Reading InputStream from a batch file's process skips to next line

Question

I am trying to run a batch file with Runtime.exec() and then output its InputStream into a JTextArea. What I have works, but only partially. What happens is the batch file runs, but if it executes a command other than something like "echo" that command immediately terminates and the next line executes. For example, let's say I try to run a simple batch file like this:

@echo off
echo hello. waiting 5 seconds.
timeout /t 5 /nobreak > NUL
echo finished. goodbye.

The batch file executes, and the JTextArea says

hello. waiting 5 seconds.
finished. goodbye.

but it doesn't wait for 5 seconds in the middle.

I can't figure out why it's doing this. Here's what I use to run the batch file and read its InputStream.

private class ScriptRunner implements Runnable {
private final GUI.InfoGUI gui; // the name of my GUI class
private final String script;

public ScriptRunner(final GUI.InfoGUI gui, final File script) {
    this.gui = gui;
    this.script = script.getAbsolutePath();
}

@Override
public void run() {
    try {
        final Process p = Runtime.getRuntime().exec(script);
        StreamReader output = new StreamReader(p.getInputStream(), gui);
        Thread t = new Thread(output);
        t.start();
        int exit = p.waitFor();
        output.setComplete(true);
        while (t.isAlive()) {
            sleep(500);
        }
        System.out.println("Processed finished with exit code " + exit);
    } catch (final Exception e) {
        e.printStackTrace();
    }
}

}
private class StreamReader implements Runnable {
private final InputStream is;
private final GUI.InfoGUI gui;

private boolean complete = false;

public StreamReader(InputStream is, GUI.InfoGUI gui) {
    this.is = is;
    this.gui = gui;
}

@Override
public void run() {
    BufferedReader in = new BufferedReader(new InputStreamReader(is));
    try {
        while (!complete || in.ready()) {
            while (in.ready()) {
                gui.setTextAreaText(in.readLine() + "\n");
            }
            sleep(250);
        }
    } catch (final Exception e) {
        e.printStackTrace();
    } finally {
            try {
                in.close();
        } catch (final Exception e) {
                e.printStackTrace();
    }
}

public void setComplete(final boolean complete) {
    this.complete = complete;
}

}
public void sleep(final long ms) {
try {
    Thread.sleep(ms);
} catch (final InterruptedException ie) {
}
}

I know my code is pretty messy, and I'm sure it contains grammatical errors.

Thanks for anything you can do to help!

Answer 1

You're creating a Process but you're not reading from its standard error stream. The process might be writing messages to its standard error to tell you that there's a problem, but if you're not reading its standard error, you won't be able to read these messages.

You have two options here:

1. Since you already have a class that reads from a stream (StreamReader), wire up another one of these to the process's standard error stream (p.getErrorStream()) and run it in another Thread. You'll also need to call setComplete on the error StreamReader when the call to p.waitFor() returns, and wait for the Thread running it to die.

2. Replace your use of Runtime.getRuntime().exec() with a ProcessBuilder. This class is new in Java 5 and provides an alternative way to run external processes. In my opinion its most significant improvement over Runtime.getRuntime().exec() is the ability to redirect the process's standard error into its standard output, so you only have one stream to read from.

I would strongly recommend going for the second option and choosing to redirect the process's standard error into its standard output.

I took your code and replaced the line

        final Process p = Runtime.getRuntime().exec(script);

with

        final ProcessBuilder pb = new ProcessBuilder(script);
        pb.redirectErrorStream(true);
        final Process p = pb.start();

Also, I don't have your GUI code to hand, so I wrote the output of the process to System.out instead.

When I ran your code, I got the following output:

hello. waiting 5 seconds.
ERROR: Input redirection is not supported, exiting the process immediately.
finished. goodbye.
Processed finished with exit code 0

Had you seen that error message, you might have twigged that something was up with the timeout command.

Incidentally, I noticed in one of your comments that none of the commands suggested by ughzan worked. I replaced the timeout line with ping -n 5 127.0.0.1 > NUL and the script ran as expected. I couldn't reproduce a problem with this.