', java_class]
    return subprocess.Popen(cmd, shell=True, stdin=subprocess.PIPE, stdout=subprocess.PIPE)

def run_java(java_file):
    compile_java(java_file)
    process = execute_java(java_file)
    for i in range(10):
        process.stdin.write(str(i) + "\n")
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Thanx for the such a quick reply :) I am trying your advice and will post about progress :)

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