从Java运行python脚本并发送输入/输出

Question

I am writing a java program that will need to run a python script.
The script will print output which will java need to read to know the progress of the script.
To be able to pause the script while running I want it to ask for input once in a while, only when java give it input the script will keep going.
Here is my Java method:

private static void sevenTry(String[] strCommands) throws IOException {
    Object oLock1 = new Object();
    Object oLock2 = new Object();

    ProcessBuilder pBuilder = new ProcessBuilder(strCommands);
    pBuilder.redirectErrorStream(true);
    Process proc = pBuilder.start();

    Thread tReader = new Thread() {
        @Override
        public void run() {
            System.out.println("~~tReader starting~~");
            BufferedReader reader = new BufferedReader(new InputStreamReader(proc.getInputStream()));
            synchronized (oLock1) {
                try {
                    String line = reader.readLine();
                    while (line != null && !line.trim().equals("--EOF--")) {
                        System.out.println("Stdout: " + line);
                        if (line.trim().equals("--INPUT--")) {
                            synchronized (oLock2) {
                                oLock2.notify();
                            }
                            oLock1.wait();
                        }
                        line = reader.readLine();
                    }
                } catch (IOException e) {
                    System.out.println("tReader: " + e.getMessage());
                } catch (InterruptedException e) {
                    System.out.println("tReader: " + e.getMessage());
                } catch (Exception e) {
                    System.out.println("tReader: " + e.getMessage());
                }
            }
            System.out.println("~~tReader end~~");
            synchronized (oLock2) {
                oLock2.notify();
            }
        }
    };
    Thread tWriter = new Thread() {
        @Override
        public void run() {
            System.out.println("~~tWriter starting~~");
            BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(proc.getOutputStream()));

            String line, input;
            Scanner scan = new Scanner(System.in);

            synchronized (oLock2) {
                try {
                    oLock2.wait();
                } catch (InterruptedException e1) {
                    System.out.println("tWriter: " + e1.getMessage());
                }
            }
            while (tReader.isAlive()) {
                synchronized (oLock1) {
                    System.out.println("Java: insert input");
                    scan.hasNext();
                    input = scan.nextLine();
                    try {
                        writer.write(input + "\n");
                        writer.flush();
                    } catch (IOException e) {
                        System.out.println("tWriter: " + e.getMessage());
                    }
                    oLock1.notify();
                }
                try {
                    Thread.sleep(2000);
                } catch (InterruptedException e1) {
                    System.out.println("tWriter: " + e1.getMessage());
                }
            }
            System.out.println("~~tWriter end~~");
        }
    };

    tReader.start();
    tWriter.start();
    System.out.println("~~everything submitted~~");
    try {
        tReader.join();
        tWriter.join();
        System.out.println("~~finish~~");
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}

This is my python script:

# coding=utf-8
import sys

print '1'
print '--INPUT--'
inum = sys.stdin.readline()
print '2'
print '--EOF--'

I tried running my code

sevenTry("python", "C:\\Testing.py");

but on java side it get stuck inside tReader at line:

String line = reader.readLine();

The program does work if i take out the input line from the python file

inum = sys.stdin.readline()

Using

inum = raw_input()

still bring up the same problem (im using python 2.7)
The most confusing part here that i even tried to test this with a java file (instead of python)

sevenTry("java", "-classpath", "C:\\class", "CheckCMD");

and it worked even with the input lines

import java.util.Scanner;

public class CheckCMD {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        String line;
        System.out.println("1");
        System.out.println("--INPUT--");
        in.hasNext();
        line = in.nextLine();
        System.out.println("2");
        System.out.println("--EOF--");
    }
}

Answer 1

As you may have noticed, this is a problem related to Python.

As described in https://unix.stackexchange.com/questions/182537/write-python-stdout-to-file-immediately ,

" when process STDOUT is redirected to something other than a terminal, then the output is buffered into some OS-specific-sized buffer (perhaps 4k or 8k in many cases)."

So, you need to call sys.stdout.flush() after each invoke to print .

Or, as a better option, you can change the default behaviour for the process, using the -u param, to get unbuffered output.