java - Make images fetched from server display in real time

Question

I have many images that get fetched via a series of threaded HTTP network calls. I'm using Callables and Futures to manage this process. As each image comes back from the server, I want to display it on a JPanel without waiting for the other images to return.

This code works, but the UI is not updated until ALL of the images come back:

private void loadAndDisplayImages() throws InterruptedException, ExecutionException {      

    final List<Callable<Image>> partitions = new ArrayList<Callable<Image>>();

    for(final MediaFeedData data : imagesList) {
        partitions.add(new Callable<Image>() {
            public Image call() throws Exception {
                    String url = data.getImageUrl();
                    return ImageDisplayer.displayImageFromUrl(url, imageSize);
                }
            }        
        });
    }

    // for testing, use only a single thread to slow down rendering
    final ExecutorService executorPool = Executors.newFixedThreadPool(1); //numImages);    

    // run each callable, capture the results in a list of futures
    final List<Future<Image>> futureImages = 
            executorPool.invokeAll(partitions, 10000, TimeUnit.SECONDS);

    for(final Future<Image> img : futureImages) {
        Image image = img.get(); // this will block the UI

        final ImageButton imageButton = new ImageButton(image, imageSize);

        SwingUtilities.invokeLater(new Runnable(){
            @Override public void run() {
                imagesPanel.add(imageButton);
                frame.validate();
                frame.setVisible(true);
            }
        });
    }

    executorPool.shutdown();
}

Answer 1

Consider using a SwingWorker in combination with an ExecutorService...

SwingWorker...

    public class ImageLoaderWorker extends SwingWorker<Image, Image> {

        private File source;
        private JPanel container;

        public ImageLoaderWorker(File source, JPanel container) {
            this.source = source;
            this.container = container;
        }

        @Override
        protected Image doInBackground() throws Exception {
            return ImageIO.read(source);
        }

        @Override
        protected void done() {
            try {
                Image img = get();
                JLabel label = new JLabel(new ImageIcon(img));
                container.add(label);
                container.revalidate();
                container.repaint();
            } catch (InterruptedException | ExecutionException ex) {
                ex.printStackTrace();
            }
        }

    }

ExecutorService...

ExecutorService executor = Executors.newFixedThreadPool(4);
File images[] = new File("...").listFiles(new FileFilter() {
    @Override
    public boolean accept(File pathname) {
        String name = pathname.getName().toLowerCase();
        return name.endsWith(".jpg") || name.endsWith(".png");
    }
});

for (File img : images) {

    executor.submit(new ImageLoaderWorker(img, this));

}

Runnable Example...

This simply scans a directory and loads the images, but the concept is basically the same...

import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.GridLayout;
import java.awt.Image;
import java.awt.Rectangle;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.FileFilter;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import javax.imageio.ImageIO;
import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.Scrollable;
import javax.swing.SwingWorker;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class TestImageLoader {

    public static void main(String[] args) {
        new TestImageLoader();
    }

    public TestImageLoader() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                    ex.printStackTrace();
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(new JScrollPane(new TestPane()));
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel implements Scrollable {

        public TestPane() {
            setLayout(new GridLayout(0, 4));
            ExecutorService executor = Executors.newFixedThreadPool(4);
            File images[] = new File("...").listFiles(new FileFilter() {
                @Override
                public boolean accept(File pathname) {
                    String name = pathname.getName().toLowerCase();
                    return name.endsWith(".jpg") || name.endsWith(".png");
                }
            });

            for (File img : images) {

                executor.submit(new ImageLoaderWorker(img, this));

            }
        }

        @Override
        public Dimension getPreferredScrollableViewportSize() {
            return new Dimension(600, 600);
        }

        @Override
        public int getScrollableUnitIncrement(Rectangle visibleRect, int orientation, int direction) {
            return 128;
        }

        @Override
        public int getScrollableBlockIncrement(Rectangle visibleRect, int orientation, int direction) {
            return 128;
        }

        @Override
        public boolean getScrollableTracksViewportWidth() {
            return false;
        }

        @Override
        public boolean getScrollableTracksViewportHeight() {
            return false;
        }

    }

    public class ImageLoaderWorker extends SwingWorker<Image, Image> {

        private File source;
        private JPanel container;

        public ImageLoaderWorker(File source, JPanel container) {
            this.source = source;
            this.container = container;
        }

        @Override
        protected Image doInBackground() throws Exception {
            return ImageIO.read(source);
        }

        @Override
        protected void done() {
            try {
                Image img = get();
                JLabel label = new JLabel(new ImageIcon(img));
                container.add(label);
                container.revalidate();
                container.repaint();
            } catch (InterruptedException | ExecutionException ex) {
                ex.printStackTrace();
            }
        }

    }

}