How to make a java program to print both out.println() and err.println() statements?

Question

I have written the java code below, which executes another java program named "Newsworthy_RB".

Newsworthy_RB.java contains both the System.out.printlln() and System.err.println() statements.

I want both the outputs to be printed in the command prompt console.

What has to be done inorder to obtain the same.

The below program just prints the out.println() statements and not the err.println() statements.

Kindly let me know whether the code below will function as i expect?

command = "java -cp .:../sqljdbc.jar SetHash Newsworthy_RB";
Process child1 = Runtime.getRuntime().exec(command);
InputStream in1 = child1.getErrorStream();
InputStream in2 = child2.getInputStream();
while ((c = in1.read()) != -1 || (c = in2.read()) != -1) {
        System.out.print((char)c);
    }

Answer 1

First of all, the preferred way of starting external programs is through ProcessBuilder. It is even mentioned in the docs for Runtime:

ProcessBuilder.start() is now the preferred way to start a process with a modified environment.

In ProcessBuilder you have a very convenient method called redirectErrorStream:

Sets this process builder's redirectErrorStream property.

If this property is true, then any error output generated by subprocesses subsequently started by this object's start() method will be merged with the standard output, so that both can be read using the Process.getInputStream() method. This makes it easier to correlate error messages with the corresponding output. The initial value is false.

A complete example of how to output both standard error and standard out:

import java.io.*;

public class Test {
    public static void main(String... args) throws IOException {

        ProcessBuilder pb =
                new ProcessBuilder("java", "-cp", "yourClassPath", "HelloWorld");

        pb.redirectErrorStream(true);
        Process proc = pb.start();

        Reader reader = new InputStreamReader(proc.getInputStream());
        int ch;
        while ((ch = reader.read()) != -1)
            System.out.print((char) ch);
        reader.close();
    }
}

---

Response to your update: No, the code with

while ((c = in1.read()) != -1 || (c = in2.read()) != -1)

will not work, since read() is a blocking method and you only have one thread. Your only option is to use one thread per input-stream, or, (preferrably) merge the two input-streams into one, using ProcessBuilder.redirectErrorStream.

Answer 2

You need to pipe the output of both streams in a separate threads. Example code from here:

Process p = Runtime.getRuntime().exec(cmd.array());
copyInThread(p.getInputStream(), System.out);
copyInThread(p.getErrorStream(), System.err);
p.waitFor();
return p.exitValue();

private void copyInThread(final InputStream in, final OutputStream out) {
    new Thread() {
        public void run() {
            try {
                while (true) {
                    int x = in.read();
                    if (x < 0) {
                        return;
                    }
                    if (out != null) {
                        out.write(x);
                    }
                }
            } catch (Exception e) {
                throw new RuntimeException(e);
            }
        }
    } .start();
}