CODEWITHSUNDEEP
javapythonsubprocesspopen
R.W
R.W

Reputation: 560

Call java program (with arguments) from Python and return results back to Python

I had to execute a java file(passing a single argument) from Python and after execution, return the result back to the script that called it. I wrote a little java test program that takes an argument and prints out some stuff to the stdout. Here is the java program

import java.util.*;
class TestX
{
       public static void main(String args[])
       {
            String someString = "Your input";
            System.out.println(someString + " " + args[0]);
            int resultOfSomething = 45;
            String someOutput = "Program's output is " + resultOfSomething;
            System.out.println(someOutput);
       }
}

And here is the Python script that calls the java program

import os.path,subprocess
from subprocess import STDOUT,PIPE

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]
    proc = subprocess.Popen(cmd, stdout=PIPE, stderr=STDOUT)
    stdout,stderr = proc.communicate(input='SomeInputstring')
    print ('This was "' + stdout + '"')

compile_java('TestX.java')
execute_java('TestX.java')

I should mention that I am not familiar with java. I compiled the little java program by this command javac TestX.java. That created a class file. And when I run the java program from the command line using java TestX SomeTestString the tiny java program works as expected and outputs some text to the console.

Now the problem; When I run the same java program from Python using the code above - 1) There is an error

This was "Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 0

        at TestX.main(TestX.java:7)
"

When I comment out the line print ('This was "' + stdout + '"') in the Python program, then the Python program runs and seems to execute the java program, but nothing is returned to the console. There is something extremely simple that I am missing here or it's just a silly mistake that I made. Any pointers? Thanks.

I tried a few different methods to output the result, like by using printf, ,and even using this class StdOut. All of them seem to behave the same.

If I were to explain this in a nutshell, it's this; My Python script should call a java program with a single argument. The java program takes in the input, performs certain functions and returns a result which I should be able to read from the Python program that called it.

Upvotes: 3

Views: 5702

Answers (2)

Allohvk
Allohvk

Reputation: 1374

For your requirement (execute a java file passing a single argument from Python and after execution, return the result back to the script that called it), Popen is not reccomended. This is because it is non-blocking. The code does not wait for the output to be returned. Please use the subprocess.run() command for Python > 3.7.

result = subprocess.run(
    ['java', '-cp', EXTERNAL_JAR_PATHS, 'MyJavaCode.java', ARGUMENT],
    capture_output = True,
    text = True,
    cwd = PATH_TO_CODE
)

result.stdout gives the program output and result.stderr gives information on errors if any.

Why this is better than debugging the Popen code:

  • Simpler and apt for your requirement. No need to deal with streams, deadlock issues etc
  • subprocess.run is the default approach reccomended for > 3.7 python version
  • Popen creates a new process and additional logic to ensure that the new process is terminated correctly etc need to be considered

Having said that in your case the immediate issue/error is correctly solved by Elliot. My approach addresses the larger requirement outlined in your last paragraph

Upvotes: 1

Elliott Frisch
Elliott Frisch

Reputation: 201537

Command line arguments are not stdin. args[0] is a command line argument. You should be able to do it like

cmd = ['java', java_class, 'SomeInputString']

Upvotes: 3

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