Question

I am writing a process that will run continuously, getting messages on a queue which cause it to call an executable and process its output. I am using Runtime.getRuntime().exec() to invoke it, but I am only getting output the first time. To more easily show the issue, I have put it in a loop. I am using ‘seq 1 5’ as a standin for the executable, which will print the numbers 1 to 5 each on a separate line. (I have run it on both Mac 10.7 and Fedora 14.)

    public static void main(String args[]) {
    for (int i = 0; i < 4; i++) {
        try {
            Process process = Runtime.getRuntime().exec("seq 1 5");
            BufferedReader reader = new BufferedReader(new InputStreamReader(process.getInputStream()));
            BufferedReader errReader = new BufferedReader(new InputStreamReader(process.getErrorStream()));
            while (reader.ready() || errReader.ready()) {
                if (reader.ready()) {
                    System.out.println("Process output: " + reader.readLine());
                }
                if (errReader.ready()) {
                    System.err.println("Process error: " + errReader.readLine());
                }
            }
            reader.close();
            errReader.close();
            int result = process.waitFor();
            if (result != 0) {
                System.err.println("Process returned with result " + 0);
            }
            System.out.println("Finished process for iteration " + i);
        } catch (IOException e) {
            e.printStackTrace();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

Here is the result – you can see the expected output of the process only occurs in the first loop:

Process output: 1
Process output: 2
Process output: 3
Process output: 4
Process output: 5
Finished process for iteration 0
Finished process for iteration 1
Finished process for iteration 2
Finished process for iteration 3

Occasionally, perhaps one out of 10 runs, one of the later iterations will print output as well – but never all of them. Obviously there must be something that doesn’t get cleaned up correctly, hence causing all but the first exec() to misbehave, but I don’t know what it could be. The input streams both get closed.

Answer 1

import java.io.*;
public class TP {
    public static void main(String... argv) {
        ProcessBuilder pb = new ProcessBuilder(argv);
        for (int i = 0; i < 4; i++) {
            try {
                Process process = pb.start();
                int result = process.waitFor();
                BufferedReader reader = new BufferedReader(new InputStreamReader(process.getInputStream()));
                BufferedReader errReader = new BufferedReader(new InputStreamReader(process.getErrorStream()));
                while (reader.ready() || errReader.ready()) {
                    if (reader.ready()) {
                        System.out.println("Process output: " + reader.readLine());
                    }
                    if (errReader.ready()) {
                        System.err.println("Process error: " + errReader.readLine());
                    }
                }
                reader.close();
                errReader.close();
                if (result != 0) {
                    System.err.println("Process returned with result " + result);
                }
                System.out.println("Finished process for iteration " + i);
            } catch (IOException e) {
                e.printStackTrace();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

    }
}