Running the program in GUI is slow

Question

I'm creating a program to scrape a website of images and place those image file names and properties on a list, the problem is, whenever I try to fetch the data from the URL using GUI, the program takes about 20-30 seconds to display the information on my table model, but when I run it without the GUI (just the console and simple system out println) it takes only 2-4 seconds, even faster at some point. here is my GUI code:

public class ImageDownloader extends JFrame {

private JPanel contentPane;
private JTextField urlTextField;
private JButton btnCheck;
private JButton btnDownload;
private JButton btnDownloadAll;
private JTable table;

private String imgUrl;
private String url;

Document document;
Elements media;

public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
        public void run() {
            try {
                ImageDownloader frame = new ImageDownloader();
                frame.setVisible(true);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    });
}

public ImageDownloader() {
    setTitle("Image Downloader");
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setBounds(100, 100, 565, 300);
    contentPane = new JPanel();
    contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
    contentPane.setLayout(new BorderLayout(0, 0));
    setContentPane(contentPane);

    JPanel panel = new JPanel();
    FlowLayout flowLayout = (FlowLayout) panel.getLayout();
    flowLayout.setAlignment(FlowLayout.LEFT);
    contentPane.add(panel, BorderLayout.NORTH);

    JLabel lblWebsiteUrl = new JLabel("Website URL:");
    panel.add(lblWebsiteUrl);

    urlTextField = new JTextField();
    panel.add(urlTextField);
    urlTextField.setColumns(30);

    btnCheck = new JButton("Check");
    btnCheck.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            List images = new ArrayList<>();

            url = urlTextField.getText();

            if(url.isEmpty()) {
                JOptionPane.showMessageDialog(ImageDownloader.this, "Please enter a website URL", "Input Error", JOptionPane.ERROR_MESSAGE);
            } else {

                try {
                    document = Jsoup.connect(urlTextField.getText()).userAgent("Mozilla").timeout(10 * 1000).get();

                    media = document.select("[src]");

                    for(Element src : media) {
                        if(src.tagName().equals("img")) {
                            imgUrl = src.attr("abs:src");
                            URL url = new URL(imgUrl);
                            long size = url.openConnection().getContentLengthLong();
                            images.add(new Images(src.tagName(), src.attr("abs:src"), src.attr("width"), src.attr("height"), size));
                        }
                    }

                    ImageDownloaderTableModel tableModel = new ImageDownloaderTableModel(images);

                    table.setModel(tableModel);
                } catch (IOException e1) {
                    JOptionPane.showMessageDialog(ImageDownloader.this, "Error loading website, The site that you are trying to reach is either down or does not exist..", "Error Loading", JOptionPane.ERROR_MESSAGE);
                    e1.printStackTrace();
                }
            }
        }
    });
    panel.add(btnCheck);

    JPanel panel_1 = new JPanel();
    contentPane.add(panel_1, BorderLayout.SOUTH);

    btnDownloadAll = new JButton("Download All");
    btnDownloadAll.addActionListener(new ActionListener() {

        public void actionPerformed(ActionEvent arg0) {
            try {
                media = document.select("img");

                for(Element src : media) {
                    String strImgUrl = src.attr("abs:src");
                    downloadImage(strImgUrl);
                }
            } catch(Exception ex) {
                ex.printStackTrace();
            }
        }           
    });
    panel_1.add(btnDownloadAll);

    btnDownload = new JButton("Download");
    panel_1.add(btnDownload);

    JScrollPane scrollPane = new JScrollPane();
    contentPane.add(scrollPane, BorderLayout.CENTER);

    table = new JTable();
    scrollPane.setViewportView(table);
}

public static void downloadImage(String imgUrl) {
    String strImgUrl = imgUrl.substring(imgUrl.lastIndexOf("/") + 1);

    try {
        URL urlImage = new URL(imgUrl);
        InputStream in = urlImage.openStream();

        byte[] buffer = new byte[4096];
        int n = -1;

        OutputStream os = new FileOutputStream(strImgUrl);

        while((n = in.read(buffer)) != -1) {
            os.write(buffer, 0, n);
        }

        os.close();

        System.out.println("Saved..");
    } catch(IOException ex) {
        ex.printStackTrace();
    }
}
}

TT. · Accepted Answer

The documentation of the SwingWorker class has an excellent recap of your situation:

Time-consuming tasks should not be run on the Event Dispatch Thread. Otherwise the application becomes unresponsive.

In your case downloading one or more files would be that time-consuming task, and you are downloading from the Event Dispatch Thread (aka EDT).

The SwingWorker class provides a solution to your problem:

SwingWorker is designed for situations where you need to have a long running task run in a background thread and provide updates to the UI either when done, or while processing.

I would also argue that action listeners (or actions) shouldn't contain long pieces of code. It is better to wrap that in a method and call the method from the action listener (action).

And in cases where you are opening a modal dialog (you're doing that with JOptionPane.showMessageDialog) I would invokeLater that method so that all pending UI messages get handled before you open that new dialog. That is general advice that will side-step other UI related problems (focus related problems come to mind).

If invokeLater is called from the event dispatching thread -- for example, from a JButton's ActionListener -- the doRun.run() will still be deferred until all pending events have been processed.

Running the program in GUI is slow

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