java: how to both read and write to & from process through pipe (stdin/stdout)

Question

(i'm new to java) I need to start a process and receive 2 or 3 handles: for STDIN, STDOUT, (and STDERR), so I can write input to the process and receive its output, the same way command line pipes behave (e.g. "grep")

in Python this is acheived with the following code:

from subprocess import Popen, PIPE
p = Popen(cmd, shell=True, stdin=PIPE, stdout=PIPE)
(child_stdin, child_stdout) = (p.stdin, p.stdout)
child_stdin.write('Yoram Opposum\n')
child_stdin.flush()
child_stdout.readlines()

What's the Java equivalent??

I've tried so far

Process p = Runtime.getRuntime().exec(cmd);
BufferedReader inp = new BufferedReader( new InputStreamReader(p.getInputStream()) );
BufferedWriter out = new BufferedWriter( new OutputStreamWriter(p.getOutputStream()) );
out.write( "Some Text!\n\n" );
out.flush();
line = inp.readLine();
print("response1: " + line );   // that's ok
out.write( "Second Line...\n" );
out.flush();
line = inp.readLine();
print("response2: " + line );    // returns an empty string, if it returns,,,
inp.close();
out.close();

BTW the first try works only with \n\n, but doesn't work with single \n (why?)

the following code works, but all input is given in advance, not the behavior i'm looking for:

out.write( "Aaaaa\nBbbbbb\nCcccc\n" );
out.flush();
line = inp.readLine();
print("response1: " + line );
line = inp.readLine();
print("response2: " + line );
line = inp.readLine();
print("response3: " + line );
line = inp.readLine();
print("response4: " + line );

output:

response1: AAAAA
response2: 
response3: bbbbbb
response4:

the process being run looks like that:

s = sys.stdin.readline()
print s.upper()
s = sys.stdin.readline()
print s.lower()

If you could give a short but complete example for both processes, that would really help. Basically a way to reproduce the behaviour. — Jon Skeet
– Jon Skeet, Commented Nov 6, 2010 at 13:24

Berry Tsakala · Accepted Answer · 2010-11-06 20:45:58Z

12

ok, it was also my python's code fault, but opposite to @Jon's answer, there was an EXTRA newline (0xA0 to be exact, which isn't Windows' standard).

once i'm strip()ing the extra 0xA0 from the line i get from Java, python adds a single "normal" \n to Java on the way back, and things run smoothly.

for the completeness of the question and answer, here's a working Java code:

import java.io.*;
import java.util.*;

public class Main {

    public static BufferedReader inp;
    public static BufferedWriter out;

    public static void print(String s) {
    System.out.println(s);
    }

    public static String pipe(String msg) {
    String ret;

    try {
        out.write( msg + "\n" );
        out.flush();
        ret = inp.readLine();
        return ret;
    }
    catch (Exception err) {

    }
    return "";
    }



    public static void main(String[] args) {

    String s;
    String cmd = "c:\\programs\\python\\python.exe d:\\a.py";

    try {

        print(cmd);
        print(System.getProperty("user.dir"));
        Process p = Runtime.getRuntime().exec(cmd);

        inp = new BufferedReader( new InputStreamReader(p.getInputStream()) );
        out = new BufferedWriter( new OutputStreamWriter(p.getOutputStream()) );

        print( pipe("AAAaaa") );
        print( pipe("RoteM") );

        pipe("quit")
        inp.close();
        out.close();
    }

    catch (Exception err) {
        err.printStackTrace();
    }
    }
}

and this is the python code

import sys
s = sys.stdin.readline().strip()
while s not in ['break', 'quit']:
    sys.stdout.write(s.upper() + '\n')
    sys.stdout.flush()
    s = sys.stdin.readline().strip()

answered Nov 6, 2010 at 20:45

Berry Tsakala

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4 Comments

Jon Skeet Over a year ago

Ah, glad you've worked it out :) Hope our incorrect answers were still useful along the way...

Johnydep Over a year ago

@BerryTsakala, well it works but what if the returned String is spanning over multiple lines, then there seem to be problem. Also if there is wrong input, you would need to have errorStream as well. I am trying to use your idea but having some problem with different programs, i guess should use threads here, any idea??

user207421 Over a year ago

There is nothing in your Java code that sends 0xA0, and 0xA0 is not a line terminator. 0x0A is a line feed, if that's what you mean.

Samantra Over a year ago

To make the Python code a bit cleaner: duplicating s = sys.stdin.readline().strip() can be replaced by a single call using the walrus operator within the loop condition: while (s := sys.stdin.readline().strip()) not in ['break', 'quit']:.

Jon Skeet · Accepted Answer · 2010-11-06 10:10:10Z

3

I believe the problem is in the process you're calling:

s = sys.stdin.readline()
print s.upper()
s = sys.stdin.readline()
print s.lower()

I suspect that readline is going to read the line but s will not include the line terminator. You're then printing that line, but without a line terminator... Java is then blocking until it reads a line terminator, which will block forever as the process isn't giving one.

This is all a bit of a guess as it's not exactly clear to me what language your called process is in - if print actually does output a line terminator, then it's an incorrect guess. However, if not, you may need to change it to something like:

s = sys.stdin.readline()
println s.upper()
s = sys.stdin.readline()
println s.lower()

EDIT: That doesn't explain the blank lines in sample output... no idea what's going on, really, but unfortunately I can't look into it now.

edited Nov 6, 2010 at 10:10

answered Nov 6, 2010 at 9:01

Jon Skeet

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9 Comments

Sean Reilly Over a year ago

Would the second block of code (that is reported as working), work if that was the case? It doesn't look like it does anything different with line endings.

Jon Skeet Over a year ago

@Sean: The second block of code is initially sending three line feeds, so the process will terminate after writing "AaaaaBbbbbb". At that point, readLine() will see the end of the input stream, and count the data so far as a final line. Note that this also explains why "\n\n" works in the first case, but then returns an empty second string.

Sean Reilly Over a year ago

Wouldn't that cause the working example to output AAAAbbbb on a single line, if there were no line breaks being printed?

Jon Skeet Over a year ago

@Sean: Hmm, yes, you're right. No idea how that's happening. Unfortunately I can't look into it now.

Berry Tsakala Over a year ago

Python's "print" statement adds a newline automatically, and is roughly equivalent to Java's println

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