Display indeterminate JProgressBar while batch file runs

Question

I've been browsing SO and google for a while now for an answer to this question, but I can't seem to find one that really works. I'll start from the beginning:

I created a Java class with a method that runs a batch file in the background (the command window does not appear). The program works great, except that it would be a little confusing to the end user, since the batch file takes a while to complete--the user will not know if the program is still running or not. After the batch script finishes executing, a message dialog appears saying it's finished, but for the period of time between when the method is run and the dialog appears, it looks as if the program is doing nothing.

So here's my question: I would very much like to display a new frame with a text area that shows the output of the batch file. However, I understand that this is very difficult to do without creating temporary files, writing to them, reading from them, and so on. I would rather avoid that if possible. Therefore, I have decided it might be better to display an indeterminate JProgressBar while the process is running, and close it when the process is finished. Unfortunately, I don't think Swing can handle this since it would require running multiple processes at once. I have heard of a SwingWorker but am not exactly sure how that would be used in this case. I have the following SSCCE, which works, but does not have the progress bar implemented.

public myClass(){
    public static void main(String[] args){
        String[] commands = {"cmd.exe", "/C", "C:\\users\\....\\myBat.bat"};
        Process p = Runtime.getRuntime().exec(commands);
        p.waitFor()
        JOptionPane.showMessageDialog(null, "Process finished!");
    }
}

While p.waitFor() waits for the process, there is nothing on the screen. I just want something showing the user that a process is still running. Thoughts? Thanks!

Answer 1

You can run a ProcessBuilder in the background of a SwingWorker, as shown below, to get both output and a progress bar.

import java.awt.BorderLayout;
import java.awt.EventQueue;
import java.awt.event.*;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import javax.swing.*;

/**
 * @se http://stackoverflow.com/a/20603012/230513
 * @see http://stackoverflow.com/a/17763395/230513
 */
public class SwingWorkerExample {

    private final JLabel statusLabel = new JLabel("Status: ", JLabel.CENTER);
    private final JTextArea textArea = new JTextArea(20, 20);
    private JButton startButton = new JButton("Start");
    private JButton stopButton = new JButton("Stop");
    private JProgressBar bar = new JProgressBar();
    private BackgroundTask backgroundTask;
    private final ActionListener buttonActions = new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent ae) {
            JButton source = (JButton) ae.getSource();
            if (source == startButton) {
                textArea.setText(null);
                startButton.setEnabled(false);
                stopButton.setEnabled(true);
                backgroundTask = new BackgroundTask();
                backgroundTask.execute();
                bar.setIndeterminate(true);
            } else if (source == stopButton) {
                backgroundTask.cancel(true);
                backgroundTask.done();
            }
        }
    };

    private void displayGUI() {
        JFrame frame = new JFrame("Swing Worker Example");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);

        JPanel panel = new JPanel();
        panel.setBorder(
            BorderFactory.createEmptyBorder(5, 5, 5, 5));
        panel.setLayout(new BorderLayout(5, 5));

        JScrollPane sp = new JScrollPane();
        sp.setBorder(BorderFactory.createTitledBorder("Output: "));
        sp.setViewportView(textArea);

        startButton.addActionListener(buttonActions);
        stopButton.setEnabled(false);
        stopButton.addActionListener(buttonActions);
        JPanel buttonPanel = new JPanel();
        buttonPanel.add(startButton);
        buttonPanel.add(stopButton);
        buttonPanel.add(bar);

        panel.add(statusLabel, BorderLayout.PAGE_START);
        panel.add(sp, BorderLayout.CENTER);
        panel.add(buttonPanel, BorderLayout.PAGE_END);

        frame.setContentPane(panel);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }

    private class BackgroundTask extends SwingWorker<Integer, String> {

        private int status;

        public BackgroundTask() {
            statusLabel.setText((this.getState()).toString());
        }

        @Override
        protected Integer doInBackground() {
            try {
                ProcessBuilder pb = new ProcessBuilder("ls", "-lR", "/");
                pb.redirectErrorStream(true);
                Process p = pb.start();
                String s;
                BufferedReader stdout = new BufferedReader(
                    new InputStreamReader(p.getInputStream()));
                while ((s = stdout.readLine()) != null && !isCancelled()) {
                    publish(s);
                }
                if (!isCancelled()) {
                    status = p.waitFor();
                }
                p.getInputStream().close();
                p.getOutputStream().close();
                p.getErrorStream().close();
                p.destroy();
            } catch (IOException | InterruptedException ex) {
                ex.printStackTrace(System.err);
            }
            return status;
        }

        @Override
        protected void process(java.util.List<String> messages) {
            statusLabel.setText((this.getState()).toString());
            for (String message : messages) {
                textArea.append(message + "\n");
            }
        }

        @Override
        protected void done() {
            statusLabel.setText((this.getState()).toString() + " " + status);
            stopButton.setEnabled(false);
            startButton.setEnabled(true);
            bar.setIndeterminate(false);
        }

    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                new SwingWorkerExample().displayGUI();
            }
        });
    }
}