Communication between Java and Python through stdout

Question

I have a Java code calling Python through an asynchronous process, and I want the Python process to listen to stdout from Java until something printed.

Following is the java code:

import java.io.*;

public class host {
    public static void main(String[] args) throws Exception{
        System.out.println("java starts at " + System.currentTimeMillis() + " ms");
        ProcessBuilder pb = new ProcessBuilder("python","a.py");
        Process tr = pb.start();
        System.out.println("First msg");
        System.out.println("Second msg");
        Thread.sleep(3000);
        System.out.println("x");
        System.out.println("java ends at " + System.currentTimeMillis() + " ms");
    }
}

and following is the python code:

import sys
import time
if __name__=="__main__":
    fo=open("a.txt","w")
    fo.write('python starts at: %.0f\n'%(time.time()*1000))
    line = sys.stdout.readline()
    while(line != "x\n"):
        fo.write(line+"\n")
        line = sys.stdout.readline()
    fo.write('python ends at: %.0f\n'%(time.time()*1000))
    fo.close()

However, the Python process seemed not be able to capture the stdout from Java. Since I am new to Java, I am not sure whether there is something fundamentally wrong or not, and whether the above model provides an effective way between Java and Python communication.

EDIT (UPDATE)

Based on the answers/comments, I modified my Java code as follows:

import java.io.*;

public class host {
    public static void main(String[] args) throws Exception{
        System.out.println("java starts at " + System.currentTimeMillis() + " ms");
        ProcessBuilder pb = new ProcessBuilder("python","a.py");
        Process tr = pb.start();
        PrintStream ps = new PrintStream(tr.getOutputStream());
        ps.println("First msg"); ps.flush();
        ps.println("Second msg"); ps.flush();
        Thread.sleep(3000);
        ps.println("x"); ps.flush();
        System.out.println("java ends at " + System.currentTimeMillis() + " ms");
    }
}

and changed python script as follows:

import sys
import time
if __name__=="__main__":
    fo=open("a.txt","w")
    fo.write('python starts at: %.0f\n'%(time.time()*1000))
    line = sys.stdin.readline()
    while(line != "x\n"):
        fo.write(line+"\n")
        line = sys.stdin.readline()
    fo.write('python ends at: %.0f\n'%(time.time()*1000))
    fo.close()

And following is the output:

python starts at: 1453263858103
First msg

First msg

Second msg

python ends at: 1453263863103

Have you tinkered with redirect? docs.oracle.com/javase/7/docs/api/java/lang/… — Heman Gandhi
– Heman Gandhi, Commented Jan 19, 2016 at 22:25
Your ProcessBuilder and the new runtime environment it creates when starting python has its own in, out and err streams. They are different from the Syste.in/out/err. Use the ProcessBuilder.redirectInput(). — JayC667
– JayC667, Commented Jan 19, 2016 at 22:26
@ JayC667 That's what I thought also, but how do I pass Java stdout to python? — Hailiang Zhang
– Hailiang Zhang, Commented Jan 19, 2016 at 22:27
See stackoverflow.com/questions/14165517/… as a simple example — JayC667
– JayC667, Commented Jan 19, 2016 at 22:29

rotten · Accepted Answer · 2016-01-19 22:43:39Z

2

The receiving process should listen to stdin, not stdout.

Then pipe the output from java to python:

java -jar myjava.jar | python mylistener

answered Jan 19, 2016 at 22:43

rotten

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Comments

njzk2 · Accepted Answer · 2016-01-19 22:47:47Z

1

In java, write to the process you created:

ProcessBuilder pb = new ProcessBuilder("python","a.py");
Process tr = pb.start();
tr.getOutputStream().println("First msg");

In python, read from stdin:

line = sys.stdin.readline()

answered Jan 19, 2016 at 22:47

njzk2

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6 Comments

Hailiang Zhang Over a year ago

I edit my script as you suggestions with minor modifications, but still have problem (see the question)

njzk2 Over a year ago

there isn't even the first and last lines? ("python starts/ends"?)

Hailiang Zhang Over a year ago

Sorry I missed that: what do you mean by "python starts/ends"?

njzk2 Over a year ago

those lines you write at the begining and the end of your file?

Hailiang Zhang Over a year ago

I had to kill python before the output text file is getting really huge.

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