execute commands from another program from a python script

Question

Unable to communicate from a python script with java program. I have a java program that reads from standard input. Logic is:

public static void main(String[] args) {
  ...
  BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
  String cmd;
  boolean salir = faslse

  while (!salir) {
    cmd = in.readLine();
    JOptionPane.showMessageDialog(null, "run: " + cmd);
    //execute cmd
    ... 
    System.out.println(result);
    System.out.flush();
  }

}

I run the program through the console console

java -cp MyProgram.jar package.MyMainClass

And execute commands and get results, and shows the command executed in a dialogue (JOptionPane.showMessageDialog(null, "run: " + cmd); )

I need to call the program from python. Right now I'm trying with this:

#!/usr/bin/python
import subprocess

p = subprocess.Popen("java -cp MyProgram.jar package.MyMainClass", shell=True, stdout=subprocess.PIPE , stdin=subprocess.PIPE)
print '1- create ok'
p.stdin.write('comand parameter1 parameter2')
print '2- writeComand ok'
p.stdin.flush()
print '3- flush ok'
result = p.stdout.readline()  # this line spoils the script
print '4- readline ok'
print result
p.stdin.close()
p.stdout.close()
print 'end'

And the output is

1- create ok
2- writeComand ok
3- flush ok

And does not show the dialog.

however if I run:

#!/usr/bin/python
import subprocess

p = subprocess.Popen("java -cp MyProgram.jar package.MyMainClass", shell=True, stdout=subprocess.PIPE , stdin=subprocess.PIPE)
print '1- create ok'
p.stdin.write('comand parameter1 parameter2')
print '2- writeComand ok'
p.stdin.flush()
print '3- flush ok'
p.stdin.close()
p.stdout.close()
print 'end'

the output is

1- create ok
2- writeComand ok
3- flush ok
end

and show show the dialog.

the line p.stdout.readline() spoils the script, as I can fix this?

Thank you very much any help.

Alfe · Accepted Answer · 2013-05-09 00:26:52Z

1

Flush your System.out after printing just one result.

Additionally change your code to do this:

p = subprocess.Popen("java -cp MyProgram.jar package.MyMainClass",
    shell=True, stdout=subprocess.PIPE, stdin=subprocess.PIPE)
p.stdin.write(command1)
p.stdin.flush()  # this should trigger the processing in the Java process
result = p.stdout.readline()  # this only proceeds if the Java process flushes
p.stdin.write(command2)
p.stdin.flush()
result = p.stdout.readline()
# and afterwards:
p.stdin.close()
p.stdout.close()

edited May 9, 2013 at 0:26

answered May 8, 2013 at 23:30

Alfe

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13 Comments

user60108 Over a year ago

in the java code? The java code works fine. I run it from the console and get the expected results. The problem is with the python script. thanks

Alfe Over a year ago

Yes, in the Java code. When writing to a terminal, different buffering is applied by all processes. When writing to a pipe (instead of a tty), the Java process assumes that this is a bulk data connection and flushes only when the buffer is exhausted. Insert that flushing and try again.

user60108 Over a year ago

I tried that and not make it. The problem is with the python code. I can not run the commands in the java interpreter. thanks

Alfe Over a year ago

Aww, come on. That flushing is definitely necessary, believe me. Been there, done that hundreds of times. And it cannot do any harm. I'll have a look into that communicate again to see what we should use instead, just a moment.

Alfe Over a year ago

I changed my answer. Give it a try :-)

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